A method for studying the drilling process in deep high-temperature and high-pressure formations and the development of fractures in the borehole wall surrounding rock

Abstract

The present invention discloses a method for studying the drilling process in deep high-temperature and high-pressure formations and the development of fractures in borehole wall surrounding rocks, belonging to the field of deep rock and drilling engineering. The rock that has been subjected to target high-temperature cold treatment is placed in a stress loading device, and target stress conditions are loaded to simulate the rock in the deep geostress environment under the action of a deep high-temperature environment. Drilling operations are carried out on it to simulate the drilling process in deep formations, and the development activities of rock fractures during the drilling process are monitored by acoustic emission. Combining various observation techniques, the development of macro, meso, and micro cracks during the hole-forming operation of the rock after high-temperature cold treatment in the deep stress environment is observed and studied, obtaining the general laws of drilling parameters and efficiency in the deep environment, and obtaining the law of the development of fractures in the borehole wall surrounding rocks during the drilling process in deep formations, guiding the design of drilling parameters and borehole wall stability schemes for deep high-temperature and high-pressure formations. The present invention provides a research method for simulating the drilling in deep high-temperature and high-pressure formations and the development of fractures in borehole wall surrounding rocks. The test scheme is close to the actual process of deep drilling, and also focuses on studying the key factor affecting the structural strength of the borehole wall surrounding rock - the cracks in the borehole wall surrounding rock.

Description

Technical Field

[0001] The present invention belongs to the field of deep rock and drilling engineering, and is a method for studying the drilling process in deep high-temperature and high-pressure formations and the development of fractures in the borehole wall surrounding rock. Specifically, it relates to an indoor test method for realizing the study of the drilling process in deep formations and the development of fractures in the borehole wall surrounding rock. Background Art

[0002] The exploitation and development of deep geothermal resources such as hot dry rock are inseparable from drilling engineering. High temperature and high pressure are significant environmental characteristics of deep formations. The temperature of hot dry rock with development value is not lower than 200°C, but the depth of the hot dry rock formation at this temperature often exceeds 3000m, and the in-situ stress and vertical self-weight stress of the formation generally exceed 50MPa. Brittle granite is a typical energy storage rock type of hot dry rock. A large number of high-temperature and stress-strain tests on granite have proven that at temperatures above 200°C, the mechanical strength and physical and mechanical parameters of granite change to varying degrees in a direction unfavorable to the stability of the borehole wall. In addition, temperature also has a significant impact on the structure of granite. Under the action of high temperature, the micro-fractures inside granite are fully developed, and the rock structure deteriorates, which is not conducive to the stability of the deep borehole wall.

[0003] At the present stage, the key research on deep geotechnical tests focuses on mines, roadways, and tunnels, serving the mining industry. The depth of the research is lower than that of deep wells such as hot dry rock, and the geotechnical environment temperature in the mining industry is relatively low, and the geotechnical conditions are mainly sedimentary rocks. At present, the research on hard and brittle granite such as deep hot dry rock stays mainly in indoor high-temperature and high-pressure tests, lacking experimental research on the drilling of granite formations under high-pressure conditions after high temperature. The drilling of deep hot dry rock is a dynamic process. During the drilling process, the rock is broken to form a borehole wall, and there is a stress balance problem in the borehole wall under the coordination of the drilling fluid column pressure. The development of borehole wall cracks has an important impact on the stability of the borehole wall. The drilling process is a process in which the stress state of the borehole wall surrounding rock changes suddenly. At present, there is little research on the real dynamic process of deep drilling. There are few studies on the crushing tests of hard and brittle rocks such as crystalline rocks under deep high-pressure environments and the development characteristics of cracks in the borehole wall surrounding rock during the crushing and cutting of the drill bit, lacking corresponding research ideas and research methods. Obtaining the conditions of deep high-temperature and high-pressure hard and brittle formations and geothermal resources by means of drilling is an important research goal. Therefore, a method for studying the drilling process in deep high-temperature and high-pressure formations and the development of fractures in the borehole wall surrounding rock is needed.

[0004] Therefore, it is particularly important to develop a method for studying the drilling process in deep high-temperature and high-pressure formations and the development of fractures in the borehole wall surrounding rock. Through this method, the development of borehole wall cracks, the drilling footage rate, and the fracture response law of the borehole wall surrounding rock during the drilling process of hard and brittle rock formations under different stress conditions and after different high-temperature heat treatments can be studied, providing substantial engineering guidance suggestions for deep formation drilling and the stability of deep formation borehole walls. Summary of the Invention

[0005] In view of the above existing problems and requirements, the present invention aims to study the drilling efficiency, the stress deformation and instability law of the shaft wall surrounding rock, and the development characteristics of shaft wall cracks during the drilling and excavation process of deep rock formations, and to provide a method for simulating actual deep well drilling and researching possibilities. During the test process, it is possible to realize the real-time monitoring of the drilling efficiency of hard and brittle rocks under high stress, the crack development of the shaft wall surrounding rock in the unloading stress field, the shaft wall stability, the stress deformation of the shaft wall surrounding rock, and the characteristics of acoustic emission information.

[0006] To achieve the above object, the present invention provides a method for studying the drilling process in deep high-temperature and high-pressure formations and the development of fractures in the borehole wall surrounding rock, and the method includes:

[0007] (1) Select common rock types in deep formations with uniform texture and no cracks, and process them into rock specimens with appropriate sizes and shapes (such as 100mm×100mm×100mm).

[0008] (2) Design the heat treatment target temperature of the rock specimen and simulate the stress conditions of the deep formation. After the heat-treated rock specimen is cooled to room temperature, first use CT scanning to observe the development of internal fissures and cracks in the rock, and then use a stress loading device for stress assembly.

[0009] (3) Design the drilling parameters and drilling methods of the drill pressure, rotation speed, direction, borehole diameter, and bit type of the drill. The borehole diameter is 20mm, the drill pressures are 1000N, 1500N, and 2000N, and the rotation speeds are 10r·min -1 、15r·min -1 、20r·min -1 . During the drilling process, maintain a constant target drill pressure and rotation speed. The cuttings discharged from the hole mouth are recovered and processed by a proprietary absorption device. After the borehole is formed, the rock sample is slowly and uniformly unloaded. At the same time, stop drilling every 50mm of drilling to monitor and record the acoustic emission data of the fracture development of the borehole wall, so as to avoid the influence of the drill bit work on the acquisition of acoustic emission data and ensure the accuracy of the monitoring data.

[0010] (4) Use high-precision three-dimensional CT to scan and image the cross-section of the rock sample along the borehole center line before and after the borehole test, and compare the development of borehole wall fractures induced by drilling under high-temperature and high-pressure conditions.

[0011] (5) First, use a scanning electron microscope and a high-power microscope to observe the micro, fine, and macroscopic fracture distributions around the borehole and on the outer surface of the rock sample, and then symmetrically and non-invasively cut the rock sample into two halves along the borehole center line cross-section, and then observe the fracture distribution characteristics of the borehole wall.

[0012] (6) Use IPP image software and MATLAB data analysis software to obtain fracture development data from micro, fine, and macroscopic images of scanning electron microscopy, CT, and high-power microscopes, and study the mechanism of borehole wall fracture development induced by drilling under high temperature and high pressure.

[0013] Compared with the existing technologies, the beneficial effects of the present invention are as follows: it provides a research method for studying deep formation drilling and crack development affecting wellbore stability, can monitor the deep rock drilling process and crack development under high stress formation conditions in the indoor laboratory, monitor, observe, analyze, and study the macroscopic cracks, mesoscopic cracks, and microscopic cracks of the wellbore confining pressure, combine crack image processing to obtain specific parameter data of crack development, provide technical guidance for deep drilling, and provide a theoretical basis for deep wellbore stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a flow chart of the research method of the present invention;

[0015] Figure 2 It is a schematic diagram of the mesoscopic crack observation of the rock in the embodiment of the present invention;

[0016] Figure 3 It is a schematic diagram of the scanning electron microscopy observation of the microscopic cracks of the rock in the embodiment of the present invention;

[0017] Figure 4 It is a schematic diagram of the CT scan of the rock with a borehole in the embodiment of the present invention;

[0018] Figure 5 It is a schematic diagram of the acoustic emission information of crack activity of the rock under biaxial stress in the embodiment of the present invention. SPECIFIC EMBODIMENT

[0019] The following describes in detail a method for studying the deep high-temperature and high-pressure formation drilling process and borehole wall surrounding rock fracture development of the present invention in combination with the drawings and test process. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the protection scope of the present invention.

[0020] The present invention proposes a method for studying the deep high-temperature and high-pressure formation drilling process and borehole wall surrounding rock fracture development, which is described in combination with the Figure 1 drawings, and the specific implementation process is as follows:

[0021] (1) Select granite, a common rock type in deep formations with uniform texture and no cracks, and process it into rock specimens with appropriate sizes and shapes (such as 100mm×100mm×100mm).

[0022] (2) Design the target temperature for the heat treatment of the rock specimens and simulate the stress conditions of the deep formation. After the processed rock specimens are heat-treated and cooled to room temperature, first use CT scanning to observe the development of internal fissures and cracks in the rocks.

[0023] (3) Place the cooled granite specimens on the stress loading test platform and apply pressure at a stress loading rate of 1 N·s -1 to the target confining pressure (MPa / 25, 50, 75, 100, 125) and then keep the pressure constant for 48 hours.

[0024] (4) Design the drilling parameters and drilling methods for the drill pressure, rotation speed, direction, borehole diameter, and bit type of the drill. The borehole diameter is 20 mm, the drill pressures are 1000 N, 1500 N, and 2000 N, and the rotation speeds are 10 r·min -1 , 15 r·min -1 , 20 r·min -1 . During the drilling process, maintain a constant target drill pressure and rotation speed. The rock cuttings discharged at the hole mouth are recovered and processed with a proprietary absorption device. After the borehole is formed, slowly and uniformly unload the rock sample. At the same time, stop drilling every 50 mm of drilling to monitor and record the acoustic emission data of the fissure development in the borehole wall, so as to avoid the influence of the drill bit operation on the acquisition of acoustic emission data and ensure the accuracy of the monitoring data. The acoustic emission monitoring and analysis of the internal crack activities of the rock are as Figure 5 shown.

[0025] (5) Use high-precision three-dimensional CT to scan and image the cross-section of the rock sample along the borehole centerline before and after the borehole test, and compare the development of fissures induced by drilling under high temperature and high pressure. The rock CT scanning imaging is as Figure 4 shown.

[0026] (6) First, use a scanning electron microscope and a high-power microscope to observe the distribution of micro, fine, and macroscopic fissures around the borehole and on the outer surface of the rock sample. Then, symmetrically and non-invasively cut the rock sample into two halves along the borehole centerline cross-section, and then observe the distribution characteristics of the fissures in the borehole wall. The microscopic observation of rock cracks is as Figure 2 shown, and the microscopic crack observation of the rock is as Figure 3 shown.

[0027] (7) Use IPP image software and MATLAB data analysis software to obtain the data of fissure development from the micro, fine, and macroscopic images of the scanning electron microscope, CT, and high-power microscope, and study the mechanism of fissure development induced by drilling and hole formation under high temperature and high pressure.

Claims

1. A method for studying the drilling process of deep high-temperature and high-pressure formations and the development of cracks in the surrounding rock of the borehole wall, characterized by: The specific steps include: Step 1: Deep rocks are mainly crystalline rocks, and granite is a typical crystalline rock. The granite is processed into multiple specimens of the same size and shape to prepare test materials for subsequent steps; Step 2: Place the granite sample with the processed size and shape into the resistance furnace for heating. The temperature rise rate is controlled at 2℃·min -1 To avoid thermal shock to the rock sample caused by too fast heating rate, after the temperature rises to the target temperature, continue to heat the rock sample in the resistance furnace at a constant temperature for 120 minutes to ensure that the rock sample is heated evenly. After the heating operation is completed, turn off the power of the resistance furnace to allow the high-temperature rock sample to cool slowly in the resistance furnace. Step 3: Place the cooled granite sample on the stress loading test platform and -1 The stress loading rate was increased to the target confining pressure of 25MPa, 50MPa, 75MPa, 100MPa and 125MPa, and then the constant pressure was maintained for 48 hours before the borehole wall crack development test was carried out; Step 4: The drilling rig uses the water drilling method to avoid the thermal shock of the drill bit on the well wall. The drilling diameter is 20mm, the drilling pressure is 1000N, 1500N, 2000N, and the speed is 10r·min -1 、15r·min -1 、20r·min -1 During the drilling process, the target drilling pressure and speed are maintained at a constant level. The rock cuttings discharged from the hole mouth are recovered and processed using a proprietary absorption device. After the hole is formed, the rock sample is unloaded slowly and evenly. At the same time, the drilling is stopped every 50 mm to monitor and record the acoustic emission data of the hole wall crack development, so as to avoid the influence of the drill bit operation on the acoustic emission data collection and ensure the accuracy of the monitoring data.

2. The method for studying the drilling process of deep high-temperature and high-pressure formations and the development of cracks in the surrounding rock of the hole wall according to claim 1, characterized in that: The test was designed using an orthogonal design scheme, with temperature, pressure, drilling pressure, and rotational speed as four factors. The order of factors in the test was as follows: heating, pressurization, and drilling. An acoustic emission device was used to monitor and record the development of rock cracks in real time during the test.

3. The method for studying the drilling process of deep high-temperature and high-pressure formations and the development of cracks in the surrounding rock of the hole wall according to claim 1, characterized in that: High-precision three-dimensional CT was used to scan and image the rock samples along the center line of the borehole before and after the drilling test, and to compare the development of hole wall cracks induced by drilling under high temperature and high pressure.

4. The method for studying the drilling process of deep high-temperature and high-pressure formations and the development of cracks in the surrounding rock of the borehole wall according to claim 1, characterized in that: A scanning electron microscope and a high-power microscope were used to first observe the distribution of micro, fine, and macro cracks around the borehole and on the outer surface of the rock sample. The rock sample was then non-invasively cut into two halves symmetrically along the center line of the borehole, and the distribution characteristics of the cracks on the hole wall were observed.

5. The method for studying the drilling process of deep high-temperature and high-pressure formations and the development of cracks in the hole wall and surrounding rock according to claim 1, characterized in that: IPP image software and MATLAB data analysis software were used to obtain data on crack development from micro, fine and macro images of scanning electron microscope, CT and high-power microscope, and to study the mechanism of hole wall crack development induced by drilling under high temperature and high pressure.

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