Shale compressive strength calculation method based on scratch experiment

Abstract

The invention belongs to the field of shale oil and gas development, and discloses a shale compressive strength calculation method based on a scratch experiment. The method comprises the following steps: carrying out uniaxial compressive strength determination experiments on a shale sample under different confining pressure conditions, and calculating the compressive strength of the sample; carrying out a scratch experiment on the shale sample, and calculating a uniaxial compressive strength UCS value of the sample; carrying out a mineral analysis experiment on the shale sample to obtain the mineral contents of quartz and clay in the sample; taking compressive strength as a dependent variable, taking UCS, quartz mineral and clay mineral contents as independent variables, and adopting a multiple linear regression method to establish a compressive strength calculation model; and establishing a shale compressive strength calculation model under different confining pressure conditions. According to the method, the shale compressive strength is calculated through the scratch experiment, damage to a shale sample can be effectively avoided, a shale compressive strength calculation model under different peripheral pressure pieces is established with the content of UCS, quartz and clay minerals as independent variables, the shale compressive strength under any confining pressure can be obtained, and the shale compressive strength under any confining pressure can be efficiently guided for on-site fracturing of a shale oil and gas reservoir.

Description

Technical Field

[0001] The present invention belongs to the field of shale oil and gas development, and relates to a method for calculating the compressive strength of shale based on a scratch test. Background Art

[0002] Compressive strength is a very crucial rock mechanics parameter and is of great significance in shale oil and gas exploration and development. On the one hand, during the drilling process, the compressive strength of shale is closely related to engineering problems such as wellbore stability and mechanical drilling rate. Accurately calculating the compressive strength of shale is of great significance for preventing drilling accidents, improving drilling efficiency, and reducing the drilling cycle. On the other hand, accurately calculating the compressive strength of shale can scientifically guide the evaluation of reservoir compressibility, the design of fracturing construction parameters, and the selection of proppants, improve the fracturing transformation effect of shale reservoirs, and ultimately increase the recovery rate of shale oil and gas.

[0003] The currently most commonly used method for measuring the compressive strength of shale is the indoor uniaxial / triaxial test method. By applying axial stress to the water-saturated sample until it fails, the compressive strength is calculated by measuring the failure load borne by the unit bearing area of the shale sample. However, this type of method has very high requirements for the specifications of shale samples. During the exploration and development of shale oil and gas, only coring is carried out for exploration wells and key evaluation wells. Therefore, it is very difficult to obtain shale samples required for indoor uniaxial / triaxial tests. At the same time, due to the strong brittleness of shale and the well-developed bedding and fractures, it is difficult to prepare shale samples for uniaxial / triaxial tests.

[0004] The invention patent with the authorized publication number of CN 112179770 B provides a method for evaluating the uniaxial compressive strength of shale based on a micro-nano indentation test of cuttings, and predicts the macroscopic mechanical parameters of shale by establishing the relationship between the indentation test load-displacement curve and the uniaxial compressive strength of shale. The disadvantage of this method is that it does not consider the influence of confining pressure on the uniaxial compressive strength of shale. Research shows that confining pressure can effectively reduce the compaction effect of overlying pressure, and the compressive strength of shale increases significantly with the increase of confining pressure. Therefore, in order to obtain the compressive strength of shale under the true formation conditions, it is necessary to carry out research on the method for calculating the compressive strength of shale under different confining pressure conditions. Summary of the Invention

[0005] The purpose of the present invention is to provide a method that can efficiently and accurately calculate the compressive strength of shale to guide the fracturing of on-site shale oil and gas reservoirs.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is:

[0007] A method for calculating the compressive strength of shale based on a scratch test, comprising the following steps:

[0008] Step 1: Conduct uniaxial compressive strength determination experiments on shale samples under different confining pressures, and calculate the compressive strength of the samples;

[0009] Step 2: Conduct scratch experiments on shale samples, and calculate the uniaxial compressive strength (UCS) value of the samples;

[0010] Step 3: Conduct mineral analysis experiments on shale samples to obtain the quartz and clay mineral contents of the samples;

[0011] Step 4: Using the compressive strength as the dependent variable, and the UCS, quartz mineral, and clay mineral contents as independent variables, adopt the multiple linear regression method to establish a compressive strength calculation model;

[0012] Step 5: Establish a shale compressive strength calculation model under different confining pressures.

[0013] Further, in Step 1, the uniaxial compressive strength of the core is calculated using the following formula:

[0014]

[0015] In the formula: σ c is the uniaxial compressive strength, MPa; P c is the shale failure load, N; A is the cross-sectional area of the rock sample, mm 2 .

[0016] Further, in Step 1, the shale samples need to be saturated with water before the uniaxial compressive strength determination experiment;

[0017] Further, in Step 1, the different confining pressures refer to: 0 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, and 50 MPa;

[0018] Further, in Step 1, during the uniaxial compressive strength determination experiment, the axial pressure is gradually loaded at a speed of 0.5 MPa per second until the experiment ends;

[0019] Further, in Step 2, the scratch experiment includes: under the drive of a drive device controlled by a computer, the cutting tool is driven to scratch the rock surface at a certain constant rate and constant depth, and the computer real-time collects and records data such as the displacement and force of the cutting tool, and substitutes the measured force data into the scratch test theoretical model to calculate the compressive strength of the rock;

[0020] Further, in Step 2, before the formal start of the rock scratch experiment, the core surface needs to be pre-scratched to make the blade evenly contact with the core surface through scratching several times;

[0021] Further, in Step 3, the samples need to be pretreated before the mineral content determination experiment, and the specific process is as follows:

[0022] (1) Wash the shale samples with oil, and the organic solvent used for washing oil is chloroform, and process until the fluorescence is below grade four;

[0023] (2) Use a constant temperature drying oven to dry the shale samples after washing oil, the temperature is 50 °C, and the time is 24 hours;

[0024] (3) Use an agate mortar to grind the shale samples after washing oil and drying into a powder with a mesh size of 100.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] Traditional methods for calculating the compressive strength of shale require uniaxial / triaxial experiments. Uniaxial / triaxial experiments not only have requirements for the specifications of shale samples, but also cause damage to the samples after the experiments. Calculating the compressive strength of shale using a scratch experiment can effectively avoid damage to shale samples. Taking the UCS, quartz, and clay mineral content as independent variables, a calculation model for the compressive strength of shale under different confining pressures is established, and the compressive strength of shale under any confining pressure can be obtained, which is used to efficiently guide on-site fracturing of shale oil and gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a crossplot of experimentally measured compressive strength and model-calculated compressive strength;

[0028] Figure 2 is a curve diagram of the model-calculated compressive strength of shale sample 1 changing with confining pressure. DETAILED DESCRIPTION OF THE INVENTION

[0029] To make the technical means and achieved purposes adopted by the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

[0030] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified.

[0031] Refer to Figure 1 and Figure 2 , this specific embodiment adopts the following technical solution: A method for calculating the compressive strength of shale based on a scratch experiment, including the following steps:

[0032] Step 1: Conduct uniaxial compressive strength measurement experiments on shale samples under different confining pressure conditions, and calculate the compressive strength of the samples. The experimental instrument for uniaxial compressive strength measurement is an RTR-2000 type high-pressure rock uniaxial dynamic testing system;

[0033] The maximum axial pressure of the system is 2000 kN, the maximum confining pressure is 140 MPa, the maximum pore pressure is 140 MPa, and the maximum temperature is 200 °C;

[0034] Before the uniaxial compressive strength test, the shale samples need to be saturated with water;

[0035] Conduct uniaxial compressive strength tests under different confining pressures;

[0036] The confining pressures are 0 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, and 50 MPa in sequence;

[0037] The experiment is carried out according to the standard "GB / T 23561.7-2009 Determination methods for physical and mechanical properties of coal and rock - Part 7: Determination of uniaxial compressive strength and calculation method of softening coefficient".

[0038] Calculate the uniaxial compressive strength of the core by the following formula:

[0039]

[0040] In the formula: σ c is the uniaxial compressive strength, MPa; P c is the failure load of the shale, N; A is the cross-sectional area of the rock sample, mm 2 .

[0041] Gradually apply the axial pressure at a speed of 0.5 MPa per second, start the test and record the test data.

[0042] Step 2: Conduct a scratch test on the shale sample and calculate the uniaxial compressive strength UCS value of the sample;

[0043] The principle of the rock scratch test is: Under the control of a computer, the driving device drives the cutting tool to scratch the rock surface at a certain constant rate and constant depth. The computer real-time collects and records data such as the displacement and force of the cutting tool, and substitutes the measured force data into the scratch test theoretical model to calculate the compressive strength of the rock;

[0044] The instrument used in the experiment is a full-scale core strength continuous scratch test system;

[0045] This system mainly includes: ① Scratch driving device; ② Load measuring instrument; ③ Depth measuring instrument; ④ Horizontal displacement measuring instrument; ⑤ Cutting tool; ⑥ Rock sample fixing device; ⑦ Computer acquisition and control system.

[0046] The main technical parameters of the instrument are: core diameter range 25 - 200 mm, core length range 2.0 - 50 cm, force sensor range 1 - 2000 N, horizontal displacement resolution 0.1 mm, cutting depth resolution 0.001 mm, sample data resolution 10 pts / mm.

[0047] The specific operation process of the rock scratch test is as follows:

[0048] (1) Rock sample assembly: Select the scribing surface and ensure that the burial direction and scribing direction of the rock sample are consistent. Fix the core firmly with the splint.

[0049] (2) Before the formal scribing starts, the surface of the core needs to be pre-scribed. Make the blade contact the core surface evenly through scribing several times.

[0050] (3) Evaluate the strength of the rock sample to determine the scribing rate and depth. Then start the formal scribing test. The computer collects and records data such as the horizontal tangential force and vertical force received by the blade in real time.

[0051] (4) Keep the scribing starting point, scribing surface, scribing depth and scribing rate unchanged, and repeat the scribing test 2 more times.

[0052] (5) After the scribing test is completed, take the average value of the scribing data for 3 times and record the data.

[0053] Step 3: Conduct mineral analysis experiments on shale samples to obtain the quartz and clay mineral contents of the samples.

[0054] The instrument for mineral analysis experiment is the D8 DISCOVER X-ray diffractometer.

[0055] The mineral analysis experiment is carried out according to the petroleum and natural gas industry standard SY / T 5163-2010 "X-ray Diffraction Analysis Method for Clay Minerals and Common Non-Clay Minerals in Sedimentary Rocks".

[0056] Before the mineral content determination experiment, the sample needs to be pretreated. The specific process is as follows:

[0057] (1) Wash the shale sample with oil. The organic solvent used for washing oil is chloroform, and the treatment is carried out until the fluorescence level is below four.

[0058] (2) Use a constant temperature drying oven to dry the washed shale sample. The temperature is 50°C and the time is 24 hours.

[0059] (3) Use an agate mortar to grind the washed and dried shale sample into a powder of 100 mesh.

[0060] Step 4: Take the Young's modulus as the dependent variable, and the uniaxial compressive strength, quartz and clay mineral contents as the independent variables, and establish a Young's modulus calculation model by using the multiple linear regression method.

[0061] As shown in Table 1: The shale compressive strength and the corresponding calculation results of UCS, quartz and clay mineral contents under different confining pressure conditions.

[0062] Table 1

[0063]

[0064] The calculation formula for the compressive strength of shale under different confining pressures can be expressed by the following formula:

[0065] Compressive strength = a × UCS + b × Q ua + c × C la + d(2)

[0066] Where: UCS is the uniaxial compressive strength in the scratch test, ksi; Q ua is the quartz mineral content, %; C la is the clay mineral content, %; a, b, and c are formula fitting coefficients, and d is a constant, dimensionless.

[0067] As shown in Table 2, the fitting coefficients and the constant d value in formula (2) under different confining pressures are as follows;

[0068] Table 2

[0069]

[0070] According to formula (2), the compressive strengths of 12 shale samples under confining pressures were calculated, and the calculation results are shown in Table 3:

[0071] Table 3

[0072]

[0073] The relationship between the experimentally measured compressive strength and the compressive strength calculated by the model is as Figure 1 shown. There is a good linear relationship between the two, and all data points are located near Y = X. In addition, the error analysis results show (Table 4) that the average value of the relative error calculated by the model is only 2.7%, indicating that the calculation model provided by the examples of the present invention has high accuracy.

[0074] Table 4

[0075]

[0076]

[0077] Step 5: Establish a calculation model for the compressive strength of shale under different confining pressures

[0078] Taking shale sample 1 as an example, the compressive strength of this sample under different confining pressures can be calculated through formula (2), as shown in Table 5:

[0079] Table 5

[0080] Confining pressure (MPa) 0 10 20 30 40 50 Model calculated compressive strength (ksi) 152.0 211.8 255.1 280.4 316.2 345.1

[0081] Taking the confining pressure as the abscissa and the compressive strength calculated by the model as the ordinate, plot the relationship curve between the two;

[0082] As Figure 2 shown, there is a very strong linear relationship between the two, and the correlation coefficient R 2 is as high as 0.9783 and can be expressed by formula (3):

[0083] y = 3.727x + 166.93 (3)

[0084] According to formula (3), the compressive strength of shale sample 1 under any confining pressure condition can be calculated;

[0085] Taking the confining pressure of 35 MPa as an example, the calculated compressive strength of shale is 3.727×35 + 166.93 = 297.38 psi.

[0086] Although some embodiments of the present invention have been given in this article, those skilled in the art should understand that the embodiments in this article can be changed without departing from the spirit of the present invention. The above embodiments are only exemplary and should not be used as a limitation of the scope of the rights of the present invention.

Claims

1. A method for calculating the compressive strength of shale based on a scratch test, characterized in that, it includes the following steps: Step 1: Conduct uniaxial compressive strength determination experiments on shale samples under different confining pressure conditions, and calculate the compressive strength of the samples; Step 2: Conduct scratch tests on shale samples, and calculate the uniaxial compressive strength UCS value of the samples; Step 3: Conduct mineral analysis experiments on shale samples to obtain the quartz and clay mineral contents of the samples; Step 4: Taking the compressive strength as the dependent variable, and taking UCS, quartz minerals, and clay mineral contents as independent variables, use the multiple linear regression method to establish a compressive strength calculation model; Step 5: Establish a shale compressive strength calculation model under different confining pressure conditions.

2. The method for calculating the compressive strength of shale based on a scratch test according to claim 1, characterized in that, in Step 1, the uniaxial compressive strength of the core is calculated using the following formula: Where: σ c is the uniaxial compressive strength, MPa; P c is the shale failure load, N; A is the cross-sectional area of the rock sample, mm 2 .

3. The method for calculating the compressive strength of shale based on a scratch test according to claim 1, characterized in that, in Step 1, the shale samples need to be saturated with water before the uniaxial compressive strength determination experiment.

4. The method for calculating the compressive strength of shale based on a scratch test according to claim 1, characterized in that, in Step 1, the different confining pressures refer to: 0 MPa, 10 MPa, 20 MPa, 30 MPa, 40 MPa, and 50 MPa.

5. The method for calculating the compressive strength of shale based on a scratch test according to claim 1, characterized in that, in Step 1, during the uniaxial compressive strength determination experiment, the axial pressure is gradually loaded at a speed of 0.5 MPa per second until the experiment ends.

6. The method for calculating the compressive strength of shale based on a scratch test according to claim 1, characterized in that, in Step 2, the scratch includes: the driving device controlled by a computer drives the cutting tool to scratch the rock surface at a certain constant rate and constant depth, the computer real-time collects and records data such as the displacement and force of the cutting tool, and substitutes the measured force data into the scratch test theoretical model to calculate the compressive strength of the rock.

7. The method for calculating the Young's modulus of shale based on a scratch test according to claim 1, characterized in that in Step 2, before the formal start of the rock scratch test, the core surface needs to be pre-scratched, and the blade is evenly contacted with the core surface through scratching several times.

8. The method for calculating the compressive strength of shale based on a scratch test according to claim 1, characterized in that, in Step 3, the samples need to be pretreated before the mineral content determination experiment, and the specific process is as follows: (1) Wash the shale samples with oil, and the organic solvent used for washing oil is chloroform, and process until the fluorescence is below grade four; (2) Use a constant temperature drying oven to dry the shale samples after washing oil, the temperature is 50 °C, and the time is 24 hours; (3) Use an agate mortar to grind the shale samples after washing oil and drying into 100-mesh powder.

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