An evaluation method for thermal damage after low-temperature fracturing of shale

Abstract

The present invention discloses a method for evaluating the thermal damage after low-temperature fracturing of shale, which includes obtaining rock samples in the target area; drying the rock samples in an oven; conducting a physical model experiment on thermal damage of the dried rock samples; taking pictures of the rock samples after the experiment to obtain image samples, and processing the image samples to obtain the surface crack feature images of the rock samples; calculating the fractal dimension value of the surface crack feature images of the rock samples by using the differential box dimension method; then calculating the dimensionless permeability of the rock samples after the experiment according to the fractal dimension value; and finally evaluating the thermal damage of the rock samples according to the dimensionless permeability of the rock samples after the experiment. The thermal damage grading proposed by the present invention has universality and can be applied not only to the thermal damage assessment after low-temperature fracturing of shale, but also to the thermal damage assessment of other types of reservoirs. It quantifies the thermal damage, provides a quantitative index for the design and optimization of oilfield production and development plans, and has important significance for the development and utilization of deep shale gas.

Description

Technical Field

[0001] The present invention relates to a method for evaluating thermal damage after low-temperature fracturing of shale, belonging to the technical field of shale fracturing. Background Art

[0002] The exploration and development of unconventional oil and gas have great development prospects and can serve as a new growth point for the sustainable development of oil and gas in China. Shale gas is of particular importance among them. At present, shale gas has abundant reserves on the earth and is the key research target for exploration and development in various countries. It is widely distributed in China and is a popular resource for future domestic energy supply. From the current world's demand for the exploitation of oil and gas resources, the exploitation of current onshore oil and gas resources is developing from shallow to deep. And with the further consumption and demand of current energy, the exploitation of deep gas reservoir resources has become more urgent.

[0003] Meanwhile, during the deep exploitation process, the problem that the real-time temperature gradually increases with the increase of formation depth will inevitably be encountered. This makes us not only consider the influence of in-situ stress during the deep exploitation process, but also must consider the real-time influence of temperature. Although systematic research has been carried out on the exploitation technologies and equipment for shallow-buried shale gas, there are still many difficult problems to overcome for the exploitation of deep shale gas.

[0004] In China, the commercial development of shale gas is still in the initial stage of exploration. Due to the very low permeability of tight shale, the hydraulic fracturing technology is the key technology for the successful development of shale gas, but there are engineering problems such as high construction pressure and the inability to add proppants. One of the reasons for these problems hindering the development of shale gas is the unclear understanding of the formation and propagation mechanism of shale rock fractures under high temperature and high pressure.

[0005] This method is based on studying the complexity of fractures after cold shock of shale at high temperature, and adopts a method combining indoor experiments and numerical analysis. Taking shale as the research object, quantitatively understanding the destructive effect of thermal damage, analyzing the data regularly to obtain a mathematical model, summarizing the indexes for quantitatively evaluating the fracture complexity, and finally providing optimization schemes and measures for solving the engineering problems of shale gas exploitation by hydraulic fracturing method. Summary of the Invention

[0006] In order to overcome the problems in the prior art, the present invention provides a method for evaluating thermal damage after low-temperature fracturing of shale.

[0007] The technical solution provided by the present invention to solve the above technical problems is: a method for evaluating thermal damage after low-temperature fracturing of shale, comprising the following steps:

[0008] Step S1, obtaining rock samples of the target area;

[0009] Step S2, drying the rock samples in an oven;

[0010] Step S3: Conduct a physical model experiment on the thermally damaged rock sample after drying;

[0011] Step S4: Take a photo of the rock sample after the experiment to obtain an image sample, and process the image sample to obtain the surface crack feature image of the rock sample;

[0012] Step S5: Use the differential box dimension method to calculate the fractal dimension value D of the surface crack feature image of the rock sample;

[0013] Step S6: Then calculate the dimensionless permeability of the rock sample after the experiment according to the fractal dimension value;

[0014]

[0015] In the formula: D is the fractal dimension value; is the dimensionless permeability;

[0016] Step S7: Finally, evaluate the thermal damage of the rock sample according to the dimensionless permeability of the rock sample after the experiment.

[0017] A further technical solution is that the specific steps in Step S1 are:

[0018] Step S11: Obtain multiple outcrop samples in the target area;

[0019] Step S12: Compare the mineral compositions of the multiple outcrop samples with the downhole core;

[0020] Step S13: Select the outcrop sample with the mineral content closest to that of the downhole core as the rock sample.

[0021] A further technical solution is that the rock sample is a cylindrical core with a diameter of 25 mm and a length of 50 mm.

[0022] A further technical solution is that in Step S2: Put the rock sample into an oven and weigh it every 2 hours until the mass change is less than 0.01 g to ensure that the sample is completely dry.

[0023] A further technical solution is that the drying temperature of the oven in Step S2 is 60 °C.

[0024] A further technical solution is that the specific process of Step S3 is:

[0025] Step S31: Use pure nitrogen as the permeating gas for the rock sample, conduct low-permeability measurement under the conditions of a confining pressure of 10 MPa and a chamber temperature of 20 °C, and conduct permeability measurement in the initial state;

[0026] Step S32: Put the rock sample into a muffle furnace, heat it to the target temperature, and keep it at the target temperature for 3 hours for uniform heating;

[0027] Step S33: Then place the rock sample in a water bath maintained at 20°C until the core temperature drops to the cooling water temperature.

[0028] A further technical solution is that the heating rate in step S32 is set to 1°C / min.

[0029] A further technical solution is that the specific evaluation criteria in step S7 are as follows:

[0030] When the rock sample is in the first-level thermal damage;

[0031] When the rock sample is in the second-level thermal damage;

[0032] When the rock sample is in the third-level thermal damage;

[0033] When the rock sample is in the fourth-level thermal damage.

[0034] A further technical solution is that the permeability of shale with first-level thermal damage doubles in single digits, the permeability of shale with second-level thermal damage doubles in two digits, the permeability of shale with third-level thermal damage doubles in three digits, the permeability of shale with fourth-level thermal damage doubles by more than three digits, and the shale with fourth-level thermal damage shows a broken state; the higher the level of thermal damage, that is, the stronger the fluid diversion ability in deep shale.

[0035] The present invention has the following beneficial effects: The novel experimental method for simulating real deep shale cold shock given by the present invention is simple to operate, has a short process time, and the results are true and reliable. By following this experiment, recording the experimental data, calculating the fractal dimension and substituting it into the empirical equation, the value of the dimensionless permeability can be obtained; the thermal damage grading proposed by the present invention has universality, and can not only be applied to the thermal damage assessment after low-temperature fracturing of shale, but also be suitable for the thermal damage assessment of other types of reservoirs, quantifies the thermal damage, provides a quantitative index for the design and optimization of oilfield production development plans, and has important significance for the development and utilization of deep shale gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a comparison diagram of mineral compositions;

[0037] Figure 2 It is a sonic test diagram;

[0038] Figure 3 It is an image of the surface crack characteristics of the rock sample;

[0039] Figure 4 It is a schematic diagram of the number of cracks. DETAILED DESCRIPTION OF THE INVENTION

[0040] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] A method for evaluating the thermal damage of shale after low-temperature fracturing according to the present invention includes the following steps:

[0042] Step S1: Obtain rock samples from the target area;

[0043] Step S11: Obtain multiple outcrop samples from the target area;

[0044] Step S12: Compare the mineral compositions of the multiple outcrop samples with the downhole core;

[0045] Step S13: Select the outcrop sample with the mineral content closest to that of the downhole core as the rock sample;

[0046] Step S14: Grind the outcrop sample into a cylindrical rock sample with a diameter of 25 mm and a length of 50 mm;

[0047] Step S2: Place the rock sample in an oven (the drying temperature is set at 60 °C), weigh it every 2 hours until the mass change is less than 0.01 g to ensure that the sample is completely dry;

[0048] Step S3: Conduct a physical model experiment on the thermal damage of the dried rock sample;

[0049] Step S31: Use pure nitrogen as the permeating gas for the rock sample, conduct low-permeability measurement under the conditions of a confining pressure of 10 MPa and a chamber temperature of 20 °C, and conduct permeability measurement in the initial state; observe the internal structure of some samples with a MICROXCT-400 CT scanner;

[0050] Step S32: Place the rock sample in a muffle furnace and heat it to the target temperature, and at the same time set the heating rate to 1 °C / min to minimize the local thermal stress damage caused during the heating process; keep it at the target temperature for 3 hours and heat it evenly;

[0051] Step S33: Then place the rock sample in a water bath maintained at 20 °C until the core temperature drops to the cooling water temperature;

[0052] Step S34: Weigh, dry, test the permeability and internal structure of the rock sample after the experiment, and then conduct a uniaxial compression test using a GCTSRTR-1500 loading system to test the mechanical properties of the shale specimen;

[0053] Step S4: Take pictures of the rock samples after the experiment to obtain image samples, and process the image samples to obtain the surface crack feature images of the rock samples;

[0054] Step S5: Use the differential box-counting dimension method to calculate the fractal dimension value D of the surface crack feature image of the rock sample;

[0055] The differential box-counting dimension method is measured by calculating the minimum number of boxes covering the image surface. For example, for an image of size A×B, divided into an M×M grid, the gray value at the image (X, Y) is F(X, Y), and the total gray level is L (the image gray level is usually 256). The image is regarded as the surface gray cluster (X, Y, F(X, Y)) of a three-dimensional object, that is, there is an (M×M) grid on the X-Y plane, the Z-axis is the gray value of the pixels in the grid, and there are several boxes stacked on each grid, and the height of the box is (L - 1)×M / min(A, B).

[0056] If in the (I, J)th grid, the Nth box contains the minimum gray value in the grid and the N'th box contains the maximum gray value in the grid, then the number of boxes covering the (I, J)th grid is N' - N + 1. The number of boxes covering the entire image is ∑ I,J N' - N + 1; thus, through perform linear regression to find the slope of the image, that is, the fractal dimension D.

[0057] Step S6: Then calculate the dimensionless permeability of the rock sample after the experiment according to the fractal dimension value;

[0058]

[0059] In the formula: D is the fractal dimension value; is the dimensionless permeability;

[0060] Step S7: Finally, evaluate the thermal damage of the rock sample according to the dimensionless permeability of the rock sample after the experiment; when , the rock sample is in the first-level thermal damage; when , the rock sample is in the second-level thermal damage; when , the rock sample is in the third-level thermal damage; when , the rock sample is in the fourth-level thermal damage; among them, the permeability of the shale with the first-level thermal damage doubles in single digits, the permeability of the shale with the second-level thermal damage doubles in two digits, the permeability of the shale with the third-level thermal damage doubles in three digits, the permeability of the shale with the fourth-level thermal damage doubles by more than three digits, and the shale with the fourth-level thermal damage shows a broken state; the higher the thermal damage level, that is, the stronger the diversion ability of the fluid in the deep shale.

[0061] Example

[0062] Step 1: Obtain the rock samples in the Changning-Weiyuan area;

[0063] The Lower Silurian Longmaxi Formation shale in the Changning-Weiyuan area is the main shale gas producing formation in China. Considering the scarcity of downhole cores, shale outcrops in the Longmaxi Formation of the Changning area were selected for experiments. In order to select outcrops closer in lithology to downhole cores, mineral composition comparisons were made between the three obtained outcrop samples and the downhole cores of Wells W204-1 and W204-2 in the Weiyuan Gas Field. Figure 1 Mineral composition comparison of the collected outcrops and downhole cores (the ones on the horizontal axis represent outcrops, closer to the mineral content of downhole cores, and Outcrop 3 was selected as the experimental outcrop). The outcrop boreholes were ground into four cylindrical cores with a diameter of 25 mm and a length of 50 mm.

[0064] In order to ensure that the subsequent tested core samples have similar properties, all samples were subjected to acoustic wave tests, and samples with an apparent density in the range of 2570 - 2600 kg / m 3 and a longitudinal wave velocity in the range of 3550 - 3620 m / s were selected as experimental samples, as Figure 2 shown;

[0065] Step 2: The four selected cores were respectively placed in an oven (the drying temperature was set at 60 °C), and weighed every 2 hours until the mass change was less than 0.01 g to ensure that the samples were completely dry;

[0066] Step 3: Thermal damage physical model experiments were carried out on the four dried cores;

[0067] 1. Experiments were carried out according to the geological and fracturing engineering conditions of deep shale gas wells in the Weiyuan Gas Field (Table 1). The calculated maximum formation temperature and rock confining pressure of deep shale were close to 180 °C and 60 MPa respectively. A method model considering fluid rheological effects for studying wellbore heat transfer and predicting wellbore temperature distribution was used for numerical simulation. At an injection rate of 12 m 3 / min, the bottom-hole temperature was close to 35 °C. Therefore, experimental parameters were designed based on the calculated data to maximize the simulation of the underground injection environment.

[0068] Table 1 Geological engineering characteristic parameters of deep shale in the Weiyuan Gas Field

[0069]

[0070] 2. The cores were permeated with pure nitrogen gas, and low-permeability measurements were carried out under the conditions of a confining pressure of 10 MPa and a chamber temperature of 20 °C, and permeability measurements were carried out in the initial state;

[0071] 3. The four cores were respectively placed in a muffle furnace and heated to the target temperatures (100 °C, 150 °C, 200 °C, 250 °C). The heating rate was set at 1 °C / min to minimize the local thermal stress damage caused during the heating process, and kept at the target temperature for 3 hours for uniform heating;

[0072] 4. After heating, one part of each of the four cores was naturally cooled to room temperature of 20°C in air, and the other part was placed in a water bath maintained at 20°C until the core temperature dropped to the cooling water temperature.

[0073] 5. The four cores were weighed, dried, tested for permeability and internal structure, and then a uniaxial compression test was carried out using a GCTS RTR-1500 loading system to test the mechanical properties of the shale specimens.

[0074] Step 4: Photos were taken of the four cores that were naturally cooled and water-cooled respectively to obtain image samples, and each image sample was processed to obtain the surface crack characteristic images of the rock samples as Figure 3 shown.

[0075] Step 5: Using the differential box dimension method, one of the fractal dimension calculation methods, the fractal dimension values of the surface crack characteristic images of the four rock samples were calculated; as Figure 4 shown, through a large number of experiments, data fitting was carried out to obtain that when the contact temperature difference under water-cooling conditions was 80°C, 130°C, 180°C, and 230°C, the number of cracks was 0, 7, 17, and 34 respectively, and the fractal dimension D values were 1, 1.22, 1.3, and 1.46 respectively.

[0076] Step 6: Then, the dimensionless permeability of the rock samples after the experiment was calculated according to the fractal dimension values; the dimensionless permeability calculations were: 1, 12.50, 24.95, 104.13 respectively.

[0077] Step 7: The thermal damage of each rock sample was evaluated according to the dimensionless permeability of the four cores.

[0078] Therefore, the thermal damage of the core with a temperature difference of 80°C was primary thermal damage; the thermal damage of the core with a temperature difference of 130°C was secondary thermal damage; the thermal damage of the core with a temperature difference of 180°C was secondary thermal damage; the thermal damage of the core with a temperature difference of 230°C was tertiary thermal damage.

[0079] As described above, it is not any form of limitation to the present invention. Although the present invention has been disclosed through the above embodiments, it is not intended to limit the present invention. Any person skilled in the art, without departing from the scope of the technical solution of the present invention, can make some changes or modifications to the above-disclosed technical content to obtain equivalent embodiments with equivalent changes. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. An evaluation method for thermal damage after low-temperature fracturing of shale, characterized in that, it includes the following steps: Step S1: Obtain a rock sample from the target area, and the rock sample is a cylindrical core with a diameter of 25 mm and a length of 50 mm; Step S2: Place the rock sample in an oven for drying; Step S3: Conduct a physical model experiment on thermal damage of the dried rock sample; Step S4: Take pictures of the rock sample after the experiment to obtain an image sample, and process the image sample to obtain the surface crack feature image of the rock sample; Step S5: Calculate the fractal dimension value of the surface crack feature image of the rock sample using the differential box-counting dimension method D ; Step S6. Then, according to the fractal dimension value D calculate the dimensionless permeability of the rock sample after the experiment; In the formula: D is the fractal dimension value; is the dimensionless permeability; Step S7: Finally, evaluate the thermal damage of the rock sample according to the dimensionless permeability of the rock sample after the experiment; When 1 ≤ ≤ 10, the rock sample is in the first-level thermal damage; When 10 < < 104, the rock sample is in the second-level thermal damage; When 104 ≤ ≤ 892, the rock sample is in the third-level thermal damage; When 892 < , the rock sample is in the fourth - level thermal damage.

2. The evaluation method for thermal damage after low-temperature fracturing of shale according to claim 1, characterized in that, the specific steps in Step S1 are: Step S11: Obtain multiple outcrop samples from the target area; Step S12: Compare the mineral compositions of the multiple outcrop samples with the downhole core; Step S13: Select the outcrop sample with the mineral content closest to that of the downhole core as the rock sample.

3. The evaluation method for thermal damage after low-temperature fracturing of shale according to claim 1, characterized in that, in Step S2: Place the rock sample in an oven and weigh it every 2 hours until the mass change is less than 0.01 g to ensure that the sample is completely dry.

4. The evaluation method for thermal damage after low-temperature fracturing of shale according to claim 1, characterized in that, the drying temperature of the oven in Step S2 is 60 °C.

5. The evaluation method for thermal damage after low-temperature fracturing of shale according to claim 1, characterized in that, the specific process of Step S3 is: Step S31: Use pure nitrogen as the permeating gas for the rock sample, conduct low-permeability measurement under the conditions of a confining pressure of 10 MPa and a chamber temperature of 20 °C, and conduct permeability measurement in the initial state; Step S32: Place the rock sample in a muffle furnace and heat it to the target temperature, and keep it at the target temperature for 3 hours for uniform heating; Step S33: Then place the rock sample in a water bath, and keep the water bath at 20 °C until the core temperature drops to the cooling water temperature.

6. The evaluation method for thermal damage after low-temperature fracturing of shale according to claim 5, characterized in that, the heating rate in Step S32 is set to 1 °C / min.

7. The evaluation method for thermal damage after low-temperature fracturing of shale according to claim 1, characterized in that, the permeability of shale with primary thermal damage doubles in single digits, the permeability of shale with secondary thermal damage doubles in two digits, the permeability of shale with tertiary thermal damage doubles in three digits, the permeability of shale with quaternary thermal damage doubles by more than three digits, and the shale with quaternary thermal damage shows a broken state; the higher the level of thermal damage, that is, the stronger the fluid diversion ability in deep shale.

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