CN106597560A - Neutron gamma density logging method by utilizing distribution symptom of fast neutron field - Google Patents

Abstract

The invention discloses a neutron gamma density logging method by utilizing a distribution symptom of a fast neutron field. Fast neutron scattering and a gamma attenuation theory are used to represent distribution of an inelastic scattering gamma field, and mathematical relations between the gamma field distribution and parameters including the density, the free path of fast neutron scattering and a cross section of inelastic scattering are provided; on the basis of distribution of the inelastic scattering gamma field, a method for calculating the density via the free path of fast neutron scattering and inelastic scattering gammas of two detectors is obtained; the measured amount of fast neutrons is used to represent the free path of fast neutron scattering, and the new method for calculating the stratum density directly via the inelastic scattering gammas and the fast neutron field is obtained. The method avoids correction of hydrogen contained indexes, and provides technical support and theoretical guidance for gamma density logging.

Description

Neutron-gamma density logging method using fast neutron field distribution representation

Technical Field

The invention relates to the field of petroleum and natural gas development, in particular to a neutron-gamma density logging method using fast neutron field distribution representation.

Background

The neutron gamma density logging technology adopts a D-T neutron source, can realize passive measurement of the stratum density by recording inelastic scattering gamma rays generated by the action of fast neutrons and the nuclei of stratum elements, has important significance for realizing formation parameter measurement while drilling and reducing radiation hazard, and is a hotspot problem in the development of current nuclear logging.

The distribution of gamma sources and the physical process of gamma rays and the stratum in neutron gamma density logging are very complex, and the method is essentially different from the traditional density logging. Currently, neutron gamma density logging is mainly to measure formation density by detecting inelastic scattering gamma rays, and the calculation result is greatly influenced by hydrogen index, and usually needs to adopt capture gamma or thermal neutrons to correct the hydrogen index.

Disclosure of Invention

Aiming at the problem of hydrogen index correction of the existing neutron-gamma density logging, the invention provides a neutron-gamma density logging method characterized by fast neutron field distribution, which combines fast neutron scattering and gamma ray attenuation theories to characterize inelastic scattering gamma field distribution and deduce that the gamma field distribution is related to parameters such as density, fast neutron scattering free path, inelastic scattering cross section and the like, and finally obtains a new method for directly characterizing formation density by inelastic scattering gamma and fast neutron field, thereby avoiding hydrogen index correction and providing technical support and theoretical guidance for neutron-gamma density logging.

The technical solution adopted by the invention is as follows:

a neutron-gamma density logging method using fast neutron field distribution characterization comprises the following steps:

(1) the inelastic scattering gamma field distribution is characterized by combining fast neutron scattering and gamma attenuation theories, and the correlation of the inelastic scattering gamma field distribution with density, a fast neutron scattering free path and an inelastic scattering cross section is deduced;

(2) obtaining a calculation method for representing density by using inelastic scattering gamma and fast neutron scattering free path according to inelastic scattering gamma field distribution;

(3) the fast neutron scattering free path is characterized by measuring the fast neutron flux, and the method for directly calculating the stratum density by using the inelastic scattering gamma and the fast neutron field is obtained.

In the step (1): the inelastic scattering gamma field distribution characterized by combining fast neutron scattering and gamma attenuation theory is as follows

Wherein S is₀As neutron source intensity, λ_sFor fast neutron scattering free path, mu_mThe mass absorption coefficient of the inelastic scattering gamma ray is rho, the stratum density is, i is the number of gamma photons averagely released by inelastic collision of a fast neutron and a nucleus, R is the radius of a spherical detector, sigma_inIs a macroscopic inelastic scattering cross section of the stratum.

According to the structural design of the instrument, the source distance of the gamma detector is generally larger than 30cm, the contribution of the region between the source and the detector to the inelastic scattering gamma counting of the detector is up to more than 95%, and the distribution of the inelastic scattering gamma field is simplified to be

Inelastic scattering gamma field distribution transformation by the Lagrange's median theorem

Wherein a is a proportionality coefficient, and the value is related to the source distance.

In the step (2): method for obtaining single detector inelastic scattering gamma determined density by using inelastic scattering gamma field distribution type (3)

Formation density measurement by adopting single-detector inelastic scattering gamma, and simultaneously measuring inelastic scattering cross section sigma_inFast neutron scattering free path lambda_sAnd a plurality of parameters are not beneficial to the calculation of the formation density.

According to the single detector density characterization method, the formation density characterization method can be simplified by adopting double detector information combination, and the method comprises the following steps:

the source distances of the near detector and the far detector are respectively assumed to be R₁And R₂The near and far detectors inelastic scattering gamma determination density is in the form

Elimination of ln (i Σ) using near and far probe density expressions_inS₀) Item, calculation method for obtaining double-source distance inelastic scattering gamma density

Wherein the slope A is the inelastic scattering gamma ratio logarithm ln (phi)_in1/φ_in2) The inverse density sensitivity of (a); intercept ρ₀(λ_s) Is ln (phi)_in1/φ_in2) The formation density value corresponding to 0 is associated with the fast neutron scattering free path. When the source distance of the detector is fixed, the formation density is measured by adopting double-source distance inelastic scattering gamma, and only the near and far inelastic scattering gamma counts and the fast neutron scattering free path need to be measured.

In the step (3): according to the fast neutron scattering theory, the fast neutron flux phi of inelastic scattering_fSatisfies the relation with the free path of fast neutron scattering

Wherein R is₃The source distance of the fast neutron detector;

method for representing fast neutron free path by using fast neutron flux to obtain formation density represented by inelastic scattering gamma and fast neutron field

When the source distance of the instrument detector is fixed, the formula (9) can be expressed as

ρ＝A ln(φ_in1/φ_in2)+Blnφ_f+C (10)

Wherein A, B and C are constants and are spaced from the detector source only by a distance R₁、R₂、R₃And neutron intensity S₀And (4) correlating.

In practical application, the measurement of the free path of the fast neutron and the intensity of the neutron source is not needed, and the inelastic scattering gamma counting comparison density curve can be establishedLine intercept ρ₀(λ_s) The characterization of the scattering free path of the fast neutron is directly finished by the relation with the fast neutron count

ρ₀(λ_s)＝Blnφ_f+C (11)。

The beneficial technical effects of the invention are as follows:

compared with the prior art, the inelastic scattering gamma field distribution is represented by combining fast neutron scattering and gamma attenuation theories, the mathematical relation between the gamma field distribution and parameters such as density, a fast neutron scattering free path, a nonelastic scattering cross section and the like is pushed out, a new method for directly representing the density by using the inelastic scattering gamma and the fast neutron field is finally obtained, hydrogen index correction is avoided, and technical support and theoretical guidance are provided for neutron gamma density logging.

Drawings

FIG. 1 is a schematic diagram of a model for deriving inelastic scattering gamma field distribution according to the present invention.

FIG. 2 is a formation model of a neutron-gamma while drilling instrument built by a Monte Carlo numerical simulation method. 1. The device comprises an instrument shell, a D-T neutron source, a shield body 3, a fast neutron detector 4, a near gamma detector 5, a far gamma detector 6, a saturated water-stone-limestone stratum 7, borehole water 8, a drill collar 9 and a slurry diversion channel 10.

Fig. 3 is a side view of fig. 2.

FIG. 4 is the response relationship between the logarithm of the ratio of the near-inelastic scattering gamma counts to the logarithm of the far-inelastic scattering gamma counts and the density under different hydrogen-containing indexes.

FIG. 5 is response relationship between fast neutron count logarithm and density under different hydrogen index conditions.

FIG. 6 is the intercept rho of the inelastic scattering gamma count specific density curve corresponding to different hydrogen indexes₀(λ_s) And the logarithm of fast neutron counts.

FIG. 7 is a result of a neutron-gamma density calculation characterized by a fast neutron field distribution.

Detailed Description

The invention provides a neutron-gamma density logging method using fast neutron field distribution representation, which combines fast neutron scattering and gamma attenuation theories to represent inelastic scattering gamma field distribution to obtain a method using inelastic scattering gamma and fast neutron field to directly represent stratum density, thereby avoiding hydrogen index correction, and sequentially comprising the following steps:

(1) and (3) representing inelastic scattering gamma field distribution by combining fast neutron scattering and gamma attenuation theories, and deducing that the inelastic scattering gamma field distribution is related to parameters such as density, a fast neutron scattering free path, an inelastic scattering cross section and the like.

Establishing a derivation model as shown in FIG. 1; the neutron source is positioned at the central position (O point) of the uniform infinite spherical stratum and uniformly emits 14MeV fast neutrons to the stratum; taking O as a sphere center, and setting a spherical detector A with a radius of R to record inelastic scattering gamma rays from the stratum; the inelastic scattering gamma field distribution is characterized by combining the fast neutron scattering and gamma attenuation theories as follows

Wherein S is₀As neutron source intensity, λ_sFor fast neutron scattering free path (associated with fast neutron field distribution, varying with hydrogen index), μ_mIs the mass absorption coefficient of the inelastic scattering gamma ray, rho is the density of the stratum, i is the number of gamma photons averagely released by inelastic collision of a fast neutron with a nucleus, sigma_inIs a macroscopic inelastic scattering cross section of the stratum. The same symbols are used in the following formulae.

According to the structural design of the instrument, the source distance of the gamma detector is generally larger than 30cm, the contribution of the region between the source and the detector to the inelastic scattering gamma counting of the detector is up to more than 95%, and the distribution of the inelastic scattering gamma field is simplified to be

Inelastic scattering gamma field distribution transformation by the Lagrange's median theorem

Wherein a is a proportionality coefficient, and the value is related to the source distance.

(2) And obtaining a calculation method for representing density by using inelastic scattering gamma and fast neutron scattering free path according to inelastic scattering gamma field distribution.

Obtaining a form of single detector inelastic scattering gamma determined density using an inelastic scattering gamma field distribution (3)

Formation density measurement by adopting single-detector inelastic scattering gamma, and simultaneously measuring inelastic scattering cross section sigma_inFast neutron scattering free path lambda_sAnd the density calculation difficulty is increased by a plurality of parameters.

According to the single detector density characterization method, a density calculation method can be simplified by adopting a double-detector combination method, which comprises the following steps:

the source distances of the near detector and the far detector are respectively assumed to be R₁And R₂The near and far detectors inelastic scattering gamma determination density is in the form

Elimination of ln (i Σ) using near and far probe density expressions_inS₀) Item, calculation method for obtaining double-source distance inelastic scattering gamma density

Wherein the slope A is the inelastic scattering gamma ratio logarithm ln (phi)_in1/φ_in2) The inverse density sensitivity of (a); intercept ρ₀(λ_s) Is ln (phi)_in1/φ_in2) The formation density value corresponding to 0 is associated with the fast neutron scattering free path. When the source distance of the detector is fixed, the formation density is measured by adopting double-source distance inelastic scattering gamma, and only the near and far inelastic scattering gamma counts and the fast neutron scattering free path need to be measured.

(3) And (3) representing a fast neutron scattering free path by measuring the fast neutron flux to obtain a method for calculating the stratum density by using inelastic scattering gamma and a fast neutron field.

Because the fast neutron scattering free path can not be directly measured by an instrument, measurable parameters are required to be adopted for representing the fast neutron scattering free path; according to the fast neutron scattering theory, the fast neutron flux phi of inelastic scattering_fSatisfies the relation with the free path of fast neutron scattering

Wherein R is₃The source distance of the fast neutron detector.

Method for representing fast neutron free path by using fast neutron flux to obtain formation density represented by inelastic scattering gamma and fast neutron field

When the source distance of the instrument detector is fixed, the formula (9) can be expressed as

ρ＝A ln(φ_in1/φ_in2)+Blnφ_f+C (10)

Wherein A, B and C are constants and are spaced from the detector source only by a distance R₁、R₂、R₃And neutron intensity S₀And (4) correlating.

The above calculation method can be used for measuring the neutron-gamma density.

According to the method, fast neutron scattering and gamma attenuation theories are combined to represent inelastic scattering gamma field distribution, and the mathematical relationship between inelastic scattering gamma field distribution and parameters such as density, fast neutron scattering free path and inelastic scattering cross section is deduced. And then obtaining a calculation method for representing density by using inelastic scattering gamma and fast neutron scattering free path according to inelastic scattering gamma field distribution. And finally, representing a fast neutron scattering free path by measuring fast neutron count to obtain a method for directly determining the formation density by using inelastic scattering gamma and fast neutrons.

The method of the invention has no requirement on the position of the fast neutron detector and can be arranged at any position of an instrument.

The present invention is further illustrated by the following specific examples.

The neutron-gamma density logging method using fast neutron field distribution representation of the invention adopts 1 fast neutron detector and 2 gamma detector instrument structure designs, as shown in fig. 2 and 3. Wherein, the fast neutron detector is used for recording the fast neutron count with energy more than 1MeV and can be placed at any position of the instrument (in the example, the source distance is 20 cm); the near and far gamma detectors record inelastic scattering gamma counts at different source spacings, 30cm and 70cm respectively.

The neutron gamma density logging method specifically comprises the following steps:

step 1, recording inelastic scattering gamma counts at different positions through a near gamma detector and a far gamma detector, and establishing response relations between the count ratio of the inelastic scattering gamma rays and the density and the fast neutron free path under different hydrogen-containing indexes; the specific relationship is as follows:

ρ＝Aln(φ₁/φ₂)+ρ₀(λ_s) (11)

wherein phi is₁And phi₂Inelastic scattering gamma counts at different locations are recorded for the near and far gamma detectors, respectively, with ρ being the formation density.

By using the formula (11), the inelastic scattering gamma ratio logarithm and the density response curve under different hydrogen-containing indexes are subjected to linear fitting as shown in figure 4, and the slope A and the intercept rho of the inelastic scattering gamma ratio logarithm and the density response curve are obtained₀(λ_s). Wherein, the slope A of the curve corresponding to different hydrogen index is approximately the same, and the intercept rho₀(λ_s) Changes along with the hydrogen index of the stratum, and satisfies a linear relation with the reciprocal of the scattering free path of the fast neutrons.

Step 2, recording fast neutron count by using a detector, and establishing an inelastic scattering gamma ratio logarithm and density curve intercept rho₀(λ_s) And the fast neutron count is utilized to represent the influence of the scattering free path on the density measurement.

The response of the log of the fast neutron counts to the density was recorded for different hydrogen index conditions, as shown in fig. 5. Establishing the intercept of inelastic scattering gamma-ray log ratio log density curve rho₀(λ_s) And fast neutron count logarithm ln phi_fLinear relation of (1)

ρ₀(λ_s)＝Blnφ_f+C (12)

Using equation (12), for rho under different hydrogen index conditions₀(λ_s) And fast neutron count logarithm ln phi_fThe curve is linearly fitted as shown in FIG. 6 to obtain ρ₀(λ_s) Logarithm of fast neutron count ln phi_fSlope B and intercept C of the curve.

And 3, representing the fast neutron scattering free path in density calculation by using the measured fast neutron count to obtain a method for directly representing density by using inelastic scattering gamma and fast neutrons.

ρ＝Aln(φ_in1/φ_in2)+Blnφ_f+C (13)

Using the above embodiment, coefficients A, B and C were obtained by linear fitting, and the density calculation results are shown in fig. 7; the formation density directly calculated by gamma and fast neutrons is matched with the actual density, and the calculation result is not influenced by the hydrogen index any more.

Compared with the prior art, the inelastic scattering gamma field distribution is represented by combining fast neutron scattering and gamma attenuation theories, the mathematical relation between the gamma field distribution and parameters such as density, a fast neutron scattering free path, a nonelastic scattering cross section and the like is pushed out, a new method for directly representing the density by using the inelastic scattering gamma and the fast neutron field is finally obtained, hydrogen index correction is avoided, and technical support and theoretical guidance are provided for neutron gamma density logging.

Claims (7)

1. A neutron-gamma density logging method using fast neutron field distribution characterization is characterized by comprising the following steps:

(1) the inelastic scattering gamma field distribution is characterized by combining fast neutron scattering and gamma attenuation theories, and the correlation of the inelastic scattering gamma field distribution with density, a fast neutron scattering free path and an inelastic scattering cross section is deduced;

(2) obtaining a calculation method for representing density by using inelastic scattering gamma and fast neutron scattering free path according to inelastic scattering gamma field distribution;

(3) and (3) representing a fast neutron scattering free path by measuring the fast neutron flux to obtain a method for calculating the stratum density by using inelastic scattering gamma and a fast neutron field.

2. The method for neutron-gamma density logging characterized by fast neutron field distribution according to claim 1, wherein in step (1): the inelastic scattering gamma field distribution is characterized by combining the fast neutron scattering and gamma attenuation theories as follows

${\mathrm{\φ}}_{in}\left(R\right)=\frac{{\∫}_0^{\mathrm{\∞}}i{\Σ}_{in}{S}_0{e}^{-r/{\mathrm{\λ}}_s}{e}^{-{\mathrm{\ρ\μ}}_m\left|r-R\right|}dr}{4{\mathrm{\πR}}^2}=\frac{i{\Σ}_{in}{S}_0}{4{\mathrm{\πR}}^2}\[\frac{2{\mathrm{\ρ\μ}}_m{e}^{-R/{\mathrm{\λ}}_s}-({\mathrm{\ρ\μ}}_m+1/{\mathrm{\λ}}_s)e^{-{\mathrm{\ρ\μ}}_{m}R}}{{\left({\mathrm{\ρ\μ}}_m\right)}^2-1/{\mathrm{\λ}}_s^2}\]---\left(1\right)$

Wherein,S₀as neutron source intensity, λ_sFor fast neutron scattering free path, mu_mThe mass absorption coefficient of the inelastic scattering gamma ray is rho, the stratum density is, i is the number of gamma photons averagely released by inelastic collision of a fast neutron and a nucleus, R is the radius of a spherical detector, sigma_inIs a macroscopic inelastic scattering cross section of the stratum.

3. The method of claim 2, wherein the method comprises the steps of: inelastic scattering gamma field distribution is simplified to

${\mathrm{\φ}}_{in}\left(R\right)\≈\frac{i{\Σ}_{in}{S}_0{\∫}_0^R{e}^{-r/{\mathrm{\λ}}_s}{e}^{-{\mathrm{\ρ\μ}}_m(R-r)}dr}{4{\mathrm{\πR}}^2}=\frac{i{\Σ}_{in}{S}_0}{4{\mathrm{\πR}}^2}\left(\frac{e^{-R/{\mathrm{\λ}}_s}-e^{{\mathrm{\ρ\μ}}_{m}R}}{{\mathrm{\ρ\μ}}_m-1/{\mathrm{\λ}}_s}\right)---\left(2\right)$

Inelastic scattering gamma field distribution transformation by the Lagrange's median theorem

${\mathrm{\φ}}_{in}\left(R\right)=\frac{i{\Σ}_{in}{S}_0}{4\mathrm{\π}R}{e}^{-R\[(1-a)(1/{\mathrm{\λ}}_s)+{\mathrm{a\ρ\μ}}_m\]}---\left(3\right)$

Wherein a is a proportionality coefficient, and the value is related to the source distance.

4. A method for neutron-gamma density logging characterized by fast neutron field distribution according to claim 3, wherein in step (2): obtaining a form of single detector inelastic scattering gamma determined density using an inelastic scattering gamma field distribution (3)

$\mathrm{\ρ}=-\frac{1}{{\mathrm{Ra\μ}}_m}{\mathrm{ln\φ}}_{in}-\frac{R(1-a)}{{\mathrm{Ra\μ}}_m}\frac{1}{{\mathrm{\λ}}_s}+\frac{1}{{\mathrm{Ra\μ}}_m}\ln \left(i{\Σ}_{in}{S}_0\right)-\frac{\ln \left(4\mathrm{\π}R\right)}{{\mathrm{Ra\μ}}_m}---\left(4\right).$

5. The neutron-gamma density logging method using fast neutron field distribution characterization according to claim 4, characterized in that according to the single detector density characterization method, the density calculation method is simplified by using a dual detector combination method, specifically as follows:

the source distances of the near detector and the far detector are respectively assumed to be R₁And R₂The near and far detectors inelastic scattering gamma determination density is in the form

$\mathrm{\ρ}=-\frac{1}{R_1{a}_1{\mathrm{\μ}}_m}{\mathrm{ln\φ}}_{in1}-\frac{R_1(1-a_1)}{R_1{a}_1{\mathrm{\μ}}_m}\frac{1}{{\mathrm{\λ}}_s}+\frac{1}{R_1{a}_1{\mathrm{\μ}}_m}\ln \left(i{\Σ}_{in}{S}_0\right)-\frac{\ln \left(4{\mathrm{\πR}}_1\right)}{R_1{a}_1{\mathrm{\μ}}_m}---\left(5\right)$

$\mathrm{\ρ}=-\frac{1}{R_2{a}_2{\mathrm{\μ}}_m}{\mathrm{ln\φ}}_{in2}-\frac{R_2(1-a_2)}{R_2{a}_2{\mathrm{\μ}}_m}\frac{1}{{\mathrm{\λ}}_s}+\frac{1}{R_2{a}_2{\mathrm{\μ}}_m}\ln \left(i{\Σ}_{in}{S}_0\right)-\frac{\ln \left(4{\mathrm{\πR}}_2\right)}{R_2{a}_2{\mathrm{\μ}}_m}---\left(6\right)$

Elimination of ln (i Σ) using near and far probe density expressions_inS₀) Item, calculation method for obtaining double-source distance inelastic scattering gamma density

$\mathrm{\ρ}=-\frac{1}{(R_1{a}_1-R_2{a}_2)u_m}\ln \left({\mathrm{\φ}}_{in1}/{\mathrm{\φ}}_{in2}\right)-\frac{\[R_1(1-a_1)-R_2(1-a_2)\]}{(R_1{a}_1-R_2{a}_2)u_m}\frac{1}{{\mathrm{\λ}}_s}+\frac{\ln (R_2/R_1)}{(R_1{a}_1-R_2{a}_2)u_m}=A\ln \left({\mathrm{\φ}}_{in1}/{\mathrm{\φ}}_{in2}\right)+{\mathrm{\ρ}}_0\left({\mathrm{\λ}}_s\right)---\left(7\right)$

Wherein the slope A is the inelastic scattering gamma ratio logarithm ln (phi)_in1/φ_in2) The inverse density sensitivity of (a); intercept ρ₀(λ_s) Is ln (phi)_in1/φ_in2) The formation density value corresponding to 0 is associated with the fast neutron scattering free path.

6. The method for neutron-gamma density logging characterized by fast neutron field distribution according to claim 5, wherein in step (3): according to the fast neutron scattering theory, the fast neutron flux phi of inelastic scattering_fSatisfies the relation with the free path of fast neutron scattering

${\mathrm{\φ}}_f\left(R_3\right)=\frac{S_0}{4{\mathrm{\πR}}_3^2}{e}^{-R_3/{\mathrm{\λ}}_s}---\left(8\right)$

Wherein R is₃The source distance of the fast neutron detector;

method for representing fast neutron free path by using fast neutron flux to obtain formation density represented by inelastic scattering gamma and fast neutron field

$\mathrm{\ρ}=-\frac{\ln \left({\mathrm{\φ}}_{in1}/{\mathrm{\φ}}_{in2}\right)}{(R_1{a}_1-R_2{a}_2)u_m}+\frac{\[R_1(1-a_1)-R_2(1-a_2)\]}{(R_1{a}_1-R_2{a}_2)u_m}{\mathrm{ln\φ}}_f+\frac{\[R_1(1-a_1)-R_2(1-a_2)\]}{(R_1{a}_1-R_2{a}_2)u_m}\ln \frac{4{\mathrm{\πR}}_3^2}{S_0}+\frac{\ln (R_2/R_1)}{(R_1{a}_1-R_2{a}_2)u_m}---\left(9\right)$

When the source distance of the instrument detector is fixed, the formula (9) can be expressed as

ρ＝A ln(φ_in1/φ_in2)+Blnφ_f+C (10)

Wherein A, B and C are constants and are spaced from the detector source only by a distance R₁、R₂、R₃And neutron intensity S₀And (4) correlating.

7. The method of claim 1, wherein the method comprises the steps of: by establishing the intercept of inelastic scattering gamma-ray log ratio log density curve rho₀(λ_s) The characterization of the scattering free path of the fast neutron is directly finished by the relation with the fast neutron count

ρ₀(λ_s)＝Blnφ_f+C (11)。