阅读说明：本技术 一种基于脉冲中子测井的快中子散射截面表征方法 (Fast neutron scattering cross section characterization method based on pulse neutron logging ) 是由 张锋 范继林 田立立 陈前 张笑瑒 于 2020-06-22 设计创作，主要内容包括：本发明公开了一种基于脉冲中子测井的快中子散射截面表征方法,涉及石油天然气开发领域。包括以下步骤：通过对非弹伽马场分布的公式变形,获得快中子散射截面的表征形式,得出影响快中子散射截面表征的影响因素包括地层密度和探测器源距；根据选定的脉冲中子测井仪器参数,通过建立地层密度的表征方法,得到利用非弹伽马通量以及非弹、俘获伽马通量比信息表征快中子散射截面的数学表达式；建立三维MCNP数值计算模型,模拟得到多探测器伽马通量信息数据库；利用最小二乘方法对不同地层条件的快中子散射截面与伽马通量和通量比信息做三元线性回归分析,得到快中子散射截面的刻度公式的数学形式,最终形成快中子散射截面的表征方法。(The invention discloses a fast neutron scattering cross section characterization method based on pulse neutron logging, and relates to the field of petroleum and natural gas development. The method comprises the following steps: obtaining a representation form of a fast neutron scattering cross section through formula deformation of non-elastic gamma field distribution, and obtaining influence factors influencing fast neutron scattering cross section representation, wherein the influence factors comprise stratum density and detector source distance; according to the selected parameters of the pulse neutron logging instrument, a mathematical expression for representing the fast neutron scattering cross section by using the non-bullet gamma flux and the non-bullet and capture gamma flux ratio information is obtained by establishing a formation density representation method; establishing a three-dimensional MCNP numerical calculation model, and simulating to obtain a multi-detector gamma flux information database; and (3) performing ternary linear regression analysis on the fast neutron scattering cross section and the gamma flux and flux ratio information under different formation conditions by using a least square method to obtain a mathematical form of a scale formula of the fast neutron scattering cross section, and finally forming a characterization method of the fast neutron scattering cross section.)

1. A fast neutron scattering cross section characterization method based on pulse neutron logging is suitable for a D-T source pulse neutron logging instrument and is characterized by comprising the following steps:

step 1: obtaining a representation form of a fast neutron scattering cross section through formula deformation of non-elastic gamma field distribution, and obtaining influence factors influencing fast neutron scattering cross section representation, wherein the influence factors comprise stratum density and detector source distance;

step 2: substituting a characterization formula of the fast neutron scattering cross section into a characterization method of the formation density according to the selected parameters of the pulse neutron logging instrument to obtain a mathematical expression for characterizing the fast neutron scattering cross section by using the non-bomb gamma flux and the non-bomb and capture gamma flux ratio information;

and step 3: establishing a three-dimensional MCNP numerical calculation model, and simulating to obtain a multi-detector gamma flux information database;

and 4, step 4: and (3) performing ternary linear regression analysis on the fast neutron scattering cross section and the gamma flux and flux ratio information under different formation conditions by using a least square method to obtain a mathematical form of a scale formula of the fast neutron scattering cross section, and finally forming a characterization method of the fast neutron scattering cross section.

2. The method for characterizing the fast neutron scattering cross section based on the pulsed neutron logging as recited in claim 1, wherein the fast neutron scattering cross section expression obtained by formula deformation of the non-elastic gamma field distribution in the step 1 is as shown in formula (1):

FNXS is a fast neutron scattering cross section; r is a source distance; phi is a_in(R) is the non-ballistic gamma ray flux, [ rho ] is the formation density value, [ mu ] α is constant at given pulsed neutron instrument parameters_mIs the formation attenuation coefficient phi₀D-T Source intensity, i is the average number of photons produced by a neutron colliding with a nucleus in the formation, ∑_inThe scattering cross section is inelastic scattering cross section of fast neutron, and is far smaller than the scattering cross section of fast neutron, and all four are regarded as constants.

3. The method for characterizing the fast neutron scattering cross section based on the pulsed neutron logging as claimed in claim 1, wherein the pulsed neutron instrument selected in step 2 comprises a D-T neutron source and two gamma detector pulsed neutron instruments.

4. The method for characterizing a fast neutron scattering cross section based on pulsed neutron logging as claimed in claim 1, wherein the expression for characterizing the density in step 2 is as follows:

rho is a formation density value; phi is a_in1、φ_in2Non-ballistic gamma flux at near and far source distances; phi is a_cap1、φ_cap2For capture gamma flux at near and far source distances, A, B and C are constant coefficients.

5. The method for characterizing the fast neutron scattering cross section based on the pulsed neutron logging as claimed in claim 1, wherein the MCNP simulation is utilized to obtain a multi-detector non-elastic and capture gamma counting flux database under the conditions of different lithologies, porosities and hydrocarbon saturations.

6. The fast neutron scattering cross section characterization method based on pulsed neutron logging as claimed in claim 1, wherein the coefficients in the fast neutron scattering cross section characterization formula obtained in step 4 are linear multiple regression.

Technical Field

The invention relates to the field of petroleum and natural gas development, in particular to a fast neutron scattering cross section characterization method based on pulse neutron logging.

Background

The fast neutron scattering cross section is a formation nuclear characteristic, is a physical parameter for measuring the interaction capacity of the formation and the fast neutrons, and is mainly used for evaluating the oil-gas saturation of the formation. In neutron logging, conventional saturation evaluation methods are susceptible to neutron interactions with certain elements, e.g., B, Cl and Gd for Sigma measurements, while H is used for porosity measurements. The fast neutron scattering cross section is only sensitive to the change of gas-liquid two-phase fluid, and the application range of the evaluation on the saturation degree is wider. Meanwhile, when gamma-gamma density measurement cannot be performed (for example, the case of prohibiting the use of a radioactive isotope source), the fast neutron scattering cross section can also be used for open hole density measurement.

Currently, the count of fast neutron scattering cross sections is measured mainly by the non-elastic gamma flux of a long source distance detector of a neutron logging instrument. However, due to the limitation of the logging instrument and the counting statistics of gamma rays, certain errors exist in the measurement of the fast neutron scattering cross section, and the requirements of field oil and gas evaluation are difficult to meet.

Disclosure of Invention

The invention aims to overcome the defects and provides a method for representing a fast neutron scattering cross section by utilizing gamma information combination of multiple detectors in a pulse neutron logging technology.

The invention specifically adopts the following technical scheme:

a fast neutron scattering cross section characterization method based on pulse neutron logging is suitable for a D-T source pulse neutron logging instrument and comprises the following steps:

step 1: obtaining a representation form of a fast neutron scattering cross section through formula deformation of non-elastic gamma field distribution, and obtaining influence factors influencing fast neutron scattering cross section representation, wherein the influence factors comprise stratum density and detector source distance;

step 2: substituting a characterization formula of the fast neutron scattering cross section into a characterization method of the formation density according to the selected parameters of the pulse neutron logging instrument to obtain a mathematical expression for characterizing the fast neutron scattering cross section by using the non-bomb gamma flux and the non-bomb and capture gamma flux ratio information;

and step 3: establishing a three-dimensional MCNP numerical calculation model, and simulating to obtain a multi-detector gamma flux information database;

and 4, step 4: and (3) performing ternary linear regression analysis on the fast neutron scattering cross section and the gamma flux and flux ratio information under different formation conditions by using a least square method to obtain a mathematical form of a scale formula of the fast neutron scattering cross section, and finally forming a characterization method of the fast neutron scattering cross section.

Preferably, the fast neutron scattering cross-section expression obtained by transforming the formula for the non-elastic gamma field distribution in step 1 is as shown in formula (1):

FNXS is a fast neutron scattering cross section; r is a source distance; phi is a_in(R) is the non-ballistic gamma ray flux, [ rho ] is the formation density value, [ mu ] α is constant at given pulsed neutron instrument parameters_mIs the formation attenuation coefficient phi₀D-T Source intensity, i is the average number of photons produced by a neutron colliding with a nucleus in the formation, ∑_inThe scattering cross section is inelastic scattering cross section of fast neutron, and is far smaller than the scattering cross section of fast neutron, and all four are regarded as constants.

Preferably, the selected pulsed neutron instrument in step 2 comprises a D-T neutron source and two gamma detector pulsed neutron instruments.

Preferably, the characterization expression for density in step 2 is as follows:

rho is a formation density value; phi is a_in1、φ_in2Non-ballistic gamma flux at near and far source distances; phi is a_cap1、φ_cap2For capture gamma flux at near and far source distances, A, B and C are constant coefficients.

Preferably, the MCNP simulation is utilized in the step 3 to obtain a multi-detector non-elastic and capture gamma counting flux database under different lithology, porosity and oil-gas saturation conditions.

Preferably, linear multiple regression is adopted to calculate the coefficients in the fast neutron scattering cross section characterization formula in step 4.

The invention has the following beneficial effects:

the method starts from a neutron-gamma coupling theory, eliminates the influence of formation density on the fast neutron scattering cross section by the mathematical representation of the fast neutron scattering cross section and the combination of gamma information of double detectors, and obtains the mathematical expression form of the fast neutron scattering cross section. And calculating the gamma flux of the multi-detector with various lithology and oil-gas containing properties by combining MCNP, obtaining the constant coefficient of a scale formula of the fast neutron scattering cross section and gamma information by data analysis fitting, and finally forming the characterization method of the fast neutron scattering cross section. The method has high calculation precision and wide universality, is slightly influenced by the lithology and the oil-gas content of the stratum, and makes up the defect of utilizing a single detector to perform fast neutron scattering cross section characterization.

Drawings

FIG. 1 is a schematic diagram of an instrument stratigraphic model architecture;

FIG. 2 is a comparison of a calculated value of a fast neutron scattering cross section of a sandstone formation and a real value;

FIG. 3 is a comparison of a calculated value of a fast neutron scattering cross section of a limestone formation with a true value;

FIG. 4 is a comparison of calculated values and actual values of fast neutron scattering cross sections of a dolomite formation;

FIG. 5 is a diagram of the effect of fast neutron scattering cross-section borehole measurements, comparing the real value with the calculated value of the fast neutron scattering cross-section of each interval.

Detailed Description

The following description of the embodiments of the present invention will be made with reference to the accompanying drawings:

a fast neutron scattering cross section characterization method based on pulse neutron logging is suitable for a D-T source pulse neutron logging instrument, adopts a D-T neutron source and at least two gamma detectors (the axial array detector is also suitable), and comprises the following steps:

step 1: and (3) obtaining a characterization form of the fast neutron scattering cross section through formula deformation of non-elastic gamma field distribution, and obtaining influence factors influencing the fast neutron scattering cross section characterization, wherein the influence factors comprise formation density and detector source distance.

The non-elastic gamma field distribution formula can be expressed as:

FNXS is a fast neutron scattering cross section; r is a source distance; phi is a_in(R) is the non-ballistic gamma ray flux, [ rho ] is the formation density value, [ mu ] α is constant at given pulsed neutron instrument parameters_mIs the formation attenuation coefficient phi₀D-T Source intensity, i is the average number of photons produced by a neutron colliding with a nucleus in the formation, ∑_inThe scattering cross section is inelastic scattering cross section of fast neutron, and is far smaller than the scattering cross section of fast neutron, and all four are regarded as constants.

The fast neutron scattering cross section expression obtained by deforming the formula (3) for the non-elastic gamma field distribution is shown as the formula (1):

FNXS is a fast neutron scattering cross section; r is a source distance; phi is a_in(R) is the non-ballistic gamma ray flux, [ rho ] is the formation density value, [ mu ] α is constant at given pulsed neutron instrument parameters_mIs the formation attenuation coefficient phi₀D-T Source intensity, i is the average number of photons produced by a neutron colliding with a nucleus in the formation, ∑_inThe scattering cross section is inelastic scattering cross section of fast neutron, and is far smaller than the scattering cross section of fast neutron, and all four are regarded as constants.

Step 2: and substituting the selected parameters of the pulse neutron logging instrument into a characterization formula of the fast neutron scattering cross section by establishing a characterization method of the stratum density to obtain a mathematical expression for characterizing the fast neutron scattering cross section by using the non-bomb gamma flux and the non-bomb and capture gamma flux ratio information.

The selected pulsed neutron instrument in the step 2 comprises a D-T neutron source and two gamma detector pulsed neutron instruments, but the method is suitable for all cable, drilling and multi-detector pulsed neutron instruments.

The characterization expression for density is as follows:

rho is a formation density value; phi is a_in1、φ_in2Non-ballistic gamma flux at near and far source distances; phi is a_cap1、φ_cap2For capture gamma flux at near and far source distances, A, B and C are constant coefficients.

In conjunction with equations (1) and (2), the fast neutron scattering cross-section can be characterized as:

K. l, M and N are both constant coefficients, and

and step 3: establishing a three-dimensional MCNP numerical calculation model, simulating to obtain a multi-detector gamma flux information database, and simulating to obtain a multi-detector non-elastic and capture gamma counting flux database under the conditions of different lithologies, porosities and oil-gas saturation by using MCNP. Taking a double-source-distance pulse neutron logging instrument as an example, an MCNP numerical calculation model is established, strata are set to have different lithology, porosity and oil-gas saturation, the strata are divided into annular grid cells with the radial length of 1.0cm multiplied by the axial length of 1.0cm, and the non-elastic and capture gamma fluxes and the flux ratio of the non-elastic and capture gamma fluxes of a near detector and a far detector are obtained through simulation.

As shown in figure 1, the stratum model of the dual-source-distance pulse neutron logging instrument comprises a D-T neutron source 1, a tungsten-nickel-iron shield 2, a near detector 3, a tungsten-nickel-iron shield 4, a far detector 5, a stratum 6, a borehole 7 and an instrument shell 8. In the simulation process, the stratum is divided into annular grid cells with the radial length of 1.0cm multiplied by the axial length of 1.0cm so as to ensure the statistical accuracy of the simulation, the stratum is set to be sandstone, limestone and dolomite formations with the interval of 0-30% and the porosity of 5%, the conditions of the stratum containing oil and gas are set, and the non-elastic and capture gamma fluxes and the flux ratio of the near detector and the far detector are obtained through simulation.

And 4, step 4: and (3) performing ternary linear regression analysis on the fast neutron scattering cross section and the gamma flux and flux ratio information under different formation conditions by using a least square method to obtain a mathematical form of a scale formula of the fast neutron scattering cross section, and finally forming a characterization method of the fast neutron scattering cross section. And solving coefficients in the fast neutron scattering cross section characterization formula by adopting linear multiple regression, solving coefficients K, L, M and N in the formula (4), and finally obtaining a calculation formula for characterizing the fast neutron scattering cross section by using gamma information of a multi-detector in the pulse neutron logging technology. And comparing the fast neutron scattering cross section value calculated by using a detector formula under three lithological conditions with the actual fast neutron scattering cross section value of the stratum, wherein the absolute error of the two is 0.2m^-1As shown in fig. 2, 3 and 4.

As shown in fig. 5, which is a diagram of the effect of the fast neutron scattering cross section downhole measurement of the present invention, the first path is depth; the second is a lithology section of the stratum, the third is the porosity of the stratum, the fourth is the gas saturation of the stratum, and the fifth is a fast neutron scattering section (dotted line) obtained by calculation and a theoretical fast neutron scattering section (gray filling).

It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.