WO2011133363A2 - Neutron porosity downhole tool with improved precision and reduced lithology effects - Google Patents
Neutron porosity downhole tool with improved precision and reduced lithology effects Download PDFInfo
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- WO2011133363A2 WO2011133363A2 PCT/US2011/032210 US2011032210W WO2011133363A2 WO 2011133363 A2 WO2011133363 A2 WO 2011133363A2 US 2011032210 W US2011032210 W US 2011032210W WO 2011133363 A2 WO2011133363 A2 WO 2011133363A2
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- neutrons
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V5/00—Prospecting or detecting by the use of nuclear radiation, e.g. of natural or induced radioactivity
- G01V5/04—Prospecting or detecting by the use of nuclear radiation, e.g. of natural or induced radioactivity specially adapted for well-logging
- G01V5/08—Prospecting or detecting by the use of nuclear radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays
- G01V5/10—Prospecting or detecting by the use of nuclear radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays using neutron sources
- G01V5/107—Prospecting or detecting by the use of nuclear radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays using neutron sources and detecting reflected or back-scattered neutrons
Definitions
- the present disclosure relates generally to neutron well logging and, more particularly, to a neutron porosity downhole tool employing a neutron monitor for improved precision and reduced lithology effects.
- Downhole tools for neutron well logging have been used in oilfield settings for many years to measure formation porosity and as gas and lithology indicators.
- These downhole tools have historically included a radioisotopic neutron source, such as AmBe, which emits neutrons into the surrounding formation.
- the neutrons may interact with the formation before being subsequently detected in neutron count rates by one or more neutron detectors.
- the neutron count rates may be sensitive to hydrogen in formation pore spaces. In general, the more hydrogen there is in the formation, the fewer neutrons arrive at the detector. Since formation porosity is generally water or hydrocarbon-filled, the neutron count rates may be employed to determine a porosity of the formation.
- porosity may be determined based on the detector count rate normalized to the neutron output of the source, which may be predictable. Indeed, it may be sufficient to perform a one-time calibration to determine the neutron output of the radioisotopic source if it has a sufficiently long half life as in the case of 241 AmBe. Thereafter, any change in the future can, in principle, be predicted from the known half-life of the radioisotopic material.
- a radioisotopic neutron source may be undesirable for a variety of reasons. For example, the use of a radioisotopic source may involve negotiating burdensome regulations and the sources may have limited useful lives (e.g., 1 to 15 years).
- radioisotopic sources are becoming more expensive and more difficult to obtain.
- neutron generators may be used in place of a radioisotopic neutron source in a neutron porosity tool.
- the output of an electronic neutron generator may be difficult or impossible to predict from the operating parameters of the tool.
- neutron generator-based devices may generally determine porosity from a ratio of count rates from detectors at different spacing (e.g., a near/far count rate ratio) in order to cancel out any variations in the output of the neutron source.
- While this method may achieve its stated goal and may introduce certain other positive effects (e.g., reducing the device's sensitivity to several borehole effects unrelated to porosity), it also may reduce the porosity sensitivity of the tool, since some of the individual detectors' sensitivity also may be canceled out in the ratio.
- This reduction in sensitivity may be especially problematic when using the deuterium-tritium (d-T) reaction-based neutron generators generally employed in the oilfield. These generators produce 14 MeV neutrons, more than twice the average energy of neutrons from an AmBe source. Used in a device with typical near and far source-detector spacings of around 1 and 2 feet, respectively, this higher neutron source energy results in a dramatic drop in porosity sensitivity at high porosity compared to an AmBe-based device.
- d-T deuterium-tritium
- a downhole neutron porosity tool may include a neutron source, a neutron monitor, a neutron detector, and data processing circuitry.
- the neutron source may emit neutrons into a subterranean formation while the neutron monitor detects a count of neutrons proportional to the neutrons emitted.
- the neutron detector may detect a count of neutrons that scatters off the subterranean formation.
- the data processing circuitry may determine an environmentally corrected porosity of the subterranean formation based at least in part on the count rate of neutrons scattered off the subterranean formation normalized to the count rate of neutrons proportional to the neutrons emitted by the neutron source.
- FIG. 1 is a schematic block diagram of a neutron porosity well logging system, in accordance with an embodiment
- FIG. 2 is a schematic diagram illustrating a neutron porosity well logging process using the system of FIG. 1, in accordance with an embodiment
- FIG. 3 is a flowchart describing an embodiment of a method for performing the neutron porosity well logging process of FIG. 2;
- FIG. 4 is a flowchart describing an embodiment of another method for performing the neutron porosity well logging process of FIG. 2;
- FIGS. 5-10 are plots modeling exemplary neutron counts obtained from individual neutron detectors and ratios of neutron detectors, in accordance with an embodiment;
- FIGS. 11 and 12 are plots modeling porosity sensitivity associated with the neutron counts of FIGS. 5-10, in accordance with an embodiment
- FIGS. 13 and 14 are plots modeling porosity precision associated with the neutron counts of FIGS. 5-10, in accordance with an embodiment.
- FIGS. 15 and 16 are plots modeling lithology effects arising from a single epithermal far neutron detector and from a ratio of epithermal near and far neutron detectors, respectively, in accordance with embodiments.
- present embodiments relate to downhole neutron porosity tools having improved precision and reduced lithology effects through the use of a neutron monitor.
- the neutron monitor may be a neutron detector employed by the downhole neutron porosity tool for the express purpose of measuring the neutron source output.
- traditional neutron porosity devices employ low-energy neutron detectors, since it is through the very process of the neutrons slowing down from high to low energy that the porosity sensitivity arises.
- a high-energy neutron monitor e.g., a neutron detector capable of detecting neutrons of greater than 1 MeV
- its count rate can be used as a direct measure of the source neutron output.
- the count rate from the (high-energy) neutron monitor may be essentially unperturbed by environmental effects from the borehole and the formation, since almost all of the neutrons counted by the detector will have undergone no scattering before reaching the neutron monitor.
- neutron porosity devices may first compute an apparent porosity from the measured count rates or count rate ratios assuming a standard set of downhole conditions (e.g., calcite formation, 8 inch borehole, fresh water borehole and formation fluids, 20 C, 1 atm, etc.). Any deviations between this apparent porosity and the true porosity are referred to as environmental effects. In particular, deviations caused by the presence of a formation lithology other than the nominal one, often taken to be calcite, may be referred to as the lithology effect.
- a standard set of downhole conditions e.g., calcite formation, 8 inch borehole, fresh water borehole and formation fluids, 20 C, 1 atm, etc.
- This lithology effect is dependent on the source- detector spacing and can be reduced for epithermal detectors by choosing an appropriate spacing.
- a traditional porosity measurement using the count rate ratio of two low-energy neutron detectors, only one of the low-energy neutron detectors will be at the optimal spacing.
- a lithology effect may be reintroduced through the non-optimally spaced detector in the ratio.
- the use of a neutron monitor to normalize the neutron generator output also may allow certain vertical resolution enhancing techniques in combination with a downhole neutron porosity tool having a variable-output neutron source, such as an electronic neutron generator.
- the vertical resolution enhancing techniques may involve alpha processing, which is described in detail in U.S. Patent No. 4,786,796 to Flaum et al., assigned to
- determining porosity based on a neutron detector count rate normalized to a neutron monitor count rate may provide a dramatic improvement in operational efficiency.
- the presently disclosed techniques may reduce statistical error in porosity precision and by a statistical factor of 2. Since otherwise reducing the statistical error by a factor of 2 may require increasing the measurement time by a factor of 4, the improvement in precision utilizing the present techniques may be considered equivalent to a potential increase in logging speed by a factor of 4 to achieve the same statistical precision that would have been achieved using a ratio porosity measurement.
- FIG. 1 illustrates a neutron well logging system 10 for determining a porosity of a subterranean formation with high precision as well as reduced lithology effects.
- the neutron well logging system 10 may include a downhole tool 12 and a data processing system 14. Although the downhole tool 12 and the data processing system 14 are illustrated as separate from one another, the data processing system 14 may be incorporated into the downhole tool 12 in certain embodiments.
- the downhole tool 12 may be a slickline or wireline tool for logging an existing well, or may be installed in a borehole assembly (BHA) for logging while drilling (LWD).
- BHA borehole assembly
- the downhole tool 12 may be encased within a housing 16 that houses, among other things, a neutron source 18.
- the neutron source 18 may include a neutron source capable of emitting relatively high-energy neutrons, such as 14 MeV neutrons.
- the neutron source 18 may be an electronic neutron source, such as a MinitronTM by Schlumberger Technology Corporation, which may produce pulses of neutrons or a continuous stream of neutrons via d-T reactions.
- the neutron source 18 may include a radioisotopic source, which may or may not emit neutrons at a variable or unpredictable rate.
- Neutron shielding 20 may separate the neutron source 18 from other components of the downhole tool 12.
- the downhole tool 12 may include a neutron monitor 22.
- the neutron monitor 22 may measure the output of the neutron source 18 to provide a basis for normalizing the neutron counts detected by other neutron detectors 24, as discussed below, which may measure neutrons scattered by a surrounding formation.
- the neutron monitor 22 may be any suitable neutron detector in any suitable configuration within the downhole tool 12 that effectively measures substantially only neutrons emitted by the neutron source 18 that have not been scattered by the surrounding formation.
- the neutron monitor 22 may be sensitive only to high-energy neutrons (e.g., of greater than 1 MeV and/or of energy levels roughly equal to that emitted by the electronic neutron source 18), may be located very close to the neutron source 18, and/or may be well-shielded from neutrons returning to the downhole tool 12 from the surrounding formation.
- the neutron monitor 22 may include a plastic scintillator coupled to a photomultiplier. Such a plastic scintillator may be described by U.S. Patent No. 6,884,994 to Simonetti et al., assigned to Schlumberger
- the neutron monitor 22 may include other detectors of fast neutrons, such as He-4 gas counters, hydrogen proportional counters, liquid scintillators, or solid state detectors such as SiC or diamond.
- the downhole tool 12 may include at least one low-energy neutron detector 24.
- a "near" neutron detector 24 is located more closely to the neutron source 18 than a similar "far” neutron detector 24.
- the downhole tool 12 may also contain additional low-energy neutron detectors 24 at intermediate spacings between "near" and "far.” If the downhole tool 12 includes only one neutron detector 24, a "far" neutron detector 24 spaced around 1 foot from the neutron source 18 generally may provide a more accurate measurement of porosity. If the downhole tool 12 includes both near and far neutron detectors 24, certain vertical resolution enhancing techniques, such as alpha processing, may be employed by the data processing system 14.
- the downhole tool 12 may further include one or more borehole-facing neutron detectors 25 in certain embodiments.
- the borehole-facing neutron detector 25 may be more sensitive to neutrons that arrive via the borehole than the near and far neutron detectors 24.
- neutron count rates obtained from the borehole-facing neutron detector 25 may be employed to correct for environmental effects related to borehole fluid composition and/or borehole geometry, as discussed in greater detail in U.S. Patent Application 12/729,384 (Atty. Docket 49.0363), filed on March 23, 2010 and assigned to Schlumberger Technology Corporation, which is incorporated by reference herein in its entirety.
- the downhole tool 12 may not include the borehole-facing neutron detector 25, but may still employ other techniques to correct for environmental effects associated with the borehole. Such techniques may include, among other things, measuring the epithermal slowing down time to correct for standoff, which may correlate with certain environmental effects. These other techniques may be employed alone or in combination with measurements from the borehole-facing neutron detector 25 to reduce the environmental effects to produce a meaningful porosity measurement with reduced lithology effects.
- neutron shields 20 also may be placed between the neutron detectors 24 and the borehole-facing side of the downhole tool 12. These neutron shields 20 may reduce the number of neutrons that may reach the neutron detectors 24 via the borehole, versus those reaching the detector via the formation, thereby increasing the sensitivity of the downhole tool 12 to formation properties versus those of the borehole. Additionally, the placement of the neutron shields 20 may increase the number of neutrons that may reach the borehole-facing neutron detector 25 via the borehole.
- the neutron detectors 24 and 25 may be any neutron detectors capable of detecting thermal and/or epithermal neutrons. In some embodiments, the neutron detectors 24 and 25 may also be relatively insensitive to high energy neutrons, such as those emitted by the neutron source 18. In general, the neutron detectors 24 and 25 may be configured substantially not to detect neutrons having an energy, for example, of 1 keV or greater. In some embodiments, the neutron detectors 24 and 25 may be 3 He neutron detectors. In certain other embodiments, the neutron detectors 24 and 25 may be capable of detecting epithermal neutrons, but similarly may be relatively insensitive to the high energy neutrons emitted by the neutron source 18.
- the near neutron detector 24 may have a "near spacing” measured from the neutron source 18 to the face of the active region of the near neutron detector 24 nearest to the neutron source 18, and the far neutron detector 24 may have a "far spacing” measured from the neutron source 18 to the face of the active region of the far neutron detector 24 nearest to the neutron source 18.
- the borehole-facing neutron detector 25 may have a "back spacing" that is nearer to the neutron source 18 than either the front spacing or the back spacing.
- the far spacing may be selected such that porosities computed based on the far neutron detector 24 count rate normalized to the neutron monitor 22 have a relatively high accuracy under a standard set of conditions (e.g., calcite formation, 8 inch borehole, fresh water borehole and formation fluids, 20 C, 1 atm, etc.), upon which an apparent porosity relationship may be based.
- a far spacing may be approximately 2 feet.
- the near neutron detector 22 may have a near spacing that enables the extraction of enhanced vertical resolution information when a porosity computed based on its normalized count rate is employed in combination with the porosity computed from that of the far neutron detector 24.
- such a near spacing may be approximately 1 foot.
- the near spacing may be much closer than many traditional configurations. Indeed, in such embodiments, the near spacing may be chosen such that, at low porosities, many of the neutrons that reach the near neutron detector 22 either directly from the neutron source or after interacting with the subterranean formation, borehole and/or within the device itself have energies too high to detect. At relatively higher porosities, due to the additional scattering off of hydrogen nuclei, the number of lower-energy, detectable neutrons may increase, as the distance the neutrons travel before being slowed to these energies decreases.
- the additional scattering off hydrogen may eventually reduce the number of neutrons of any energy that reach the detector, but not before resulting in a porosity response that is relatively flat or even increasing over part of the porosity range.
- the exact optimal spacing will depend on specific details of the design of the downhole tool 12, including the size and efficiency versus energy of the neutron detector 24, and where, what kind, and how much neutron shielding is used.
- the near neutron detector 24 may be spaced such that its porosity response may be relatively flat and/or increasing as porosity increases. Such a spacing may enable a high porosity sensitivity, as discussed in greater detail in U.S. Provisional Patent Application 61/115,670 (Atty. Docket 49.0357), filed on November 17, 2009 and assigned to Schlumberger Technology Corporation, which is incorporated by reference herein in its entirety.
- the neutron detectors 24 may detect a quantity of neutrons that varies depending on the output of the neutron source 18 and the porosity of the formation, among other things.
- the responses of the neutron monitor 22 and the neutron detectors 24 may be transferred as data 26 to the data processing system 14.
- the porosity sensitivity of the one of the neutron detectors 24 may be preserved in the computed porosity.
- the porosity may be corrected for environmental effects based on a variety of techniques, such as by measuring epithermal neutron slowing down time to correct for the standoff of the downhole tool 12 or by measuring neutrons from the borehole-facing neutron detector 25.
- the data processing system 14 may include a general-purpose computer, such as a personal computer, configured to run a variety of software, including software implementing all or part of the present techniques.
- the data processing system 14 may include, among other things, a mainframe computer, a distributed computing system, or an application-specific computer or workstation configured to implement all or part of present techniques based on specialized software and/or hardware provided as part of the system.
- the data processing system 14 may include either a single processor or a plurality of processors to facilitate
- processing may take place at least in part by an embedded processor in the downhole tool 12.
- the data processing system 14 may include data acquisition circuitry 28 and data processing circuitry 30.
- the data processing circuitry 30 may be a microcontroller or microprocessor, such as a central processing unit (CPU), which may execute various routines and processing functions.
- the data processing circuitry 30 may execute various operating system instructions as well as software routines configured to effect certain processes.
- These instructions and/or routines may be stored in or provided by an article of manufacture, which may include a computer readable-medium, such as a memory device (e.g., a random access memory (RAM) of a personal computer) or one or more mass storage devices (e.g., an internal or external hard drive, a solid-state storage device, CD-ROM, DVD, or other storage device).
- the data processing circuitry 30 may process data provided as inputs for various routines or software programs, including the data 26.
- Such data associated with the present techniques may be stored in, or provided by, a memory or mass storage device of the data processing system 14.
- data may be provided to the data processing circuitry 30 of the data processing system 14 via one or more input devices.
- data acquisition circuitry 28 may represent one such input device; however, the input devices may also include manual input devices, such as a keyboard, a mouse, or the like.
- the input devices may include a network device, such as a wired or wireless Ethernet card, a wireless network adapter, or any of various ports or devices configured to facilitate communication with other devices via any suitable communications network, such as a local area network or the Internet.
- the data processing system 14 may exchange data and communicate with other networked electronic systems, whether proximate to or remote from the system.
- the network may include various components that facilitate communication, including switches, routers, servers or other computers, network adapters, communications cables, and so forth.
- the downhole tool 12 may transmit the data 26 to the data acquisition circuitry 28 of the data processing system 14 via, for example, internal connections within the downhole tool 12 or the downhole tool 12 string, a telemetry system communication to the surface (uplink) through a cable or other means of downhole- to-surface communication, or a communication cable or other communication link that may connect the surface unit to a unit in a different location.
- the data acquisition circuitry 28 may transmit the data 26 to the data processing circuitry 30.
- the data processing circuitry 30 may process the data 26 to ascertain one or more properties of a subterranean formation surrounding the downhole tool 12, such as porosity. Such processing may involve, for example, determining a porosity based on the neutron count of the far neutron detector normalized to the neutron count of the neutron monitor. Additionally or alternatively, the processing may involve performing a vertical resolution enhancing technique. The vertical resolution enhancing technique may include alpha processing a porosity computed based on the normalized near detector count rate.
- the data processing circuitry 30 may thereafter output a report 32 indicating the one or more ascertained properties of the formation.
- the report 32 may be stored in memory or may be provided to an operator via one or more output devices, such as an electronic display and/or a printer.
- FIG. 2 represents a well logging operation 34 using the downhole tool 12 to ascertain a porosity of a subterranean formation 36.
- the downhole tool 12 may be lowered into a borehole 38 in the subterranean formation 36, which may or may not be cased in a casing 40.
- the borehole 38 may have a diameter D, which may impact the neutron counts detected by the downhole tool 12, as discussed below.
- a neutron emission 42 from the neutron source 18 may have various interactions 44 with elements of the subterranean formation 36 and/or the borehole 38.
- the neutron emission 42 may be a neutron burst containing 14-MeV neutrons.
- the neutron monitor 22 may obtain a count of emitted neutrons that has not substantially interacted 44 with the subterranean formation 36. This count of emitted neutrons, which may be proportional to the total neutron emission 42, may form a basis upon which to normalize counts subsequently obtained by other neutron detectors 24 of the downhole tool 12.
- the interactions 44 of the neutron emission 42 with elements of the subterranean formation 36 and/or the borehole 38 may include, for example, inelastic scattering, elastic scattering, and neutron capture. These interactions 44 may result in neutrons 46 from the neutron emission 42 traveling through the subterranean formation 36 or borehole 38 and reaching the neutron detectors 24 at lower energies than when first emitted. Depending on the composition of the subterranean formation 36, the borehole 38, and/or the downhole tool 12 itself, the interactions 44 may vary. For example, hydrogen atoms may cause elastic scattering.
- chlorine atoms found in salt in the subterranean formation 36 or borehole fluid may cause neutron capture events 48 for certain of the neutrons 46 after the neutrons 46 have reduced in energy below approximately 0.1 eV.
- the numbers and energies of the neutrons 46 that reach the neutron detectors 24 at different distances from the neutron source 18 may thus vary based in part on properties of the subterranean formation 34, including, among other things, a porosity of the subterranean formation 36.
- the data processing system 14 may ascertain the porosity of the subterranean formation 36 using any suitable technique.
- FIG. 3 is a flow chart 50 representing an embodiment of a method for performing the neutron well logging operation 34 of FIG. 2.
- the downhole tool 12 may be deployed into the subterranean formation 36 on a wireline, slickline, or while the borehole 38 is being drilled by a borehole assembly (BHA).
- the neutron source 18 may emit neutrons (illustrated as neutron emission 42 in FIG. 2) into the surrounding subterranean formation 36.
- the neutron emission 42 may be in bursts of neutrons or as a continuous stream of neutrons.
- the neutron monitor 22 may detect a count of the emitted neutrons that is proportional to the total neutron emission 42 in step 56.
- the data processing system 14 may normalize the response of the at least one neutron detector 24 to the response of the neutron monitor 22 to obtain a normalized neutron count and, in step 62, the data processing system 14 may determine a porosity of the subterranean formation 36 based on the normalized neutron count. As noted above, the data processing system 14 may determine the porosity using any suitable technique.
- the data processing system 14 may determine an apparent porosity from the normalized neutron count, upon which specific environmental corrections may be applied to determine the actual porosity. Additionally or alternatively, the data processing system 14 may determine the porosity at least in part using a transform derived from modeled and/or experimental data that relates the normalized neutron count to the porosity of a subterranean formation.
- the apparent porosity determined in step 62 may many undesirable environmental effects. These environmental effects may be much greater than those of porosities based on ratios of neutron count rates. Thus, in step 63, the data processing system 14 may undertake one or more correction schemes to eliminate some of the environmental effects from the porosity determined in step 62.
- the data processing system 14 may perform step 63 by applying environmental corrections based on distinctions between the neutron count rates from the front-facing neutron detector(s) 24 and the borehole-facing neutron detector 25. For example, the data processing system 14 may determine, based on neutron count rates obtained from the borehole-facing neutron detector 25 a back apparent porosity (p back using any suitable techniques for computing porosity.
- the data processing system 14 may compute a corrected porosity ⁇ p corr based on a relationship between the near apparent porosity (p near and the back apparent porosity 3 ⁇ 4 d and a corresponding true porosity.
- a relationship may include, for example, a polynomial in the apparent porosities: i+j ⁇ n
- Equation (1) represents a polynomial function, it should be understood that any suitable functional form may be employed to compute the corrected porosity (p corr in the manner described above. Additionally or alternatively, the data processing system 14 may determine the porosity directly from the neutron count rates using a transform derived from modeled and/or experimental data relating the epithermal neutron count rates to various borehole and formation conditions. Additionally or alternatively, the data processing system 14 may determine the corrected porosity by an inversion of a forward model giving the expected count rates (or apparent porosities) as a function of the true porosity and other formation 36 and borehole 38 conditions.
- the data processing system 14 may apply environmental corrections such as for standoff to the porosity determined in step 62 by computing an epithermal neutron slowing down time.
- a flowchart 64 illustrated in FIG. 4 describes an embodiment of a method for obtaining a high vertical resolution porosity using the downhole tool 12 when the downhole tool 12 includes the near neutron detector 24 and the far neutron detector 24.
- an "accurate" porosity may be determined based on the response of the far neutron detector 24 in the manner described above with reference to the flowchart 50 of FIG. 3. This porosity measurement may be termed “accurate” because the porosity from the far neutron detector 24 is likely to be more accurate than the porosity from the near neutron detector 24, but may have poorer vertical resolution.
- a "less accurate” porosity with high vertical resolution may be determined based on the response of the near neutron detector 24 in a like manner. This porosity measurement may be termed "higher vertical resolution” because the porosity from the near neutron detector 24 is likely to have a higher vertical resolution than the porosity from the far neutron detector 24.
- the data processing system 14 may perform alpha processing using the porosity measurements determined in steps 66 and 68 to obtain a high vertical resolution porosity measurement.
- the techniques by which the data processing system 14 may perform alpha processing may be described in U.S. Patent No. 4,786,796 to Flaum et al., assigned to Schlumberger Technology Corporation, which is incorporated by reference herein in its entirety.
- FIGS. 5-16 represent plots comparing measurements associated with porosity based on a traditional ratio of near/far neutron detector 24 counts and based on the individual near and/or far neutron detector 24 counts normalized to the output of the neutron source 18 using the neutron monitor 22. These plots are intended to illustrate that the disclosed techniques involving normalized neutron detector 24 counts may provide porosity measurements with improved precision and reduced lithology effects as compared to traditional techniques.
- the plots illustrated in FIGS. 5-16 have been modeled using the Monte Carlo N-Particle transport code, (MCNP), a leading nuclear Monte Carlo modeling code.
- MCNP Monte Carlo N-Particle transport code
- detector size, neutron source strength, and shielding may vary in different embodiments of the well logging system 10; however, the use of the neutron monitor 22 to normalize the responses of the neutron detectors 24 generally may provide the benefits disclosed herein. As such, while these variables may influence the absolute count rates modeled by the plots of FIGS. 5-16, the relative shape of the responses generally may remain the same.
- FIGS. 5-10 represent modeled plots of count rates from the near and far neutron detectors 24 and ratios of the near and far neutron detectors 24 relative to the porosity of the subterranean formation 36.
- FIGS. 5-7 represent count rates when the neutron detectors 24 are thermal neutron detectors
- FIGS. 8-10 represent count rates when the neutron detectors 24 are epithermal neutron detectors.
- the near and far source-detector spacings are approximately 1 and 2 feet, respectively. Most of the region between the neutron source 18 and the end of far detector 24 not occupied by the detectors 24 themselves is occupied by neutron shielding 20.
- a plot 80 includes an ordinate 82 representing count rate in units of counts per second (cps) and an abscissa 84 representing porosity in porosity units (p.u.) for a thermal near neutron detector 24 spaced approximately 1 foot from the neutron source 18.
- a curve 86 illustrates a relationship between count rate and porosity obtained by the thermal near neutron detector 24 alone.
- FIG. 6 illustrates a plot 88, which includes an ordinate 90 of count rate in units of counts per second (cps) and an abscissa 92 of porosity in porosity units (p.u.) for a thermal far neutron detector 24 spaced approximately 2 feet from the neutron source 18.
- a curve 94 illustrates a relationship between count rate and porosity obtained by the thermal far neutron detector 24 alone.
- FIG. 7 illustrates a plot 96, which includes an ordinate 98 of a ratio of count rates and an abscissa 100 of porosity in porosity units (p.u.) for a ratio of the counts of the thermal near neutron detector 24 to those of the thermal far neutron detector 24.
- a curve 102 illustrates a relationship between the ratio of count rates and the porosity of the subterranean formation 36.
- a plot 104 includes an ordinate 106 representing count rate in units of counts per second (cps) and an abscissa 108 representing porosity in porosity units (p.u.) for an epithermal near neutron detector 24 spaced approximately 1 foot from the neutron source 18.
- a curve 110 illustrates a relationship between count rate and porosity obtained by the epithermal near neutron detector 24 alone.
- FIG. 9 illustrates a plot 112, which includes an ordinate 114 of count rate in units of counts per second (cps) and an abscissa 116 of porosity in porosity units (p.u.) for an epithermal far neutron detector 24 spaced approximately 2 feet from the neutron source 18.
- a curve 118 illustrates a relationship between count rate and porosity obtained by the epithermal far neutron detector 24 alone.
- FIG. 10 illustrates a plot 120, which includes an ordinate 122 of a ratio of count rates in units of counts per second (cps) and an abscissa 124 of porosity in porosity units (p.u.) for a ratio of the counts of the epithermal near neutron detector 24 to those of the epithermal far neutron detector 24.
- a curve 126 illustrates a relationship between the ratio of count rates and the porosity of the subterranean formation 36.
- Porosity may be calculated based on the count rates plotted in FIGS. 5-10. However, as illustrated by FIGS. 11 and 12, the porosity sensitivity may vary dramatically depending upon which of these count rates is used in determining the porosity.
- a plot 128 illustrates porosity sensitivity from the neutron count rates of thermal neutron detectors 24 illustrated in FIGS. 5-7.
- the plot 128 includes an ordinate 130 representing porosity sensitivity in units of percent per porosity percentage unit (p.u.) (i.e., ) and an abscissa 132
- Curves 134, 136, and 138 respectively illustrate porosity sensitivity derived from the thermal near neutron detector 24 count rate of FIG. 5 normalized to a count rate of the neutron monitor 22, the thermal far neutron detector 24 count rate of FIG. 6 normalized to the count rate of the neutron monitor 22, and the ratio of count rates of the thermal neutron detectors 24 of FIG. 7.
- a plot 140 illustrates porosity sensitivity from the neutron count rates of epithermal neutron detectors 24 illustrated in FIGS. 8-10.
- the plot 140 includes an ordinate 142 representing porosity sensitivity in units of percent per porosity percentage unit (p.u.) and an abscissa 144 representing porosity in units of porosity units (p.u.).
- Curves 146, 148, and 150 respectively illustrate porosity sensitivity deriving from the epithermal near neutron detector 24 count rate of FIG. 8 normalized to a count rate of the neutron monitor 22, the epithermal far neutron detector 24 count rate of FIG. 9 normalized to a count rate of the neutron monitor 22, and the ratio of count rates of the epithermal neutron detectors 24 of FIG. 10. (Note that in the near and far porosity sensitivity calculations, the neutron monitor 22 count rate used for normalization doesn't depend on porosity and thus cancels in ratio.)
- the plots 128 and 140 illustrate that the normalized far neutron detector 24 counts provide higher porosity sensitivities than corresponding ratios of neutron counts from similar detectors.
- the thermal far neutron detector 24 porosity sensitivity curve 136 is higher at all porosities than the thermal neutron detector 24 ratio porosity sensitivity curve 138.
- the epithermal far neutron detector 24 porosity sensitivity curve 148 is higher at all porosities than the epithermal neutron detector 24 ratio porosity sensitivity curve 150.
- the near neutron detector 24 porosity sensitivity curves 134 and 146 show a higher porosity sensitivity than the respective ratio porosity sensitivity curves 138 and 150 at porosities higher than approximately 25 p.u.
- certain of the normalized counts of the neutron detectors 24 also may have improved porosity precision over the ratio of counts of the neutron detectors 24.
- Exemplary plots of the porosity precision of the neutron counts of FIGS. 5-10 appear in FIGS. 13 and 14. These plots show the corresponding one standard deviation porosity precision achieved with one second of data acquisition, assuming typical detector 24 sizes and source neutron 18 output.
- the near and far neutron detector 24 precisions are equivalent to the precision for the count rates normalized by the neutron monitor 22 count rate if it is assumed that the count rate in the neutron monitor 22 can be made much larger than the count rates in the low energy detectors 24. This is generally achievable due to the close proximity to the neutron source 18 of the neutron monitor 22.
- FIG. 13 is a plot 152, which represents porosity precision when porosity is determined from the neutron count rates of thermal neutron detectors 24, as illustrated in FIGS. 5-7.
- An ordinate 154 represents porosity precision (statistical error referred to as precision) in units of porosity units (p.u.) at one second, and an abscissa 156 represents porosity in units of porosity units (p.u.).
- Curves 158, 160, and 162 respectively illustrate porosity precision derived from the thermal near neutron detector 24 count rate of FIG. 5 normalized to a count rate of the neutron monitor 22, the thermal far neutron detector 24 count rate of FIG.
- FIG. 14 illustrates a plot 164, which represents porosity precision when porosity is determined from the neutron count rates of epithermal neutron detectors 24, as illustrated in FIGS. 8-10.
- An ordinate 166 represents porosity precision in units of porosity units (p.u.) at one second
- an abscissa 168 represents porosity in units of porosity units (p.u.).
- Curves 170, 172, and 174 respectively illustrate porosity precision derived from the epithermal near neutron detector 24 count rate of FIG. 8 normalized to a count rate of the neutron monitor 22, the epithermal far neutron detector 24 count rate of FIG. 9 normalized to a count rate of the neutron monitor 22, and the ratio of count rates of the epithermal neutron detectors 24 of FIG. 10.
- the porosity precision obtained from normalized count rates of the neutron detectors 24 may be significantly better than the porosity precision obtained from ratios of count rates of the neutron detectors 24.
- the thermal far neutron detector 24 porosity precision curve 160 is equal to or better at all porosities than the thermal neutron detector 24 ratio porosity precision curve 162.
- the epithermal far neutron detector 24 porosity precision curve 172 and epithermal near neutron detector 24 porosity precision curve 170 may be better at all porosities than the epithermal neutron detector 24 ratio porosity precision curve 174.
- obtaining porosity from a normalized count rate of neutrons from a single neutron detector 24 rather than from a ratio of neutron detectors 24 may provide improved porosity sensitivity and improved porosity precision.
- obtaining porosity from a normalized count rate of neutrons from a single neutron detector 24 may result in a reduction of lithology effects as compared to obtaining porosity from a ratio of neutron detectors 24.
- FIGS. 15 and 16 respectively offer plots illustrating the lithology effect for porosity obtained from the neutron count rate of a single epithermal far neutron detector 24 and for porosity obtained from a ratio of epithermal near neutron detector 24 count rate to the epithermal far neutron detector 24 count rate.
- a plot 176 includes an ordinate 178, which represents lithology error compared to a calcite formation in units of porosity units (p.u.), and an abscissa 180, which represents porosity in units of porosity units (p.u.).
- This lithology error is the deviation between the true porosity of a formation and the one observed assuming that the formation is composed of calcite.
- FIG. 16 presents a similar plot 182, which includes an ordinate 184 representing lithology error in units of porosity units (p.u.), and an abscissa 186 representing porosity in units of porosity units (p.u.).
- the lithology effects introduced by the minerals anhydrite, corumdum, dolomite, and quartz appear in curves of the plot 182.
- certain minerals may introduce significantly greater lithology effects at higher porosities when porosity is determined from a ratio of counts of the neutron detectors 24 than when porosity is determined from counts of the neutron detectors 24 normalized to counts of the neutron monitor 22.
- the single "optimally-spaced" neutron detector 24 of the plot 176 has much less lithology effect than the near/far ratio of the plot 182. Choosing the "other" detector 24 to be farther from the neutron source 18 (e.g., at 2 feet), rather than closer, may result in an even larger lithology error.
- the porosity can be computed directly from the normalized count rate of a single epithermal or thermal neutron detector 24 by using a transform derived from modeled and/or experimental data that relates the measured normalized count rate to the porosity of the formation 36.
Abstract
Description
Claims
Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
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RU2012149430/28A RU2515111C1 (en) | 2010-04-21 | 2011-04-13 | Neutron downhole device for measurement of porosity with increased accuracy and reduced lithological effects |
MX2012012127A MX2012012127A (en) | 2010-04-21 | 2011-04-13 | Neutron porosity downhole tool with improved precision and reduced lithology effects. |
CA2795421A CA2795421A1 (en) | 2010-04-21 | 2011-04-13 | Neutron porosity downhole tool with improved precision and reduced lithology effects |
EP11772446.8A EP2548053A4 (en) | 2010-04-21 | 2011-04-13 | Neutron porosity downhole tool with improved precision and reduced lithology effects |
BR112012026673A BR112012026673A2 (en) | 2010-04-21 | 2011-04-13 | neutron porosity downhole tool, method, downhole tool, and neutron porosity system |
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US12/764,814 | 2010-04-21 | ||
US12/764,814 US9372277B2 (en) | 2010-04-21 | 2010-04-21 | Neutron porosity downhole tool with improved precision and reduced lithology effects |
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WO2011133363A2 true WO2011133363A2 (en) | 2011-10-27 |
WO2011133363A3 WO2011133363A3 (en) | 2012-02-02 |
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PCT/US2011/032210 WO2011133363A2 (en) | 2010-04-21 | 2011-04-13 | Neutron porosity downhole tool with improved precision and reduced lithology effects |
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US (1) | US9372277B2 (en) |
EP (1) | EP2548053A4 (en) |
BR (1) | BR112012026673A2 (en) |
CA (1) | CA2795421A1 (en) |
MX (1) | MX2012012127A (en) |
RU (1) | RU2515111C1 (en) |
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Cited By (1)
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US9341737B2 (en) | 2014-02-11 | 2016-05-17 | Baker Hughes Incorporated | Measuring total, epithermal and thermal neutron formation porosities with one single set of neutron detectors and a pulsed neutron generator |
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US20130304396A1 (en) * | 2012-05-08 | 2013-11-14 | Sean E. Walston | Online statistical analysis of neutron time intervals using bayesian probability analysis |
US9069095B1 (en) * | 2013-12-16 | 2015-06-30 | Schlumberger Technology Corporation | Monitoring the output of a radiation generator |
US9274245B2 (en) | 2014-05-30 | 2016-03-01 | Baker Hughes Incorporated | Measurement technique utilizing novel radiation detectors in and near pulsed neutron generator tubes for well logging applications using solid state materials |
RU2624996C1 (en) * | 2016-06-03 | 2017-07-11 | Федеральное государственное унитарное предприятие "Всероссийский научно-исследовательский институт автоматики им. Н.Л. Духова" (ФГУП "ВНИИА") | Downhole device for measurement of neutron porosity |
WO2018057035A1 (en) | 2016-09-26 | 2018-03-29 | Halliburton Energy Services, Inc. | Neutron porosity log casing thickness corrections |
RU2690095C1 (en) * | 2018-01-24 | 2019-05-30 | Федеральное Государственное Унитарное Предприятие "Всероссийский Научно-Исследовательский Институт Автоматики Им.Н.Л.Духова" (Фгуп "Внииа") | Device for neutron porosity determination |
CN110439545B (en) * | 2019-08-02 | 2023-04-25 | 中国石油天然气集团有限公司 | Environment correction method for logging instrument with neutron porosity of controllable source while drilling |
CN116500694B (en) * | 2023-06-28 | 2023-09-01 | 中海油田服务股份有限公司 | Post-sleeve physical quantity inversion method, post-sleeve physical quantity inversion device, computing equipment and storage medium |
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- 2011-04-13 EP EP11772446.8A patent/EP2548053A4/en not_active Withdrawn
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WO2011133363A3 (en) | 2012-02-02 |
MX2012012127A (en) | 2012-11-22 |
EP2548053A2 (en) | 2013-01-23 |
US9372277B2 (en) | 2016-06-21 |
US20110260044A1 (en) | 2011-10-27 |
BR112012026673A2 (en) | 2017-10-10 |
EP2548053A4 (en) | 2013-10-30 |
CA2795421A1 (en) | 2011-10-27 |
RU2515111C1 (en) | 2014-05-10 |
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