WO2014049015A1

WO2014049015A1 - Method for the recovery of natural gas and natural gas condensate from subterranean gas condensate reservoirs

Info

Publication number: WO2014049015A1
Application number: PCT/EP2013/070007
Authority: WO
Inventors: Vladimir Stehle
Original assignee: Wintershall Holding GmbH
Priority date: 2012-09-27
Filing date: 2013-09-25
Publication date: 2014-04-03
Also published as: EP2900792A1; CA2882932A1; EA201590646A1; US20150240608A1

Abstract

The invention relates to a method for the recovery of natural gas and/or natural gas condensate from a subterranean gas condensate reservoir, which contains a gas mixture having retrograde condensation behavior, said method comprising at least the following method steps: a) sinking at least one production bore into the subterranean gas condensate reservoir and recovering natural gas and/or natural gas condensate from the subterranean production well by means of the at least one production bore; b) injecting a solution (L), which comprises a solvent and urea, through the at least one production bore into the subterranean gas condensate reservoir; c) inserting an idle phase, in which the urea contained in the solution (L) is hydrolyzed; d) recovering natural gas and/or natural gas condensate from the subterranean gas condensate reservoir by the at least one production bore.

Description

Process for extracting natural gas and natural gas condensate from underground gas condensate deposits

description

The present invention relates to a process for the production of natural gas and / or natural gas condensate from underground gas condensate deposits containing a gas mixture with retrograde condensation behavior. Gas mixtures with retrograde (retrogressive) condensation behavior, coming from the gas phase, undergo a partial condensation in the event of an isothermal reduction in pressure and return to the gas phase on further lowering of the pressure. In general, a retrograde condensation behavior occurs in a gas mixture whose temperature is above the critical temperature of the gas mixture. Natural gas mixtures containing, for example, besides methane, ethane, propanes and butanes, 2 to 20% by volume of heavy hydrocarbons (C ₅ +, such as, for example, pentanes and hexanes) generally have a retrograde condensation behavior. The phase behavior of gas mixtures with retrograde condensation behavior is shown by way of example in FIG.

In the development of gas condensate / deposits having gas mixtures with retrograde condensation behavior (also referred to as retrograde gas condensate deposits), the condensation behavior of the above-described retrograde gas mixtures leads to problems. The removal of natural gas and / or natural gas condensate from such reservoirs through a production well reduces pressure in the reservoir, with the temperature of the reservoir remaining largely unchanged. This quasi-isothermal pressure reduction in the deposit leads to a partial condensation of the natural gas contained in the deposit. The pressure reduction is most pronounced in the vicinity of the production well (Bohrlochnahzone). Due to the partial condensation, in particular in the region of the borehole near zone, a liquid gas condensate is formed. This liquid gas condensate can block the Bohrlochnahzone, the delivery rate of natural gas and / or natural gas condensate through the production wells decreases sharply or even completely comes to a standstill. This effect is particularly pronounced in the production of natural gas and / or natural gas condensate deposits, which have a low permeability. By blocking the porous rocks in the area of the borehole near zone, the inflow of natural gas and / or natural gas condensate to the production well is severely restricted or even completely stopped. Depending on the geological characteristics of the deposit and on the pressure and temperature conditions in the deposit, the area where the liquid gas condensate blocks the flow of natural gas and / or natural gas condensate to the production well may be 5 to 100 m wide. The region in which the blocking is brought about by the liquid gas condensate generally has a quasi-cylindrical shape in the center of which the production bore lies. The lowering of the reservoir ruck occurring due to the conveyance and the consequent blocking with liquid gas condensate can in some cases even lead to the loss of the reservoir.

Methods are described in the prior art which lead to a reduction in the formation of liquid gas condensate and to an improvement in the production of natural gas and / or natural gas condensate from a gas condensate reservoir.

RU 2018639 describes a method for preventively preventing the formation of liquid gas condensate in a gas condensate reservoir. The process described there is also known as "cycling-process." During gas production, the heavy hydrocarbons (C ₅ +) are separated from light hydrocarbons (such as methane, ethane, and propanes) by light hydrocarbons Gas "pressed back into the deposit to increase the reservoir pressure. The "cyc // ng" process is very time-consuming and cost-intensive and, with this process, the formation of liquid gas condensate in gas condensate deposits can not be reliably prevented.

SU 605429 describes a process for the development of gas condensate deposits. In this process, the deposit is flooded with highly mineralized water. The high mineralization prevents the solution of gases in the flood water and thus allows the displacement of the natural gas and the natural gas condensate from the area of the borehole near the production well. A disadvantage of this method is the massive dilution of the deposit by the injected flood water. In addition, the injected flood water itself can lead to a blockage of the Bohrlochnahzone. This method does not allow for an effective increase in production rates.

SU 1596081 and RU 2064572 disclose methods of treating the gas condensate reservoir with seismic waves. The seismic waves should thereby lead to an increase in the delivery rate from the gas condensate deposit. This process is not very efficient, especially in low-lying deposits. RU 2415257 describes a method of stimulating the rates of delivery of gas condensate deposits by electromagnetic waves. This method is also unsuitable, especially for low-lying deposits.

RU 2245997 discloses a method in which solvents are injected into the well area at cyclic intervals to dissolve the liquid condensate. The solvents used for this purpose are aqueous mixtures of acetone and methanol, chloroform and methanol or acetone and chloroform. A disadvantage of this method is that the introduced aqueous mixtures also lead to a dilution of Bohrlochnahbereichs. In addition, the process is associated with the organic solvents used at an enormous cost. The organic solvents used also cause environmental problems due to their toxicity.

The present invention is therefore based on the object to provide an improved method for the production of natural gas and / or natural gas condensate from underground gas condensate deposits containing a gas mixture having a retrograde condensation behavior. The method should not or only to a lesser extent have the disadvantages of the prior art described above. The inventive method should be inexpensive and easy to carry out and lead to an effective increase in the delivery rate of natural gas and / or natural gas condensate from gas condensate deposits after the well zone was at least partially blocked by liquid gas condensate.

The object is achieved by a method for the production of natural gas and / or natural gas condensate from a subterranean gas condensate deposit containing a gas mixture with retrograde condensation behavior, comprising at least the process steps a) Lowering at least one production well into the underground gas condensate deposit and natural gas and / or natural gas condensate from the underground production well through the at least one production well, b) injecting a solution (L) containing a solvent and urea through the at least one production well into the subterranean gas condensate reservoir, c) placing a quiescent phase in which the urea contained in the solution (L) is hydrolyzed, (d) extraction of natural gas and / or natural gas condensate from the underground gas condensate reservoir through the at least one production well.

The object is further achieved by a method for the production of natural gas and / or natural gas condensate from a subterranean gas condensate deposit containing a gas mixture with retrograde condensation behavior, comprising at least the process steps a) Lowering at least one production well into the underground gas condensate deposit and promotion of Natural gas and / or

B) injecting a solution (L) containing a solvent and urea through the at least one production well into the subterranean gas condensate deposit, c) placing a quiescent phase in which the at least one production well is withdrawn from the underground gas condensate deposit; hydrolyzing urea contained in the solution (L), d) conveying natural gas and / or natural gas condensate from the underground gas condensate reservoir through the at least one production well.

The method according to the invention makes it possible to effectively increase the delivery rate of natural gas and / or natural gas condensate from a gas condensate deposit in which the well area is blocked by liquid natural gas condensate. The method according to the invention has the advantage that it manages with cost-effective and toxicologically harmless substances. The inventive method prevents dilution of the Bohrlochnahzone the gas condensate deposit.

Process step a)

In process step a) at least one production well is drilled into the underground gas condensate deposit. The downcasting of the at least one production well into the underground gas condensate deposit takes place by conventional methods known to the person skilled in the art and is described, for example, in EP 0 952 300. The production hole can be a vertical, horizontal or a deflected hole. Preferably, the production well is a deflected well that includes a quasi-vertical and a quasi-horizontal section. The gas condensate deposit contains a gas mixture with a retrograde condensation behavior. Such gas condensate deposits are also referred to as retrograde gas condensate deposits. The gas mixture contained in the underground gas condensate deposit generally contains from 80 to 98% by volume of light hydrocarbons and from 2 to 20% by volume of heavy hydrocarbons. By light hydrocarbons are understood according to the invention methane, ethane, propanes and butanes. Hydrocarbons according to the invention are understood to mean hydrocarbons having 5 or more carbon atoms, for example pentanes, hexanes and heptanes and optionally higher hydrocarbons. The terms propanes, butanes, pentanes, hexanes and heptanes are understood in the present case to mean both the unbranched hydrocarbon compounds and also all branched isomers of the abovementioned hydrocarbon compounds. The properties of gas mixtures with retrograde condensation behavior are shown purely by way of example in FIG. The area denoted by (al) describes the single-phase region in which the gas mixture is present exclusively in liquid form. The single-phase region marked with (av) shows the region in which the gas mixture is exclusively gaseous. The region marked (l + v) shows the biphasic region in which one part of the gas mixture is in liquid form and another part is in gaseous form. (CP) shows the critical point of the gas mixture connecting the bubble point curve (bpc) to the dew point curve (dpc). The Bubble Point Curve (bpc) is also referred to as the bubble-po / nf curve, and the dew point curve (dpc) is also called the dew-point curve.

The bubble point curve (bpc) separates the single-phase liquid region (a1) from the biphasic region (l + v). On the Bubble Point Curve (bpc), the gas mixture is virtually 100% liquid and contains only infinitesimal amounts of gas.

The dew point curve (dpc) separates the single-phase gaseous region (av) from the two-phase region (c + v). On the dew point curve (dpc), the gas mixture is virtually 100% gaseous and contains only infinitesimal amounts of liquid. On the horizontal axis is the temperature (T), on the vertical axis the pressure (P) is plotted.

A gas mixture with a retrograde condensation behavior undergoes a partial condensation in the event of an isothermal reduction in pressure and reverts to the gas phase on further lowering of the pressure. The retrograde condensation behavior usually occurs at temperatures which are above the critical point (CP) of the gas mixture. The following is an example of this Behavior of a mixture described at a predetermined temperature, which is above the critical point (CP).

At a given temperature (T-i), the gas mixture with retrograde 5 condensation behavior at point (A) is gaseous and single-phase. In the isothermal pressure reduction (indicated by the dashed line in FIG. 1), the gas mixture at point (B) reaches the dew point curve (dpc). At this point, the gas mixture is virtually 100% gaseous, but an infinitesimal amount of liquid begins to form. With further pressure reduction that goes

10 gas mixture in the two-phase range (l + v), in which forms a liquid phase in addition to the gas phase by partial condensation. Thus, at point (C), natural gas and liquid natural gas condensate are juxtaposed in a two-phase system. If the pressure is further reduced isothermally, the gas mixture again reaches the dew point curve (dpc) (in FIG. 1 by point (D)).

15 marked). When the dew point curve (dpc) is exceeded, the gas mixture reverts to the single-phase gaseous state. At point (E) in Figure 1, the gas mixture is again in gaseous and single-phase. The illustration in FIG. 1 merely serves to explain the condensation behavior of retrograde gas mixtures without restricting the present invention.

20

The reservoir temperature T _{L of} the gas condensate deposits from which natural gas and / or natural gas condensate are conveyed by the process according to the invention is usually in the range from 60 to 200 ° C., preferably in the range from 70 to 150 ° C., particularly preferably in the region of 80 to 140 ° C and in particular in the range of 25 85 ° C to 120 ° C.

The storage temperature T _{L of} the gas condensate deposits must meet the following conditions:

1) T _L is higher than the crystallization temperature of the solution

30 2) T _L must allow the complete hydrolysis of the urea in a relatively short time, for example within 1 to 20 days.

The subject matter of the present invention is therefore also a process in which the underground gas condensate deposit has a reservoir temperature (T _L ) in the range 35 from 60 to 200 ° C, preferably in the range from 70 to 150 ° C, particularly preferably in the range from 80 to 140 ° C and in particular in the range of 85 to 120 ° C.

The initial reservoir pressure, that is, the pressure of prior to carrying out the process of the invention, is usually in the range of 80 to 1500 bar, 40 normally the initial reservoir pressure at gas condensate reservoirs is 300 to 600 bar. The permeability of the underground gas condensate deposits is generally in the range of 0.01 to 10 mD (MiliDarcy).

The porosity of the underground gas condensate deposits is generally in the range of 0.1 to 30%.

After sinking the production well into the underground deposit, reservoir pressure is usually sufficient to pump natural gas and / or natural gas condensate through production wells by conventional methods. Of course, the terms natural gas and natural gas condensate do not in this context mean a pure hydrocarbon mixture. The natural gas and / or natural gas condensate can of course also contain other substances in addition to methane, ethane, propanes, butanes, hexanes and heptanes and optionally higher hydrocarbons.

Other substances may be, for example, sulfur-containing hydrocarbons or formation water. In the present case, formation water is understood to mean water which is originally present in the deposit, and water which has been introduced into the deposit through secondary and tertiary production process steps, for example so-called floodwater. The formation water also includes water which has optionally been introduced into the gas condensate deposit by the process according to the invention.

For example, a gas mixture with a retrograde condensation behavior has the following composition (data in mol%):

Methane 74.6%

Ethane 8.9%

Propane 3.8%

Butane 1, 8%

Pentane 6.4%

Nitrogen 4.5%

Original density 0.745 g / cm ³

In the present case, natural gas is understood as meaning gaseous gas mixtures which are conveyed from the gas condensate deposit. Natural gas condensate is understood as meaning liquid mixtures which are conveyed from the gas condensate reservoir. The aggregate state of the mixtures extracted from the gas condensate deposit depends on the temperature and the pressure in the deposit or in the production well. According to the method of the invention, it is possible to exclusively feed natural gas through the production well. In addition, it is possible to promote only natural gas condensate through the production well. It is also possible to have one Promote mixture of natural gas and natural gas condensate through the production well. The state of aggregation of the further substances optionally present in the natural gas or in the natural gas condensate likewise depends on the pressure and the temperature in the deposit or in the production well. The other substances may likewise be present in liquid form or in gaseous form in the mixture conveyed through the production well.

In the event that after deposition of the production well (process step a)), the reservoir pressure is sufficient to deliver natural gas and / or natural gas condensate from the reservoir through the production well, this is done by conventional production methods. The subject matter of the present invention is thus also a method in which, after the sinking, the at least one production well is introduced into the underground gas condensate deposit (process step a) and before the solution (L) is injected into the underground gas condensate reservoir (process step b)). first, natural gas and / or natural gas condensate (by conventional methods) is conveyed through the at least one production well.

However, this is not mandatory. It is also possible to carry out process step b) directly after the production well has been lowered in order to prevent the formation of natural gas condensate.

As a rule, after process step a), however, natural gas and / or natural gas condensate are first conveyed by conventional methods from the gas condensate deposit. By promoting natural gas and / or natural gas condensate from the gas condensate reservoir, the pressure in the gas condensate reservoir decreases, with the temperature of the gas condensate reservoir remaining largely unchanged. Thus, the production of natural gas and / or natural gas condensate from the gas condensate reservoir results in isothermal pressure reduction. Under isothermal is understood in the present case that the temperature of the gas condensate storage in the implementation of the method according to the invention remains largely constant, that is, that the temperature of the gas condensate deposit by a maximum of +/- 20 ° C, preferably by +/- 10 ° C, and more preferably by +/- 5 ° C when carrying out the method according to the invention compared to the initial storage temperature before carrying out the method according to the invention changes.

The pressure reduction is most pronounced near the production well and decreases with increasing distance from the production well. FIG. 2 shows by way of example the pressure curve in the underground gas condensate deposit as a function of the distance to the production well. The distance to the production bore is plotted on the horizontal axis in meters. The reservoir jerk (P) is plotted on the dashed vertical axis. In one certain distance from the production well, the reservoir pressure (P) reaches a value at which the partial condensation of the retrograde gas mixture begins. This distance is shown by the vertical dotted line in FIG. At point (B) on the dashed reservoir pressure curve (P), formation of a biphasic mixture containing natural gas and natural gas condensate begins. Point (B) on the dashed reservoir pressure curve (P) corresponds to point (B) in Figure 1. To the left of the dotted line, the gas mixture is in two-phase (range (l + v)). To the right of the dotted line, the gas mixture is in single phase (area (av)). With the onset of partial condensation, the proportion of liquid natural gas condensate increases. The proportion of liquid natural gas condensate is plotted on the vertical axis (KG) and is shown by the solid curve (KG) in Figure 2. Above a certain concentration of liquefied natural gas condensate, the well zone is blocked, which reduces or completely stops the production rates of natural gas and / or natural gas condensate from the gas condensate reservoir. This critical area is represented by the gray-shaded area (KB) in FIG. The critical concentration of the liquid natural gas condensate in the gas mixture is represented by the point (KS) on the curve (KG) in FIG. FIG. 2 merely illustrates, by way of example, the conditions in a gas condensate deposit which has a gas mixture with retrograde condensation behavior, without limiting the present invention thereto.

The production of natural gas and / or natural gas condensate from the underground gas condensate reservoir through the at least one production well is generally carried out until a reduction in the production rate of natural gas and / or natural gas condensate is registered.

The reduction of the delivery rate is due to the formation of the critical area (KB), which is at least partially blocked by liquid natural gas condensate.

The subject matter of the present invention is therefore also a process in which the underground gas condensate deposit before the implementation of process step b) has a critical area (KB) which is at least partially blocked by liquid natural gas condensate.

Before injecting the solution (L) in process step b), the production of natural gas and / or natural gas condensate is generally stopped.

The subject matter of the present invention is therefore also a process in which the process step a) involves the downsizing of at least one production well into the underground gas condensate deposit, the production of natural gas and / or natural gas condensate from the underground gas condensate deposit until formation a critical region (KB) which is at least partially blocked by liquid natural gas condensate and which comprises adjusting the production of natural gas and / or natural gas condensate from the underground gas condensate reservoir through the at least one production well.

Process step b)

In step b), a solution (L) containing a solvent and urea is injected through the production well into the underground gas condensate deposit.

Usually, the solution (L) contains 50 to 79% by weight of urea and 21 to 50% by weight of solvent, the solvent containing water, alcohol or a mixture of water and alcohol, based on the total weight of the solution (L). The solution (L) preferably contains from 60 to 78% by weight of urea and from 22 to 40% by weight of solvent, based on the total weight of the solution (L). The solution (L) particularly preferably contains 65 to 77% by weight of urea and 23 to 35% by weight of solvent, based on the total weight of the solution (L). In a further particularly preferred embodiment, the solution (L) contains 75 to 77 wt .-% urea and 23 to 25 wt .-% of solvent, based on the total weight of the solution (L).

The present invention thus also provides a process in which the solution (L) contains 50 to 79% by weight of urea and 21 to 50% by weight of solvent, the solvent containing water, alcohol or a mixture of water and alcohol , in each case based on the total weight of the solution (L).

As a solvent, therefore, only water can be used. It is also possible to use only alcohol as solvent. The use of a solvent containing only alcohol is possible if there is sufficient formation water for the hydrolysis of the urea in the production well and / or the underground gas condensate reservoir. In addition, it is possible to use as solvent a mixture of water and alcohol. As alcohol, exactly one alcohol can be used. It is also possible to use a mixture of two or more alcohols. As the alcohol, methanol, ethanol, 1-propanol, 2-propanol or a mixture of two or more of these alcohols can be used. Preferred alcohol is methanol.

In a further particularly preferred embodiment, the solution (L) contains urea and water in a stoichiometric ratio of water to urea of 1: 1. At this ratio, the urea contained in the solution (L) completely converts to water with ammonia and carbon dioxide. As a result, the water contained in the solution (L) is completely consumed and prevents contamination of the gas condensate reservoir by water. In the event that the solution (L) contains only water as a solvent, the solution (L) then contains water and urea in a wt .-% ratio of 23.1 wt .-% water to 76.9 wt .-% urea.

The solution (L) can consist only of solvent and urea, the above statements and preferences apply accordingly. However, it is also possible to add at least one surface-active component (surfactant) to the solution (L). In this case, the solution (L) preferably contains 0.1 to 5 wt .-%, particularly preferably 0.5 to 1 wt .-% of at least one surfactant, based on the total weight of the solution (L).

As surface-active components it is possible to use anionic, cationic and nonionic surfactants.

Common nonionic surfactants are, for example, ethoxylated mono-, di- and trialkylphenols, ethoxylated fatty alcohols and polyalkylene oxides. In addition to the unmixed polyalkylene oxides, preferably C ₂ -C ₄ -alkylene oxides and phenylsubstituted C ₂ -C ₄ -alkylene oxides, in particular polyethyleneoxides, polypropyleneoxides and poly (phenylethyleneoxides), especially block copolymers, in particular polypropylene oxide and polyethylene oxide blocks or poly (phenylethylene oxide) and Polyethylene oxide blocks having polymers, and also random copolymers of these alkylene oxides suitable. Such Alkylenoxidblockcopolymerisate are known and commercially z. Examples of suitable anionic surfactants are alkali metal and ammonium salts of alkyl sulfates (alkyl radical: C ₈ -C ₁₂ ), of sulfuric monoesters of ethoxylated alkanols (alkyl radical: C ₁₂ -C ₁₈ ) and ethoxylated alkylphenols (US Pat. Alkyl radicals: C ₄ -C ₁₂ ) and of alkylsulfonic acids (alkyl radical: Ci ₂ -Ci ₈ ). Suitable cationic surfactants are for example C ₆ -C having ₈ alkyl, alkylaryl, or heterocyclic radicals, primary, secondary, tertiary or quaternary ammonium salts, pyridinium salts, imidazolinium salts, Oxozoliniumsalze, morpholinium, Propyliumsalze, sulfonium salts and phosphonium salts. For example, its dodecylammonium acetate or the corresponding sulfate, disulfates or acetates of the various 2- (N, N, N-trimethylammonium) ethylparaffine esters, N-cetylpyridinium sulfate and N-laurylpyridinium salts,

Cetyltrimethylammoniumbromid and sodium lauryl sulfate called.

The use of surface-active components in the solution (L) lowers the surface tension of the solution (L). As a result, the solution (L), better penetrate the areas of the Bohrlochnahzone blocked by the natural gas condensate and displace the natural gas condensate. Urea converts to ammonia and carbon dioxide in the presence of water by hydrolysis according to the following equation: H ₂ N-CO-NH ₂ + H ₂ O -> 2NH ₃ + C0 ₂

One mole of urea and one mole of water form two moles of ammonia and one mole of carbon dioxide. The hydrolysis of urea with water under the action of heat is also referred to as thermohydrolysis. From a temperature of about 60 ° C, the hydrolysis of urea and water proceeds sufficiently quickly to completely hydrolyze the urea and water to carbon dioxide and ammonia in economically meaningful periods. The rate of hydrolysis of the urea contained in the solution (L) increases with increasing temperature. The solution (L) is usually provided above ground by dissolving the urea in the solvent. Optionally, it is also possible to add further additives, for example surface-active components (surfactants). The urea is usually used as granules. To accelerate the dissolution of the urea in the solvent and the preparation of the solution (L), the solution (L) can be heated. The subject matter of the present invention is thus also a method in which the solution (L) is heated before or during the injection according to method step b). The subject matter of the present invention is thus also a method in which the solution (L) is heated before or during the injection according to method step b).

The solution (L) can be used as a true solution (L). It is also possible to use as solution (L) a mixture containing solvent and urea in dissolved form and urea in undissolved form, for example in the form of crystals. For the inventive method is sufficient if the solution (L) can be pumped by conventional pumps in the gas condensate reservoir. Preferably, a true solution is used as solution (L). The dissolution behavior of urea in water is shown in the phase diagram in FIG. On the horizontal axis, the urea content of the solution (L) is given in% by weight, based on the total weight of the solution (L). On the right vertical axis, the temperature is given in ° C. On the left vertical axis and through the dotted curve (1), the proportion of remaining after the hydrolysis of the urea residual water (RW) is given, based on the total weight of the solution used (L). The dashed vertical line (2) in Figure 3 indicates the urea concentration (76.9% by weight) at which the water contained in the solution (L) is completely consumed in the hydrolysis of the urea, that is, the proportion of the remaining residual water (RW) after hydrolysis of the urea is equal to 0. In the event that solutions (L) are used with lower urea concentrations, residual water (RW) remains after the hydrolysis of the urea. The amount of remaining residual water (RW) as a function of the urea concentration of the solution used (L) is shown in Figure 3 by the dotted curve (1). In the event that only water is used as the solvent, residual water remaining after urea hydrolysis (RW) can be calculated by the following formula:

RW = 100 wt .-% - (KH · 1, 3)

RW indicates therein the proportion of residual water (RW) remaining after hydrolysis of the urea in% by weight, based on the total weight of the solution (L) used, in the event that only water is used as the solvent. KH indicates therein the urea fraction of the solution (L) used in wt .-%, based on the total weight of the solution used (L).

In the case that as the solution (L), a solution containing 60 wt .-% urea (that is, KH = 60 wt .-%) and 40 wt .-% water (based on the total weight of the solution (L )), the proportion of remaining after hydrolysis residual water (RW) results

RW = 100% by weight (60% by weight * 1, 3) = 22% by weight In a preferred embodiment, starting from a hypothetical solution (L) containing only urea and water, the proportion of hypothetical Residual water (RW) calculated in wt .-%, which would remain in the urea hydrolysis of this hypothetical solution (L). Subsequently, the proportion of the calculated hypothetical residual water (RW) in the solution (L) is replaced by the corresponding amount by weight of an alcohol. Suitable alcohols are methanol, ethanol or mixtures of ethanol and methanol, with methanol being preferred. In the event that the solution (L) contains less than 76.9% by weight of urea, based on the total weight of the solution (L), the solvent (L) solution preferably contains a mixture of water and alcohol. The preferred amount of alcohol corresponds to the proportion of hypothetical residual water (RW) and is calculated by the following formula: KA = 100% by weight - (KH * 1, 3).

KA indicates the preferred amount of the alcohol contained in the solution (L).

KH indicates therein the urea content of the solution (L) in% by weight.

At urea concentrations of 50% by weight, the solution (L) preferably contains 50% by weight of urea, 35% by weight of alcohol and 15% by weight of water.

At a urea concentration of 55% by weight, the solution (L) preferably contains 28.5% by weight of alcohol and 16.5% by weight of water.

At a urea concentration of 60% by weight, the solution (L) preferably contains 22% by weight of alcohol and 18% by weight of water.

At a urea concentration of 65% by weight, the solution (L) preferably contains 15.5% by weight of alcohol and 19.5% by weight of water.

At a urea concentration of 70% by weight, the solution (L) preferably contains 9% by weight of alcohol and 21% by weight of water.

At a urea concentration of 75% by weight, the solution (L) preferably contains 2.5% by weight of alcohol and 22.5% by weight of water.

The subject matter of the present invention is thus also a process in which the solution (L)

From 50 to <76.9% by weight of urea,

> 0 to 35 wt .-% alcohol and

Contains 15 to 50 wt .-% water.

The sum of urea, alcohol and water preferably gives 100 wt .-%.

The present invention furthermore relates to a process in which the solution (L)

From 50 to <76.9% by weight of urea,

KA wt .-% alcohol as well

Contains 15 to 50% by weight of water,

where KA is given by the following formula:

KA = 100 wt .-% - (KH * 1, 3), in the KA the amount of alcohol contained in the solution (L) in wt .-% and KH indicates the amount of urea contained in the solution (L) in Indicates wt .-%.

The sum of urea, alcohol and water preferably gives 100 wt .-%. For the solution (L) used in process step b), the urea concentration is preferably selected so that the crystallization temperature (T _K ) of the solution (L) is below the reservoir temperature (T _L ) of the underground gas condensate reservoir, wherein below crystallization temperature (T _K ) the temperature is understood to crystallize below the dissolved urea in the solution (L), so that the solution (L) contains water, urea in dissolved form and urea in undissolved form.

In other words, the reservoir temperature T _{L is} preferably above the crystallization temperature T _{K of} the solution (L) used. The crystallization temperature T _{K of} the solution (L) corresponds in FIG. 1 to the curve which separates the greyed area "solution" from the area "solution + crystals". In the case where T _{L is} larger than T _K , the crystallization of urea from the solution (L) in the underground gas condensate deposit can be surely avoided. The crystallization of urea in the underground gas condensate deposit could result in the blockage of the wellbore near the underground gas condensate deposit.

The subject matter of the present invention is therefore also a process in which the solution (L) has a crystallization temperature (T _K ) which is below the reservoir temperature (T _L ) of the underground gas condensate reservoir.

The subject of the present invention is further a method in which the reservoir temperature (T _L ) of the underground gas condensate deposit is higher than the crystallization temperature (T _K ) of the solution (L).

Preferred solutions (L) used are solutions having a urea concentration in the range from 50 to 76.9% by weight, based on the total weight of the solution (L). At these urea concentrations, 60 to 100% by weight of the water originally contained in the solution (L) is consumed in the hydrolysis of the urea. This prevents or at least reduces the contamination of the underground gas condensate reservoir with water.

The present invention furthermore relates to a process in which the duration of the quiescent phase is selected so that the urea originally contained in the solution (L) in the underground gas condensate deposit is completely hydrolyzed to carbon dioxide and ammonia and 60 to 100% by weight. % of the water originally contained in the Lösunt (L).

The present invention thus also provides a process in which the solution (L) contains 50 to 76.9% by weight of urea and 23.1 to 50% by weight of water, in each case based on the total weight of the solution (L) , In a preferred embodiment, a solution (L) with 65 to 72 wt .-% urea, preferably with 69 to 71 wt .-% urea, based on the total weight of the solution (L) is used. As can be seen in Figure 3, these amounts of urea can be prepared at temperatures in the range of 50 to 55 ° C to form a true solution 5 (L). The relatively low temperatures of 50 to 55 ° C have the advantage that at these temperatures, the hydrolysis of the urea proceeds very slowly, so that no appreciable amounts of ammonia and carbon dioxide are formed. The heating of the solution (L) is carried out by conventional heating elements, such as an electric heater. As container for the preparation of the solution 10 (L), for example, stirred tank with propeller mixer can be used.

In addition, it is possible to produce first day a solution (L) which contains water, urea in dissolved form and urea in undissolved form, for example in the form of crystals. This solution can subsequently by the

15 production wells are introduced into the gas condensate deposit. In this embodiment, a heating element is mounted in the production well above the gas condensate reservoir to dissolve the urea present in the solution (L) in undissolved form. However, such a heating element is not mandatory. It is sufficient as stated above, if the

20 reservoir temperature T _{L is} above the crystallization temperature T _{K of} the solution (L). The complete dissolution of the urea in undissolved form then takes place directly in the deposit. This embodiment may be selected when injecting solution (L) into fracture gaps in the underground gas condensate reservoir (see, for example, Figure 4c, reference 5). The optionally filled with proppant

25 Frackspalte has a very high permeability and porosity and can "absorb" the crystals.

In a further preferred embodiment, a solution (L) having a urea content in the range of> 72 wt .-% to 76.9 wt .-%, preferably in the range of

30 75 wt .-% to 76.9 wt .-% urea, each based on the total weight of the solution (L) used. For the preparation of a true solution (L) temperatures in the range of 60 to 70 ° C are necessary for this purpose. At these temperatures, there is already a marked hydrolysis of the urea to form ammonia and carbon dioxide. To minimize gas formation, it is possible to manufacture

35 of the solution (L) to heat them in the short term to the temperatures necessary for the solution and subsequently to cool to temperatures in the range of 50 to 55 ° C. Short-term warming minimizes the formation of carbon dioxide and ammonia. The supersaturated solution (L) thus formed is stable for a long time since the process of crystallization of urea from the supersaturated solution (L) is slow

40 runs. The supersaturated solution (L) may be prepared as described above and then injected through the production well into the underground gas condensate reservoir. In addition, it is also possible to dissolve the urea in the solution (L) obertage only partially, so that the solution (L) contains water, urea in dissolved form and urea in undissolved form. This solution (L) is injected as described above, subsequently through the production well into the underground gas condensate deposit. In this case, it is again possible to install a heating element in the production well above the deposit, so that the urea is dissolved in undissolved form in the production well in the solution (L). However, this is not mandatory. It is also possible to inject the solution (L) containing water, urea in dissolved form and urea in undissolved form into the underground gas condensate reservoir. In this embodiment, the urea dissolves in undissolved form in the solution (L) in the underground gas condensate reservoir. The above statements always apply on condition that the reservoir temperature T _{L is} higher than the crystallization temperature T _{K of} the solution (L).

The use of aqueous urea solutions for the development of oil reservoirs, which have viscous petroleum, is described in the not yet disclosed patent application EP 121 72571.

The amount of solution (L) injected in step b) depends on the geological parameters of the underground gas condensate deposit, including the permeability of the deposit and the size of the area (critical area of Figure 2) in which the well zone is liquid Natural gas condensate is blocked. Preferably, the solution (L) is injected in volumes corresponding at most to the pore volume of the critical area (KB) blocked by the liquid natural gas condensate. Suitable volumes of the solution (L) injected in process step b) are in the range of 1 to 10 m ³ per 1 m of the production well, which is surrounded by the critical region (KB), preferably in the range of 2 to 8 m ³ , more preferably in Range from 3 to 7 m ³ .

The present invention thus also provides a process in which the solution (L) in process step b) is injected in volumes which, in the hydrolysis of urea, lead to a gas volume of carbon dioxide and ammonia which is at least the pore volume of the critical region (KB ) corresponds.

In the final phase of the injection of the solution (L) according to process step b), methanol may be added to the solution (L). The term "final phase" as used herein means that at least 90% by weight of the solution (L) has been injected, based on the total weight of the solution (L) injected in process step b). It is also possible to completely inject solution (L) and subsequently inject methanol.

This will fill the production well with methanol. This facilitates the resumption of the production of natural gas and / or natural gas condensate in process step d).

The present invention thus also relates to a process in which, together with the injection of the solution (L) or after the injection of the solution (L) according to process step b), methanol is injected into the production well.

The described solution (L) can also be used for flooding gas condensate deposits. In this case, at least one hole is used as a continuous injection well. The solution (L) is injected into this hole. The solution (L) forms gases in the deposit. This process can be used particularly efficiently in the development of deposits which were shut down due to the massive failure of a retrograde gas condensate.

Process step c)

After injecting the solution (L), a quiescent phase is generally set up in which the urea in the underground gas condensate deposit is hydrolyzed to ammonia and carbon dioxide. In a preferred embodiment, the duration of this quiescent phase is chosen so that a complete hydrolysis of the urea takes place.

The rate at which hydrolysis of the urea occurs depends on the reservoir temperature T _{L of} the underground gas condensate reservoir and the temperature at which the solution (L) is injected in process step b). At high storage temperatures T _L , the hydrolysis is correspondingly faster, so that the rest phase can be chosen shorter. The period of rest is generally in the range of 1 to 10 days. At reservoir temperatures T _L of> 100 ° C, the rest phase may be shorter, for example 1 to 5 days. At reservoir temperatures T _L in the range of 80 to <100 ° C, the range of rest period is 5 to 10 days. In the event that the reservoir temperature T _{L is} in the range of 60 to <80 ° C, the rest phase must be chosen correspondingly longer, for example in the range of 15 to 20 days.

By placing the quiescent phase, the urea contained in the solution (L) in the underground gas condensate deposit is completely hydrolyzed. During the rest phase, the production well is closed in a preferred embodiment. This can be done by conventional means, such as packers. By closing the production well, the pressure in the critical region of the underground gas condensate deposit increases, whereby the efficiency of the method according to the invention is increased.

The subject matter of the present invention is thus also a method in which the at least one production well is closed during the quiescent phase according to step c).

The resulting carbon dioxide is partially dissolved in natural gas and mainly in the liquid natural gas condensate. As a result, the viscosity of the liquid natural gas condensate is lowered, whereby the mobility of the liquid natural gas condensate in the critical range (KB) of the gas condensate reservoir is significantly increased. The resulting ammonia dissolves in the formation water present in the deposit as well as in the water injected with the solution (L) and forms an alkaline ammonia buffer system having a pH of 9-10. If the deposit is slightly diluted, highly alkaline solutions are formed: Under certain conditions ammonia may also partially liquefy in the deposit. Liquid ammonia and aqueous ammonia solutions are very good solvents. This additionally increases the mobility of the gas condensate.

This buffer system has a surfactant-like action in the underground gas condensate reservoir. As a result, the interfacial tension between the phases, i. reduced between the natural gas phase and the liquid natural gas condensate phase and optionally the Formationswasserphase. The formation of the gases (ammonia and carbon dioxide) in the underground gas condensate deposit also has a purely mechanical displacement effect of the liquid natural gas condensate. By lowering the viscosity of the liquid natural gas condensate and increasing the mobility of the liquid natural gas condensate, the production of natural gas and liquefied natural gas condensate from the underground gas condensate reservoir is facilitated. As a result, the delivery rate increases significantly. In the production of natural gas, natural gas flushes the liquid natural gas condensate present in the critical area (KB) of the underground gas condensate deposit in the direction of the production well. This leads to a further increase in the production rate.

In a preferred embodiment, in process step b) the solution (L) is introduced in such amounts that the volume of gas produced during the hydrolysis of urea corresponds at least to the pore volume of the critical region of the underground gas condensate reservoir. The present invention thus also relates to the use of a solution (L) containing water and urea as a means for increasing the delivery rates of natural gas and / or natural gas condensate from a gas condensate reservoir containing a gas mixture with retrograde condensation behavior. For the use of the solution (L) as a means for increasing the delivery rates, the above statements and preferences with regard to the solution (L) apply accordingly.

Process step d) In process step d) natural gas and / or natural gas condensate from the underground gas condensate deposit is promoted, that is resumed. The promotion takes place according to conventional methods. The natural gas and the natural gas condensate can be conveyed through the production well through which, in process step b), the solution (L) was injected into the underground gas condensate reservoir. It is also possible to drill additional wells into the underground gas condensate deposit. The production of natural gas and natural gas condensate can then be carried out through the production well or through further drilling. The production bore can also fulfill the function of an injection well through which a flood medium is pressed into the underground gas condensate deposit, the actual promotion then takes place via the one or more further holes. It is also possible to inject a flood medium via the one or more further holes into the underground gas condensate deposit and carry out the production through the production through which in step b) the solution (L) was injected.

The production of natural gas and / or natural gas condensate from the underground gas condensate deposit is continued in accordance with process step d) until the resulting pressure reduction in the underground gas condensate reservoir leads to the formation of liquid natural gas condensate, whereby the critical region (KB) is formed and the delivery rates decrease significantly. In this case, steps b) and c) are performed again. The steps b) and c) of the method according to the invention are thus always carried out when a critical area (KB) forms again in the underground gas condensate deposit which is blocked by liquid natural gas condensate.

The subject matter of the present invention is therefore also the use of a solution (L) as a means for increasing the delivery rates of natural gas and / or natural gas condensate from a subterranean gas condensate deposit having a gas mixture with retrograde condensation behavior. The present invention is further illustrated by the following Example and Figures 1, 2, 3 and 4, without being limited thereto. In the figures, reference numerals have the following meanings: al single-phase liquid region bpc bubble point curve l + v two-phase region dpc dew point curve

CP critical point av single-phase gaseous region

A, B, C, D and E points in the isothermal pressure reduction of the retrograde

Gas mixture KG Concentration of the liquid natural gas condensate in the gas mixture

KB critical area

KS critical concentration of liquid natural gas condensate in the

mixture of gases

P pressure

T temperature

(1) concentration of residual water after hydrolysis of the

Urea in the solution used (L)

(2) Concentration of urea at which the water in the solution

(L) is completely consumed in the hydrolysis

3 production well

4 critical area (KB), which is blocked with liquid natural gas condensate

5 Frack column in the underground gas condensate deposit The figures show in detail: FIG. 1

The phase behavior of gas mixtures with retrograde condensation behavior. FIG. 2

The pressure curve and the concentration of liquid natural gas condensate in a 10 underground gas condensate deposit as a function of the distance to the production well.

FIG. 3

The phase diagram of an aqueous urea solution.

15

FIGS. 4a, 4b, 4c

Various embodiments of the production bore 3.

Figures 1, 2 and 3 are described in the description of the present invention 20.

Figure 4 shows different embodiments of a drilled hole 3. Figure 4a shows a vertical production bore. Region 4 represents the area which is blocked by liquid natural gas condensate. Figure 4b shows a 25 embodiment in which a deflected bore has been drilled. FIG. 4 c shows an embodiment in which a deflected bore has been drilled and in which the underground gas condensate deposit has a fracking gap 5.

Example:

30

For the development of a gas condensate deposit which is stored at a depth in the range of 3400 to 3700 m, a deflected production well 3 according to FIG. 4b or 4c is drilled. The thickness of the productive layer is 50 to 80 m. The reservoir temperature T _L is 105 ° C. The reservoir is about

35 650 atm (658.6 bar). The permeability of the deposit is low and is between 0.5 and 5.0 mD. After the deflection of the deflected production bore 3, it is fractured in the region of the productive layer, whereby a fracture zone 5 is formed. The porosity of the gas condensate deposit ranges from 0.2 to 0.25%. After the sinking and fracing of the production well 3 is with the

40 promotion of natural gas and / or natural gas condensate started by conventional methods. After a year of extraction of natural gas and / or natural gas condensate, a significant reduction in the production rate is registered. The reduction of Delivery rate is due to a blockage of the well area by liquid natural gas condensate. The critical area 4, in which the blocking by liquid natural gas condensate has occurred, is estimated at a radius of about 20 m. The area has a cylindrical shape, in the center of which the production bore 3 is located. To dissolve the blocking, 100 m ^{3 of} the solution (L) containing urea and water, with a urea concentration of 70 wt .-%, based on the total weight of the solution (L) through the production bore 3 in the critical region 4 of the gas condensate Deposit pressed in. To prepare the solution (L), 70 t of urea are dissolved in 30 t of water. After injecting the solution (L) into the gas condensate reservoir, the urea in the gas condensate reservoir is hydrolyzed to form about 85,000 m ^{3 of} gases (ammonia and carbon dioxide). To prepare the solution (L) urea is used in granular form. To prepare the solution (L), it is heated using conventional heaters. Immediately before injection of the aqueous solution (L), the solution (L) has a temperature of 50 ° C to prevent the crystallization of urea from the solution (L). The injection of the solution (L) by means of conventional pumps. After injecting the solution (L) into the gas condensate reservoir, a resting phase is introduced. The rest period is 3 to 5 days. During the quiescent phase, the urea in the underground gas condensate deposit is completely hydrolyzed. During the resting phase, the production well is closed. This increases the pressure in the critical region of the underground gas condensate deposit, which increases the efficiency of the process according to the invention. In the final phase of the injection, additional methanol can be used. This will fill the production well with methanol during the dormant phase. This subsequently facilitates the resumption of the promotion. Hydrolysis of the urea almost completely consumes the water injected with the solution (L) into the underground gas condensate reservoir. Blockage of the Bohrlochnahzone by water is thereby prevented.

After the period of rest, support will be resumed using conventional methods. The hydrolysis of the urea in the underground gas condensate reservoir significantly increases the mobility of the gas mixture contained in the reservoir. The subsequently conveyed natural gas flushes the possibly still existing liquid natural gas condensate also in the direction of the production well. This further reduces the blockage of the critical area. After the quiescent phase, natural gas and liquid natural gas condensate are extracted from the underground gas condensate deposit.

Claims

claims

1. A method for the production of natural gas and / or natural gas condensate from a subterranean gas condensate deposit, which is a gas mixture with retrograde

Condensation behavior, comprising at least the process steps a) Lowering at least one production well into the underground gas condensate reservoir and promoting natural gas and / or natural gas condensate from the underground production well through the at least one production well, b) injecting a solution (L) containing a solvent and C) introducing a quiescent phase in which the urea contained in the solution (L) is hydrolyzed; d) conveying natural gas and / or natural gas condensate from the underground gas condensate reservoir through the at least one production well.

2. The method according to claim 1, characterized in that the underground gas condensate deposit a reservoir temperature (T _L ) in the range of 60 to 200 ° C, preferably in the range of 70 to 150 ° C, particularly preferably in the range of 80 to 140 ° C and in particular in the range of 85 to 120 ° C. 3. The method according to claim 1 or 2, characterized in that the solution (L) 50 to 79 wt .-% urea and 21 to 50 wt .-% solvent, wherein the solvent is water, alcohol or a mixture of water and alcohol contains, in each case based on the total weight of the solution (L). 4. The method according to any one of claims 1 to 3, characterized in that the solution (L) has a crystallization temperature (T _K ), which is below the reservoir temperature (T _L ) of the underground gas condensate deposit.

Method according to one of claims 1 to 4, characterized in that

the solution (L) is heated before or during the injection according to method step b).

6. The method according to any one of claims 1 to 5, characterized in that the deposit temperature (T _L ) of the underground gas condensate deposit is higher than the crystallization temperature (T _K ) of the solution (L). 7. The method according to any one of claims 1 to 6, characterized in that the solution (L) is injected in step b) with a temperature which is higher than the crystallization temperature (T _K ), and the reservoir temperature (T _L ) of the underground Gas condensate deposit is higher than the crystallization temperature (T _K ) of the solution (L).

8. The method according to any one of claims 1 to 7, characterized in that the underground gas condensate deposit before carrying out the process step b) has a critical area (KB), which is at least partially blocked by liquid natural gas condensate.

9. The method according to any one of claims 1 to 8, characterized in that the duration of the quiescent phase is selected so that in the solution (L) originally contained urea in the underground gas condensate deposit is completely hydrolyzed to carbon dioxide and ammonia and 60 bis 100% by weight of the water originally contained in the solution (L) is consumed.

10. The method according to any one of claims 1 to 9, characterized in that the at least one production bore during the resting phase in step c) is closed.

1 1. Method according to one of claims 1 to 10, characterized in that the solution (L)

From 50 to <76.9% by weight of urea,

> 0 to 35 wt .-% alcohol and

Contains 15 to 50 wt .-% water.

12. The method according to any one of claims 1 to 1 1, characterized in that the solution (L)

From 50 to <76.9% by weight of urea,

KA wt .-% alcohol as well

Contains 15 to 50 wt .-% water, wherein KA is given by the following formula:

KA = 100% by weight - (KH * 1, 3), in the KA the amount of alcohol contained in the solution (L) in wt .-% and KH indicates the amount of urea contained in the solution (L) in% by weight.

13. The method according to any one of claims 1 to 12, characterized in that the solution (L) in step b) is injected in volume quantities which correspond to a maximum of the pore volume of the critical area (KB).

14. The method according to any one of claims 1 to 13, characterized in 10 that the solution (L) is injected in step b) in volume quantities, which result in the hydrolysis of urea to a gas volume of carbon dioxide and ammonia, at least the pore volume of the critical area (KB).

Use of a solution (L) containing water and urea as a means of

Increasing the production rates of natural gas and / or natural gas condensate from a gas condensate reservoir containing a gas mixture with retrograde condensation behavior.