CN106950115B - The full-hole core hydrofracturing ultrasonic detection method of axial stress independent loads - Google Patents

Abstract

The invention relates to a full-diameter rock core hydraulic fracturing ultrasonic detection method under the condition of independent axial stress loading. The axial loading system is designed as two semi-cylindrical anvils, which can provide two sets of independent axial load. The axial loading system is provided with pressure by a hydraulic pump, which transmits hydraulic pressure to the two semicircular anvils in the holder through controllable valves. The semicircular anvils are movable parts that can generate displacement in the axial direction. Two sets of axial stresses can be induced in the core within the holder. Ultrasonic generating/receiving probes are installed on the end face of the anvil. After the core is deformed by force, fracturing fluid is injected into the core hole to generate cracks of a certain shape. The approximate extension direction and extension range of the cracks can be detected by ultrasonic technology. The invention is an evaluation method suitable for studying hydraulic cracks and natural crack expansion law under complicated ground stress state. The method has scientific and reasonable design, accurate evaluation method and high creativity.

Description

Ultrasonic testing method for hydraulic fracturing of full-diameter rock cores independently loaded by axial stress

technical field

The invention belongs to the field of petroleum and natural gas engineering, and relates to indoor experimental evaluation of oil and gas field exploitation technology, in particular to an ultrasonic detection method for hydraulic fracturing of a full-diameter rock core independently loaded by axial stress.

Background technique

The principle of conventional hydraulic fracturing simulation experiments is to inject fracturing fluid into the core through the fracturing fluid injection pipeline under confining pressure and axial pressure, and at the same time, external sensors monitor the load stress and pump pressure, and record the pump pressure under different triaxial stress test conditions. The curve of pressure and time is used to study the relationship between triaxial stress and hydraulic fracturing pressure.

Conventional methods can only provide a set of axial compression loading on the end face, but the actual geological conditions are disturbed by natural fractures, the in-situ stress is not a certain value, and the actual stress state of the rock in the formation may be complex situations such as twisting and shearing, so Traditional technical methods can only simulate the single stress state of the rock, and cannot simulate the rock distortion or shear stress state.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, to provide a full-diameter rock core hydraulic fracturing ultrasonic detection system and its evaluation method independently loaded by axial stress, so as to measure the water pressure of the full-diameter rock core under the triaxial stress state. Acoustic parameters of hydraulic fractures during fracturing.

The technical scheme that the present invention solves technical problem adopts is:

An ultrasonic testing method for hydraulic fracturing of full-diameter rock cores independently loaded by axial stress. More than two sets of different loads are applied on the same core end face. The value and ratio of each set of loads are adjusted independently, and each set of loads is connected to an ultrasonic probe. , using an ultrasonic probe to detect the extension of hydraulic fractures within the full diameter of the core.

The detection system for realizing the detection method includes a core holder, an ultrasonic detection system, a hydraulic loading system and a fracturing fluid injection system. The hydraulic loading system includes two sets of axial pressure loading devices, a set of confining pressure loading devices and The hydraulic pump is connected to two sets of axial pressure loading devices and one set of confining pressure loading devices respectively. The axial pressure loading devices are a pair of semicircular anvils, which are respectively symmetrically installed on the cylindrical core in the core holder. The top surface and the bottom surface of the half; two sets of axial pressure loading devices can just cover the entire end surface of the rock core, and a groove is formed on the contact surface of the anvil and the rock core, and an ultrasonic probe is embedded in the groove.

An ultrasonic testing method for hydraulic fracturing of a full-diameter rock core independently loaded by axial stress, characterized in that the specific steps are as follows:

(1) Drill a small hole on the end face of the rock core, insert the fracturing fluid injection pipe and fix it, apply coupling agent on the ultrasonic probe on the end face of the bottom anvil, put it into the core holder, and then put it into the rock core;

(2) Apply an appropriate amount of coupling agent to the ultrasonic probe on the end surface of the top anvil, and put the top anvil into the core holder to ensure that the fracturing fluid injection pipeline fixed in the core can pass through the center hole of the top anvil;

(3) Connect hydraulic and pneumatic pipelines with ultrasonic transmitting and receiving cables and fracturing fluid injection pipelines;

(4) Operate the controllable valve, load a small amount of axial pressure on both ends of the core, and then open the air pressure pipeline. Axial compression to the set load, let the core rest for 1 hour, until the core is fully deformed;

(5) Turn on the ultrasonic transmitting/receiving system, adjust the oscilloscope until the characteristic waveform can be clearly displayed, and then start the advection pump to pump fracturing fluid into the core at a constant flow rate. When the core is pressed open, the excess fracturing fluid It will flow out from the small hole in the center of the bottom anvil;

(6) Record the curve of triaxial stress changing with time, directly read the fracture pressure of the core under the triaxial stress condition, and at the same time, according to the records of transverse and longitudinal wave velocities and waveform amplitudes in the oscilloscope at different time points, describe the The process of hydraulic fractures from forming to completely penetrating the core.

Advantage and positive effect of the present invention are:

1. Compared with the conventional full-diameter rock core hydraulic fracturing experiment, the present invention does not directly apply a single axial load to the end face of the rock core, but uses two semicircular anvils with independent hydraulic supply channels to achieve Two sets of different loads are applied on the end face of the same rock core, and the values and ratios of the two sets of loads can be adjusted arbitrarily, which fills the blank that the prior art cannot realize independent loading of the rock core under axial pressure.

2. The present invention installs ultrasonic excitation\reception probes at the end of each semicircular anvil, and a total of four ultrasonic excitation\reception dual-purpose probes ensure that the sound beams cover the entire diameter of the core while ensuring that the sound beams are opposite to each other. Concentrated to meet the needs of horizontal resolution.

3. This experimental system and evaluation method introduces the core physical experiment method, and expands the traditional triaxial stress hydraulic fracturing simulation experiment from the two aspects of stress loading mode and crack detection method. It can not only realize the average loading of the core end face, but also Two sets of independent axial loads are provided to simulate the state of the rock under the condition of shear force or torsional force.

4. The experimental system and evaluation method can realize the simulation of the stress transfer mode in the formation with relatively developed fractures through the independent axial compression loading method.

5. The present invention is a full-diameter rock core hydraulic fracturing ultrasonic detection system and evaluation method independently loaded by axial stress. The design of the system is scientific and reasonable, the evaluation method is accurate, and it has high creativity.

Description of drawings

Figure 1 is a schematic diagram of the system connection;

Figure 2 is a schematic diagram of the control area of the four probes after loading;

Fig. 3 is the sound amplitude data detected by the coaxial pressure loading probe 1;

Fig. 4 is the sound amplitude data that coaxial pressure loading probe 2 detects;

Fig. 5 is the sound amplitude data that coaxial pressure loading probe 3 detects;

Fig. 6 is the sound amplitude data that coaxial pressure loading probe 4 detects;

Fig. 7 is the sound amplitude data detected by the independent axial pressure loading probe 1;

Fig. 8 is the sound amplitude data detected by the independent axial pressure loading probe 2;

Fig. 9 is the sound amplitude data detected by the independent axial pressure loading probe 3;

FIG. 10 is the sound amplitude data detected by the independent axial pressure loading probe 4 .

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and through specific embodiments. The following embodiments are only descriptive, not restrictive, and cannot limit the protection scope of the present invention.

A full-diameter rock core hydraulic fracturing ultrasonic detection system independently loaded by axial stress, including a core holder 3, an ultrasonic detection system, a hydraulic loading system, and a fracturing fluid injection system.

The ultrasonic testing system includes a computer 1, an oscilloscope 2, an ultrasonic signal cable 11 and an ultrasonic probe 10, the computer is connected to the oscilloscope, and the oscilloscope is connected to four ultrasonic probes through ultrasonic cables.

The fracturing fluid injection system includes an advection pump 5 and a fracturing fluid injection pipe 12, the advection pump is connected to the fracturing fluid injection pipe, and the fracturing fluid injection pipe is inserted into the core from the center of the top surface of the rock core.

The hydraulic loading system includes two sets of axial pressure loading devices, a set of confining pressure loading devices and a hydraulic pump 4, the hydraulic pumps are respectively connected to two sets of axial pressure loading devices, a set of confining pressure loading devices, and the confining pressure loading The device is to set the rubber cylinder 6 on the outer cylindrical surface of the core holder, and the rubber cylinder is connected to the hydraulic pump through the hydraulic valve 8 and the hydraulic pipeline 7. The axial pressure loading device is a pair of semicircular anvils 9, which are symmetrical It is installed on the top surface and the bottom surface of the cylindrical core half in the core holder; two sets of axial pressure loading devices can just cover the end surface of the entire core. The anvil is connected to the hydraulic pump through a hydraulic valve and a hydraulic pipeline. A groove is formed on the surface of each anvil in contact with the rock core, and an ultrasonic probe is embedded in the groove.

This system simulates the triaxial stress condition, the advection pump 5 delivers the fracturing fluid to the core through the fracturing fluid injection pipeline and generates cracks, the ultrasonic probe 10 detects the cracks, transmits them to the oscilloscope 2 through the signal cable 11, and finally transmits them to the connected The computer 1 also records the data, and the liquid for fracturing the rock core is finally discharged through the venting pipeline 13 .

The innovation point of the present invention is:

1. Two sets of axial pressure loading devices can generate two sets of different axial pressures on the same core end face, and the values and ratios of the two sets of axial pressures can be adjusted arbitrarily. When the axial pressure is applied, by operating the hydraulic valve, the upper and lower anvils 9 on the same side will produce a small amount of relative displacement under the action of hydraulic pressure, and a certain load will be applied to the core, and then the hydraulic pump can be connected with the other by operating the hydraulic valve. The upper and lower anvils on one side generate another set of axial loads on the end face of the other side of the core.

2. Grooves are processed on the end surface of the anvil 9, and an ultrasonic probe 10 is embedded. The ultrasonic probe is a dual-purpose probe for excitation and reception, and can excite and receive transverse waves and longitudinal waves. Complementary software implementation. Because two ultrasonic probes 10 are used on the same end face, it not only ensures the lateral coverage of the ultrasonic beam on the core diameter direction, but also ensures that the single beam is relatively concentrated to improve the lateral resolution, which is important for the monitoring of hydraulic fracture formation. significance. The ultrasonic probe cannot be in excitation and reception modes at the same time, nor can it emit transverse waves and longitudinal waves at the same time.

The frocks used in the present invention are conventional devices in the field, so their structures are not described in detail.

A method for testing a full-diameter rock core hydraulic fracturing ultrasonic detection system independently loaded by axial stress, the specific steps of which are as follows:

1. Process the experimental core according to the size specification of 10cm in diameter and 10-15cm in length, drill a liquid injection hole with a diameter of 0.7cm on the core section, and then install the liquid injection pipe and fix it.

2. Apply an appropriate water-based couplant to the ultrasonic probe 4 on the end face of the anvil 9, install the bottom anvil on the bottom of the core holder 3, and then load the core.

3. Install the top anvil and connect the hydraulic pipeline and ultrasonic signal cable.

4. Use a hydraulic pump to transmit hydraulic pressure to the anvil, apply a small amount of axial load to the core to further couple the end face of the anvil to the end face of the core, then load the design confining pressure and axial pressure, and wait for the core to fully deform after stabilizing for 1 hour.

5. Turn on the ultrasonic excitation\receiving probe 10, control and switch the transverse and longitudinal wave transmission and reception of the two sets of probes through the computer 1, observe the waveform changes on the oscilloscope, adjust the waveform phase and amplitude display, and read the transverse and longitudinal wave velocity and amplitude of the two sets of probes , and record the data.

Figures 3 to 10 reflect the relationship between the ultrasonic amplitude and stress within the sweeping range of each probe. When the stress is enhanced, the ultrasonic velocity and amplitude both increase with the increase of stress, but if the core strength is insufficient or there are natural internal defects, rupture will occur under high stress conditions, and the sound velocity and amplitude of the sound wave will both decrease significantly.

When coaxial loading, as shown in Figure 3-6, the stress loaded on the four probe faces is the same, among which probe 1 and probe 2 reflect that the ultrasonic amplitude increases first and then decreases with the increase of stress, which means that when the stress is loaded to about 20MPa At this time, cracks appeared inside the core, which hindered the propagation of sound waves; on the side of probes 3 and 4, the amplitude of the sound wave increased steadily with the increase of stress, so the damage was only concentrated on the side of probes 1 and 2.

When loading independently, the stresses on both sides are different. Probes 1 and 2 and probes 3 and 4 respectively reflect the integrity of the core under different stress conditions. It can be seen from Figure 7-10 that the stress on the side of the probes 1 and 2 is relatively low, and the load will not increase until it reaches 15MPa. On the side of probes 3 and 4, when the stress is loaded to about 20 MPa, the amplitude of the acoustic wave decreases significantly, indicating that on the side of high stress, damage occurs inside the core.

Claims (2)

1. a kind of full-hole core hydrofracturing ultrasonic detection method of axial stress independent loads, it is characterised in that: same Apply two sets of different load on one rock core end face, the numerical value and ratio of every set load individually adjusts, every set load with ultrasound Wave probe connection detects the extension situation of hydraulic fracture in the full diameter range of rock core by ultrasonic probe, and then more accurate Ground judgement destroys the region concentrated, and realizes that the detection system of the detection method includes core holding unit, ultrasonic wave detecting system, liquid Pressure load G system and pressure break liquid injection system, the hydraulic loaded G system includes that two sets of axis press loading device, a set of Confining pressure loading device and hydraulic pump, hydraulic pump are separately connected two sets of axis pressure loading devices, a set of confining pressure loading device, the axis Pressure loading device is a pair of semicircular anvil, is respectively symmetrically mounted on the top surface and bottom of core holding unit interior cylindrical rock core half portion Face；Two sets of axis pressure loading devices be able to cover the end face of entire rock core, fluted in the wheat flour that anvil is contacted with rock core, in groove Interior embedded ultrasonic probe.

2. the full-hole core hydrofracturing ultrasonic detection method of axial stress independent loads according to claim 1, It is characterized by: specific step is as follows:

(1) aperture is drilled through in rock core end face, is inserted into fracturing fluid injection pipe and fixation, the ultrasonic probe on the anvil end face of bottom It is packed into core holding unit after place's daubing coupling agent, is reloaded into rock core；

(2) top anvil is packed into core holding unit by the appropriate daubing coupling agent at the ultrasonic probe on the anvil end face of top, Top anvil central aperture can be passed through by ensuring to be fixed on the fracturing fluid injection pipeline in rock core；

(3) hydraulic, air pressure pipeline and ultrasonic wave transmitting are connected, receives cable and fracturing fluid injection pipeline；

(4) controllable valve is operated, loads a small amount of axis pressure at rock core both ends, then open air pressure pipeline, sonic probe is in gas pressure Under can with the further good coupling in rock core end face, then load confining pressure to pressure is set, last operation valve, loading axis, which is depressed into, to be set Determine load, rock core 1 hour is stood, to rock core fully deformed；

(5) open ultrasonic wave transmitting receive system, adjustment oscillograph is until apparent signature waveform can be shown, then starts flat Stream pump, is pumped into fracturing fluid into rock core with constant flow rate, after rock core is pressed off, extra fracturing fluid can be by bottom anvil center The aperture at position flows out；

(6) curve that record triaxial stress changes over time, directly fracture pressure of the reading rock core under the condition of triaxial stress, Simultaneously according to record horizontal, longitudinal wave velocity and amplitude of wave form in different time points oscillograph, describes waterpower in different time sections and split Seam is from forming up to completely through the process of rock core.