The invention is related to the field of methods and devices for determining the nature of the formation at the head of a drilling tool. More specifically, the invention concerns methods and devices for determining the nature of a formation at the well bottom during drilling operations, so as to be able to adapt the drilling trajectory in relation to the results obtained.
PRIOR STATE OF THE ART
When drilling, it is preferable to be able to bring data on the formation located around the bottom of the drilling well back to the ground surface as fast as possible.
In effect, by determining as quickly as possible the nature of the formation at the level of the drilling tool positioned at the bottom end of a drill string, it is optionally possible to alter the direction of the drilling tool so that it moves into an area rich in hydrocarbons, or quite simply to prevent the tool from moving toward formations basically containing water.
In this technical field, several implementations have already been proposed.
In general, the drilling of a well is performed using a drilling tool caused to rotate with a drill string. The latter is made up of an assembly of hollow rods screwed one to another. A drilling fluid circulating within the well from inside to outside the drill string, also called drilling sludge, is used in particular for cooling and lubricating the drilling tool, and more particularly for removing formation debris cut away by the drilling tool.
Concerning the removal function of formation debris cut away by the drilling tool, a first type of method for determining the nature of a formation at the bottom of a well is known. Basically, this method consists of analysing the composition of the drilling fluid when it has carried out a complete cycle within the well, and it has reached ground level.
Since the drilling fluid is extracted continuously from the drilling well, automatic or manual collection of samples of this fluid is conducted on the surface. These samples are used to perform tests so as to determine one or more physicochemical properties of the constituent formation of the bottom of the well, by analysing, for example, some of the debris extracted from the fluid derived directly from the formation through which the drilling tool has cut.
Nevertheless, this type of method has a major drawback in connection with drilling depth, which is often considerable possibly reaching several kilometres.
When the drilling fluid is discharged from the drill string at the level of the drilling tool, it takes in debris derived from the formation through which the tool has drilled and then moves back towards ground level. In very deep drill holes, the fluid containing the debris able to provide information on the nature of the drilled formation sometimes only reaches ground level some hours after passing through the drilling tool. Consequently, the information does not reach ground level fast enough for its real time use in adapting the drilling trajectory.
A second type of method is also known for determining the nature of a formation at the bottom of a well.
In this type of method, measuring means are assembled on the drill string in the vicinity of the drilling tool. It is to be specified that the distance between the drilling tool and the measuring means is sufficient to prevent any damage to the measuring means since the latter is subjected to impacts and vibrations caused partly by the drilling tool. Due to the nonexistence of satisfactory mechanical protection, it is therefore recommended that the measuring means be installed a certain distance away from the drilling tool. By way of example, the measuring means are usually positioned approximately ten or twenty metres from the drilling tool, or at an even greater distance. The measuring means may be of any kind. Means are known which enable ‘Logging While Drilling’ techniques, which may, for example, measure the resistivity or the nuclear density of the formations drilled by the drilling tool.
In one such method, the data measured is generally transmitted to ground level using sound waves, which makes it possible to correct the direction of drilling, if necessary, almost instantaneously after the measurement has been taken.
However, one essential drawback remains at the time of implementing this type of method.
While it is true that the transmission of the physicochemical data measured is very fast, these data nevertheless concern physicochemical properties relating to the formations located at the level of the measuring means. On this account, the measurements made do not concern the formation forming the bottom of the well directly in contact with the drilling tool, but only concern the formation or formations located at some tens of metres, even at over one hundred metres above the bottom of the drilling well.
Consequently, when information on a physicochemical property of a formation reaches the ground surface, the drilling tool is already quite distant from that formation. Therefore, the drilling tool may be located in a formation having a totally different nature to that of the formation for which data is available. In such event, the contact of the drilling tool with a hostile formation would not be noted at ground level until some time afterwards, which may be as long as several hours.
Furthermore, this drawback is even more restrictive when hydrocarbon-rich formations are not very thick, for example, just a few metres deep, with the margin of error in the drilling trajectory then becoming more restricted.
The methods of the prior art using conventional drilling fluids therefore do not provide real time information on the physicochemical properties of the formation at the bottom of the well.
This situation not only causes substantial extra drilling costs through a non-optimised drilling trajectory, but also additional costs due to the drilling equipment used. Since a formation rich in hydrocarbons is generally in contact with an underlying formation mainly made up by water, if a drilling tool mistakenly reaches a water source, the hydrocarbons come into contact with and are mixed with the water. This then causes the formation of a hydrocarbon/water mixture making the material to be extracted from the drilling well considerably heavier. One possible consequence of this trajectory error is to make the well non-eruptive, requiring heavy pump-type means to pump out the hydrocarbon/water mixture from the well, together with costly means for separating the water from the hydrocarbons and re-injecting the water into the ground.
Also, it is to be pointed out that the lack of real-time control over the drilling trajectory may also represent a major explosion risk, should the drilling tool perforate a cavity containing gas under heavy pressure.
DESCRIPTION OF THE INVENTION
The purpose of the invention is therefore to present a method and a device for determining the nature of a formation at the head of a drilling tool, thereby overcoming, at least in part, the above-described drawbacks relating to the implementations of the prior art.
The present invention also sets out to propose a method and device enabling almost real-time delivery of information on the formation that makes up the bottom of the drilling well.
To achieve these purposes, the subject of the invention is firstly a method for determining the nature of a formation at the bottom of a drilling well, the well being equipped with a drill string fitted with a drilling tool and filled with a drilling fluid circulating within the well from inside to outside said drill string. According to the invention, the method comprises the following steps:
differential measurement, between the outside and the inside of the drill string in the vicinity of the drilling tool, of at least one physicochemical property of the drilling fluid;
using the differential measurement to determine the nature of the formation at the bottom of the drilling well.
Advantageously, the method according to the invention, allows very fast determination of the nature of the formation that makes up the bottom of the drilling well using a single measurement of an appropriate physicochemical property or properties of the drilling fluid.
In effect, the time required for the drilling fluid to circulate between the formation at the bottom of the well, where it takes in debris, and the measuring means positioned in the vicinity of the drilling tool, remains short and does not exceed a few minutes.
Thus, information is available with which to determine the nature of the formation at the bottom of the well in distinctly quicker manner than with the methods of the prior art.
In addition, with the differential measurement obtained it is possible to overcome variations in measurement due to changes in measuring conditions caused, for example, by a temperature increase in the drilling fluid as and when the drilling well increases in depth.
According to a first preferred embodiment, the determination of the nature of the formation at the bottom of the drilling well consists of qualitatively determining at least one physicochemical property of the formation at the bottom of the well. In this way, a simple comparison between the physicochemical properties of the drilling fluid and those of the formation at the bottom of the well, lead to easy determination of the nature of this formation.
Also in a second preferred embodiment of the invention, provision may also be made so that determination of the nature of the formation at the bottom of the drilling well consists of a quantitative determination of at least one physicochemical property of the formation at the bottom of the well. In this case, in addition to qualitative knowledge of at least one physicochemical property of the formation at the bottom of the well, it is also possible to deduce the value of these physicochemical properties relative to the formation.
According to a third preferred embodiment of the invention, the determination of the nature of the formation may consist of comparing at least one measured physicochemical property of the drilling fluid with pre-set values. Advantageously, these pre-set values are able to provide direct information on the nature of the formations encountered by the drilling tool.
It is to be specified that the method may include a data-transmission step for the transmission of information from the bottom to the top surface of the well, and this may be conducted continuously when drilling.
A further subject of the invention is a device for determining the nature of a formation at the bottom of a drilling well, the well being equipped with a drill string fitted with a drilling tool and filled with a drilling fluid circulating within the well from inside to outside said drill string, said device comprising primary measuring means and secondary measuring means mounted on the drill string in the vicinity of the drilling tool. According to the invention, the primary measuring means and the secondary measuring means are suitable for measuring at least one physicochemical property of the drilling fluid respectively located outside and inside said drill string, so as to obtain a differential measurement of each measured physicochemical property, at least one physicochemical property measured enabling determination of the nature of the formation at the bottom of the drilling well.
Further, the primary measuring means are able to measure at least one of the physicochemical properties of the drilling fluid chosen from among the group comprising impedance, pH, nuclear density, electric voltage and chemical tracers which react with the fluid contained in the formation.
Other characteristics and advantages of the invention will become apparent in the non-restrictive description given below.