CN109521487B - Method for identifying gas layer by using element gamma energy spectrum logging - Google Patents
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Abstract
The invention discloses a method for identifying a gas layer by utilizing element gamma energy spectrum logging, which selects a pulse neutron source and a gamma detector, determines the stratum gas saturation according to the ratio of non-bomb gamma counting and capture gamma counting recorded by the gamma detector and evaluates the stratum gas saturation. The invention provides a method for identifying a gas layer by utilizing element gamma energy spectrum logging, which determines the stratum gas saturation according to the non-bomb gamma and capture gamma counting ratio recorded by a gamma detector, provides a solution for gas layer identification and quantitative evaluation, and simultaneously expands the application of the element gamma energy spectrum logging technology.
Description
Technical Field
The invention relates to a method for identifying a gas reservoir by utilizing element gamma energy spectrum logging, belonging to the technical field of geophysical logging in a mine field.
Background
With the exploration and development of oil and gas fields, the evaluation and quantitative calculation of the gas saturation of the stratum become more important. The traditional methods for evaluating the gas content of the stratum comprise a pulsed neutron curve superposition technology, lithologic density logging, a three-porosity overlapping method, thermal neutron decay time logging, resistivity logging and imaging logging. With the development of pulse neutron logging technology in recent years, the neutron and gamma time spectrum information detected by an instrument is utilized to evaluate the stratum gas saturation degree, and the method plays an important role in oil and gas identification.
Compared with a conventional reservoir, unconventional reservoirs such as coal rocks and shales have the characteristics of complicated reservoir lithology, poor reservoir physical properties (the porosity is less than 10 percent, the permeability is less than 1.0mD), deeper burial and the like, and the oil and gas exploration and development difficulty is higher. The element gamma energy spectrum logging technology determines the element content and divides the lithology by utilizing the information of gamma rays emitted by the action of neutrons and the atomic nuclei of stratum elements, and has wide application prospect in unconventional reservoir evaluation.
The existing element gamma energy spectrum instrument using a pulse neutron source utilizes information of a D-T neutron source and a single detector to complete stratum element evaluation and organic carbon detection in actual exploration and development, and no specific solution exists for evaluating stratum gas saturation by utilizing gamma information obtained by detection of the element gamma energy spectrum instrument.
Disclosure of Invention
Based on the technical problems, the invention provides a method for identifying a gas reservoir by utilizing element gamma energy spectrum logging, which provides a solution for gas reservoir identification and quantitative evaluation and simultaneously expands the application of an element energy spectrum logging technology.
The technical solution adopted by the invention is as follows:
a method for identifying a gas layer by utilizing element gamma energy spectrum logging selects a pulse neutron source and a gamma detector, determines the saturation of stratum gas according to the ratio of non-bomb gamma counting and capture gamma counting recorded by the gamma detector, and evaluates the stratum gas content.
The method specifically comprises the following steps:
selecting a formation element gamma energy spectrum measuring device, wherein the device comprises a pulse neutron source and a gamma detector;
secondly, high-energy fast neutrons emitted by the underground pulse neutron source and stratum materials generate inelastic scattering and radiation capture effects to generate non-elastic gamma rays and capture gamma rays, and the gamma rays are received and recorded by a gamma detector;
thirdly, simulating to obtain a response relation between the ratio of the non-bomb gamma count to the capture gamma count and the stratum gas saturation under different porosity conditions, and establishing a stratum gas saturation evaluation model:
Sg＝(2145-176.6*φ+5.93*φ2-0.066*φ3)+(-2635+233.7*φ-7.85，φ2+0.088*φ3)*R
wherein R is the non-bullet capture ratio, namely the ratio of the non-bullet gamma count to the capture gamma count; phi is formation porosity (%); sg is the stratum gas saturation (%);
and step four, utilizing the non-elastic gamma count and capture gamma count ratio R detected by the detector to combine with the formation porosity parameter to complete formation gas saturation evaluation.
The distance between the gamma detector and the pulse neutron source is preferably 60-70 cm.
Preferably, the gamma detector is a BGO crystal detector.
Preferably, the pulse neutron source is a D-T controllable neutron source, the pulse period of the D-T controllable neutron source is 400 mu s, in one pulse period, fast neutrons are emitted at 0-40 mu s, and the emission of the fast neutrons is stopped at 40-400 mu s.
The beneficial technical effects of the invention are as follows:
the invention provides a method for identifying a gas layer by utilizing element gamma energy spectrum logging, which determines the stratum gas saturation according to the non-bomb gamma and capture gamma counting ratio recorded by a gamma detector, provides a solution for gas layer identification and quantitative evaluation, and simultaneously expands the application of the element gamma energy spectrum logging technology.
Drawings
FIG. 1 is a schematic diagram of a gamma spectrum measuring device for formation elements used in the present invention; in the figure: 1-pulsed neutron source, 2-gamma detector, 3-formation, 4-wellbore fluid;
FIG. 2 shows sandstone formations (porosity 10% and 30%), fast neutron moderation length LsAnd thermal neutron diffusion length LtStratospheric gas saturation SgThe variation relationship of (a);
FIG. 3 is a plot of non-ballistic capture count ratio R versus formation gas saturation for different formation conditions;
FIG. 4 is an illustration of an X-well application of the present invention.
Detailed Description
The invention is further described with reference to the following figures and specific embodiments:
the invention provides a method for identifying a gas layer by utilizing element gamma energy spectrum logging, which selects a pulse neutron source and a gamma detector, determines the stratum gas saturation according to the ratio of non-bomb gamma counting and capture gamma counting recorded by the gamma detector and evaluates the stratum gas saturation.
The method specifically comprises the following steps:
firstly, a formation element gamma energy spectrum measuring device is selected, and as shown in figure 1, the device comprises a pulse neutron source 1 and a gamma detector 2.
And step two, high-energy fast neutrons emitted by the underground pulse neutron source and stratum materials generate inelastic scattering and radiation capture effects to generate non-elastic gamma rays and capture gamma rays, and the gamma rays are received and recorded by a gamma detector.
Thirdly, simulating by using a Monte Carlo numerical simulation method to obtain a response relation between the ratio of the non-bomb gamma count to the capture gamma count and the stratum gas saturation under different porosity conditions, and establishing a stratum gas saturation evaluation model:
Sg＝(2145-176.6*φ+5.93*φ2-0.066*φ3)+(-2635+233.7*φ-7.85*φ2+0.088φ3)*R
wherein R is the non-bullet capture ratio, namely the ratio of the non-bullet gamma count to the capture gamma count; phi is formation porosity (%); sg is the formation gas saturation (%).
And step four, utilizing the non-elastic gamma count and capture gamma count ratio R detected by the detector to combine with the formation porosity parameter to complete formation gas saturation evaluation.
In the above step, the distance between the gamma detector and the pulse neutron source is 60-70 cm.
In the above step, the gamma detector is a BGO crystal detector.
In the above steps, the pulse neutron source is a D-T controllable neutron source, the pulse period of the D-T controllable neutron source is 400 mus, in one pulse period, fast neutrons are emitted at 0-40 mus, and the emission of the fast neutrons is stopped at 40-400 mus.
The invention is further described below in conjunction with the following principle derivation and application examples.
According to the neutron diffusion theory, the source intensity S is assumed0The neutron source emits fast neutrons to the stratum, the intensity of gamma rays emitted by inelastic scattering of the fast neutrons and one atomic nucleus is i, and the intensity of non-elastic gamma rays generated by a stratum volume element dV at a detector in unit time is as follows:
equation of Chinese ∑inIs a non-elastic scattering cross section phifThe flux distribution of fast neutrons is shown, mu is the linear absorption coefficient of gamma rays, Ls is the deceleration length of the fast neutrons, and r and X are the distances from a volume element dV to a neutron source and a detector respectively. dC (direct current)inThe intensity of non-elastic gamma rays generated by a formation volume element dV at a detector in unit time is obtained by integrationin。
Similarly, the capture gamma rays generated by the capture reaction between the thermal neutrons and the atomic nuclei are as follows:
wherein i' is the intensity of capture gamma ray released by the capture reaction of thermal neutron and atomic nucleus, LtIs the diffusion length of the thermal neutrons.
The ratio R of the non-bomb gamma count to the capture gamma count recorded by the detector is therefore:
in the formula, the ratio R is only equal to i, i' and the fast neutron deceleration length LsAnd thermal neutron diffusion length LtCorrelation; wherein i and i ' are related to the nuclei of the material in the formation, and considering that i and i ' exist in the form of a ratio (i/i ') in R, the two counteract each other to a certain extent with the change of the nuclei of the material, and the influence thereof is relative to LsAnd LtThe resulting effect is negligible; therefore, the effect of the ratio (i/i') of the non-ballistic and capture gamma ray intensities produced by the action of the nuclei of the formation material on the neutrons is not taken into account.
Non-bomb capture count ratio R is mainly subject to fast neutron deceleration length LsAnd thermal neutron diffusion length LtIs mainly dependent on the formation porosity and gas saturation Sg. Sandstone stratum (porosity 10% and 30%), fast neutron moderation length and thermal neutron diffusion length with stratum gas saturation SgThe variation relationship of the neutron moderation length and the thermal neutron diffusion length is shown in fig. 2, and it can be seen that when the porosity of the stratum is certain, the fast neutron moderation length and the thermal neutron diffusion length both increase with the increase of the saturation of the gas in the stratum, but the fast neutron moderation length is influenced by the saturation of the gas in the stratum to change more greatly. Since the distribution of non-ballistic and capture gamma rays generated in the formation is related to the deceleration length of fast neutrons and the diffusion length of thermal neutrons, the gas saturation S is differentgThe non-bomb capture count ratio R of the formation is different, so the non-bomb capture gamma count ratio R recorded by the same detector can be used to determine the formation gas saturation.
FIG. 3 is a graph showing the relationship of the non-ballistic capture count ratio R with the formation gas saturation at different formation conditions when the formation gas saturation is 0%, 30%, 50%, 70% and 100% and the formation porosity is changed from 0% to 40%. It can be seen that changes in the non-bomb and capture gamma ratio R can reflect the magnitude of the formation gas saturation. When the porosity of the stratum is certain, the non-ballistic and capture gamma counts are increased along with the increase of the gas saturation of the stratum, the calculated non-ballistic capture ratio R is linearly reduced along with the increase of the gas saturation of the stratum, and the larger the porosity of the stratum is, the larger the difference of R caused by different gas saturation is. When the formation gas saturation is constant, the non-elastic capture ratio R increases with the increase of the formation porosity, and the smaller the formation gas saturation, the more drastic the change of the non-elastic capture ratio R is.
According to the response relation between the non-elastic capture ratio R and the stratum gas saturation under different porosity conditions in the figure 3, a stratum gas saturation evaluation model is established as follows:
Sg＝(2145-176.6*φ+5.93*φ2-0.066*φ3)+(-2635+233.7*φ-7.85*φ2+0.088*φ3)*R
wherein R is the non-elastic capture ratio; phi is formation porosity (%); sgIs the formation gas saturation (%).
From the formula, the formation gas saturation SgAnd the non-elastic and capture gamma counting ratio R detected by the detector is combined with the formation porosity parameter to complete the formation gas saturation evaluation.
Calculating the inelastic scattering gamma counts and the capture gamma counts obtained by actual measurement of an elemental energy spectrum logging instrument in a cased hole, calculating the nonelastic capture ratio R of measured data, and performing gas saturation evaluation on the measured data by combining a gas saturation response model, as shown in FIG. 4, it can be seen that when the depth is 5320-5327 meters, the sound wave time difference curve has cycle skip, the compensation density curve indicates that the density value of the stratum is reduced, at the moment, the nonelastic capture ratio R is obtained by calculating the nonelastic gamma counts and the capture gamma counts, the nonelastic capture ratio R displays a low value, the gas saturation of the stratum is obtained by combining the gas saturation response model, and the stratum with higher gas saturation is obtained; when the depth is 5327.5-5329 m, the formation gamma value is high gamma, the lithology shows that the stratum is completely argillaceous, the stratum is a mudstone layer, the non-ballistic capture ratio is much higher than that of a gas-bearing reservoir at the moment, and the evaluation of the gas-bearing property shows that the stratum does not contain gas.
It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.
Claims (4)
1. A method for identifying a gas layer by utilizing element gamma energy spectrum logging is characterized by comprising the following steps: selecting a pulse neutron source and a gamma detector, determining stratum gas saturation according to the ratio of non-bomb gamma counting and capture gamma counting recorded by the gamma detector, and evaluating the stratum gas saturation, wherein the method specifically comprises the following steps:
selecting a formation element gamma energy spectrum measuring device, wherein the device comprises a pulse neutron source and a gamma detector;
secondly, high-energy fast neutrons emitted by the underground pulse neutron source and stratum materials generate inelastic scattering and radiation capture effects to generate non-elastic gamma rays and capture gamma rays, and the gamma rays are received and recorded by a gamma detector;
thirdly, simulating to obtain a response relation between the ratio of the non-bomb gamma count to the capture gamma count and the stratum gas saturation under different porosity conditions, and establishing a stratum gas saturation evaluation model:
Sg＝(2145-176.6*φ+5.93*φ2-0.066*φ3)+(-2635+233.7*φ-7.85*φ2+0.088*φ3)*R
wherein R is the non-bullet capture ratio, namely the ratio of the non-bullet gamma count to the capture gamma count; phi is formation porosity,%; sgFormation gas saturation,%;
and step four, utilizing the non-elastic gamma count and capture gamma count ratio R detected by the detector to combine with the formation porosity parameter to complete formation gas saturation evaluation.
2. The method of identifying a gas formation using elemental gamma spectrometry logging, according to claim 1, wherein: the distance between the gamma detector and the pulse neutron source is 60-70 cm.
3. The method of identifying a gas formation using elemental gamma spectrometry logging, according to claim 1, wherein: the gamma detector adopts a BGO crystal detector.
4. The method of identifying a gas formation using elemental gamma spectrometry logging, according to claim 1, wherein: the pulse neutron source is a D-T controllable neutron source, the pulse period of the D-T controllable neutron source is 400 mu s, in one pulse period, fast neutrons are emitted at 0-40 mu s, and the emission of the fast neutrons at 40-400 mu s is stopped.
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| CN111123378B (en) * | 2019-12-25 | 2022-06-03 | 中国石油天然气股份有限公司 | Method and device for determining gamma ray intensity critical value for dividing lithology type |
| CN112034524B (en) * | 2020-09-01 | 2023-07-04 | 中国石油大学(华东) | Double-detector well stratum capture section calculation method |
| CN112593923B (en) * | 2020-12-16 | 2023-10-20 | 中国海洋石油集团有限公司 | Method and device for predicting gas saturation based on pulse neutrons |
| CN113123779B (en) * | 2021-04-06 | 2023-08-18 | 长江大学 | Gas while drilling layer identification device and method based on iron inelastic scattering gamma |
| CN115267928B (en) * | 2022-09-28 | 2022-12-23 | 中石化经纬有限公司 | Intelligent energy spectrum processing method for logging while drilling element |
| CN118309418A (en) * | 2023-04-23 | 2024-07-09 | 山东科技大学 | Quantitative evaluation method for gas saturation of tight sandstone reservoir |
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