A formation porosity calculation method and system based on effective reaction cross section

Abstract

The present invention provides a method and system for calculating formation porosity based on an effective reaction cross section. The method comprises the following steps: determining a theoretical calculation formula for the effective reaction cross section based on fast neutron transport and gamma decay theory; determining the effective reaction cross sections and coefficients of the relationship for different materials by solving an overdetermined system of equations based on the theoretical calculation formula; measuring the effective reaction cross section using the inelastic gamma ratio and epithermal neutron ratio, and calibrating the measurement formula using the effective reaction cross sections of each material; and using the measured effective reaction cross sections to determine the porosity of the formation to be tested. Compared to traditional neutron porosity, the new parameter effective reaction cross section obtained by this method has higher porosity sensitivity, its logging response conforms to the volume physics model, is unaffected by the excavation effect, and is more suitable for porosity evaluation in complex reservoirs.

Description

Technical Field

[0001] The present invention relates to the technical field of well logging in oil and gas exploration, and in particular to a formation porosity calculation method and system based on an effective reaction cross section. Background Art

[0002] Porosity is a crucial geological parameter in formation evaluation. Accurately measuring reservoir porosity is crucial for oil and gas development and exploration. Neutron logging has long been a primary method for measuring formation porosity. The development of pulsed neutron logging technology has not only addressed safety and contamination issues associated with traditional isotope neutron sources, but also enabled multi-mode pulsed neutron logging to simultaneously measure multiple parameters, making it a key method for formation evaluation. The development of multifunctional, integrated pulsed neutron logging instruments is a current focus of nuclear logging research, with porosity logging being a key component. While neutron porosity logging offers an advantage over other logging methods in being less affected by pore structure, its response is affected by the mineralogy and fluid properties of the skeletal structure, making it challenging to accurately measure formation porosity in complex reservoirs, particularly gas-bearing formations. Due to the excavation effect, the response can even decrease with increasing porosity. Summary of the Invention

[0003] In response to the above problems, the present invention provides a formation porosity calculation method and system based on effective reaction cross section, which can improve the detection sensitivity of formation porosity.

[0004] The present invention discloses a formation porosity calculation method based on an effective reaction cross section, comprising the following steps:

[0005] Measure and obtain the effective reaction cross section of the formation to be tested;

[0006] The effective reaction cross section is used to determine the porosity of the formation to be tested.

[0007] Furthermore, in the step of measuring and obtaining the effective reaction cross section of the formation to be detected, the effective reaction cross section measurement formula is:

[0008]

[0009] Where: ERCS is the effective reaction cross section; R is the ratio of epithermal neutrons, thermal neutrons or captured gamma rays; R i is the inelastic gamma ratio; a1~a6 are coefficients.

[0010] Furthermore, in the step of determining the porosity of the formation to be detected by using the effective reaction cross section, the porosity calculation formula of the formation to be detected based on the effective reaction cross section is:

[0011]

[0012] Where, is the porosity calculated based on the effective reaction cross section, V sh is the volume content of mud, ERCS is the effective reaction cross section of the formation, and ERCS ma is the effective reaction cross section value of the skeleton, ERCS sh is the effective reaction cross section value of mud, ERCS f is the effective reaction cross section value of the fluid.

[0013] Furthermore, before the step of measuring and obtaining the effective reaction cross section of the formation to be detected, the method further includes the following steps:

[0014] Obtain inelastic gamma ratio and epithermal neutron ratio through formation measurements;

[0015] Determine the theoretical calculation formula for the effective reaction cross section. By setting up a variety of formations with different porosities, determine the relationship between the inelastic gamma ratio and epithermal neutron ratio and the effective reaction cross section.

[0016] Based on the effective reaction cross section theoretical calculation formula, an effective reaction cross section measurement formula is determined.

[0017] Furthermore, the step of determining the theoretical calculation formula of the effective reaction cross section includes:

[0018] Based on the fast neutron transport theory and gamma decay theory, the relationship between the inelastic gamma ratio and the deceleration length, mass attenuation coefficient and formation density is obtained:

[0019]

[0020] is the inelastic gamma ratio recorded by the gamma detector at the source distances r1 and r2, r1<r2, L e is the deceleration length, μ m is the mass attenuation coefficient, ρ is the density;

[0021] Based on Lagrange's mean value theorem, the transformation is:

[0022]

[0023] b1 and b2 are in the interval [0,1], and the deceleration length L e The scattering cross section is expressed as a function of the mass number of the atoms in the formation. For the same formation, the neutron deceleration length L e for:

[0024]

[0025] Among them, R f is the neutron deceleration distance, the macroscopic scattering cross section of the formation σ i is the microscopic scattering cross section of the i-th element in the formation, W i is the mass percentage of the i-th element in the stratigraphic framework, A is the mass number, and A i is the atomic mass of the i-th element, N A is Avogadro's constant, f(A) is a function related to the formation mass number;

[0026] make is the effective reaction cross section ERCS, then:

[0027]

[0028] Where C is a constant related to the source distances r1 and r2; f(R) is a function of the ratio of epithermal neutrons, thermal neutrons, or captured gamma rays; taking into account the mass attenuation coefficient μ m In the logging range, the changes of various materials are small, so μ m After the influence, the theoretical calculation formula of the effective reaction cross section is converted:

[0029]

[0030] Where f'(R) is a function composed of f(R) and C.

[0031] Furthermore, the step of determining the relationship between the inelastic gamma ratio and the epithermal neutron ratio and the effective reaction cross section by setting a plurality of strata with different porosities includes:

[0032] The theoretical calculation formula of the effective reaction cross section is converted into coefficient expression:

[0033]

[0034] Among them, c1~c4 are coefficients, i is the i-th substance, n is the total number of substances, v i is the volume content of the i-th substance;

[0035] Set up a variety of formations with different porosities, simulate and obtain the inelastic gamma ratio, formation density and epithermal neutron ratio, and substitute them into the theoretical calculation formula of effective reaction cross section to establish an overdetermined equation system;

[0036] The overdetermined set of equations is solved to obtain coefficient values in the theoretical calculation formula of the effective reaction cross section under the current simulation conditions, thereby determining the theoretical calculation formula of the effective reaction cross section.

[0037] Furthermore, after the step of measuring and obtaining the effective reaction cross section of the formation to be detected, the method further includes the following steps:

[0038] The formation to be detected is evaluated based on the effective reaction cross section.

[0039] The present invention also discloses a formation porosity calculation system based on an effective reaction cross section, comprising:

[0040] A formation effective reaction cross-section acquisition device, used to measure and acquire the effective reaction cross-section of the formation to be detected;

[0041] The porosity calculation module is used to determine the porosity of the formation to be detected using the effective reaction cross section.

[0042] Furthermore, when measuring and obtaining the effective reaction cross-section of the formation to be detected, the formation effective reaction cross-section obtaining device is used to obtain the effective reaction cross-section of the formation to be detected according to the effective reaction cross-section measurement formula:

[0043] The effective reaction cross section measurement formula is:

[0044]

[0045] Where: ERCS is the effective reaction cross section; R is the ratio of epithermal neutrons to thermal neutrons or captured gamma rays; R i is the inelastic gamma ratio; a1~a6 are coefficients.

[0046] Furthermore, when the porosity calculation module determines the porosity of the formation to be detected using the effective reaction cross section, the porosity calculation formula of the formation to be detected based on the effective reaction cross section is:

[0047]

[0048] Where, is the porosity calculated based on the effective reaction cross section, V sh is the volume content of mud, ERCS is the effective reaction cross section of the formation, and ERCS ma is the effective reaction cross section value of the skeleton, ERCS sh is the effective reaction cross section value of mud, ERCS f is the effective reaction cross section value of the fluid.

[0049] Furthermore, the formation porosity calculation system further includes a formation measurement device for transmitting the measured data to the formation effective reaction cross section acquisition device for calculating the effective reaction cross section of the formation to be detected;

[0050] The formation measurement device includes a neutron source, an epithermal neutron detector and a gamma detector, wherein:

[0051] The neutron source is arranged in a wellbore of a drilled well, the epithermal neutron detector includes a near epithermal neutron detector and a far epithermal neutron detector, and the gamma detector includes a near gamma detector and a far gamma detector. The near epithermal neutron detector, the near gamma detector, the near epithermal neutron detector and the far epithermal neutron detector are arranged in sequence from the neutron source toward the surface.

[0052] Furthermore, a first shielding body made of boron carbide is provided around the epithermal neutron detector to shield neutrons that directly enter the epithermal neutron detector without passing through the formation to be detected; a second shielding body made of tungsten nickel iron is provided around the gamma detector to shield gamma photons that directly enter the gamma detector without passing through the formation to be detected.

[0053] The present invention provides a method and system for calculating formation porosity based on the effective reaction cross section. This method calculates formation porosity by designing a new parameter, the effective reaction area. The effective reaction cross section is dependent solely on the elements that make up a substance and remains constant for the same substance. The method comprises the following steps: obtaining a theoretical calculation formula for the effective reaction cross section based on fast neutron transport and gamma decay theory; measuring the effective reaction cross section using the inelastic gamma ratio and epithermal neutron ratio; and determining the porosity of the formation to be tested using the effective reaction cross section. Compared to traditional neutron porosity, this parameter has higher porosity sensitivity, its logging response conforms to a volumetric physics model, is unaffected by the mining effect, and is more suitable for evaluating the porosity of complex reservoirs. This provides theoretical and technical support for obtaining more accurate formation porosity using multifunctional integrated pulsed neutron logging. The present invention's formation porosity calculation system based on the effective reaction cross section is used to implement the present invention's formation porosity calculation based on the effective reaction cross section. The instrumentation used is conventional in the field, making it easy to operate, implement, and test.

[0054] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 A schematic structural diagram of a formation measurement device provided by an embodiment of the present invention;

[0056] Figure 2 A schematic flow chart of a formation porosity calculation method based on an effective reaction cross section provided by an embodiment of the present invention;

[0057] Figure 3The k value variation range when the source distance is 30 cm provided by the embodiment of the present invention;

[0058] Figure 4 The k value variation range when the source distance is 60 cm provided by the embodiment of the present invention;

[0059] Figure 5 The effective reaction cross section measurement effect provided by the embodiment of the present invention;

[0060] Figure 6 The response characteristics of the effective reaction cross section provided by the embodiment of the present invention under different lithological conditions;

[0061] Figure 7 The response characteristics of the effective reaction cross section provided by the embodiment of the present invention under different mud content conditions;

[0062] Figure 8 The response characteristics of the effective reaction cross section provided by the embodiment of the present invention under different fluid property conditions;

[0063] Figure 9 This is the application effect of the effective reaction cross section provided by the embodiment of the present invention in calculating the porosity of the formation to be tested. DETAILED DESCRIPTION

[0064] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0065] Example 1:

[0066] This embodiment provides a formation porosity calculation method based on the effective reaction cross section, and the steps are as follows: Figure 2 As shown, the specific steps include:

[0067] S1. Based on the theory of fast neutron transport and gamma decay, determine the theoretical calculation formula of the effective reaction cross section;

[0068] S2. Based on the effective reaction cross section theoretical calculation formula, the effective reaction cross sections and relationship coefficients of different substances are determined by solving the overdetermined equation system;

[0069] S3. Use the inelastic gamma ratio and epithermal neutron ratio to measure the effective reaction cross section, and use the effective reaction cross section of each substance to calibrate the measurement formula;

[0070] S4. Determine the porosity of the formation to be tested using the measured effective reaction cross section.

[0071] In step S1, based on the fast neutron transport and gamma decay theory, the effective reaction cross section theoretical calculation formula is determined. The specific derivation steps of the effective reaction cross section theoretical calculation formula are as follows:

[0072] 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, ∑ in is the inelastic scattering macroscopic cross section, Q is the source intensity, L e is the deceleration length; ρ is the density, D n is the fast neutron diffusion coefficient, D r is the gamma diffusion coefficient, r is the distance between the measuring point and the gamma source. In one embodiment of the present invention, a deuterium-tritium neutron generator (DT) is used as both a neutron source and a gamma source.

[0073]

[0074] μ m is the mass attenuation coefficient, and its expression is σ j The microscopic scattering cross section of the jth element in the formation, Z i is the atomic number of the i-th element, A i is the atomic mass of the i-th element, N A is Avogadro's constant, m is the total number of substances in the formation, and n is the total number of elements in the jth substance.

[0075] For the inelastic gamma ratio recorded by gamma detectors at source distances r1 and r2, r1 < r2, as shown in Formula 2, the inelastic gamma ratio is the ratio of the inelastic gamma fluxes, obtained by the near gamma detector and the far gamma detector.

[0076]

[0077] Based on Lagrange's mean value theorem, Formula 2 can be transformed into Formula 3.

[0078]

[0079] Among them: b1 and b2 are coefficients in the interval [0,1], L e is the deceleration length, and its expression is Formula 4.

[0080]

[0081] For the neutron deceleration distance R f If the medium is composed of light nuclei, then R f It can be expressed as shown in Formula 5.

[0082]

[0083] Where, ∑ s is the macroscopic scattering cross section of the formation, A is the mass number, ξ is the average logarithmic energy loss of the formation, E0 is the initial neutron energy, E f is the energy of the neutron after deceleration. If the medium is composed of nuclides with larger mass numbers, the mean square value of the deceleration distance is Calculated by formula 6.

[0084]

[0085] Regardless of whether the formation is composed of light nuclei or heavy nuclei with larger mass numbers, for the same formation, the neutron deceleration length can be calculated by formula 7.

[0086]

[0087] Wherein, f(A) is a function related to the formation mass number, and can be determined according to Formula 5 and Formula 6.

[0088] Macroscopic scattering cross section of the formation ∑ s It can be expressed by formula 8.

[0089]

[0090] Among them, σ i The microscopic scattering cross section of the i-th element in the formation, W i is the mass percentage of the i-th element in the stratigraphic framework, A i is the atomic mass of the i-th element, N A is Avogadro's constant. Let f(A) is the effective reaction cross section ERCS. Formula 3 can be expressed as shown in Formula 9.

[0091]

[0092] Assume k = (b2r2-b1r1), the range of k value is as follows Figure 3 and Figure 4 As shown in Figure 2 , in limestone formations containing methane or water in their pores, the k value increases with the formation hydrogen index, regardless of whether the hydrogen atoms are present in the water or methane medium, and is independent of the fluid type. Therefore, similar to the response of thermal neutrons to the formation, the k value can be characterized by the epithermal neutron count rate ratio.

[0093] Therefore, Formula 9 can be expressed as Formula 10.

[0094]

[0095] Where C is a constant related to the source distances r1 and r2. f(R) is a function of the ratio of epithermal neutrons to thermal neutrons or captured gamma rays. The mass attenuation coefficient μ is m The changes of various materials within the logging range are small, so the effective reaction cross section ERCS can be expressed as Equation 11.

[0096]

[0097] Where f'(R) is a function composed of f(R) and C.

[0098] In step S2, based on the effective reaction cross section theoretical calculation formula, the effective reaction cross sections and relationship coefficients of different substances are determined by solving the overdetermined equations. Specifically:

[0099] According to Equation 11, the effective reaction cross section can be calculated from the inelastic gamma ratio, density, and epithermal neutron ratio. By setting up various formations with varying porosities, such as limestone saturated with water, oil, or gas, and sandstone saturated with water, oil, or gas, the effective reaction cross section and formula coefficients for the current measurement instrument can be obtained using an overdetermined system of equations. The theoretical calculation formula for the effective reaction cross section is shown in Equation 12.

[0100]

[0101] Among them, c1~c4 are coefficients, i is the i-th substance, n is the total number of substances, v i is the volume content of the i-th substance. By substituting data from various formations with varying porosities and performing coefficient fitting calculations, we ultimately obtain the theoretical effective reaction cross section formula determined by the formation coefficient. This allows direct calculation of the theoretical effective reaction cross section using Equation 12 for unknown formations or substances not involved in solving the overdetermined equations. Calculating the effective reaction cross section using Equation 12 requires the corresponding inelastic gamma ratio, density, and epithermal neutron ratio.

[0102] In step S3, the effective reaction cross section is measured using the inelastic gamma ratio and the epithermal neutron ratio, and the measurement formula is calibrated using the effective reaction cross section of each substance. Specifically:

[0103] The effective reaction cross section (ERCS) depends solely on the elements that make up a substance. For a given substance, its effective reaction cross section remains constant. The theoretical effective reaction cross section (ERCS) calculation is dependent on the inelastic gamma ratio, density, and epithermal neutron ratio. Since density can be calculated from the inelastic gamma ratio and epithermal neutron ratio, the effective reaction cross section (ERCS) can be calculated using only the inelastic gamma ratio and epithermal neutron ratio. Based on the obtained effective reaction cross section (ERCS), a simulation was conducted to simulate various formations with varying porosities, studying the relationship between the inelastic gamma ratio and epithermal neutron ratio and the effective reaction cross section. This led to the determination of a formula for measuring the effective reaction cross section, as shown in Equation 13.

[0104]

[0105] Where R is the ratio of epithermal neutrons, thermal neutrons, or captured gamma rays; R i is the inelastic gamma ratio, a1~a6 are coefficients. The measurement effect of formula 13 in different formations is as follows Figure 5 shown.

[0106] The difference between the effective reaction cross section measurement formula and the theoretical effective reaction cross section calculation formula is that the influencing variable, formation density, is omitted in the measurement formula. When calculating the coefficients in Formula 13, the inelastic gamma ratio and epithermal neutron ratio measured by the actual instrument are used in conjunction with the effective reaction cross section calculated using Formula 12 to fit and determine the values of coefficients a1-a6. This determines the calculation method for the effective reaction cross section for the current measuring instrument (different inelastic gamma ratio and epithermal neutron ratio measuring instruments will have different fitted coefficients c1-c4 and a1-a6). It should be understood that when the measuring instrument is replaced, all coefficients in the effective reaction cross section measurement formula and the theoretical effective reaction cross section calculation formula need to be recalculated.

[0107] In combination with actual measurement data, the corresponding effective reaction cross-section value can be calculated through the effective reaction cross-section measurement formula and the effective reaction cross-section theoretical calculation formula. When the logging density is unknown, the effective reaction cross-section measurement formula can be used for calculation.

[0108] In step S4, the porosity of the formation to be tested is determined using the measured effective reaction cross section. Specifically:

[0109] The logging response of the effective reaction cross section conforms to the volumetric physical model, e.g. Figure 6 、 Figure 7 and Figure 8 shown. Figures 6 to 8 These are the response characteristics of the effective reaction cross section measured by the method provided in this embodiment under different formation conditions. Figures 6 to 8 It can be seen that under different lithology, shale content, and fluid property conditions, the effective reaction cross section logging response conforms to the volumetric physics model. Therefore, the effective reaction cross section can be combined with other parameters for formation evaluation, or directly used for porosity calculation. Using the volumetric physics model method, the effective reaction cross section is used to calculate formation porosity. The calculation formula is as follows:

[0110]

[0111] Where, is the porosity calculated based on the effective reaction cross section, ERCS is the effective reaction cross section of the formation, ma is the effective reaction cross section value of the skeleton, V shis the mud content, Φ shc is the effective reaction cross section value of mud, ERCS f is the effective reaction cross section value of the fluid.

[0112] The effective reaction cross section calculation data is obtained through formation measurement combined with the effective reaction cross section measurement formula or the effective reaction cross section theoretical calculation formula.

[0113] The effective reaction cross section (ERC) provided by this invention is a new parameter. It is essentially a function of the macroscopic scattering cross section, minus the influence of density, and the mass number of the atoms that make up the formation. The above derivation indicates that the ERC is dependent solely on the elements that make up the substance and remains constant for the same substance.

[0114] Example 2:

[0115] This embodiment also provides a formation porosity calculation system based on an effective reaction cross section, which is used to implement the method described in the first embodiment. The system includes a formation effective reaction cross section acquisition device, which is used to acquire the effective reaction cross section of the formation to be tested; wherein the effective reaction cross section is determined based on the macroscopic scattering cross section after removing the influence of density and the mass number of the atoms constituting the formation;

[0116] The porosity calculation module is used to determine the porosity of the formation to be tested using the measured effective reaction cross section.

[0117] Furthermore, when acquiring the effective reaction cross-section of the formation to be detected, the formation effective reaction cross-section acquisition device is used to:

[0118] By setting up a variety of formations with different porosities through simulation, the relationship between the inelastic gamma ratio and the epithermal neutron ratio and the effective reaction cross section is studied, and the effective reaction cross section measurement formula is determined:

[0119]

[0120] Where: ERCS is the effective reaction cross section; R is the ratio of epithermal neutrons to thermal neutrons or captured gamma rays; R i is the inelastic gamma ratio; a1~a6 are coefficients.

[0121] Furthermore, when the porosity calculation module determines the porosity of the formation to be detected using the effective reaction cross section,

[0122] The porosity calculation method of the formation to be tested based on the effective reaction cross section is:

[0123]

[0124] Where, is the apparent porosity calculated based on the effective reaction cross section, ERCS is the effective reaction cross section of the formation, ma is the effective reaction cross section value of the skeleton, V sh is the mud content, Φ shc is the effective reaction cross section value of mud, ERCS f is the effective reaction cross section value of the fluid.

[0125] Furthermore, the formation porosity calculation system based on effective reaction cross section of the present invention further comprises a formation measurement device for transmitting measured data to the formation effective reaction cross section acquisition device;

[0126] The standard well model was constructed by software to simulate and verify the formation porosity calculation method of the present invention. Figure 1 The formation measurement device in the simulation is used to perform formation measurement, and the inelastic gamma ratio and epithermal neutron ratio are calculated based on the measured data. The effective reaction cross section of the formation is measured using Formula 13, and the porosity is calculated using Formula 14. Figure 9 This is the effect of calculating porosity under different formation conditions using the effective reaction cross section measured by the method provided in this embodiment. Figure 9 It can be seen that the effective reaction cross section is not affected by the excavation effect. Regardless of whether the formation contains gas or water, the porosity calculated based on the effective reaction cross section is close to the formation porosity. Therefore, the method of the present invention can obtain very accurate formation porosity values, thus laying the foundation for formation evaluation.

[0127] Specifically, in the above calculation method, the formation to be tested can be a complex formation containing different framework minerals and fluid properties. The inelastic gamma ratio of the formation to be tested can be obtained using the inelastic gamma counts detected by the near gamma detector and the far gamma detector. The epithermal neutron ratio of the formation to be tested can be obtained using the counts of the near epithermal neutron detector and the far epithermal neutron detector.

[0128] The wellbore is a wellbore formed by the drill bit drilling from the surface to the completed wellbore depth. It is generally cylindrical. In the specific implementation of the present invention, a neutron source, two gamma detectors and two epithermal neutron detectors are all placed in the wellbore along the drilling axis. The four detectors are at different distances from the neutron source, such as Figure 1 As shown, for example, the near epithermal neutron detector can be arranged between the pulsed neutron source (i.e., the neutron source and gamma source described in the embodiment of the present invention) and the far epithermal neutron detector, the near epithermal neutron detector is between the pulsed neutron source and the near gamma detector, and the far epithermal neutron detector is between the near gamma detector and the far gamma detector.

[0129] In this embodiment, the detectors (epithermal neutron detector and gamma detector) and neutron source can be conventional instruments in the art. For example, the neutron source can be a currently commonly used deuterium-tritium neutron generator (DT); the two gamma detectors used can be lanthanum bromide (LaBr3) detectors; the two epithermal neutron detectors used can be helium triple tubes ( 3 He) detector.

[0130] Furthermore, in an embodiment of the present application, neutrons and gamma photons that do not enter the formation to be detected are shielded. By varying the thickness of the shield and then measuring the neutron and gamma photon counts, neutrons are prevented from being directly received by the detector without passing through the formation, thereby improving the reliability of the calculation results. In a specific implementation of the present invention, a first shield made of boron carbide is disposed around the epithermal neutron detector to shield neutrons that do not pass through the formation to be detected and directly enter the epithermal neutron detector. A second shield made of tungsten-nickel-iron is disposed around the gamma detector to shield gamma photons that do not pass through the formation to be detected and directly enter the gamma detector.

[0131] The principle of the present invention is as follows: Based on the fast neutron transport theory and the gamma decay theory, the relationship between the inelastic gamma ratio and the deceleration length, the mass attenuation coefficient and the formation density is obtained; the deceleration length is further expressed as a function related to the scattering cross section and the mass number of the atoms constituting the formation, wherein for the same material constituting the formation with a certain atomic mass number, the scattering cross section can be expressed as the density and the microscopic cross section of each element of the constituent material; thereby, the density can be separated from the scattering cross section, and the function related to the macroscopic scattering cross section and the mass number of the atoms constituting the formation after removing the influence of density is considered to be a new parameter (i.e., the effective reaction cross section in the present invention). Considering that the attenuation coefficient of each material within the logging range varies little, the above relationship can be further converted into a relationship between the effective reaction cross section, the inelastic gamma ratio and the density, thereby determining the theoretical calculation formula for the effective reaction cross section.

[0132] Formation density can be calculated using the inelastic gamma ratio and epithermal neutron ratio, and the theoretical effective reaction cross section can be measured using the inelastic gamma ratio and epithermal neutron ratio. Using the Monte Carlo method, a variety of formations with varying porosities are set up, and the inelastic gamma ratio and epithermal neutron ratio are measured. Based on the theoretical effective reaction cross section calculation formula, the effective reaction cross section measurement formula based on the inelastic gamma ratio and epithermal neutron ratio is determined. The effective reaction cross section formula coefficient is then determined based on reference well data. This allows for porosity analysis of formations based on the effective reaction cross section.

[0133] The above analysis shows that the effective reaction cross section (ERC) is solely dependent on the elements of the material (both the inelastic gamma ratio and density are related to the elemental composition of the material in the formation). For the same material, the ERC remains constant. By using a rock volume physics model and setting up multiple formation models with varying porosity, we simulated the ERC, formation density, and epithermal neutron ratio. By solving an overdetermined system of equations, we calibrated the theoretical ERC calculation formula (i.e., the correlation coefficient in the formula) and obtained theoretical ERC values for various materials. In addition to directly obtaining theoretical ERC values using the formation model, we can also use the calibrated ERC calculation formula.

[0134] The calculation principle of porosity is that porosity is a macroscopic manifestation of formation fluid. The greater the difference between the measured value of skeleton minerals and the measured value of fluid, the more suitable it is for porosity measurement.

[0135] Calculations based on the present invention indicate that the effective reaction cross-sections of framework minerals are similar, with the framework's effective reaction cross-section being approximately twice that of oil and water, and the oil-water's reaction cross-section being approximately twice that of gas. The logging response of the effective reaction cross-sections conforms to the rock volume physics model. Therefore, using the effective reaction cross-sections for formation porosity analysis can avoid the influence of the mining effect, providing theoretical and technical support for obtaining more accurate formation porosity using multifunctional integrated pulsed neutron logging.

[0136] The effective reaction cross section (ERC)-based formation porosity calculation method and system presented in this application are more suitable for evaluating porosity in complex reservoirs. Compared to traditional neutron porosity logging, the ERC is more suitable for determining the porosity of gas-bearing formations. Because the ERC conforms to a volumetric physics model, it can be combined with other parameters for formation evaluation or used alone to calculate formation porosity.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A formation porosity calculation method based on effective reaction cross section, characterized in that: The following steps are included: Measure and obtain the effective reaction cross section of the formation to be tested; Determining the porosity of the formation to be tested using the effective reaction cross section; In the step of measuring and obtaining the effective reaction cross section of the formation to be detected, the effective reaction cross section measurement formula is: in: ERCS is the effective reaction cross section; is the ratio of epithermal neutrons, thermal neutrons or captured gammas; is the non-elastic gamma ratio; is the coefficient.

2. The formation porosity calculation method based on effective reaction cross section according to claim 1, characterized in that: In the step of determining the porosity of the formation to be detected by using the effective reaction cross section, the porosity calculation formula of the formation to be detected based on the effective reaction cross section is: Where, is the porosity calculated based on the effective reaction cross section, is the volume content of mud, is the effective reaction cross section of the formation, is the effective reaction cross-section value of the skeleton, is the effective reaction cross section value of mud, is the effective reaction cross section value of the fluid.

3. The formation porosity calculation method based on effective reaction cross section according to claim 1, characterized in that: Before the step of measuring and obtaining the effective reaction cross section of the formation to be detected, the method further includes the following steps: Obtain inelastic gamma ratio and epithermal neutron ratio through formation measurements; Determine the theoretical calculation formula for the effective reaction cross section. By setting up a variety of formations with different porosities, determine the relationship between the inelastic gamma ratio and epithermal neutron ratio and the effective reaction cross section. Based on the effective reaction cross section theoretical calculation formula, an effective reaction cross section measurement formula is determined.

4. The formation porosity calculation method based on effective reaction cross section according to claim 3, characterized in that: The steps of determining the theoretical calculation formula of the effective reaction cross section include: Based on the fast neutron transport theory and gamma decay theory, the relationship between the inelastic gamma ratio and the deceleration length, mass attenuation coefficient and formation density is obtained: The source distances are , The inelastic gamma ratio recorded by the gamma detector is < , is the deceleration length, is the mass attenuation coefficient, is the density; Based on Lagrange's mean value theorem, the transformation is: and The value is in the interval [0,1], the deceleration length The scattering cross section is expressed as a function of the mass number of the atoms in the formation. For the same formation, the neutron deceleration length for: in, is the neutron deceleration distance, the macroscopic scattering cross section of the formation ， For the stratum i The microscopic scattering cross section of the element, For the i The mass percentage of the elements in the stratigraphic framework, A is the mass number ， For the i The atomic mass of an element, is Avogadro's constant, It is a function related to the size of the formation mass number; make is the effective reaction cross section ERCS , we get: in, The distance from the source and Related constants; is a function of the ratio of epithermal neutrons, thermal neutrons, or captured gamma rays; taking into account the mass attenuation coefficient The changes of various materials within the logging range are small and can be ignored. After the influence, the theoretical calculation formula of the effective reaction cross section is converted: in, for and Composition function.

5. The formation porosity calculation method based on effective reaction cross section according to claim 4, characterized in that: The step of determining the relationship between the inelastic gamma ratio and the epithermal neutron ratio and the effective reaction cross section by setting a plurality of formations with different porosities includes: The theoretical calculation formula of the effective reaction cross section is converted into coefficient expression: = in, is the coefficient, i For the i species of substances, n is the total amount of matter, For the i Volume content of the substance; Set up a variety of formations with different porosities, simulate and obtain the inelastic gamma ratio, formation density and epithermal neutron ratio, and substitute them into the theoretical calculation formula of effective reaction cross section to establish an overdetermined equation system; The overdetermined set of equations is solved to obtain coefficient values in the theoretical calculation formula of the effective reaction cross section under the current simulation conditions, thereby determining the theoretical calculation formula of the effective reaction cross section.

6. The formation porosity calculation method based on effective reaction cross section according to any one of claims 1 to 5, characterized in that: After the step of measuring and obtaining the effective reaction cross section of the formation to be detected, the method further includes the following steps: The formation to be detected is evaluated based on the effective reaction cross section.

7. A formation porosity calculation system based on effective reaction cross section, characterized in that: include: A formation effective reaction cross-section acquisition device, used to measure and acquire the effective reaction cross-section of the formation to be detected; a porosity calculation module, configured to determine the porosity of the formation to be detected using the effective reaction cross section; When measuring and obtaining the effective reaction cross section of the formation to be detected, the formation effective reaction cross section obtaining device is used to obtain the effective reaction cross section of the formation to be detected according to the effective reaction cross section measurement formula: The effective reaction cross section measurement formula is: in: ERCS is the effective reaction cross section; is the ratio of epithermal neutrons or thermal neutrons or captured gammas; is the non-elastic gamma ratio; is the coefficient.

8. The formation porosity calculation system according to claim 7, characterized in that: When the porosity calculation module determines the porosity of the formation to be detected using the effective reaction cross section, the porosity calculation formula of the formation to be detected based on the effective reaction cross section is: Where, is the porosity calculated based on the effective reaction cross section, is the volume content of mud, is the effective reaction cross section of the formation, is the effective reaction cross-section value of the skeleton, is the effective reaction cross section value of mud, is the effective reaction cross section value of the fluid.

9. The formation porosity calculation system according to claim 7, characterized in that: It also includes a formation measurement device for transmitting the measured data to the formation effective reaction cross section acquisition device for calculating the effective reaction cross 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 wellbore of a drilled well, the epithermal neutron detector includes a near epithermal neutron detector and a far epithermal neutron detector, and the gamma detector includes a near gamma detector and a far gamma detector. The near epithermal neutron detector, the near gamma detector, the near epithermal neutron detector and the far epithermal neutron detector are arranged in sequence from the neutron source toward the surface.

10. The formation porosity calculation system according to claim 9, characterized in that: A first shielding body made of boron carbide is arranged around the epithermal neutron detector to shield neutrons that directly enter the epithermal neutron detector without passing through the formation to be detected; a second shielding body made of tungsten nickel iron is arranged around the gamma detector to shield gamma photons that directly enter the gamma detector without passing through the formation to be detected.

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