CN116357301A - Stratum porosity calculation method and system based on effective reaction section - Google Patents
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Abstract
The invention provides a stratum porosity calculation method and system based on an effective reaction section, wherein the neutralization method comprises the following steps: determining an effective reaction section theoretical calculation formula based on a fast neutron transport and gamma attenuation theory; based on an effective reaction section theoretical calculation formula, determining effective reaction sections and relation coefficients of different substances in a mode of solving an overdetermined equation set; measuring an effective reaction section by adopting a non-elastic gamma ratio and a epithermal neutron ratio, and calibrating a measurement formula by utilizing the effective reaction section of each substance; and using the measured effective reaction section to detect the porosity of the stratum. Compared with the traditional neutron porosity, the effective reaction cross section of the new parameters obtained by the method has higher porosity sensitivity, well logging response accords with a volume physical model, is not influenced by the mining effect, and is more suitable for complex reservoir porosity evaluation.
Description
Technical Field
The invention relates to the technical field of logging in oil and gas exploration, in particular to a stratum porosity calculation method and system based on an effective reaction section.
Background
Porosity is a very important geological parameter in stratum evaluation, and accurate measurement of porosity of oil reservoirs has extremely important significance for oil and gas development and exploration. Neutron logging has been one of the main methods of formation porosity measurement. Along with the development of pulse neutron logging technology, the problems of safety, pollution and the like of a traditional isotope neutron source are solved, and the multi-mode pulse neutron logging can measure a plurality of parameters simultaneously, so that the method is one of the important methods for evaluating the stratum at present. Development of multifunctional integrated pulse neutron logging instruments is the current nuclear logging research direction, and porosity logging is one of the necessary logging projects. While neutron porosity logging has advantages over other logging methods in that it is less affected by the pore structure, its logging response is affected by skeletal mineral and fluid properties, making it difficult for neutron porosity logging to accurately measure formation porosity, particularly gas bearing formations, in complex reservoirs, and even decreasing logging response values as porosity increases due to the effects of the mining effect.
Disclosure of Invention
Aiming at the problems, the invention provides a stratum porosity calculation method and system based on an effective reaction section, which can improve the detection sensitivity of stratum porosity.
The invention discloses a stratum porosity calculation method based on an effective reaction section, which comprises the following steps:
measuring and obtaining an effective reaction section of a stratum to be detected;
and determining the porosity of the stratum to be detected by using the effective reaction section.
Further, in the step of measuring and obtaining the effective reaction cross section of the stratum to be detected, the effective reaction cross section measurement formula is as follows:
wherein: ERCS is the effective reaction cross section; r is the ratio of epithermal neutrons, thermal neutrons or capture gamma; r is R i Is a non-elastic gamma ratio; a, a 1 ～a 6 Is a coefficient.
Further, in the step of determining the porosity of the stratum to be detected by using the effective reaction section, a porosity calculation formula of the stratum to be detected based on the effective reaction section is as follows:
in the method, in the process of the invention,
for the porosity calculated based on the effective reaction cross section, V sh ERCS is the effective reaction cross section of the stratum, the content of the argillaceous volume ma ERCS is the effective reaction section value of the skeleton sh Effective reaction section value for muddy, ERCS f Is the effective reaction cross-section value of the fluid.
Further, before the step of measuring and acquiring the effective reaction cross section of the stratum to be detected, the method further comprises the steps of:
acquiring a non-elastic gamma ratio and an epithermal neutron ratio through stratum measurement;
determining an effective reaction section theoretical calculation formula, and determining the relationship between a non-elastic gamma ratio and a epithermal neutron ratio and an effective reaction section by setting a plurality of formations with different porosities;
and determining an effective reaction section measurement formula based on the effective reaction section theoretical calculation formula.
Further, the step of determining the effective reaction section theoretical calculation formula includes:
based on a fast neutron transport theory and a gamma attenuation theory, acquiring the relationship between a non-elastic gamma ratio and a deceleration length, a mass attenuation coefficient and formation density:
based on the Lagrangian median theorem, the conversion results in:
b 1 and b 2 At [0,1]Taking value in interval and reducing length L e The moderation length L of neutrons in the same stratum is expressed by a function of the scattering cross section and the atomic mass number of the constituent stratum e The method comprises the following steps:
wherein R is f Macroscopic scattering cross section of the formation for neutron moderation distance
σ i Microcosmic scattering cross section of the ith element of the stratum, W i Is the mass percent of the ith element in the stratum skeleton, A is the mass number, A i Is the atomic mass of the ith element, N A For the avogalileo constant, f (a) is a function related to the magnitude of the formation mass number;
order the
For an effective reaction cross section ERCS, then we obtain:
wherein C is the distance from the source r 1 And r 2 A related constant; f (R) is a function of the ratio of epithermal neutrons, thermal neutrons or capture gamma; taking into account the mass attenuation coefficient mu m The change of each substance in the logging range is small, and mu is ignored m After the influence, the effective reaction section theoretical calculation formula is obtained through conversion:
wherein f' (R) is a function of f (R) and C.
Further, the step of determining the relationship between the non-elastic gamma ratio and the epithermal neutron ratio and the effective reaction section by arranging a plurality of formations with different porosities comprises the following steps:
converting the effective reaction section theoretical calculation formula into coefficient expression:
wherein c 1 ～c 4 Is the coefficient, i is the i-th substance, n is the total number of substances, v i Is the volume content of the i-th substance;
setting a plurality of stratums with different porosities, obtaining a non-elastic gamma ratio, a stratums density and an epithermal neutron ratio in a simulation manner, and substituting the non-elastic gamma ratio, the stratums density and the epithermal neutron ratio into an effective reaction section theoretical calculation formula to establish an overdetermined equation set;
and solving the overdetermined equation set to obtain a coefficient value in an effective reaction section theoretical calculation formula under the current simulation condition, thereby determining the effective reaction section theoretical calculation formula.
Further, after the step of measuring and acquiring the effective reaction cross section of the stratum to be detected, the method further comprises the steps of:
and carrying out formation evaluation on the stratum to be detected based on the effective reaction section.
The invention also discloses a stratum porosity calculation system based on the effective reaction section, which comprises:
the stratum effective reaction section obtaining device is used for measuring and obtaining an effective reaction section of the stratum to be detected;
and the porosity calculation module is used for determining the porosity of the stratum to be detected by using the effective reaction section.
Further, the stratum effective reaction section obtaining device is used for obtaining the effective reaction section of the stratum to be detected according to an effective reaction section measurement formula when measuring and obtaining the effective reaction section of the stratum to be detected:
the effective reaction section measurement formula is as follows:
wherein: ERCS is the effective reaction cross section; r is the ratio of epithermal neutrons or thermal neutrons or capture gamma; r is R i Is a non-elastic gamma ratio; a, a 1 ～a 6 Is a coefficient.
Further, when the porosity of the stratum to be detected is determined by the porosity calculation module through the effective reaction section, a porosity calculation formula of the stratum to be detected based on the effective reaction section is as follows:
in the method, in the process of the invention,
for the porosity calculated based on the effective reaction cross section, V sh ERCS is the effective reaction cross section of the stratum, the content of the argillaceous volume ma ERCS is the effective reaction section value of the skeleton sh Effective reaction section value for muddy, ERCS f Is the effective reaction cross-section value of the fluid.
Further, the stratum porosity calculation system also comprises stratum measurement device, which is used for transmitting the measured data to the stratum effective reaction section acquisition device to calculate the effective reaction section of the stratum to be detected;
the formation measurement device includes a neutron source, an epithermal neutron detector, and a gamma detector, wherein,
the neutron source is arranged in a well bore of a well, the epithermal neutron detector comprises a near epithermal neutron detector and a far epithermal neutron detector, the gamma detector comprises a near gamma detector and a far gamma detector, and the near epithermal neutron detector, the near gamma detector, the near epithermal neutron detector and the far epithermal neutron detector are sequentially arranged from the neutron source to the surface direction.
Further, a first shielding body made of boron carbide is arranged around the epithermal neutron detector; for shielding neutrons directly entering the epithermal neutron detector without passing through the formation to be detected; and a second shielding body made of tungsten-nickel-iron is arranged around the gamma detector and is used for shielding gamma photons which directly enter the gamma detector without passing through the stratum to be detected.
According to the stratum porosity calculation method and system based on the effective reaction section, the stratum porosity is calculated by designing a new parameter, namely the effective reaction area. The effective reaction cross section is only related to the elements constituting the substance, and is a certain value for the same substance. The stratum porosity calculation method based on the effective reaction section comprises the following steps: based on fast neutron transport and gamma attenuation theory, a theoretical calculation formula of an effective reaction section is obtained, the effective reaction section is measured by utilizing a non-elastic gamma ratio and a epithermal neutron ratio, and the porosity of the stratum to be detected is determined by utilizing the effective reaction section. Compared with the traditional neutron porosity, the parameter has higher porosity sensitivity, well logging response accords with a volume physical model, is not influenced by the mining effect, and is more suitable for complex reservoir porosity evaluation. Thereby providing theoretical support and technical support for obtaining more accurate stratum porosity by multifunctional integrated pulse neutron logging. The stratum porosity calculation system based on the effective reaction section is used for realizing stratum porosity calculation based on the effective reaction section, and the used instruments and equipment are conventional measuring instruments and equipment in the field, so that the operation, implementation and test are convenient.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
Drawings
FIG. 1 is a schematic diagram of a formation measurement apparatus according to an embodiment of the present invention;
FIG. 2 is a schematic flow chart of a method for calculating formation porosity based on an effective reaction section according to an embodiment of the present invention;
FIG. 3 is a graph showing the range of k values for a source distance of 30cm provided by an embodiment of the present invention;
FIG. 4 is a graph showing the range of k values for a source distance of 60cm provided by an embodiment of the present invention;
FIG. 5 shows the effect of measuring the effective reaction cross section provided by the embodiment of the invention;
FIG. 6 is a graph showing response characteristics of effective reaction cross sections under different lithology conditions provided by embodiments of the present invention;
FIG. 7 is a graph showing response characteristics of effective reaction cross sections under different clay content conditions provided by embodiments of the present invention;
FIG. 8 is a graph showing response characteristics of an effective reaction section provided by an embodiment of the present invention under different fluid property conditions;
fig. 9 shows an application effect of an effective reaction section provided in an embodiment of the present invention in calculating the porosity of a formation to be detected.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Embodiment one:
the embodiment provides a stratum porosity calculation method based on an effective reaction section, which has a step flow shown in fig. 2 and specifically comprises the following steps:
s1, determining an effective reaction section theoretical calculation formula based on fast neutron transport and gamma attenuation theory;
s2, determining effective reaction sections and relation coefficients of different substances in a mode of solving an overdetermined equation set based on an effective reaction section theoretical calculation formula;
s3, measuring an effective reaction section by adopting a non-elastic gamma ratio and a epithermal neutron ratio, and calibrating a measurement formula by utilizing the effective reaction section of each substance;
s4, determining the porosity of the stratum to be detected by using the measured effective reaction section.
In step S1, an effective reaction section theoretical calculation formula is determined based on fast neutron transport and gamma attenuation theory, and the derivation of the effective reaction section theoretical calculation formula is as follows:
according to the fast neutron transport and gamma decay theory, the inelastic gamma flux can be calculated using equation 1. Where p is the average number of gamma photons per inelastic collision, Σ in Is a non-elastic scattering macroscopic section, Q is a strong source, L e Is the deceleration length; ρ is density, D n For fast neutron diffusivity, D r For the gamma diffusion coefficient, r is the distance between the measurement point and the gamma source, in one embodiment of the invention, a deuterium-tritium neutron generator (D-T) is used as both neutron source and gamma source.
μ m Is a mass attenuation coefficient, and the expression is
σ j Microscopic scattering section, Z, of the j-th element of the formation i Is the atomic number of the ith element, A i Is the atomic mass of the ith element, N A For the Avofila constant, m is the total number of materials in the formation and n is the total number of elements of the j-th material.
For source distances r respectively 1 ，r 2 The non-elastic gamma ratio recorded by the gamma detector, r 1 ＜r 2 As shown in formula 2, the inelastic gamma ratio is the ratio of inelastic gamma fluxes obtained by the near gamma detector and the far gamma detector.
Based on the lagrangian median theorem, equation 2 may be converted to equation 3.
Wherein: b 1 And b 2 Is of the coefficient of [0,1 ]]Take value in interval L e The expression is equation 4 for the deceleration length.
For neutron moderating distance R f If the medium is a substance consisting of light nuclei, R f Can be expressed as shown in equation 5.
In Sigma s Is the macroscopic scattering section of the stratum, A is the mass number, ζ is the average logarithmic energy loss of the stratum, E 0 For neutron initiation energy, E f Is the energy of neutrons after deceleration. If the medium is composed of nuclides with larger mass numbers, the mean square value of the deceleration distance
Calculated by equation 6.
Whether the formation is composed of light nuclei or heavy nuclei of a larger mass number, the moderation length of neutrons for the same formation can be calculated by equation 7.
Where f (A) is a function related to the magnitude of the formation mass number, f (A) can be determined according to equations 5 and 6.
Macroscopic scattering cross section Sigma of the formation s Can be represented by equation 8.
Wherein sigma i Microcosmic scattering cross section, W, of the ith element of the formation i Is the mass percent of the ith element in the stratum skeleton, A i Is the atomic mass of the ith element, N A Is the avogalileo constant. Order the
f (A) is the effective reaction cross section ERCS. Equation 3 may be expressed as shown in equation 9.
Let k= (b) 2 r 2 -b 1 r 1 ) The k value variation range is shown in fig. 3 and 4. The k value of a limestone formation containing methane or water in the pores, whether hydrogen atoms are present in the medium water or methane, increases with increasing hydrogen index of the formation and is independent of the fluid type. Thus, similar to the response of thermal neutrons to the formation, the k value may be characterized by a epithermal neutron count rate ratio.
Thus, equation 9 can be expressed by equation 10.
Wherein C is the source distance r 1 And r 2 Related constants. f (R) is a function of the ratio of epithermal neutrons or thermal neutrons or capture gamma. Mass attenuation coefficient mu m The change in each substance is small in the logging range, and thus the effective reaction section ERCS can be expressed as formula 11.
Wherein f' (R) is a function of f (R) and C.
In step S2, based on an effective reaction section theoretical calculation formula, the effective reaction sections and relation coefficients of different substances are determined in a mode of solving an overdetermined equation set. The method comprises the following steps:
as can be seen from equation 11, the effective reaction cross section can be calculated from the inelastic gamma ratio, density and epithermal neutron ratio. By setting various formations with different porosities, such as limestone saturated with water, oil or gas, sandstone saturated with water, oil or gas and other formations with different porosities, the effective reaction section and formula coefficient under the current measuring instrument are obtained by means of an overdetermined equation set. The theoretical calculation formula of the effective reaction section is shown in formula 12.
Wherein c 1 ～c 4 Is the coefficient, i is the i-th substance, n is the total number of substances, v i Is the volume content of the i-th substance. And substituting various stratum data with different porosities to perform coefficient fitting calculation to finally obtain a theoretical reactive section calculation formula determined by the stratum coefficients. Thus, the theoretical effective reaction section is directly calculated by using the formula 12 for unknown stratum or substances which do not participate in the solution of the overdetermined equation set. When the effective reaction section is calculated by using the formula 12, the corresponding non-elastic gamma ratio, density and epithermal neutron ratio are required to be obtained.
In step S3, the effective reaction section is measured by adopting a non-elastic gamma ratio and a epithermal neutron ratio, and the effective reaction section of each substance is used for calibrating a measurement formula. The method comprises the following steps:
the effective reaction cross section is only related to the elements constituting the substance, and is a certain value for the same substance. The theoretical effective reaction cross-section calculations are related to the non-elastic gamma ratio, density and epithermal neutron ratio. Since the density can be calculated from the non-elastic gamma ratio and the epithermal neutron ratio, the effective reaction cross section can be calculated using only the non-elastic gamma ratio and the epithermal neutron ratios. According to the obtained effective reaction section ERCS, a plurality of formations with different porosities are set in a simulation mode, the relation between the non-elastic gamma ratio and the epithermal neutron ratio and the effective reaction section is researched, and an effective reaction section measurement formula is determined, wherein the specific formula is shown in formula 13.
Wherein R is the ratio of epithermal neutrons or thermal neutrons or capture gamma; r is R i A is a non-elastic gamma ratio 1 ～a 6 Is a coefficient. The effect of equation 13 on various formation measurements is shown in fig. 5.
The difference between the effective reaction section measurement formula and the effective reaction section theoretical calculation formula is that the influence variable of formation density is omitted in the measurement formula, and when the coefficient in the formula 13 is calculated, the coefficient a is determined by adopting the joint fitting calculation of the non-elastic gamma ratio and the epithermal neutron ratio measured by actual instruments and the effective reaction section calculated by the formula 12 1 ～a 6 Is a value of (2). Thereby determining the coefficient c fitted by the current measuring instrument (measuring instruments with different non-elastic gamma ratios and epithermal neutron ratios) 1 ～c 4 A) 1 ～a 6 There are differences) in the effective reaction cross-section. It should be appreciated that when the measuring instrument is replaced, each coefficient in the effective reaction cross section measurement formula and the effective reaction cross section theoretical calculation formula needs to be recalculated.
And by combining with actual measurement data, corresponding effective reaction section values can be calculated through an effective reaction section measurement formula and an effective reaction section theoretical calculation formula, and when the logging density is unknown, the effective reaction section measurement formula can be applied to calculation.
In step S4, the porosity of the formation to be detected is determined using the measured effective reaction cross-section. The method comprises the following steps:
the log response of the effective reaction cross section conforms to a volumetric physical model as shown in fig. 6, 7 and 8. Figures 6-8 are response characteristics of effective reaction cross sections under different formation conditions measured in accordance with the methods provided in this example. From fig. 6 to 8, it can be seen that the effective reaction cross-sectional log response conforms to the volumetric physical model under different lithology, argillaceous content and fluid properties. Therefore, the effective reaction section can be used for stratum evaluation in combination with other parameters, and also can be used for directly calculating the porosity. And calculating the formation porosity by using an effective reaction section by adopting a volume physical model method, wherein the calculation formula is as follows:
in the method, in the process of the invention,
for porosity calculated based on effective reaction cross section, ERCS is the effective reaction cross section of the formation, ERCS ma Effective reaction section value of skeleton, V sh Is of muddy content, phi shc Effective reaction section value for muddy, ERCS f Is the effective reaction cross-section value of the fluid.
The effective reaction section calculation data is obtained by combining formation measurement with an effective reaction section measurement formula or an effective reaction section theoretical calculation formula.
The effective reaction section provided by the invention is a new parameter, and the essence of the effective reaction section is as follows: the macroscopic scattering cross section, which removes the effect of density, is a new parameter that is composed of a function of the number of atomic masses that make up the formation. From the above derivation, it can be seen that: the effective reaction cross section is only related to the elements constituting the substance, and is a certain value for the same substance.
Embodiment two:
the embodiment also provides a stratum porosity calculation system based on an effective reaction section, which is used for realizing the method described in the first embodiment, and comprises a stratum effective reaction section obtaining device, a stratum detection device and a stratum detection device, wherein the stratum effective reaction section obtaining device is used for obtaining an effective reaction section of a stratum to be detected; wherein the effective reaction section is determined according to macroscopic scattering section after removing density influence and the atomic mass number of the formation;
and the porosity calculation module is used for determining the porosity of the stratum to be detected by using the measured effective reaction section.
Further, the stratum effective reaction section obtaining device is used for obtaining the effective reaction section of the stratum to be detected:
multiple formations with different porosities are arranged in a simulation mode, the relation between the non-elastic gamma ratio and the epithermal neutron ratio and the effective reaction section is researched, and an effective reaction section measurement formula is determined:
wherein: ERCS is the effective reaction cross section; r is the ratio of epithermal neutrons or thermal neutrons or capture gamma; r is R i Is a non-elastic gamma ratio; a, a 1 ～a 6 Is a coefficient.
Further, the porosity calculation module, when determining the porosity of the formation to be detected using the effective reaction section,
the porosity calculation method of the stratum to be detected based on the effective reaction section comprises the following steps:
in the method, in the process of the invention,
for apparent porosity calculated based on effective reaction cross-section, ERCS is the effective reaction cross-section of the formation, ERCS ma Effective reaction section value of skeleton, V sh Is of muddy content, phi shc Effective reaction section value for muddy, ERCS f Is the effective reaction cross-section value of the fluid.
Further, the stratum porosity calculation system based on the effective reaction section also comprises stratum measurement device, and is used for transmitting measured data to the stratum effective reaction section acquisition device;
and constructing a standard well model through software, and performing simulation verification on the stratum porosity calculation method. Specifically, formation measurements were simulated using the formation measurement device of fig. 1, non-elastic gamma-to-epithermal neutron ratios were calculated from the measured data, the effective reaction cross-section of the formation was measured using equation 13, and the porosity calculation was performed using equation 14. Fig. 9 is an effect of effective reaction cross-section on calculation of porosity under different formation conditions measured according to the method provided in this example. As can be seen from fig. 9, the effective reaction section is not affected by the digging effect, and the porosity calculated based on the effective reaction section is similar to the formation porosity, whether the formation contains gas or water. Therefore, the method of the invention can obtain very accurate stratum porosity value, thereby laying foundation for stratum evaluation.
In particular, in the above-described calculation method, the formation to be detected may be a complex containing different framework minerals and fluid properties. The non-elastic gamma ratio of the formation to be detected may be obtained using non-elastic gamma counts detected by the near gamma detector and the far gamma detector. The epithermal neutron ratio of the formation to be detected can be obtained by counting the near epithermal neutron detector and the far epithermal neutron detector.
The borehole channel between the ground and the earth formed by the drill bit from the earth's surface to the depth of the well is a borehole, and is generally columnar. In the implementation of the present invention, the neutron source, the two gamma detectors and the two epithermal neutron detectors are all disposed in the borehole along the drilling axis, where the four detectors have different distances from the neutron source, as shown in fig. 1, for example, a near epithermal neutron detector may be disposed between the pulsed neutron source (i.e., the neutron source and the gamma source described in the embodiments of the present invention) and a far epithermal neutron detector, a near epithermal neutron detector between the pulsed neutron source and the near gamma detector, and a far epithermal neutron detector between the near gamma detector and the far gamma detector.
In this embodiment, the detector (epithermal neutron detector and gamma detector) and neutron source may be conventional in the art, for example, the neutron source may be specifically a deuterium-tritium neutron generator (D-T) commonly used at present; both gamma detectors used can be made of lanthanum bromide (LaBr 3 ) A detector; both epithermal neutron detectors can be wrapped with two millimeter cadmium sheetsHelium triple tube 3 He) detector.
Furthermore, in the embodiment of the application, the neutron and the gamma photon which do not enter the stratum to be detected are shielded, and the neutron count and the gamma photon count are measured by changing the thickness of the shielding body, so that the neutrons are prevented from being directly received by the detector without passing through the stratum, and the reliability of the calculation result is improved. In the implementation process of the invention, a first shielding body made of boron carbide is arranged around the epithermal neutron detector; for shielding neutrons directly entering the epithermal neutron detector without passing through the formation to be detected; and a second shielding body made of tungsten-nickel-iron is arranged around the gamma detector and is used for shielding gamma photons which directly enter the gamma detector without passing through the stratum to be detected.
The principle of the invention is as follows: based on a fast neutron transport theory and a gamma attenuation theory, obtaining a relation between a non-elastic gamma ratio and a deceleration length, a mass attenuation coefficient and a stratum density; the deceleration length is further expressed by a function of the scattering cross section and the atomic mass number of the formation, wherein the atomic mass number of the formation is a certain value for the same substance, and the scattering cross section can be expressed by the density and the microscopic cross section of each element of the composition substance; the density can thus be separated from the scattering cross-section and the macroscopic scattering cross-section after removal of the effect of the density is considered to be a new parameter (i.e. the effective reaction cross-section in the present invention) as a function of the mass number of constituent atoms of the formation. The variation of the attenuation coefficient of each substance is considered to be small in the logging range. Therefore, the above relation can be further converted into a relation among the effective reaction section, the non-elastic gamma ratio and the density, thereby determining the theoretical calculation formula of the effective reaction section.
The stratum density can be calculated by using a non-elastic gamma ratio and epithermal neutrons, and the theoretical effective reaction section can be measured by using the non-elastic gamma ratio and the epithermal neutron ratio. And setting various formations with different porosities by using a Monte Carlo method, measuring a non-elastic gamma ratio and a epithermal neutron ratio, determining an effective reaction section measurement formula form based on the non-elastic gamma ratio and the epithermal neutron ratio according to a theoretical effective reaction section calculation formula, and determining an effective reaction section formula coefficient according to standard well data. Thereby realizing the analysis of the porosity of the stratum based on the effective reaction section.
From the above analysis, it is known that the effective reaction section is only related to the elements constituting the substance (both the non-elastic gamma ratio and the density are related to the kind of elements of the substance in the formation), and the effective reaction section is a certain value for the same substance. According to the rock volume physical model, a non-elastic gamma ratio, a stratum density and an epithermal neutron ratio are obtained through simulation by setting stratum models with different porosities, the effective reaction section theory is calculated by means of solving an overdetermined equation set to obtain scales (namely correlation coefficients in formulas), the effective reaction section theory values of various substances are obtained, and besides the fact that the stratum models are directly set to obtain the effective reaction section theory values, the effective reaction section theory calculation formulas with good scales can be used for calculation.
The porosity is calculated by the following principle: porosity is a macroscopic representation of the formation fluid, the greater the difference between the measured values of the framework minerals and the measured values of the fluid, the more suitable the porosity measurement.
Calculation according to an embodiment of the invention reveals that: the effective reaction cross sections among the framework minerals are similar, the effective reaction cross section of the framework is approximately 2 times of that of the oil water, and the reaction cross section of the oil water is approximately 2 times of that of the gas. The logging response of the effective reaction section accords with the rock volume physical model, so that the analysis of the formation porosity by adopting the effective reaction section can avoid being influenced by the mining effect, thereby providing theoretical support and technical support for obtaining more accurate formation porosity by the multifunctional integrated pulse neutron logging.
The stratum porosity calculation method and system based on the effective reaction section are more suitable for complex reservoir porosity evaluation. Compared with the traditional neutron porosity logging, the effective reaction section is more suitable for solving the porosity of the gas-containing stratum. Because the effective reaction section accords with the volume physical model, the effective reaction section can be used for evaluating the stratum in a mode of combining with other parameters, and can also be independently used for calculating the stratum porosity.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.
Claims (12)
1. The stratum porosity calculation method based on the effective reaction section is characterized by comprising the following steps of:
measuring and obtaining an effective reaction section of a stratum to be detected;
and determining the porosity of the stratum to be detected by using the effective reaction section.
2. The method of claim 1, wherein in the step of measuring and obtaining an effective reaction cross section of the formation to be detected, the effective reaction cross section measurement formula is:
wherein: ERCS is the effective reaction cross section; r is the ratio of epithermal neutrons, thermal neutrons or capture gamma; r is R i Is a non-elastic gamma ratio; a, a 1 ～a 6 Is a coefficient.
3. The method according to claim 2, wherein in the step of determining the porosity of the formation to be detected using the effective reaction section, the porosity calculation formula of the formation to be detected based on the effective reaction section is:
in the method, in the process of the invention,
for the porosity calculated based on the effective reaction cross section, V sh ERCS is the effective reaction cross section of the stratum, the content of the argillaceous volume ma ERCS is the effective reaction section value of the skeleton sh Effective reaction section value for muddy, ERCS f Is the effective reaction cross-section value of the fluid.
4. The method of calculating the formation porosity based on the effective reaction section according to claim 1, further comprising, before the step of measuring and acquiring the effective reaction section of the formation to be detected, the steps of:
acquiring a non-elastic gamma ratio and an epithermal neutron ratio through stratum measurement;
determining an effective reaction section theoretical calculation formula, and determining the relationship between a non-elastic gamma ratio and a epithermal neutron ratio and an effective reaction section by setting a plurality of formations with different porosities;
and determining an effective reaction section measurement formula based on the effective reaction section theoretical calculation formula.
5. The method of claim 4, wherein the step of determining an effective reaction cross-section theoretical calculation formula comprises:
based on a fast neutron transport theory and a gamma attenuation theory, acquiring the relationship between a non-elastic gamma ratio and a deceleration length, a mass attenuation coefficient and formation density:
based on the Lagrangian median theorem, the conversion results in:
b 1 and b 2 At [0,1]Taking value in interval and reducing length L e The moderation length L of neutrons in the same stratum is expressed by a function of the scattering cross section and the atomic mass number of the constituent stratum e The method comprises the following steps:
wherein R is f Macroscopic scattering cross section of the formation for neutron moderation distance
σ i Microcosmic scattering cross section of the ith element of the stratum, W i Is the mass percent of the ith element in the stratum skeleton, A is the mass number, A i Is the atomic mass of the ith element, N A For the avogalileo constant, f (a) is a function related to the magnitude of the formation mass number;
order the
For an effective reaction cross section ERCS, then we obtain:
wherein C is the distance from the source r 1 And r 2 A related constant; f (R) is epithermal neutron, thermal neutron or trapped gammaA function of the horse's ratio; taking into account the mass attenuation coefficient mu m The change of each substance in the logging range is small, and mu is ignored m After the influence, the effective reaction section theoretical calculation formula is obtained through conversion:
wherein f' (R) is a function of f (R) and C.
6. The method of claim 5, wherein the determining the relationship between the non-elastic gamma ratio and the epithermal neutron ratio and the effective reaction section by setting a plurality of formations with different porosities comprises:
converting the effective reaction section theoretical calculation formula into coefficient expression:
wherein c 1 ～c 4 Is the coefficient, i is the i-th substance, n is the total number of substances, v i Is the volume content of the i-th substance;
setting a plurality of stratums with different porosities, obtaining a non-elastic gamma ratio, a stratums density and an epithermal neutron ratio in a simulation manner, and substituting the non-elastic gamma ratio, the stratums density and the epithermal neutron ratio into an effective reaction section theoretical calculation formula to establish an overdetermined equation set;
and solving the overdetermined equation set to obtain a coefficient value in an effective reaction section theoretical calculation formula under the current simulation condition, thereby determining the effective reaction section theoretical calculation formula.
7. The method for calculating the formation porosity based on the effective reaction section according to any one of claims 1 to 6, further comprising, after the step of measuring and acquiring the effective reaction section of the formation to be detected, the steps of:
and carrying out formation evaluation on the stratum to be detected based on the effective reaction section.
8. A formation porosity calculation system based on an effective reaction cross-section, comprising:
the stratum effective reaction section obtaining device is used for measuring and obtaining an effective reaction section of the stratum to be detected;
and the porosity calculation module is used for determining the porosity of the stratum to be detected by using the effective reaction section.
9. The formation porosity calculation system according to claim 8, wherein the formation effective reaction section obtaining device is configured to obtain an effective reaction section of the formation to be detected according to an effective reaction section measurement formula when measuring and obtaining the effective reaction section of the formation to be detected:
the effective reaction section measurement formula is as follows:
wherein: ERCS is the effective reaction cross section; r is the ratio of epithermal neutrons or thermal neutrons or capture gamma; r is R i Is a non-elastic gamma ratio; a, a 1 ～a 6 Is a coefficient.
10. The formation porosity calculation system according to claim 8, wherein the porosity calculation module, when determining the porosity of the formation to be detected using the effective reaction section, calculates a porosity calculation formula of the formation to be detected based on the effective reaction section as:
in the method, in the process of the invention,
for the porosity calculated based on the effective reaction cross section, V sh ERCS is the effective reaction cross section of the stratum, the content of the argillaceous volume ma ERCS is the effective reaction section value of the skeleton sh Effective reaction section value for muddy, ERCS f Is the effective reaction cross-section value of the fluid.
11. The formation porosity calculation system according to claim 8, further comprising formation measurement means for transmitting the measured data to the formation effective reaction section acquisition means for calculating an effective reaction section of the formation to be detected;
the formation measurement device includes a neutron source, an epithermal neutron detector, and a gamma detector, wherein,
the neutron source is arranged in a well bore of a well, the epithermal neutron detector comprises a near epithermal neutron detector and a far epithermal neutron detector, the gamma detector comprises a near gamma detector and a far gamma detector, and the near epithermal neutron detector, the near gamma detector, the near epithermal neutron detector and the far epithermal neutron detector are sequentially arranged from the neutron source to the surface direction.
12. The formation porosity calculation system according to claim 11, wherein a first shield of boron carbide is disposed around the epithermal neutron detector; for shielding neutrons directly entering the epithermal neutron detector without passing through the formation to be detected; and a second shielding body made of tungsten-nickel-iron is arranged around the gamma detector and is used for shielding gamma photons which directly enter the gamma detector without passing through the stratum to be detected.
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