CN112034524B - Double-detector well stratum capture section calculation method - Google Patents

Abstract

The invention discloses a method for measuring a stratum capture section in a double-detector well. The invention adopts a measuring device consisting of a D-T controllable neutron source, a W-Ni-Fe shielding body and two LaBr3 gamma detectors to measure in a borehole; the emission and closing of a neutron source are controlled through pulse time sequence, and non-bullet and gamma information is measured by utilizing a gamma detector respectively, so that capture gamma time spectrums of a near detector and a far detector, capture gamma counts and non-bullet gamma counts of the near detector are obtained; calculating a view capture section at a near and far detector using a gamma time spectrum; then, according to the gamma counts, respectively calculating the capture gamma count ratio RCAP at the near detector and the far detector and the non-elastic capture gamma count ratio RIC at the near detector, and determining the equivalent capture cross sections of the diffusion effect at the near detector and the far detector; the corrected formation capture section is calculated in combination with the apparent capture section and the diffusion effect equivalent capture section. The method improves the calculation accuracy of the stratum capture section, and is beneficial to accurately evaluating the oil-gas characteristic of the stratum.

Description

A Calculation Method of Formation Capture Cross Section in Double Detector Well

technical field

The invention belongs to the technical field of oil and gas exploration and development, and in particular relates to a method for calculating a formation capture cross section in a dual-detector well.

Background technique

At present, the measurement of the capture cross-section of the formation is mainly through the pulsed neutron source emitting periodic neutron pulses to the formation. During the pulse off period, the gamma detector is used to measure the gamma time spectrum. Combined, the formation capture cross-section is obtained, and finally the formation capture cross-section is interpreted by using the rock volume physical model to calculate the oil or gas saturation of the formation.

After the neutron pulse is turned off, the attenuation of the gamma time spectrum is not only affected by the formation capture cross section, but also by the diffusion effect. Generally, when the neutron deceleration and capture ability of the formation is weak, most of the neutrons in the detection area are reduced by diffusion, and the diffusion effect has a positive impact on the capture cross section. The smaller the source distance of the detector, the The stronger this positive effect is; when the deceleration and capture ability of neutrons in the formation is stronger, the neutrons in the detection area are basically reduced by being absorbed, and even when the number of neutrons is small, the rate of neutron reduction is not enough In order to fully reflect the absorption capacity of the formation, the diffusion effect has a negative impact on the capture cross section, and the smaller the source distance of the detector, the stronger the negative impact.

In previous capture cross-section measurements, the influence of diffusion effects was often ignored. By measuring the capture cross-section at a fixed source distance, it is considered that this position is the equilibrium point of the diffusion effect, which can roughly eliminate the influence of the diffusion effect. However, the equilibrium point of the diffusion effect itself is greatly affected by the neutron deceleration and absorption capacity of the formation, and cannot be accurately fixed. The capture cross section obtained by using a fixed source distance is affected by the forward diffusion effect in low-salinity and low-porosity oil-bearing formations (weak neutron deceleration and absorption capacity), and the formation capture cross-section value is too large, resulting in the calculation of oil saturation. The result is too small; in high salinity and high porosity oil-bearing formations (strong neutron deceleration and absorption capacity), affected by the negative diffusion effect, the formation capture cross-section value is too small, resulting in too large oil saturation calculation results.

Contents of the invention

Aiming at the above-mentioned deficiencies, the present invention proposes a method for calculating the capture cross-section of formation in a dual-detector well, which uses dual source distances to measure captured gamma counts and non-elastic gamma counts, and utilizes the ratio of captured gamma counts and non-elastic capture gamma counts Reflect the neutron deceleration and absorption capacity of the formation in real time, and correct the possible positive or negative effects of the diffusion effect, so as to accurately calculate the capture cross section.

In order to achieve the above object, the present invention adopts the following technical solutions:

A method for calculating the formation capture cross section in a dual-detector well, using a measuring device composed of a D-T controllable neutron source, a tungsten nickel iron shield and two LaBr3 gamma detectors to measure the accurate formation capture cross section in the borehole, Specifically include the following steps:

Step 1: The D-T steerable neutron source emits fast neutrons in pulse form, the near LaBr3 gamma detector and the far LaBr3 gamma detector measure the non-elastic gamma count during the pulse emission and the captured gamma count and gamma time spectrum;

Step 2: Using the gamma time spectra measured by the near LaBr3 gamma detector and the far LaBr3 gamma detector, calculate the apparent formation capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector respectively;

Step 3: Using the captured gamma counts measured by the near LaBr3 gamma detector and the far LaBr3 gamma detector, calculate the near-far capture gamma count ratio RCAP, and then use the non-elastic gamma count measured by the near LaBr3 gamma detector and capture gamma counts, calculate non-bullet capture gamma count ratio RIC;

Step 4: According to the near and far capture gamma count ratio RCAP and the non-ballistic capture gamma count ratio RIC, calculate the diffusion effect equivalent capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector;

Step 5: Correct the apparent formation capture cross section based on the diffusion effect equivalent capture cross section to obtain an accurate formation capture cross section.

Preferably, in the first step, the average energy of neutrons emitted by the D-T controllable neutron source is about 14 MeV, and the emission pulse period is 1800 μs. Within one pulse period, neutrons are emitted at 0-200 μs, and neutron emission stops at 200-1800 μs.

Preferably, the near LaBr3 gamma detector and the far LaBr3 gamma detector in the first step record non-elastic gamma counts at 0-200 μs, and capture gamma counts and capture gamma time spectra at 400-1800 μs.

Preferably, in the step 4, the formula for calculating the equivalent capture cross section of formation diffusion effect is as follows:

Σ _diff = α · RCAP + β · RIC (1)

In the formula, Σ _diff represents the equivalent capture cross-section of the diffusion effect, and the unit is cu; α and β represent the source distance scale coefficients.

Preferably, in the fifth step, an accurate formation capture cross section is obtained by weighting the corrected near and far LaBr3 gamma detector capture cross sections.

Beneficial technical effects brought by the present invention:

The invention uses a dual-source gamma detector to measure captured gamma counts, non-elastic gamma counts and captured gamma time spectra, and reflects formation alignment in real time according to the ratio of near and far captured gamma counts and the ratio of non-bullet captured gamma counts The deceleration and absorption capacity of electrons can correct the influence of diffusion effect on the capture cross-section; the present invention obtains accurate capture cross-section by weighting the corrected near and far LaBr3 gamma detector capture cross-section, and improves formation oil-bearing or gas-bearing saturation. The accuracy of calculation is beneficial to improve the efficiency of exploration and development of oil and gas resources.

Description of drawings

Fig. 1 is a schematic diagram of the measurement principle of formation capture cross section in a dual-detector well. In the figure: 1 is the D-T controllable neutron source, 2 is the tungsten-nickel-iron shielding body, 3 is the near LaBr3 gamma detector, 4 is the far LaBr3 gamma detector, 5 is the formation capture cross section measurement device in the double detector well, 6 is the borehole, and 7 is the formation material.

Fig. 2 shows the response law of non-elastic capture gamma count ratio under different porosity and salinity conditions.

Fig. 3 shows the response law of near- and far-captured gamma count ratios under different porosity and salinity conditions.

Figure 4 shows the comparison between the apparent capture section and the real section of near and far detectors.

Figure 5 shows the comparison between the corrected capture section and the real section.

Detailed ways

Below in conjunction with accompanying drawing and specific embodiment the present invention is described in further detail:

The present invention proposes a method for calculating the capture cross-section of formations in a double-detector well, using a measuring device composed of a D-T controllable neutron source, a tungsten-nickel-iron shielding body and two LaBr3 gamma detectors, as shown in Figure 1. The average energy of neutrons emitted by the D-T controllable neutron source in the measuring device is about 14MeV. Using the measuring device to measure the accurate formation capture cross section in the borehole, specifically includes the following steps:

Step 1: The D-T controllable neutron source emits fast neutrons with an average energy of about 14MeV in the form of pulses, and the emission pulse period is 1800μs. Within a pulse period, neutrons are emitted at 0-200μs, and neutrons stop at 200-1800μs; through The near LaBr3 gamma detector and the far LaBr3 gamma detector measure the non-elastic gamma counts during pulse launch and the captured gamma counts and gamma time spectra during pulse off, i.e. near LaBr3 gamma detector and far LaBr3 gamma The detector records non-elastic gamma counts at 0-200 μs, and capture gamma counts and capture gamma time spectra at 400-1800 μs.

Step 2: Using the gamma time spectra measured by the near LaBr3 gamma detector and the far LaBr3 gamma detector, respectively calculate the apparent formation capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector, as shown in formula (2 ) as shown:

N(t)＝A _BH ×exp(-t·νΣ _BH )+A _FORM ×exp(-t·νΣ _FORM ) (2)

In the formula, N(t) represents the captured gamma counts at different time intervals; A _BH and A _FORM represent the contribution of borehole and formation to the gamma counts respectively; ν represents the thermal neutron migration velocity, ν=2200m/s; Σ _BH and Σ _FORM represent the apparent capture cross-section values of borehole and formation, respectively, in cu.

Step 3: Using the captured gamma counts measured by the near LaBr3 gamma detector and the far LaBr3 gamma detector, calculate the near-far capture gamma count ratio RCAP, and then use the non-elastic gamma count measured by the near LaBr3 gamma detector and capture gamma counts, calculate the non-capture gamma count ratio RIC.

Figure 2 shows the variation law of the non-elastic capture gamma count ratio RIC with porosity. The count ratio RIC is greatly affected by the formation's ability to absorb neutrons.

Fig. 3 shows the change law of the near and far capture gamma count ratio RCAP with porosity. From Fig. 3, the near and far capture gamma count ratio RCAP has little change under different salinity conditions, indicating that near and far The far-capture gamma count ratio RCAP is less affected by the formation’s ability to absorb neutrons, while the near- and far-capture gamma count ratio RCAP is mainly affected by the formation’s ability to decelerate neutrons. Therefore, using the non-bullet capture gamma count ratio RIC The combination of near and far capture gamma count ratio RCAP can reflect the deceleration and absorption capacity of the formation in real time, and calculate the diffusion effect equivalent capture cross section caused by the diffusion effect on the formation capture cross section.

Step 4: According to the near and far capture gamma count ratio RCAP and the non-ballistic capture gamma count ratio RIC, respectively calculate the diffusion effect equivalent capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector, as shown in the formula ( 1) as shown:

Σ _diff = α · RCAP + β · RIC (1)

In the formula, Σ _diff represents the equivalent capture cross-section of the diffusion effect, and the unit is cu; α and β represent the source distance scale coefficients.

Figure 4 shows that the formation lithology is sandstone and limestone, the porosity ranges from 0% to 40%, and the salinity ranges from 0g/L to 200g/L, measured by near and far LaBr3 gamma detectors The comparison between the apparent formation capture cross-section and the real formation capture cross-section can be seen from Fig. 4. When the diffusion effect is not corrected, the formation capture cross-section value directly calculated by the near and far LaBr3 gamma detectors has a large gap with the real cross-section value, and, The performance of the gap between the two is also different under different formation conditions. In some formation conditions, the apparent capture cross-section value is greater than the real capture cross-section value, and in some formation conditions, the apparent capture cross-section value is smaller than the real capture cross-section value. Therefore, the diffusion effect has a positive impact on the capture cross-section and Negatively affects both situations.

Step 5: Correct the apparent formation capture cross-section based on the equivalent capture cross-section of the diffusion effect, and calculate the accurate formation capture cross-section by weighting the corrected near and far LaBr3 gamma detector capture cross-sections.

Figure 5 shows the comparison results of the capture cross-section values corrected by using the near- and far-capture gamma count ratio RCAP and the non-bullet capture gamma count ratio RIC and the real capture cross-section value. It can be seen from Fig. 5 that the corrected capture cross-section The difference between the value and the real capture cross section is small; by calculating the relative error of each measurement point, the average relative error between the apparent capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector and the real capture cross section is 21.98%, After correction, the average relative error between the capture cross section values at the near LaBr3 gamma detector and the far LaBr3 gamma detector and the real capture cross section is 2.36%.

Of course, the above descriptions are not intended to limit the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or replacements made by those skilled in the art within the scope of the present invention shall also belong to the present invention. protection scope of the invention.

Claims (3)

1. A method for calculating the formation capture cross section in a double detector well, using a measuring device composed of a D-T controllable neutron source, a tungsten-nickel-iron shielding body and two LaBr3 gamma detectors, characterized in that, in the wellbore Accurate formation capture cross section, specifically includes the following steps: Step 1: The D-T steerable neutron source emits fast neutrons in pulse form, the near LaBr3 gamma detector and the far LaBr3 gamma detector measure the non-elastic gamma count during the pulse emission and the captured gamma count and gamma time spectrum; Step 2: Using the gamma time spectra measured by the near LaBr3 gamma detector and the far LaBr3 gamma detector, calculate the apparent formation capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector respectively; Step 3: Using the captured gamma counts measured by the near LaBr3 gamma detector and the far LaBr3 gamma detector, calculate the near-far capture gamma count ratio RCAP, and then use the non-elastic gamma count measured by the near LaBr3 gamma detector and capture gamma counts, calculate non-bullet capture gamma count ratio RIC; Step 4: According to the near and far capture gamma count ratio RCAP and the non-ballistic capture gamma count ratio RIC, calculate the diffusion effect equivalent capture cross section at the near LaBr3 gamma detector and the far LaBr3 gamma detector; Step 5: Correct the apparent formation capture cross-section based on the equivalent capture cross-section of the diffusion effect to obtain an accurate formation capture cross-section; In the fourth step, the formula for calculating the equivalent capture cross section of formation diffusion effect is as follows: Σ _diff = α · RCAP + β · RIC (1) In the formula, Σ _diff represents the equivalent capture cross-section of the diffusion effect, and the unit is cu; α and β represent the source-distance scale coefficients; In the fifth step, the accurate stratum capture cross section is obtained by weighting and averaging the corrected near and far LaBr3 gamma detector capture cross sections. 2. The method for calculating the capture cross-section of formations in a dual-detector well according to claim 1, wherein the average energy of neutrons emitted by the D-T controllable neutron source in said step 1 is about 14MeV, and the emission pulse period is 1800μs, within a pulse period, 0-200μs emits neutrons, and 200-1800μs neutrons stop emitting. 3. The method for calculating the formation capture cross section in a dual-detector well according to claim 1, wherein the near LaBr3 gamma detector and the far LaBr3 gamma detector in said step 1 record at 0-200 μs Non-elastic gamma counts, capture gamma counts and capture gamma time spectra were recorded at 400-1800 μs.

Images