CN114166871B - A method for evaluating the brittleness of continental shale oil reservoirs - Google Patents

Abstract

The invention relates to the technical field of evaluation of shale oil reservoirs, in particular to a method for evaluating the brittleness of continental shale oil reservoirs, which comprises the following steps: (1) sample collection; (2) types and contents of minerals in the samples analysis; (3) analysis of sample mineral crystal morphology; (4) screening of brittle mineral types; (5) calculation of brittle mineral content; (6) calculation of brittleness index. The present invention designs a new shale oil reservoir brittleness index calculation method according to the actual situation of the continental shale oil reservoir, and comprehensively considers factors such as mineral content, crystal shape and occurrence. The fracturing performance is evaluated to guide the production of shale oil.

Description

A method for evaluating the brittleness of continental shale oil reservoirs

technical field

The invention relates to the technical field of evaluation of shale oil reservoirs, in particular to a method for evaluating the brittleness of continental shale oil reservoirs.

Background technique

With the rapid development of the world economy, conventional oil and gas can no longer meet the global energy demand, and shale oil is currently one of the most likely energy sources to replace conventional oil and gas. However, shale oil exploration is very different from conventional oil and gas, and has the characteristics of integrated source and reservoir and continuous distribution.

Shale is the source rock and reservoir of shale oil, and the brittleness index is an important parameter reflecting the quality of fracturing, which largely determines the difficulty of fracturing and the shape of fracturing fractures formed. Key parameters of reservoir quality. The better the brittleness of the shale, the more complex the fracture network after fracturing, and the more ideal the fracturing effect is in production.

At present, there are two types of methods for calculating shale brittleness index: mineral brittleness index method and elastic parameter brittleness index method. The calculation of elastic parameter brittleness index requires compressional and shear wave velocity and density data, which are often obtained through array acoustic logging and density logging. However, it is more difficult to obtain; and the mineral brittleness index can be directly calculated only based on the rock and mineral test data, which is more practical.

However, all the current methods for calculating shale brittleness index based on mineral content only consider the content of brittle minerals, and do not consider the crystal shape and occurrence of shale brittle minerals.

SUMMARY OF THE INVENTION

The content of the present invention is to provide a method for evaluating the brittleness of continental shale oil reservoirs, which calculates the brittleness index of shale oil reservoirs after comprehensively considering factors such as mineral content, crystal form and occurrence.

A method for evaluating the brittleness of continental shale oil reservoirs according to the present invention, comprising the following steps:

Step 1. Sample collection;

Step 2. Mineral type and content analysis in the sample;

Step 3. Sample mineral crystal morphology analysis;

Step 4. Screen brittle mineral types;

Step 5. Calculate the content of brittle minerals;

Step 6. Calculate the brittleness index.

Preferably, in step 1, first collect the logging data of the target horizon in the study area, preselect the depth of the sample according to the stratigraphic horizon, lithology and resistivity displayed by the logging data, observe the core on site, and collect different depths, Shale samples from different well locations.

Preferably, in step 2, after the sample is collected, XRD whole-rock analysis is performed on the sample, and after the polycrystalline sample powder is analyzed by X-ray diffraction, the content, order degree, polymorphism of the mineral isomorphic replacement component are determined. Phase composition of mineral mixtures and quantitative estimation of the content of each phase.

Preferably, in step 3, the collected samples are observed by microscopic slices and scanning electron microscopes. After the samples are ground into mineral slices, the distribution, crystal morphology and distribution of minerals are observed through an optical microscope to determine the fractability of mineral particles.

Preferably, in step 4, after observing the mineral characteristics of the sample, the target brittle mineral type in the study area is screened out; the shell calcite has a sheet-like, fibrous structure and is arranged horizontally. The shell calcite and authigenic quartz in rock oil reservoirs cannot be used as brittleness indicators, and the brittle minerals are detrital quartz, dolomite and pyrite.

Preferably, in step 5, observe the pictures obtained by electron microscope in step 3, use image analysis software to count the area of shell calcite and authigenic quartz inside the shell, and obtain the ratio of shell calcite to authigenic quartz; combined with the sample mineral group determined in step 2 points to obtain the brittle mineral content.

Preferably, in step 6, the formula for calculating the brittleness index is as follows:

BI=(W _{detrital quartz} +W _dolomite +W _pyrite )/W _total ;

W _{detrital quartz} =W _quartz -X*W _calcite ;

X is the ratio of authigenic quartz to shell area, BI is the brittleness index, W is the mass fraction of detrital _quartz , W is the mass fraction of _dolomite , W is the mass fraction of _pyrite , W is the mass fraction of pyrite, and W is the mass fraction of pyrite. _Quartz is the mass fraction of quartz, W _calcite is the mass fraction of calcite, and W is the _sum of the mass fractions of all minerals; the brittleness index is equal to the sum of the mass fractions of detrital quartz, dolomite and pyrite compared to the mass fraction of all minerals sum of scores.

The present invention designs a new shale oil reservoir brittleness index calculation method based on the actual situation of the continental shale oil reservoir, and comprehensively considers factors such as mineral content, crystal shape and occurrence. The fracturing performance is evaluated to guide the production of shale oil.

Description of drawings

FIG. 1 is a flow chart of a method for evaluating the brittleness of continental shale oil reservoirs in Example 1. FIG.

Detailed ways

In order to further understand the content of the present invention, the present invention will be described in detail with reference to the accompanying drawings and embodiments. It should be understood that the embodiments are only for explaining the present invention and not for limiting.

Example 1

As shown in FIG. 1 , this embodiment provides a method for evaluating the brittleness of continental shale oil reservoirs, which includes the following steps:

(1) Sample collection;

First, collect the logging data of the target horizon in the study area, pre-select the sample depth according to the stratigraphic horizon, lithology, resistivity, etc. displayed by the logging data, observe the core on site, and collect the shale of different depths and different well positions according to the accuracy required by the research. sample. Systematic sampling is to ensure that the samples taken conform to the actual situation of the research horizon and the analysis data are representative.

(2) Mineral type and content analysis in the sample;

After the samples were collected, the samples were subjected to XRD whole-rock analysis. Minerals are mostly crystals. The wavelength of X-rays is close to the intercrystalline atomic spacing of minerals. X-rays will be diffracted into diffraction patterns with different intensities after passing through the crystals. After the polycrystalline sample powder is analyzed by X-ray diffraction, minerals can be determined The content of the replacement components, the degree of order, the phase composition of the multi-mineral mixture, and the quantitative (or semi-quantitative) estimation of each phase content. By analyzing the sample powder by X-ray diffractometer, the mineral type and content of the sample can be determined in preparation for subsequent analysis.

After being detected by X-ray diffraction analyzer, the XRD whole rock analysis of 10 samples is shown in Table 1, and the mineral content of each component can be known.

Table 1 Mineral content of each component

(3) Crystal morphology analysis of sample minerals; (thin section and electron microscope analysis of crystal morphology and occurrence of different minerals)

The samples were prepared as mineral flakes and SEM samples, wherein the SEM samples were prepared as 2cm×2cm block regular samples, and were polished and gold sprayed. The collected samples were observed by microscopic thin section and scanning electron microscope. After grinding the samples into mineral thin sections, the distribution, crystal morphology and distribution of minerals can be observed by optical microscope to determine the fractability of mineral particles. High-resolution scanning electron microscopy can clearly observe the different growth forms of the same mineral. This step is to observe the morphology and occurrence of minerals, and prepare for the screening of brittleness evaluation index minerals.

(4) Screening the types of brittle minerals; (Establish screening criteria: select granular brittle minerals, these minerals can be distributed in laminar or dispersed form. Flake and fiberglass calcite should be eliminated and replaced with flaky calcite. Autogenous quartz to be removed)

After observing the mineral characteristics of the samples, the target brittle mineral types in the study area can be screened out. It has been observed that the intermediate crustal calcite in continental shale oil reservoirs is flaky, fibrous and arranged horizontally, which is nearly perpendicular to the current fracturing direction, which is not conducive to fracture creation. Therefore, calcite cannot be used as a brittleness indicator; Calcite metasomatism is formed, and it cannot be used for shale brittleness evaluation. However, detrital quartz, dolomite and pyrite have high Young's modulus and low Poisson's ratio, and the cracking direction is compatible with the engineering fracturing direction. Therefore, the brittle minerals are determined to be detrital quartz, dolomite and pyrite. After determining the brittle mineral type in this step, the brittleness index (BI) of the shale can be calculated.

(5) Calculate the content of brittle minerals;

Step (2) After XRD whole-rock analysis, the mineral composition of the sample was determined, and the contents of quartz (authigenic quartz + detrital quartz), dolomite and pyrite were obtained; The area of the shell calcite and the authigenic quartz inside the shell can be counted by the image analysis software, and the ratio of the shell calcite to the authigenic quartz can be obtained. This step is to synthesize the data obtained in the previous steps to obtain the selected brittle mineral content, which is prepared for the calculation of the brittleness index.

(6) Calculate the brittleness index.

Mineral composition analysis of continental shale oil reservoirs shows that the content of detrital quartz, dolomite and pyrite can be used to calculate the brittleness index of shale reservoirs. After obtaining the ineffective quartz content in the previous step, the detrital quartz, that is, the effective part in the evaluation of brittleness, is equal to the total quartz content minus the ineffective quartz content. The brittleness index is equal to the sum of the mass fractions of detrital quartz, dolomite and pyrite and the sum of the mass fractions of all minerals.

In this step, the brittleness index of the reservoir is calculated based on the authigenic quartz ore content and XRD data obtained in the previous steps; the formula is as follows:

BI=(W _{detrital quartz} +W _dolomite +W _pyrite )/W _total ;

W _{detrital quartz} =W _quartz -X*W _calcite ;

X is the ratio of authigenic quartz to shell area, BI is the brittleness index, W is the mass fraction of detrital _quartz , W is the mass fraction of _dolomite , W is the mass fraction of _pyrite , W is the mass fraction of pyrite, and W is the mass fraction of pyrite. _Quartz is the mass fraction of quartz, W _calcite is the mass fraction of calcite, and W is the _sum of the mass fractions of all minerals.

In this example, a new shale oil reservoir brittleness index calculation method is designed according to the actual situation of continental shale oil reservoirs and comprehensive consideration of factors such as mineral content, crystal shape and occurrence. Fracability is evaluated to guide shale oil production.

The present invention and its embodiments have been described above schematically, and the description is not restrictive, and what is shown in the accompanying drawings is only one of the embodiments of the present invention, and the actual structure is not limited thereto. Therefore, if those of ordinary skill in the art are inspired by it, without departing from the purpose of the present invention, any structural modes and embodiments similar to this technical solution are designed without creativity, which shall belong to the protection scope of the present invention. .

Claims (4)

1. A method for evaluating brittleness of a continental facies shale oil reservoir is characterized by comprising the following steps: the method comprises the following steps:

step 1, collecting a sample;

step 2, analyzing the types and the contents of minerals in the samples;

step 3, analyzing the mineral crystal morphology of the sample;

step 4, screening the brittle mineral types;

step 4, after observing the mineral characteristics of the sample, screening out the target brittle mineral type in the research area; the mesochite calcite is in a sheet-shaped and fiber-shaped structure and is horizontally arranged, and the authigenic quartz is formed by the intercrossing of the calcite inside the mesochite, so the mesochite calcite and the authigenic quartz in the continental-phase shale oil reservoir can not be used as brittleness indexes, and the brittleness minerals are clastic quartz, dolomite and pyrite;

step 5, calculating the content of brittle minerals;

step 5, carrying out electron microscope observation on the obtained picture in the step 3, and using image analysis software to count the areas of the mesochite calcite and the authigenic quartz in the mesochite, so as to obtain the ratio of the mesochite calcite to the authigenic quartz; combining the sample mineral components determined in the step 2 to obtain the content of brittle minerals;

step 6, calculating a brittleness index;

in step 6, the brittleness index calculation formula is as follows:

BI＝(W_{scrap quartz}+W_Dolomite+W_Pyrite)/W_{General assembly}；

W_{Scrap quartz}＝W_Quartz-X*W_Calcite；

X is the ratio of the authigenic quartz to the mesoscale area, BI is the brittleness index, W_{Scrap quartz}Is the mass fraction of the crushed quartz, W_DolomiteIs the mass fraction of dolomite, W_PyriteIs the mass fraction of pyrite, W_QuartzIs the mass fraction of quartz, W_CalciteIs the mass fraction of calcite, W_{General assembly}Is the sum of the mass fractions of all minerals; the brittleness index is equal to the sum of the mass fractions of the three minerals clastic quartz, dolomite and pyrite, compared to the sum of the mass fractions of the whole minerals.

2. The method for evaluating the brittleness of a continental shale oil reservoir according to claim 1, wherein the method comprises the following steps: in the step 1, logging information of a target layer position in a research area is collected, the sample depth is preselected according to the stratum position, the lithology and the resistivity displayed by the logging information, a rock core is observed on site, and shale samples with different depths and different well positions are collected according to the precision required by research.

3. The method for evaluating the brittleness of a continental shale oil reservoir according to claim 2, wherein the method comprises the following steps: in the step 2, after the sample is collected, XRD full rock analysis is carried out on the sample, and after the polycrystalline sample powder is subjected to X-ray diffraction analysis, the content and the order degree of the mineral isomorphism substitute components and the phase composition of the multi-mineral mixture are determined and the content of each phase is quantitatively estimated.

4. The method for evaluating the brittleness of a continental shale oil reservoir according to claim 3, wherein the method comprises the following steps: and 3, performing microscopic slice and scanning electron microscope observation on the collected sample, grinding the sample into a mineral slice, and observing the distribution, crystal morphology and distribution of the mineral through an optical microscope to determine the compressibility of the mineral particles.

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