Method for characterizing the distribution of primary water in a rock and its applications

Abstract

This invention relates to a method for characterizing the distribution of primordial water in rocks and its application. The method includes: preparing particles from fresh rock cores as fresh samples and particles from dried rock cores as dried samples; calculating the scattering length density of the rock to be tested based on the dried sample, and preparing a deuterated toluene contrast-matching fluid with the same scattering length density as the rock to be tested; immersing the fresh sample and the dried sample in the deuterated toluene contrast-matching fluid respectively to obtain immersion samples; performing small-angle neutron scattering (SANS) experiments on the dried sample and the immersion samples respectively; obtaining pore structure information before and after immersion based on the experimental results; and determining the distribution of primordial water in the rock to be tested based on the pore structure information. This invention, based on SANS experiments, provides a novel experimental technique for characterizing the distribution and water saturation of primordial water in sedimentary rocks, with advantages such as being non-destructive, efficient, time-saving, and highly accurate.

Description

Technical Field

[0001] This invention relates to a method for characterizing the original water distribution in rocks and its application. Background Technology

[0002] Extensive exploration and development practices have shown that sedimentary rock reservoirs contain primary water and a large amount of fracturing fluid mixed with primary water. The retained liquid water is often stored in the pores of sedimentary rocks in an adsorbed or free state, which can significantly affect the content, storage capacity, gas diffusion, and permeability of free gas in the reservoir. Therefore, understanding the retention mechanism, distribution, and water-rock interaction mechanism of water in the reservoir pore space at the microscale is of great guiding significance for production.

[0003] However, the distribution of reservoir native water and fracturing fluid in the pores is still unclear, and there is a lack of effective technical means to characterize the microscopic distribution and occurrence state of water. Summary of the Invention

[0004] To address the problem of characterizing the original water distribution and the original water saturation in interconnected pores of sedimentary rocks, embodiments of the present invention provide a method for characterizing the original water distribution in sedimentary rocks using small-angle neutron scattering (SANS) technology and its application.

[0005] This invention provides a method for characterizing the original water distribution in rocks, comprising the following steps:

[0006] Particles from fresh rock cores are used as fresh samples, and particles from dried rock cores are used as dried samples.

[0007] The scattering length density of the rock to be tested was calculated based on the dried sample, and a deuterated toluene contrast-matching fluid with the same scattering length density as the rock to be tested was prepared.

[0008] The fresh sample and the dried sample were respectively immersed in the deuterated toluene contrast-matching fluid to obtain the immersed sample;

[0009] Small-angle neutron scattering experiments were performed on the dried sample and the impregnated sample, respectively.

[0010] Based on the experimental results, information on the pore structure before and after impregnation was obtained.

[0011] The original water distribution in the rock to be tested is determined based on the pore structure information.

[0012] Another aspect of the present invention provides an application of the above-described method in characterizing the occurrence and distribution of water in porous media.

[0013] This invention provides a novel experimental technique for characterizing the distribution and water saturation of original water in sedimentary rocks based on small-angle neutron scattering experiments. It has the advantages of being non-destructive, efficient, time-saving, and highly accurate.

[0014] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures described in the written description, claims, and drawings.

[0015] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0017] Figure 1 A schematic diagram illustrating the principle of small-angle neutron scattering experiments using deuterated toluene as a contrast-matched fluid.

[0018] Figure 2 A schematic diagram illustrating the calculation principle and processing procedure for water saturation data in connected pores. Detailed Implementation

[0019] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0020] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the invention as detailed in the appended claims.

[0021] The inventor discovered after research:

[0022] Nuclear magnetic resonance relaxation T2 spectra can characterize the pore structure parameters of porous media, with signal intensity representing the volume of water. However, accurately obtaining the pore structure parameters of porous media requires pressurized water saturation of the sample; therefore, current techniques have not yet been able to directly measure the in-situ water distribution in porous media.

[0023] Small-angle neutron scattering (SANS) can determine the total porosity of shale within a certain pore size range without destructive testing, including open pores, blind pores, and closed pores. It is not limited by fluid accessibility or water content, whereas current pore structure characterization methods such as gas adsorption, mercury porosimetry, and scanning electron microscopy all require completely dry samples. Therefore, we designed a method based on SANS to characterize the distribution of original water in sedimentary rocks. This method includes the following steps:

[0024] S1: Prepare fresh rock core particles as fresh samples, and prepare dried rock core particles as dried samples.

[0025] The fresh samples were preserved by wrapping them in PVC film and storing them at -18°C to ensure the in-situ presence of core water.

[0026] The specific preparation process of the dried sample is as follows: a sample of the rock to be tested, such as a shale sample, is selected, ground into shale particles using a grinder, and then dried. The particle size of the shale particles is 177–500 μm (35–80 mesh); the drying temperature is 60°C, and the sample is considered completely dry until its mass no longer changes.

[0027] S2: Calculate the scattering length density (SLD) of the rock to be tested based on the dry sample, and prepare a deuterated toluene contrast-matching fluid identical to the rock SLD.

[0028] The dried sample is interpreted as a two-phase random system (pore space and solid matrix). The composition of the solid matrix, i.e., the percentage content of various minerals, can be obtained through XRD experiments. The scattering density lengths of various individual minerals, such as quartz, feldspar, calcite, illite, and montmorillonite, are known. Therefore, the scattering density length of the solid matrix of the dried sample is equal to the weighted average of the known scattering density lengths of each mineral component, weighted by the percentage content of the minerals.

[0029] Specifically, toluene and deuterated toluene are mixed in a certain proportion according to the SLD value of toluene and the SLD value of deuterated toluene to obtain the deuterated toluene contrast-matched fluid.

[0030] S3: Set up three groups of pretreatment control samples with the following conditions: dried sample, dried sample + deuterated toluene, and fresh sample + deuterated toluene.

[0031] The specific operation of sample pretreatment is as follows: In addition to drying the sample, the experimental group sample is fully immersed in deuterated toluene so that the sample pores are filled with the fluid, thus becoming the immersed sample.

[0032] Specifically, the fluid, fresh sample, and dry sample of the shale particles to be tested are placed into a quartz sample cell, and the shale particles to be tested are fully immersed in the fluid. The inner diameter of the quartz sample cell is preferably 1 mm.

[0033] S4: Small-angle neutron scattering experiments were performed on the three pretreated samples in S3 to obtain the raw data.

[0034] The original data is a two-dimensional scattering image of scattering vectors and scattering intensities.

[0035] S5: Preprocess the raw data in S4.

[0036] Specifically, the preprocessing can be as follows: import the SLD value of the rock to be tested obtained in step S2 and the original data obtained in step S4 into the IRENA plugin based on IGOR software, use Guinier and Porod fitting to realize one-dimensional processing of the two-dimensional scattering image obtained from the small-angle neutron scattering experiment to obtain a one-dimensional scattering intensity curve, and perform background subtraction and normalization processing on the one-dimensional scattering intensity curve.

[0037] In the background subtraction process, an empty quartz sample cell is used as the background for the subtraction. The inner diameter of the quartz sample cell is preferably 1 mm.

[0038] S6: Based on the experimental results, obtain information on the pore structure before and after impregnation.

[0039] The pore structure information may include information such as porosity, pore size distribution, and pore volume.

[0040] Specifically, the pore structure information of the sample before and after wetting can be obtained by calculating the preprocessed data using the Irena macros plugin in IGOR Pro software.

[0041] Among them, IGOR Pro software is a powerful and scalable tool for plotting, data analysis, image processing, and programming; the Irena macros plugin can analyze scattering intensity information using a polydisperse sphere model to obtain the pore structure parameters of the scatterer.

[0042] S7: By comparing the pore structure information of the sample before and after wetting, the original water distribution in the rock microstructure is obtained.

[0043] The beneficial effect of this embodiment is that by injecting a fluid with a SLD that matches the sedimentary rock matrix into the pores of the sample, the pores filled with this fluid, which is identical to the matrix's SLD, will appear as non-porous in small-angle neutron scattering experiments, thus not contributing any additional scattering signal. Since the primary water in sedimentary rocks is immiscible with toluene and will occupy a certain amount of pore space, comparing the differences in their pore size distributions can quantitatively characterize the distribution and water saturation of the primary water in the sedimentary rocks.

[0044] The method described in this embodiment is based on small-angle neutron scattering experiments and provides a novel experimental technique for characterizing the distribution and water saturation of original water in sedimentary rocks. It has the advantages of being non-destructive, efficient, time-saving, and highly accurate.

[0045] The method described in this embodiment is applicable to the characterization of water occurrence and distribution in porous media such as porous materials and sedimentary rocks, with a characterization range of 1-100 nm.

[0046] The following examples are illustrated using experimental data:

[0047] S1 prepares granular samples of both fresh and dried samples. The specific steps are as follows: select the sedimentary rock sample to be tested and grind it into particles using a grinder. In this embodiment, the particle size of the sedimentary particles is 177–500 μm, and they are dried in an oven at 60°C until their mass no longer changes. The purpose is to remove free and bound water from the dried sample and improve the accuracy of the experiment.

[0048] S2 calculates the scattering length density (SLD) of the rock and prepares a deuterated toluene contrast-matching fluid with the same SLD as the rock. In this embodiment, the fluid is prepared by mixing toluene (C7H8) and deuterated toluene (C7D8), and the SLD value of toluene is... SLD values ​​of deuterated toluene Despite their differences, they possess the same fluid chemistry, and mixing them in proportion can yield a scatterer with an average contrast close to that of a sedimentary rock matrix.

[0049] S3 sets up three groups of pretreated control samples with the following conditions: dry sample, dry sample + deuterated toluene, and fresh sample + deuterated toluene. The specific sample pretreatment procedure is as follows: except for the dry sample, the experimental group samples are fully immersed in deuterated toluene to fill the sample pores with the fluid. In this embodiment, the fluid and the shale particle sample to be tested are placed in a quartz sample cell, and the shale particle sample to be tested is fully immersed in the fluid for 48 hours.

[0050] S4 performs small-angle neutron scattering experiments on the three pretreated samples from S3 to obtain raw data. In this embodiment, the small-angle neutron scattering experiment is conducted at the China Spallation Neutron Source (CSNS) small-angle spectrometer station. Before conducting the small-angle neutron scattering experiment, the staff needs to complete detector efficiency calibration, wavelength calibration, background calibration, and transmittance measurement. During the sample testing process, the sample thickness, testing time, and wavelength of the neutron beam used need to be recorded for subsequent data processing.

[0051] In this experiment, granular shale samples were selected to obtain information on the average orientation of the pore structure, facilitating small-angle neutron scattering (SANS) experiments after water absorption. The particles were screened to a size of 177–500 μm and loaded into a quartz sample cell with an inner diameter of 1 mm. The granular samples were then molded under pressure, with maximal compaction for SANS experiments. The neutron wavelength used in this experiment was [wavelength value missing]. By changing the position where the detector receives the signal, the range of the scattering vector (q) can be covered. For polydisperse porous media, the relationship between pore radius (r) and scattering vector (q) is r≈2.5 / q, which allows us to calculate that the experimentally detectable sample pore diameter range is 5~97nm.

[0052] S5 preprocesses the raw data from S4. The raw data is a two-dimensional scattering image of scattering vectors and scattering intensities. Specifically, the raw data is imported into the IRENA plugin based on IGOR, and using Guinier and Porod fitting, the two-dimensional scattering image obtained from the small-angle neutron scattering experiment is transformed into a one-dimensional scattering intensity curve. Background subtraction and normalization are then performed on the one-dimensional scattering intensity curve. A fluid with zero scattering length density is placed in a quartz sample cell as a background for background subtraction.

[0053] S6 uses the Irena macros plugin in IGOR Pro software to calculate the processed data, thereby obtaining the pore structure information of the shale particle sample before and after wetting. The pore structure information includes porosity, pore size distribution, pore volume, etc.

[0054] S7 obtains the original water distribution in the rock's microstructure by comparing the pore structure information of the sample before and after wetting. The data processing principle and method are as follows:

[0055] • Small-angle neutron scattering test of dry sample: full aperture distribution V total

[0056] • Dry sample + deuterated toluene small-angle neutron scattering test: closed-pore distribution V closed

[0057] • Fresh sample + deuterated toluene small-angle neutron scattering test: original water volume fraction V in closed pores + connected pores closed +V water

[0058] Therefore, the water saturation of connected pores

[0059] A schematic diagram of the principle of the small-angle neutron scattering experiment of deuterated toluene with contrast-matched fluid is shown below. Figure 1 As shown; the data processing example in this embodiment is as follows. Figure 2 As shown.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for characterizing the original water distribution in rocks, characterized in that, Includes the following steps: Particles from fresh rock cores are used as fresh samples, and particles from dried rock cores are used as dried samples. The scattering length density of the rock to be tested was calculated based on the dried sample, and a deuterated toluene contrast-matching fluid with the same scattering length density as the rock to be tested was prepared. The fresh sample and the dried sample were respectively immersed in the deuterated toluene contrast-matching fluid to obtain the immersed sample; Small-angle neutron scattering experiments were performed on the dried sample and the impregnated sample, respectively. Based on the experimental results, information on the pore structure before and after impregnation was obtained. The original water distribution in the rock to be tested is determined based on the pore structure information. The wetting process includes: placing the deuterated toluene contrast-matching fluid and the dried sample into a quartz sample cell, and fully wetting the fresh sample and the dried sample in the deuterated toluene contrast-matching fluid; The fresh samples were preserved by wrapping them in PVC film and refrigerating them at -18°C. Determining the original water distribution includes: Small-angle neutron scattering (SANS) tests were performed on the dried sample to obtain the full aperture distribution. ; Small-angle neutron scattering (SANS) tests were performed on the dried sample and the deuterated toluene-matched fluid to obtain the closed-pore distribution. ; Small-angle neutron scattering (SANS) tests were performed on the fresh sample and the deuterated toluene-matched fluid to obtain the original water volume fraction in closed and connected pores. ; Calculate the water saturation of connected pores .

2. The method according to claim 1, characterized in that, The small-angle neutron scattering experiment includes: The two-dimensional scattering image of the scattering vector and scattering intensity obtained from the small-angle neutron scattering experiment is used as the raw data; The original data is processed to obtain a one-dimensional scattering intensity curve; and The experimental results were obtained by performing background subtraction and normalization on the one-dimensional scattering intensity curve.

3. The method according to claim 2, characterized in that, The background subtraction process includes using an empty quartz sample cell as a background for background subtraction processing.

4. The method according to claim 3, characterized in that: The inner diameter of the quartz sample cell is 1 mm.

5. The method according to claim 1, characterized in that, The preparation of the dried sample includes: selecting the rock to be tested, grinding it into rock particles, and drying it to obtain the particles of the dried rock core.

6. The method according to claim 5, characterized in that, The drying temperature is 60°C.

7. The method according to claim 5, characterized in that, The rock particles are shale particles with a particle size of 177~500μm and a mesh size of 35-80.

8. The method according to claim 1, characterized in that, The preparation of the deuterated toluene contrast-matching fluid includes: preparing the deuterated toluene contrast-matching fluid by mixing toluene and deuterated toluene in a certain proportion according to the scattering length density of toluene and the scattering length density of deuterated toluene.

9. An application of the method described in any one of claims 1 to 8 in characterizing the occurrence and distribution of water in porous media.

10. The application according to claim 9, characterized in that, The range of the characterization is 1–100 nm.

Images