Method for determining horizontal stress of rock stratum based on digital drilling technology

Abstract

The application discloses a rock stratum horizontal stress measuring method based on a digital drilling technology, and steps of the method comprise the following steps: step 1, a rock digital drilling test is carried out by using a digital drilling device, a relationship between a cutting stress and a feed force is obtained, and then a contact friction angle θ between a cutting surface of a diamond blade and a compression crushing zone is calculated; step 2, the contact friction angle θ obtained in step 1 is used to solve an internal friction angle φ of the rock; and step 3, α, θ, φ', φ obtained in steps 1 and 2 and a rock cohesion c measured are used to solve a confining pressure P of surrounding rock f The method has the advantages of low cost, simple calculation and high precision.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of energy geological exploration, and relates to a rock stratum horizontal ground stress determination method based on digital drilling technology. BACKGROUND

[0002] With the rapid development of the world economy, science and technology and society, the supply of traditional fossil energy is becoming increasingly scarce, and the resources of the shallow layer of the earth are becoming exhausted day by day. The proportion of deep energy in the energy consumption structure will increase, so that the development of deep energy resources is attracting attention. At present, the high-risk disasters induced by deep high ground stress rock mass engineering resources production are serious, which leads to high safety cost, frequent disasters and accidents, and great difficulty in production of deep resource development. With the continuous exploitation of oil, natural gas and other resources, deep geological stratum energy production has great environmental protection potential.

[0003] In order to successfully carry out the deep high ground stress rock mass engineering and resource development, reduce the safety cost and disaster accidents caused by high ground stress, and accurately and quickly determine the ground stress, there is important theoretical value and practical value. Although the ground stress measured by using the existing instrument is relatively accurate, the data quantity is small and the cost is high, and it is difficult to continuously monitor the ground stress data in a large range.

[0004] The digital drilling data has the characteristics of deep measurement depth, large information quantity and relatively continuous data, therefore, developing a rock stratum horizontal ground stress determination method based on the existing ground stress test method and the digital drilling technology, and realizing a new continuous and reliable ground stress test method are technical problems to be solved in the field of rock mechanics and engineering. SUMMARY

[0005] The purpose of the application is to provide a horizontal ground stress determination method based on digital drilling technology, which solves the problems of great limitation, complex test process and large determination result error of the ground stress determination method of the prior art.

[0006] The technical solution adopted by the application is a rock stratum horizontal ground stress determination method based on digital drilling technology, which is implemented according to the following steps:

[0007] Step 1, rock digital drilling test is carried out by using a digital drilling device to obtain the relationship between the cutting stress and the feed force, and then the contact friction angle θ between the cutting surface of the diamond blade and the compression crushing zone is calculated;

[0008] Step 2, the internal friction angle of the rock is solved by using the contact friction angle θ obtained in step 1

[0009] Step 3, α and θ obtained in steps 1 and 2, and the rock cohesion c obtained by measurement are used to solve the confining pressure P of the surrounding rock f That is, it is completed.

[0010] The present application has the advantages of low cost, simple calculation and high precision, compared with the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1a is a curve of the cutting force of shale and the feed force; Figure 1b is a curve of the cutting force of red sandstone and the feed force; Figure 1c is a curve of the cutting force of granite and the feed force; Figure 1d is a curve of the cutting force of limestone and the feed force; Figure 1e is a curve of the cutting force of sandstone and the feed force; Figure 1f is a curve of the cutting force of diorite and the feed force;

[0012] Figure 2 is a comparison diagram of the actual horizontal stress and the horizontal stress estimated by the method of the present application. DETAILED DESCRIPTION

[0013] The present application will be described in detail below in combination with the drawings and specific embodiments.

[0014] The horizontal stress measurement method of the present application combines the digital drilling technology and is implemented according to the following steps:

[0015] Step 1: Perform rock digital drilling test by using the digital drilling equipment to obtain the relationship between the cutting stress and the feed force, and further calculate the contact friction angle θ between the cutting surface of the diamond blade and the compression crushing zone.

[0016] During the rock digital drilling test by the digital drilling equipment, the test is divided into two stages according to the different dominant forces, i.e. the cutting stage and the friction stage. When the cutting force is greater than the feed force, the cutting force occupies the dominant position, belonging to the cutting stage. When the feed force is greater than the cutting force, the feed force occupies the dominant position, belonging to the friction stage.

[0017] Condition 1: When the cutting force is greater than the feed force, the function expression between the cutting force and the feed force is:

[0018]

[0019] wherein F is the cutting force, F is the feed force, α is the drill bit rake angle, θ is the contact friction angle of the cutting surface, F is the friction component of F t n t w t ​​​​

[0020] After eliminating the effective contact area between the drill bit and the rock, equation (1) is transformed into equation (2) as follows:

[0021]

[0022] where S t is the cutting stress, S n is the feed force, R is the outer diameter of the drill bit, and r is the inner diameter of the drill bit.

[0023] Using Kalantari's model, the intercept in equation (2) is ignored, and equation (2) is transformed into equation (3) as follows:

[0024]

[0025] Condition 2: When the cutting force is less than the feed force, the functional expression of the cutting force and the feed force is as follows:

[0026]

[0027] where P f is the confining pressure; c is the rock cohesion; is the internal friction angle of the rock; is the contact friction angle between the broken zone and the intact rock;

[0028] After eliminating the effective cutting area in equation (4), equation (5) is obtained as follows:

[0029]

[0030] From the data of S t and S n , the functional relationship between S t and S n can be obtained, and from equation (3), the slope of both is tan (α+θ). Knowing the value of the rake angle α, the contact friction angle θ between the cutting surface of the core drill bit and the compression broken zone can be obtained.

[0031] Step 2: Using the contact friction angle θ obtained in step 1, the internal friction angle of the rock

[0032] is solved. The expression of the internal friction angle of the intact rock is as follows:

[0033]

[0034]

[0035] The contact friction angle θ and the bit rake angle α obtained in step 1 are brought into formula (6) and formula (7), and the contact friction angle of the broken zone and the intact rock is obtained and the internal friction angle of the rock

[0036] Step 3, α, θ, obtained from step 1 and step 2, and the measured rock cohesion c are used to solve the confining pressure P of the surrounding rock f (that is, the horizontal stress),

[0037] The confining pressure P f is calculated as follows:

[0038]

[0039] The θ, obtained above, and the known values of α, c, S t , S n are substituted into formula (8) to obtain the confining pressure P f , that is, the formation.

[0040] Example 1, the rock type is granite, the steps of the method are implemented according to the foregoing method of the application, and the relevant data and the comparison of the results are described with reference to Tables 1, 2 and 3.

[0041] Example 2, the rock type is diorite, the steps of the method are implemented according to the foregoing method of the application, and the relevant data and the comparison of the results are described with reference to Tables 1, 2 and 3.

[0042] Example 3, the rock type is sandstone, the steps of the method are implemented according to the foregoing method of the application, and the relevant data and the comparison of the results are described with reference to Tables 1, 2 and 3.

[0043] Example 4, the rock type is red sandstone, the steps of the method are implemented according to the foregoing method of the application, and the relevant data and the comparison of the results are described with reference to Tables 1, 2 and 3.

[0044] Example 5, the rock type is shale, the steps of the method are implemented according to the foregoing method of the application, and the relevant data and the comparison of the results are described with reference to Tables 1, 2 and 3.

[0045] Example 6, the rock type is limestone, the steps of the method are implemented according to the foregoing method of the application, and the relevant data and the comparison of the results are described with reference to Tables 1, 2 and 3.

[0046] Experimental verification

[0047] 1) Perform a digital drilling test and collect relevant data.

[0048] Digital borehole testing follows the ISRM recommended testing procedure and precautions. The data acquisition rate is set to 0.02 s / point during the experiment. The maximum horizontal stress that the hydraulic system can provide is 80 MPa. Before the experiment, 3-5 pre-drilling should be performed to observe whether the digital borehole parameters are within the normal value range. This embodiment uses a diamond hollow drill bit, and the inner diameter and outer diameter of the drill bit are 20 mm and 25 mm, respectively. The drilling speed is 0.1-1.2 mm / min, and the rotation speed is 200-600 rpm. In order to protect the test system, when the drilling speed exceeds 1 mm / min, the rotation speed is controlled to be above 200 rpm. The horizontal pressure of 0 MPa, 5 MPa, 10 MPa, 15 MPa, and 20 MPa is applied to six different rock samples (i.e., shale, red sandstone, granite, limestone, sandstone, and diorite), and then drilling tests are respectively performed on each rock sample. Thus, the linear relationship curves of the cutting force and the feed force of the six kinds of rocks under different horizontal stress levels are obtained, as shown in FIGS. 1-6, and the data of the cutting point of the digital drill of different rocks are shown in Table 1. Figure 1a 、 Figure 1b 、 Figure 1c 、 Figure 1d 、 Figure 1e 、 Figure 1f

[0049] Table 1, DPMA measured cutting point of digital drill of different rocks

[0050]

[0051] 2) Perform a conventional triaxial compression test and collect relevant data.

[0052] The triaxial compression test follows the recommended standard of ISRM. When performing a conventional triaxial compression test on a WDT-1500 type multifunctional rock material compression testing machine, the sample is wrapped with a flexible rubber film to prevent pressure oil from penetrating. In order to minimize the error caused by the uneven bottom surface of the sample, a spherical base is used at the top. In the test, two heat-treated hardened smooth plates are placed between the two parts of the sample to ensure no friction. In order to eliminate the error caused by the heterogeneity of the rock, the measured data of each rock is averaged. The physical information and strength parameters of the rock obtained by the conventional triaxial compression test are shown in Table 2.

[0053] Table 2, physical information and strength parameters of rock obtained by conventional triaxial compression test

[0054]

[0055] 3) Calculate the contact friction angle θ of the cutting surface and compare and select the optimal one.

[0056] ​According to the three steps of the method, the contact friction angle of the cutting surface is solved, and the numerical comparison is shown in Table 3, and it can be seen from the table that the method is based on the result of the digital drilling and the laboratory test result is more and more close to the increase value of the ground stress, which fully shows that the prediction result of the method is more accurate, and the reliability meets the technical requirements on the site. Figure 2 As shown in the table, it can be seen that the method is based on the result of the digital drilling and the laboratory test result is more and more close to the increase value of the ground stress, which fully shows that the prediction result of the method is more accurate, and the reliability meets the technical requirements on the site.

[0057] Table 3, actual horizontal ground stress and model predicted horizontal ground stress

[0058]

[0059] The application is based on the rock drilling test on the site, the relationship curve of the cutting force and the feed force is obtained, the horizontal ground stress of the rock is calculated in combination with the corresponding theory, the method only needs manual calculation, the calculation process is simple, with the increase of the ground stress, the accuracy is higher and higher, the error is within 12.5% when the ground stress is 10MPa, and the error is within 5% when the ground stress is 20MPa. In the actual engineering, there is basically no ground stress, so most of the errors can be basically ignored.

[0060] In summary, the method for calculating the parameters only needs the test from the site while drilling, does not need the traditional sampling for indoor test, the process is simple, the survey cost is saved, and the application prospect is wide.

Claims

1. A method for determining horizontal in-situ stress in rock strata based on digital drilling technology, characterized in that, The following steps are implemented: Step 1, rock digital drilling test is carried out by using digital drilling equipment to obtain the relationship between cutting stress and feed force, and the contact friction angle between the cutting surface of the diamond blade and the compression crushing zone is calculated θ , The specific process is: Condition 1: When the cutting force is greater than the feed force, the functional expression between the cutting force and the feed force is: （1） wherein F t is the cutting force, F n is the feed force, α is the drill point angle, θ is the contact friction angle of the cutting face, F t w is the F t friction component, After eliminating the effective contact area between the drill bit and the rock, equation (1) is transformed into the following equation (2): （2） wherein, S t is the cutting stress, S n is the feed force, R is the drill bit outer diameter, r is the drill bit inner diameter, Using Kalantari's model, the intercept in equation (2) is ignored, and equation (2) is transformed into the following equation (3): （3） Condition 2: When the cutting force is less than the feed force, the functional expression between the cutting force and the feed force is: （4） wherein, P f is the confining pressure; c is the rock cohesion; φ is the internal friction angle of the rock; φ’ is the contact friction angle between the fractured zone and the intact rock, After eliminating the effective cutting area from equation (4), the following equation (5) is obtained: （5） From S t and S n data can be derived S t and S n function relationship, by equation (3) can be obtained both of the slope tan( α + θ ), given the bit rake angle α the value of ， can be derived from the core bit cutting surface and the contact friction angle between the compression crushing zone θ ; Step 2, using the contact angle obtained in Step 1 θ , solving for the internal friction angle of the rock φ , The specific process is: Solving the contact friction angle of fractured zone and intact rock φ’、 Internal friction angle of intact rock The expression is as follows: （6） （7） The contact friction angle θ and the bit rake angle α are inserted into equations (6) and (7), then the contact friction angle φ’ between the broken zone and the intact rock is obtained φ and the internal friction angle of the rock Step 3, from Step 1 and Step 2 α 、 θ 、 φ’ 、 φ , and the measured rock cohesion c , solve for the confining pressure of the surrounding rock P f , The specific process is: Calculating confining pressure P f The expression is as follows: （8） The values of the foregoing θ , φ’ , φ and the known α , c , S t , S n are substituted into equation (8) to obtain the confining pressure P f , i.e.

2. The method of claim 1, wherein the method is characterized by, In step 1, when testing with digital drilling, the data acquisition rate is set to 0.02 s / time; a diamond hollow drill bit is used, with an inner diameter and an outer diameter of 20 mm and 25 mm, respectively; the drilling speed is 0.1-1.2 mm / min, and the rotation speed is 200-600 rpm.

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