CN104849433A - Experimental device and method for testing magnitude of crustal stress of cylindrical rock core - Google Patents

Abstract

The invention discloses an experimental device and an experimental method for testing the magnitude of in-situ stress by using a cylindrical rock core. The experimental device is composed of cylindrical specimen holding device, pressure kettle, liquid booster pump, digital signal conversion device, displacement sensor, pressure sensor and other components. The experimental method of the present invention mainly includes five steps: core processing, wave velocity anisotropy determination of horizontal principal stress azimuth, specimen brushing, cylindrical specimen differential strain experiment, and data processing. The experimental method of the invention is an extension of the conventional differential strain experiment, has the advantages of strong practicability, simplicity, convenience, high reliability, and the like, and has a wider application range than the conventional differential strain experiment.

Description

An experimental device and method for testing the in-situ stress of a cylindrical rock core

technical field

The invention discloses an experimental device and method for testing the in-situ stress of rock cores, which are used for in-situ stress testing of small-diameter cylindrical specimens, and are used for small-diameter drilling and coring of ultra-deep wells and part of small-diameter exploration wells It is of great significance to study the magnitude of the in-situ stress.

Background technique

The force that exists inside the crust is called in-situ stress, which is caused by the gravity and horizontal movement of crustal rocks and other factors. The force per unit area inside the formation medium is the magnitude of the in-situ stress. In-situ stress exists in crustal rocks. The magnitude and direction of in-situ stress change with time and space. The magnitude and direction of in-situ stress are related to geological structure and geological tectonic movement. Oil and gas field geological exploration, oil and gas field development and fracturing stimulation measures, etc. Both are closely related to the magnitude and direction of the ground stress. Therefore, the ability to accurately understand the geostress state of the underground rock mass is of great significance to the effective development of oil and gas field resources and the rationality of the design and construction of hydraulic fractures.

At present, there are mainly four kinds of indoor ground stress measurement methods commonly used: ① conventional differential strain method; ② Kessel acoustic emission method; ③ stress relief method; ④ inelastic strain recovery method. These methods have relatively high requirements on cores, and the consumption of cores in the experimental process is relatively large. For downhole drilling and coring samples, especially ultra-deep well drilling or geological slim hole drilling and coring, conventional in-situ stress test experiments Requirements for specimens may limit the testing of in-situ stress chamber experiments. For example, the ultra-deep carbonate wells in Xinjiang are drilled to coring, and the vertical core is 5.5cm. If the Kaiser acoustic emission experiment is used, the small rock required for the Kaiser experiment needs to be drilled on the same full-diameter core or adjacent full-diameter cores. It is difficult to drill small-diameter cores, and it consumes too much core. The experiment needs to consume a full-diameter core of more than 15 cm; if the inelastic strain recovery test is used, the minimum diameter of the core is generally required to be about 7.5 cm. The diameter of the rock core is too small to meet the size requirements of the inelastic strain recovery test; if the conventional differential strain test is used, the core needs to be processed into a cube of about 6.0 cm. For a full-diameter core with a diameter of only 5.5-6.0 cm, processing more difficult.

The conventional differential strain test is a commonly used test method for in-situ stress. It is theoretically rigorous and reliable. Generally, the core needs to be processed into a cube with a side length of about 6 cm. However, some drilling cores cannot guarantee the size of the specimen. The differential strain experiment needs to measure 12 strains, and the consistency requirements for each strain sensor are high, and the probability of failure of the experiment is high.

Contents of the invention

The invention discloses a differential strain testing experimental device and method for cylindrical core testing. Conventional drilling and coring is suitable for this method. The small-diameter cores drilled by exploration drilling are also applicable to the experimental method of the present invention, which can reduce the size requirements and limitations of conventional differential strain experiments on experimental specimens, and simultaneously utilize high-pressure resistant displacement sensors to measure the displacement of the specimens. The experimental precision is high, which can fully meet the requirements of experimental precision.

In order to achieve the above object, the present invention has designed a set of special indoor experiment device and special cylinder specimen rock core holder; During the experiment, high-precision displacement sensor is used to measure the displacement in the experiment process, and the traditional strain gauge measurement experiment is abandoned. The strain in the process; during the experiment, only the displacement in the direction of the three principal stresses is measured, which are the vertical direction, the direction of the maximum principal stress in the horizontal direction and the direction of the minimum principal stress in the horizontal direction.

The experimental device designed by the present invention mainly includes: a cylindrical rock core holding device, which is used to fix the experimental rock core and install the displacement sensor. It consists of a base and an upper frame. The upper frame is fixed on the base by screws, and the middle of the upper frame is used to clamp the experimental specimen. Space.

The displacement sensor is used to measure the displacement deformation of the experimental specimen during the loading process. There are three pairs of displacement sensors, including the vertical principal stress direction displacement sensor, the horizontal minimum principal stress direction displacement sensor and the horizontal maximum principal stress direction displacement sensor. The positions corresponding to the three directions of the experimental specimen core on the upper frame.

The pressure kettle is used to install the core holding device and seal the pressure to ensure that the test piece is in the preset pressure environment.

The liquid booster pump is connected with the autoclave through pipelines to provide pressure for the experiment.

A pressure sensor is installed in the autoclave to timely monitor the pressure change in the autoclave.

The digital signal converter is connected to the displacement sensor through the signal line, and converts the signal of the displacement sensor and the pressure sensor into a digital signal recognized by the control computer;

   Control the computer, record the displacement change value during the experiment, and process the data.

The method of the present invention comprises the following steps:

(1) Processing of core specimens. Carry out simple core processing according to the cores taken, and process the cores into cylinders with a diameter of 5.5-6.0 cm and a height of 6.0 cm. The cylinders are required to be homogeneous and must not contain natural cracks or horizontal bedding. It should be smooth and flat; the parallelism of the two end faces is less than 0.1mm, and the existence of natural cracks will affect the experimental results;

(2) Wave velocity anisotropy experiment. Using the difference characteristics of ultrasonic waves in different directions of horizontal stress, determine the initial directions of the horizontal maximum principal stress and horizontal minimum principal stress of the cylindrical core on the experimental specimen, and mark them;

(3) Brush the experimental specimen with glue. In order to prevent the liquid from penetrating into the test piece during the compression process, apply epoxy resin evenly around the test piece to isolate the pressurized liquid from the test piece. During the process of applying epoxy resin, the application of epoxy resin must be ensured. Uniform, reduce the influence of epoxy resin on the experimental results;

(4) Improved differential strain experiment. For the cylindrical experimental specimen, use the holding device to fix the cylindrical core, combine the horizontal principal stress orientation marked on the cylindrical core, and install the displacement sensor. The specific installation steps are as follows: (1) When installing the displacement sensor, first move the holder In the core specimen, make the marking line of the horizontal maximum principal stress direction on the core align with the installation point of the sensor on the holder; (2) install the displacement sensor in the vertical principal stress direction, and fix the displacement with the installation locking screw Sensor, during the installation process, use the control computer to determine that the initial value of the displacement sensor is near 0, so as to ensure that the displacement measured during the experiment is always within the range of the displacement sensor; (3) After installing the vertical displacement sensor, install the horizontal The installation method of the displacement sensor in the direction of the maximum principal stress and the direction of the minimum principal stress in the horizontal direction is similar to that of the vertical displacement sensor, so that the initial value of the displacement sensor is near the zero value of the displacement sensor to ensure that the displacement sensor is always in the range during the experiment. (4) After installing the 3 pairs of displacement sensors, according to the initial displacement values of the 3 pairs of displacement sensors, some displacement sensors whose initial displacement values deviate greatly from 0 are tested to ensure that the measurement results during the experiment are always within their range Inside.

Put the holding device equipped with the experimental specimen into the geostress test autoclave, connect the data transmission line of the displacement sensor, and seal the autoclave; turn on the liquid booster pump, use the computer to control the pressure loading, and perform the pressure test after the liquid is full of the autoclave. Loading, at the same time record the displacement value and pressure value data of three pairs of sensors in a timely manner, and stop loading after the experimental pressure reaches the experimental preset pressure value;

(5) Data processing. According to the results of compressive stress and strain obtained in laboratory experiments, data processing is carried out to obtain the ratio relationship between vertical principal stress, horizontal maximum principal stress and horizontal minimum principal stress; The core depth is used to calculate the vertical principal stress, and the horizontal maximum principal stress and the horizontal minimum principal stress are calculated according to the ratio of the obtained vertical principal stress to the three-dimensional principal stress.

The specific differences between the differential strain test method of the present invention and the conventional differential strain test method are as follows:

(1) The shapes of the specimens used in the experiment are different. The experimental specimen of the conventional differential strain experiment is a cube of about 6.0 cm, while the specimen size of the inventive method is a cylinder with a diameter of 5.5 to 6.0 cm and a height of 6.0 cm. The size requirement reduces the amount of rock core used in the experiment process, and for some small-diameter rock cores that cannot be tested for conventional in-situ stress, the in-situ stress indoor experiment of the present invention can be carried out by simple processing.

(2) The components used to measure strain are different. The traditional differential strain test uses resistance strain gauges. The resistance strain gauges are pasted on the three adjacent vertical surfaces of the specimen, sealed with glue, and then the differential strain test is performed. Experimental errors using resistance strain gauges are relatively large. For the cylindrical test piece, the resistance strain gauge cannot be attached to the side of the cylindrical test piece. The measuring element used in the present invention is a displacement sensor, which can conveniently measure the displacement and deformation of the test piece.

The beneficial effects of the present invention are that, for cores with relatively small diameters for drilling cores, the in-situ stress test can be carried out by the method of the present invention, which reduces the limitation of the indoor in-situ stress test on the experimental specimen; Due to the difficulty of stress experiments, it is more convenient to test the size of the ground stress indoors. This method has high experimental accuracy and is an expansion and improvement of the conventional test method of the size of the ground stress.

The differential strain test method of the cylindrical rock core of the present invention is the expansion of the conventional differential strain test, therefore, the scope of application of the differential strain test is wider than the conventional differential strain test, and the applicable rock core of the conventional differential strain test is also applicable to the present invention. Improved differential strain experiments.

Description of drawings

In order to illustrate the present invention more clearly, the accompanying drawings that need to be used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention:

Figure 1 is a schematic diagram of determining the direction of horizontal principal stress by wave velocity anisotropy;

In Figure 1: 1—experimental specimen not coated with epoxy resin; 2—ultrasonic transmitting probe, mainly used to transmit ultrasonic signals; 3—ultrasonic receiving probe; 4—ultrasonic test azimuth angle; 5—specimen rotation angle.

Figure 2 is a schematic diagram of the experimental results of wave velocity anisotropy;

In Fig. 2: 6—experimental data of wave velocity anisotropy; 7—fitting curve of wave velocity anisotropy; 8—lowest point of wave velocity, orientation of minimum horizontal principal stress on the core; 9—maximum position of wave velocity, maximum horizontal principal stress position on the core.

Figure 3 is a schematic diagram of the core holding device for differential strain experiments of cylindrical specimens.

In Figure 3: 10—vertical principal stress direction; 11—horizontal maximum principal stress direction; 12—horizontal minimum principal stress orientation; 13—cylindrical experimental specimen; 14—vertical principal stress direction displacement sensor; 15—horizontal Minimum principal stress direction displacement sensor; 16—horizontal maximum principal stress direction displacement sensor; 17—displacement sensor fixing screw; 18—rock core holding device base; 19—rock core holding device upper frame; 20—rock core holder upper frame and lower base set screws.

Figure 4 is a schematic diagram of the ground stress test.

In Figure 4: 21—pressure kettle; 22—experimental core holding device; 23—displacement and pressure transmission line; 24—pressure pipeline; 25—displacement and pressure signal converter; 26—liquid booster pump; 27—control computer

Figure 5 is a diagram of the experimental results of the present invention.

In Figure 5: 28—the curve of displacement and pressure in the direction of vertical principal stress; 29—the curve of displacement and pressure in the direction of horizontal maximum principal stress; 30—the curve of displacement and pressure in the direction of horizontal minimum principal stress.

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

In order to overcome the impact of the size of the experimental specimen on the test of the magnitude of the ground stress, the invention provides an experimental device and method for a cylindrical experimental specimen,

See Figure 4, the experimental setup includes:

Cylindrical rock core holding device 22 is used to fix the experimental rock core and install the displacement sensor. Referring to FIG. 3, it consists of a base 18 and an upper frame 19. The upper frame is fixed on the base by screws 20, and the middle of the upper frame is for clamping the experimental specimen 13 Space.

The displacement sensor is used to measure the displacement deformation during the loading process of the experimental specimen. There are three pairs of displacement sensors, including the vertical principal stress direction displacement sensor 14, the horizontal minimum principal stress direction displacement sensor 15 and the horizontal maximum principal stress direction displacement sensor 16, They are respectively installed on the upper frame 19 by screws 17 at positions corresponding to the three directions of the experimental specimen core.

The pressure kettle 21 is used to install the core holding device 22 and seal the pressure to ensure that the test piece is in a preset pressure environment;

The liquid booster pump 26 is connected with the pressure kettle 21 through the pressure pipeline 24 to provide pressure for the experiment;

Displacement and pressure signal converter 25, connect displacement sensor by signal line, the signal of displacement sensor and pressure sensor is converted into the digital signal of control computer identification;

   The control computer 27 records the displacement change value during the experiment and performs data processing.

The inventive method comprises the following steps:

Step 1: Processing of core specimens, simple core processing is carried out according to the cores taken, and the cores are processed into a cylinder with a diameter of 5.5-6.0 cm and a height of 6.0 cm. The cylinder is homogeneous and cannot contain natural cracks. The existence of natural cracks will affect the experimental results.

Step 2: Wave velocity anisotropy experiment. (1) Mark an erasable temporary marking line on the cylindrical specimen, determine that the marking line is the 0° angle of the wave velocity anisotropy test, and measure the ultrasonic velocity every 15°, as shown in schematic diagram 1 ; (2) Due to the relatively small size of the experimental specimen, there will be certain errors in the ultrasonic wave. In order to ensure the accuracy of the experiment, multiple measurements are made for each angle. Calculate the average value of the ultrasonic velocity at each angle; (3) According to the ultrasonic velocity value, perform ultrasonic velocity sine or cosine curve fitting, the fitted curve is shown in schematic diagram 2, and the angle with the maximum ultrasonic velocity is the horizontal minimum principal stress direction, such as the angle shown in 8 in schematic diagram 2, and the angle with the smallest ultrasonic velocity is the angle of the horizontal maximum principal stress direction, such as the angle corresponding to 9 in schematic diagram 2; (4) The relative orientation of the horizontal principal stress on the core is measured through experiments, according to The test azimuth calibrates the azimuth of the horizontal maximum principal stress on the core.

Step 3: Apply glue to the experimental specimen. In order to prevent the liquid from seeping into the test piece during the compression process, it is necessary to evenly brush epoxy resin around the test piece to isolate the pressurized liquid from the test piece. During the process of applying epoxy resin, the epoxy resin must be guaranteed Apply evenly to reduce the influence of epoxy resin on the experimental results.

Step 4: Modified differential strain experiment.

(1) Install the test piece on the test piece holding device 22, the displacement sensors 14, 15, 16 (one at the upper and lower ends of the cylindrical test piece, two horizontal maximum principal stress directions on the circumference of the test piece, and two horizontal minimum principal stress directions) 2) respectively installed in the directions of vertical principal stress, horizontal maximum principal stress and horizontal minimum principal stress, according to the control computer 27, adjust the initial displacement value of the displacement sensor to ensure that the displacement change of the test piece during the experiment is within the range of the sensor , use the displacement sensor fixing screw 17 to clamp the displacement sensor, after the three pairs of displacement sensors are installed, fine-tune the installation of the displacement sensor according to the initial displacement values of the three pairs of displacement sensors, so as to ensure that the initial displacement value is within a reasonable range;

(2) Put the holding device equipped with the experimental specimen into the ground stress test pressure vessel 21, connect the data transmission lines of the displacement sensors 14, 15, and 16, seal the pressure vessel, and connect the pressure pipeline and the data transmission line;

(3) Turn on the liquid booster pump 26, use the control computer 27 to control the loading and control of the pressure, carry out the pressure loading after the liquid is full of the autoclave, and use the displacement sensor and pressure sensor to record the displacement and pressure values of the three pairs of sensors in a timely manner The data is converted by the digital signal conversion device 25, and the pressure and displacement values are converted into digital signals that can be recognized by the computer, and the loading is stopped after the pressure reaches a predetermined value;

(4) Release the pressure from the autoclave, remove the experimental specimen, and prepare for the next experiment.

Step 5: Data processing.

(1) According to the measured displacement value, it is converted into the relationship curve of the displacement of the three-dimensional principal stress and the value of the pressurized pressure. The curve of the three-dimensional principal stress is shown in Figure 4;

(2) Calculate the magnitude of the vertical principal stress by using the density logging curve;

(3) Calculate the values of the horizontal maximum principal stress and the horizontal minimum principal stress according to the obtained ratio of the vertical principal stress and the triaxial principal stress.

The installation methods of the above three pairs of displacement sensors are shown in schematic diagram 3: (1) When installing the displacement sensors, first select and rotate the specimen in the core holder so that the marking line of the horizontal maximum principal stress direction on the core is aligned with the clamping (2) Install the displacement sensor in the vertical principal stress direction, and fix the displacement sensor with fixing screws 17. During the installation process, use the control computer to determine that the initial displacement value of the displacement sensor is within a reasonable range to ensure During the experiment, the measured displacement value is always within the range of the displacement sensor; (3) After installing the vertical displacement sensor, install the displacement sensor in the direction of the horizontal maximum principal stress and the direction of the horizontal minimum principal stress, the installation method and vertical displacement The installation method of the sensor is similar. The initial displacement value of the displacement sensor is always within a reasonable range to ensure that the displacement sensor is always within the range during the experiment; (4) After installing the three pairs of displacement sensors, according to the initial displacement value of the three pairs of displacement sensors Size, for some displacement sensors whose initial displacement value deviates greatly, use the fixing screw to fine-tune the position of the displacement sensor to ensure that the measurement results during the experiment are always within its range.

The experimental results of 6 ultra-deep wells carried out by the present invention are compared with the results of the lowest horizontal principal stress obtained by on-site fracturing curve back calculation, and the specific values are shown in Table 1. The minimum horizontal principal stress experimental error of the experimental method of the present invention is the largest It is 3.3%, which proves that the experimental error of the improved differential strain test method of the present invention is small, fully meets the accuracy requirement, and further confirms the accuracy of the improved differential strain test result of the present invention.

Table 1

hashtag Vertical principal stress (MPa) Horizontal maximum principal stress (MPa) Horizontal minimum principal stress (MPa) Inverse calculated horizontal minimum principal stress (MPa) error(%) 1# 164 134 105 103.45 1.4 2# 162 130 99 96.1 3.0 3# 171 126 95 95.49 0.5 4# 159 133 100 98.65 1.4 5# 157 129 97 98.6 1.6 6# 154 126 91 94.08 3.3

Claims (4)

1. a kind of experimental method of cylindrical rock core ground stress size test is characterized in that, the method may further comprise the steps: (1) Processing and processing of experimental specimens: cutting and processing for drilling cores, and processing drilling cores into cylindrical specimens with a diameter of 5.0cm-6.0cm and a height of 5.0cm-6.0cm; (2) Wave velocity anisotropy test: use the difference characteristics of ultrasonic longitudinal waves in different directions of horizontal stress to determine the initial directions of the horizontal maximum principal stress and horizontal minimum principal stress on the experimental specimen, and mark them; (3) Cylinder core brushing: evenly brush epoxy resin around the cylinder core to isolate the experimental loading liquid and the test piece, so as to prevent the liquid from penetrating into the test piece and affecting the experimental results; (4) Improved differential strain experiment: use the holding device to fix the cylindrical core, combine the horizontal principal stress orientation marked on the cylindrical core, and install 6 Two displacement sensors are installed correspondingly in each direction, and the angle of the experimental specimen is adjusted so that the two pairs of displacement sensors are respectively aligned with the directions of the horizontal maximum principal stress and the horizontal minimum principal stress of the rock core; Put it into the geostress test autoclave, connect the data transmission lines of the displacement sensor and the pressure sensor, and seal the autoclave; turn on the liquid booster pump, wait for the liquid to fill the autoclave, and carry out pressure loading, and record the displacement values and pressures of the three pairs of displacement sensors in a timely manner. pressure data from the sensor; Data processing: use the relationship curves of displacement and pressure in the three main directions to convert them into the ratio of the three-dimensional principal stress, and use the density logging curve of the research area and the coring depth of the experimental specimen to calculate the vertical principal stress of the experimental specimen , and then calculate the horizontal maximum principal stress and horizontal minimum principal stress values. 2. The method for testing the in-situ stress of a cylindrical rock core as claimed in claim 1, wherein the method for testing the wave velocity anisotropy is as follows: (1) mark an erasable temporary bar on the cylindrical specimen Marking line, confirm that the marking line is the 0° angle of the wave velocity anisotropy test, and measure the ultrasonic velocity once every 15°; (3) According to the ultrasonic velocity value, perform ultrasonic velocity sine or cosine curve fitting to obtain the fitting curve, and the angle with the maximum ultrasonic velocity is the horizontal minimum principal stress direction , the angle at which the ultrasonic velocity is the smallest is the angle of the horizontal maximum principal stress direction; (4) The relative orientation of the horizontal principal stress and the horizontal minimum principal stress on the core is measured through experiments. 3. as claimed in claim 1 or 2, the experimental method of the test of the stress size of the cylindrical rock core, is characterized in that, the cylindrical test piece can not comprise obvious natural cracks or horizontal bedding, the two end faces of the test piece The difference in parallelism cannot exceed 0.1mm. 4. realize the experimental device of the cylinder rock core ground stress size test of the described method of claim 1-3, it is characterized in that, experimental device comprises: The cylinder core holding device (22) is used to fix the experimental core and install the displacement sensor. It consists of a base (18) and an upper frame (19). The upper frame is fixed on the base by screws (20). The middle of the upper frame (19) is the space for clamping the experimental specimen (13); The displacement sensor is used to measure the displacement and deformation of the experimental specimen during the loading process. There are three pairs of displacement sensors, including the vertical principal stress direction displacement sensor (14), the horizontal minimum principal stress direction displacement sensor (15) and the horizontal maximum principal stress direction displacement sensor (15) Displacement sensors (16) are respectively installed on the upper frame (18) at positions corresponding to the three directions of the experimental specimen core; The pressure kettle (21) is used to install the core holding device and seal the pressure to ensure that the test piece is in a preset pressure environment; A liquid booster pump (26) is connected to the pressure kettle through a pipeline to provide pressure for the experiment, and a pressure sensor is installed in the pressure kettle to timely monitor the pressure change in the pressure kettle; A digital signal converter (25), connected to the displacement sensor through a signal line, converts the signals of the displacement sensor and the pressure sensor into digital signals recognized by the control computer; The control computer (27) records the displacement change value during the experiment and performs data processing.