CN115098823B - Neutron lifetime logging stratum macroscopic capture section extraction method - Google Patents
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Abstract
The invention discloses a neutron lifetime logging stratum macroscopic capture section extraction method. The method mainly realizes accurate extraction of the macroscopic capture section of the stratum from the capture time spectrum acquired by the neutron lifetime logging, and effectively peels off the influence of the borehole environment. The main flow of the invention can be summarized as follows: smoothing filtering is carried out on the capturing time spectrum to improve the signal to noise ratio; intercepting data of the tail end part of the spectral line to calculate the mean value and standard deviation of the background counting rate, and determining the background area range by taking three times of the standard deviation as a standard; selecting initial stratum attenuation data points according to the background area range, and performing linear fitting under a semi-logarithmic coordinate system to obtain a stratum attenuation curve; subtracting the stratum attenuation curve from the original spectrum to obtain borehole attenuation data, and fitting the borehole attenuation data to obtain a borehole attenuation curve; and subtracting the well attenuation curve from the original spectrum, obtaining stratum attenuation data again, fitting, iterating until the stratum attenuation curve is not changed any more, and outputting the stratum attenuation curve at the moment for calculating a capture section.
Description
Technical Field
The invention relates to a method for extracting a macroscopic capture section of a neutron lifetime logging stratum, which is mainly used for accurately extracting the macroscopic capture section of the stratum from a capture time spectrum acquired by the neutron lifetime logging, and simultaneously realizing the automatic elimination of adverse effects of borehole fluid and background gamma on the extraction of the macroscopic capture section of the stratum.
Background
Along with the gradual entering of most land oil fields in China into the middle and later stages of development, the oil fields face a series of problems of high water content, high extraction degree and the like, and the exploration and the mining of residual oil (gas) have important significance for the development of the middle and later stages of the oil field development, and are also important means for realizing stable yield and increase of the oil field in the middle and later stages and improving the recovery ratio. Neutron life logging technology is widely applied to the evaluation of the saturation of residual oil (gas) of a reservoir after the well is sleeved since the advent of the neutron life logging technology, and plays an important role in the development of the residual oil (gas) of an oil field in the middle and later stages.
The measurement object of neutron lifetime logging is the macroscopic capture section Sigma (Σ) of the stratum, which is a physical quantity reflecting the thermal neutron absorption capacity of a substance. The neutron lifetime logging instrument emits fast neutrons into the stratum through a pulse neutron source, the fast neutrons gradually slow down in the stratum, and diffusion is finally captured and capture gamma rays are released. By monitoring the change in capture gamma ray count rate over time and recording as a capture time spectrum, the average decay time of the thermal neutron count in the formation, i.e., neutron lifetime τ, can be obtained therefrom. The formation macroscopic capture section Σ can be obtained from the neutron lifetime τ. The fundamental principle of neutron life logging technology is to distinguish hydrocarbon-containing and water-containing reservoir units by utilizing the great difference of attenuation speed of thermal neutrons in hypersalinity stratum water and hydrocarbon substances, wherein the difference is derived from the fact that stratum water contains a large amount of chlorine elements with larger capture cross sections, and the attenuation speed of capture gamma rays is higher when the chlorine element content is higher, so that the macroscopic capture cross section in a hypersalinity stratum is far larger than that of an oil layer. The measurement of neutron life logging can penetrate through the development process of the whole oil-gas well, and the change of residual oil-gas in the production well along with time and the migration of an oil-gas-water interface can be obtained by utilizing the distinguishing capability of the neutron life logging on oil (gas) water, namely a time lapse monitoring technology. And neutron life logging is also used as an important monitoring technology for evaluating residual oil in a water-flooding oil reservoir.
The main interpretation parameter used for neutron lifetime logging interpretation is the stratum macroscopic capture section Σ extracted from the measured capture time spectrum. Based on the rock physical volume model, the water saturation of the stratum can be calculated according to the capture section values of the rock skeleton, the argillaceous matter, the stratum water and the oil gas in the stratum and stratum parameters such as the porosity, the argillaceous matter content and the like, and the evaluation of the reservoir after the reservoir is realized. However, since neutron lifetime logging is affected by the borehole environment at the time of measurement, this results in a measured capture cross section that is the result of the interaction of the formation with the borehole fluid. In order to eliminate the influence of the well bore environment on the calculation of the capture section, a plurality of researchers at home and abroad develop corresponding researches. Based on neutron life logging measurement theory, researchers derive boundary problem solutions under the logging model, obtain a diffusion model conforming to neutron life logging, and can better understand the correlation between thermal neutron parameters of the well bore and the stratum. Researchers build a database through a large number of actual measurement data of different lithology, porosity, borehole size, casing size and formation fluid mineralization degree to build a calibration model of neutron lifetime logging; and the neutron life logging data and the carbon-oxygen ratio logging data are combined for explanation, so that the limitation of single neutron life logging is overcome, and the method can be used for determining the saturation of the oil, gas and water phases after the well logging. With the development of numerical simulation technology, the response rule and the influence factor of the nuclear logging can be studied more deeply. The neutron life logging response rules of different borehole sizes, different stratum parameters and different borehole fluids are researched by students by adopting a Monte Carlo numerical simulation technology. The correction method from the logging value to the formation capture section is established by the learner according to the mineralization degree of the well fluid by utilizing the artificial formation experiment and Monte Carlo numerical simulation; and describing a standard model response process of neutron lifetime logging affected by the stratum or wellbore environment by utilizing Monte Carlo numerical simulation and multiple sets of diffusion equations. The scholars also calculate a large number of simulated data points under different well conditions by utilizing Monte Carlo numerical simulation, and a correction model is built by adopting the relative count rates of two detectors in different time gates, so that the capturing section or mineralization degree of the well fluid can be not required to be known in advance during interpretation. In the method for extracting the macroscopic capture section of the stratum, researchers have proposed to fit a neutron lifetime capture time spectrum by adopting a double-index model so as to separate the borehole capture section from the stratum capture section; or by providing two suitable time gates in the capture time spectrum, calculating the macroscopic capture cross section of the formation by counting the count rates in the two time gates and based on the exponential decay law.
In general, the current difficulty in extracting the macroscopic capture section of the formation from the mid-life log capture time spectrum is how to eliminate the effects of the borehole environment and background gamma, thereby obtaining the macroscopic capture section value of the formation.
Disclosure of Invention
The invention mainly overcomes the defects in the prior art, and aims to provide a method for accurately extracting the macroscopic capture section of the stratum from the log capture time spectrum of the middle service life, effectively eliminate the influence of the borehole environment and background gamma on the calculation of the capture section of the stratum and lay a foundation for the calculation of the residual oil saturation of the subsequent reservoir. In order to achieve the technical purpose, the invention adopts the following technical scheme:
In order to extract the formation macroscopic capture section values from the mid-life log capture time spectrum, the capture time spectrum needs to be analyzed first. The capture time spectrum acquired by neutron lifetime logging can be divided into three main areas, and the rate of decay of capture gamma count rate with time is different for oil and water layers due to the difference in macroscopic capture cross section, as shown in fig. 1. Furthermore, since neutrons generally decay faster in the wellbore than in the formation, the contribution of wellbore fluids is greater for the decay in the early period of the spectral line, which corresponds to the wellbore environment, which may be referred to as the wellbore decay zone; the contribution of the formation to neutron capture increases progressively over time, so this section may be referred to as a formation attenuation zone; then as time goes on, the capture time spectrum count rate changes gradually and gradually, the total count rate approaches the background value, so the segment can be called a background gamma region. The focus of neutron lifetime log interpretation is on the attenuation of thermal neutrons in the formation, so borehole attenuation effects and background gamma interference need to be removed when calculating the capture section values used for the final interpretation, so that a macroscopic capture section of the formation can be obtained.
The macroscopic capture cross-section of the formation may be calculated from thermal neutron lifetime in the formation. Thermal neutron lifetime is defined as the average time that a thermal neutron takes from production to capture absorption, and is equal to the ratio of the mean free path S of thermal neutrons to the mean velocity v of thermal neutrons. The average velocity of thermal neutrons is temperature dependent and is about 2.2X10 5 cm/s at 25 ℃. The mean diffusion free path of thermal neutrons is defined as the inverse of the macroscopic capture cross section, and therefore the relationship of thermal neutron lifetime τ to macroscopic capture cross section can be expressed as:
wherein, tau is thermal neutron life, mu s;
Σ—macroscopic capture cross section of the formation, c.u.;
v-thermal neutron velocity, cm/s.
Based on the relationship between the spatial-temporal distribution of thermal neutrons and the thermal neutron number in the formation, the thermal neutron lifetime can be calculated from the capture time spectrum, so that the macroscopic capture cross section of the formation can be calculated by using formula (1). The time-dependent law of the thermal neutron number in space can be expressed as:
Wherein, N is thermal neutron counting rate, cps;
t-time, μs;
D-diffusion coefficient function;
-a vector characterizing a position somewhere in space;
Q-thermal neutron production rate function.
The right side of the formula (2) has three items, which respectively represent the change of the thermal neutron number with time caused by thermal neutron diffusion, generation and capture at a certain position in space. Thermal neutron lifetime is only related to the third term on the right of the above equation, so the effect of the first two terms needs to be eliminated. The change in thermal neutron number caused by the diffusion effect can be described by a diffusion equation, as shown in the first term on the right. Eliminating the effects of diffusion effects can generally be achieved by optimizing the source distance or parameter compensation. In practice, a diffusion effect can only occur when two thermal neutrons have a concentration difference, and the thermal neutrons flow out cleanly at the position close to the neutron source due to the higher thermal neutron concentration; and a lower thermal neutron concentration at a location remote from the neutron source where the thermal neutrons flow in net. Therefore, it is presumed that there is a point in the space where the inflow and outflow amounts of thermal neutrons are similar to each other, and dynamic balance is achieved. The influence of the diffusion effect can be reduced by setting a proper source distance to measure the capture time spectrum at the position, or the influence of the diffusion effect on the thermal neutron service life can be determined in an experimental mode and the like, and the result caused by the diffusion effect is compensated by setting parameters. The second item on the right represents the change in quantity due to the generation of thermal neutrons. According to the measurement time sequence of the neutron life logging instrument, when the instrument collects the capture time spectrum, the pulse neutron source in the instrument stops working, so that the thermal neutron production rate is zero. The relationship between the thermal neutron number and the thermal neutron lifetime obtained by the above analysis can be expressed as formula (3). Since neutron lifetime logging reflects the change in thermal neutron count over time by recording the change in capture gamma ray count rate over time, the law of change in capture gamma count rate also obeys this equation.
Wherein, n 0 is the initial thermal neutron count rate or capture gamma count rate, cps;
And n is the thermal neutron counting rate or the capture gamma counting rate at the moment t, cps.
From the above, it can be seen from the formulas (1) and (3) that the capture gamma count rate decays exponentially with time for a single medium, and the decay rate is determined by the macroscopic capture cross section of the medium. The decay rate of the neutron lifetime log capture time spectrum is affected by both the wellbore fluid and the formation. In order to accurately calculate the macroscopic capture section of the stratum, the invention provides a novel stratum macroscopic capture section extraction method.
The method for extracting the macroscopic capture section of the neutron lifetime logging stratum is characterized by comprising the following steps of:
let the neutron lifetime log capture time spectrum sum to k channels, which can be expressed as vector s= [ n 1,n2,…,nk]T.
Step1: and (5) capturing time spectrum filtering. The influence of instrument noise and radioactive statistical fluctuation on the gamma counting rate of the capturing time spectrum is reduced by carrying out smooth filtering on the capturing time spectrum acquired by the photon life logging, so that the signal-to-noise ratio of the time spectrum is improved, the attenuation characteristic of the capturing time spectrum is more obvious, and the subsequent processing is facilitated.
Step2: capturing a partial spectrum section of the time spectrum background. According to the structural analysis of the capture time spectrum, the tail data of the capture time spectrum is background gamma, the last k-p+1 data in the capture time spectrum is intercepted to form a background vector SS= [ n p,…,nk]T ] and the mean value m and the standard deviation sigma of elements in the vector are calculated. To ensure that the selected region contains only background gamma, the background vector SS should not be too long.
Step3: capturing time spectrum background area estimation. Based on the three-time standard deviation criterion, starting from the k-p track of the capturing time spectrum, judging whether the current track address counting rate n meets the following conditions in a reverse sequence by track:
n＞m+3σ(4)
Once the above condition is met, the operation is terminated and the trace B is output, and the background region in the capture time spectrum can be represented as vector b= [ n b,…,nk]T.
Step4: initial formation attenuation data is set. All the address data between the address c and the address b in the middle of the capturing time spectrum are selected as initial stratum attenuation data.
Step5: and (5) fitting a stratum decay curve. And (3) performing linear fitting on the formation attenuation data under a semi-logarithmic coordinate system to obtain a formation attenuation curve S F.
Step6: a borehole attenuation curve is calculated. And subtracting the stratum attenuation curve S F obtained by fitting from the original time spectrum, so as to obtain borehole attenuation data, and performing linear fitting on the borehole attenuation data under a semi-logarithmic coordinate system to obtain a borehole attenuation curve S W.
Step7: the formation decay curve is re-fitted. And subtracting the wellbore attenuation curve S W obtained by fitting from the original spectral line, obtaining stratum attenuation data again, and performing linear fitting on the stratum attenuation data under a semilogarithmic coordinate system to obtain a stratum attenuation curve S F again.
Step8: and judging whether the output condition is satisfied. And comparing the difference between the stratum decay curves S F obtained by the two fitting, outputting the stratum decay curves if the accumulated deviation between the track count rates of the two is smaller than a given threshold epsilon, and returning to Step6 otherwise.
Step9: and calculating the macroscopic capture section of the stratum. According to the formulas (1) and (3), the thermal neutron life in the stratum is determined according to the stratum decay curve analytic formula obtained by final fitting, and the macroscopic capture section of the stratum is calculated by utilizing the relation between the thermal neutron life and the capture section.
In summary, a flowchart of a method for extracting a macroscopic capture section of a neutron lifetime logging stratum is shown in fig. 2.
Drawings
FIG. 1 is a schematic diagram of neutron lifetime spectrum attenuation region distribution.
Fig. 2 is a flowchart of a method for extracting a macroscopic capture section of a neutron lifetime logging stratum.
FIG. 3 is a background region determined in the capture time spectrum.
FIG. 4 is a plot of wellbore decay and formation decay extracted from a measured capture time spectrum.
FIG. 5 is a neutron lifetime log interpretation outcome plot for an example well.
Detailed Description
The present invention will be described in further detail with reference to examples of application in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific examples described herein are intended to illustrate the invention and are not intended to limit the invention.
Application example:
the example uses the stratum macroscopic capture section extraction result and the saturation evaluation result of a certain neutron lifetime logging data to illustrate the application effect of the invention.
The example well is a gas well, which is put into production in 7 months of 2000, producing gas 446121m 3 daily, producing water 0t daily, and the natural gas relative density is 0.5657g/cm 3. From the analysis and conclusion of the lithology section of the open hole well, the well has 1 main sandstone reservoir section, and the depth range of the sandstone reservoir section is 5142-5167 m. The depth of the shooting well section of the reservoir section is 5142.5-5163 m. The formation skeleton, the argillaceous and the hydrocarbon capture section values of the well can be determined to be 8c.u.,35c.u. and 15c.u., respectively, based on the main regional parameters of the well location. The mineralization degree of stratum water of the well is 168000ppm, the water type is calcium chloride, the capture section is about 90.47c.u., the well belongs to a high mineralization degree stratum, the capture section difference between stratum water and oil gas is large, and therefore neutron life logging has good applicability in the stratum of the well.
The neutron source operating frequency of the neutron lifetime logging instrument used by the well is 1KHz. The duration of each operation of the neutron source is 60 mus. The instrument acquisition system records the capture time spectrum over a period of 0-1000 mus for a total of 100 data, each representing 10 mus. Taking the capture time spectrum acquired by the measurement point of the well near 5132m depth as an example, the method for extracting the macroscopic capture section of the stratum is described in detail.
According to the steps of the invention, the spectral line is filtered to suppress noise in the spectral line, and the signal to noise ratio is improved. Meanwhile, the counting rate of 10 channels in total at the tail end of the capturing time spectrum is selected, and the average value and standard deviation of the counting rate are calculated, wherein the results are 1.08x10 3 and 310.65 respectively. According to the judgment standard of Step3, the background area is 83 rd to 100 th, as shown in fig. 3.
And then according to the steps of the invention, sequentially fitting and calculating a stratum attenuation curve and a borehole attenuation curve, and carrying out loop iteration according to Step6 to Step8 until the calculation result reaches the requirement. Finally, a borehole attenuation curve and a stratum attenuation curve are obtained, stripping of the borehole environment influence is achieved, and a calculation result is shown in fig. 4. The stratum attenuation curve analysis method obtained by the measurement point fitting is as follows:
n＝2.939×105e-6025t (5)
from this, it can be seen that thermal neutron lifetime τ=1/6025 μs in the site stratum, and the macroscopic capture cross section of the site stratum is calculated according to equation (1) based on the thermal neutron average velocity:
The invention is adopted to calculate the capture section of all the measuring points of the measuring well section and evaluate the water saturation, and the well logging interpretation result diagram is obtained, as shown in figure 5. The total number of the lines is 10, wherein the 9 th line is the calculated capture section value of each measuring point, the calculation result is combined with the capture section value of each component of the well stratum, and the calculation formula of the water saturation of the stratum can be obtained according to the petrophysical volume model, and is as follows:
in the formula, sigma-measuring macroscopic capture cross section, c.u.;
Σ w -formation water macroscopic capture cross section, c.u.;
Σ ma —rock skeleton macroscopic capture cross section, c.u.;
Σ h —macroscopic capture section of oil and gas, c.u.;
Σ sh —argillaceous macroscopic capture section, c.u.;
Phi-formation porosity, decimal;
V sh -the shale content, fractional;
s w -stratum water saturation, decimal.
The formation water saturation curve calculated according to equation (7) is shown in fig. 5 at lane 10. From the completion acoustic time difference, density and compensated neutron curve and neutron lifetime log capture cross section curve shown in fig. 5, it can be seen that the physical properties are better for the sandstone interval of the well 5142.5-5167 m, the capture cross section exhibits significantly lower values, so the interval is interpreted as a gas reservoir. Since this interval has been perforated, it can be found that the neutron lifetime after-casing water saturation is increased compared to the completion water saturation, so that the small layers located at 5142.5-5144.9 m in this layer are interpreted as bad gas layers in the after-casing interpretation conclusion, while the water saturation of the two small layers 5144.9-5152.5 m and 5152.5-5167.0 m, although increased compared to the completion water saturation, still have a higher gas saturation, and therefore are interpreted as gas layers. The interval as a whole may still be considered the main production interval of the well. The completion data and neutron lifetime logging data for the three small layers are shown in Table 1.
Table 1 example well neutron lifetime log interpretation results
Claims (1)
1. The method for extracting the macroscopic capture section of the neutron lifetime logging stratum is characterized by comprising the following steps of:
let neutron lifetime log capture time spectrum total k channels, which can be expressed as vector s= [ n 1,n2,…,nk]T;
Step1: the influence of instrument noise and radioactivity statistical fluctuation on the gamma counting rate of the capturing time spectrum is reduced by carrying out smooth filtering on the capturing time spectrum acquired by the photon life logging, so that the signal-to-noise ratio of the time spectrum is improved, the attenuation characteristic of the capturing time spectrum is more obvious, and the subsequent processing is facilitated;
Step2: intercepting the last k-p+1 data in the capture time spectrum to form a background vector SS= [ n p,…,nk]T ] and calculating the mean value m and standard deviation sigma of elements in the vector, wherein the background vector SS is not overlong in order to ensure that the selected region only contains background gamma;
Step3: starting from the k-p track of the capturing time spectrum, judging whether the current track address counting rate n meets the following conditions in a reverse sequence and track by track:
n＞m+3σ (1)
Once the above condition is met, terminating the operation and outputting the address B, wherein the background area in the capture time spectrum can be represented as a vector b= [ n b,…,nk]T;
Step4: selecting all channel address data from channel address c to channel address b in the middle of the capturing time spectrum as initial stratum attenuation data;
Step5: performing linear fitting on stratum attenuation data under a semi-logarithmic coordinate system to obtain a stratum attenuation curve S F;
step6: subtracting the stratum attenuation curve S F obtained by fitting from the original time spectrum, so as to obtain borehole attenuation data, and performing linear fitting on the borehole attenuation data under a semi-logarithmic coordinate system to obtain a borehole attenuation curve S W;
Step7: subtracting the wellbore attenuation curve S W obtained by fitting from the original spectral line, re-obtaining stratum attenuation data, and performing linear fitting on the stratum attenuation data under a semi-logarithmic coordinate system to obtain a stratum attenuation curve S F again;
Step8: comparing the difference between stratum attenuation curves S F obtained by the two fitting, outputting the stratum attenuation curves if the accumulated deviation between the counting rates of the two sites is smaller than a given threshold epsilon, otherwise returning to Step6;
step9: and determining the thermal neutron life in the stratum according to the stratum decay curve analytic style obtained by fitting, and calculating the macroscopic capture section of the stratum by utilizing the relation between the thermal neutron life and the capture section.
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| Deterministic and stochastic neutron cross-section generation for PARCS applied to VR-1 reactor;Filip Fejt;《2016 17th International Scientific Conference on Electric Power Engineering (EPE)》;20160728;1-8 * |
| 脉冲中子能谱测井解释方法研究;邓仁双;《中国优秀硕士学位论文全文数据库 工程科技I辑》;20171115;B019-13 * |
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