CN105649617A - Experimental method for monitoring expansion process of hydraulic fracture through electromagnetic signals - Google Patents

Abstract

The present invention provides a kind of experimental method that detects hydraulic crack propagation process by electromagnetic signal, and this method comprises the following steps: (a) make cylindrical artificial test piece or natural test piece, the prepared test piece is placed in Nacl solution Carry out saturation treatment; (b) after the saturation treatment of the specimen, paste the electromagnetic signal monitoring probe on the surface of the specimen; (c) place the specimen on an ordinary triaxial test frame to apply confining pressure, and use clear water as the fracturing (d) Open the electromagnetic signal detection equipment while fracturing. The method provided by the present invention monitors the expansion process of hydraulic fractures under indoor laboratory conditions. A large number of ions at the tip of the fracture dissolve in the fracturing fluid to form an electric current, which generates a magnetic field, and the electromagnetic signal detector monitors and records the strength of the electromagnetic signal . The invention can accurately monitor the expansion process of hydraulic cracks, and overcomes the disadvantages that traditional acoustic emission cannot monitor hydraulic cracks caused by stretching damage and inaccurate monitoring of cracks.

Description

An Experimental Method for Monitoring the Propagation Process of Hydraulic Fractures Using Electromagnetic Signals

technical field

The invention relates to an experimental method for monitoring the expansion of hydraulic fractures through electromagnetic signals, which is applied to monitoring the electromagnetic signals generated during the expansion of hydraulic fracturing cracks, especially the real-time monitoring of the electromagnetic signal strength at the tip of the cracks, and at the same time explains the mechanism of signal generation , and the distribution of the electromagnetic field during the crack propagation process.

Background technique

In the process of hydraulic fracturing, when fractures expand and extend in the reservoir, especially in reservoirs with high ion-containing solutions, the ions in the reservoir at the tip of the fracture are continuously dissolved into the fracturing fluid to form an electric current, thereby generating electromagnetic signals until the reservoir The state of electrical equilibrium with the fracturing fluid is reached, that is, after the fracture is formed, there will be no ion exchange between the fracture wall reservoir and the fracturing fluid, and no current will be generated, so electromagnetic signals will no longer be generated. Due to the complex working conditions of on-site fracturing and the high cost of monitoring, there are no successful implementation cases. Therefore, it is necessary to simulate the process of monitoring electromagnetic signals during fracture propagation through experiments, and to explore the mechanism of electromagnetic signals during hydraulic fracture propagation. Assuming that the fracture is uniformly expanded, the specimen is an isotropic medium, and the intensity of the electromagnetic signal is a function of the fluid pressure profile in the fracture. Since the electromagnetic signal is related to the nature of the emission source, and its propagation speed is different in different media, the strength of the electromagnetic signal also changes with the distance. By monitoring and recording the electromagnetic signals in hydraulic fractures, the characteristic information of hydraulic fractures can be obtained, which will provide theoretical support for monitoring the expansion of hydraulic fractures in the field through electromagnetic signals in the future.

At present, in the field, the expansion of cracks is mainly monitored by microseismic, the shape of cracks is inverted by P-wave and S-wave, and the deformation of the ground surface is monitored by an inclinometer to obtain the geometric shape (length, height, dip) and direction of the crack. , downhole measurement tools and surface inclinometers are the only means of monitoring on-site hydraulic fracturing today, but this method cannot monitor hydraulic fractures formed by tensile failure, and the fracture morphology obtained by measurement does not match the results of microseismic measurements. At the same time, because there are also a large number of acoustic emission signals in the filtration area, the measured fracture scale is often larger than the actual fracture shape scale. However, indoor experiments cannot accurately monitor the acoustic emission signals under triaxial pressure, especially the cracks caused by tensile failure. The invention provides an experimental method for monitoring the expansion process of hydraulic fractures through electromagnetic signals, monitors the electromagnetic signals generated during the expansion process of hydraulic fractures, explores the mechanism of electromagnetic signal generation, and guides the site to monitor the expansion process of hydraulic fracturing cracks through electromagnetic signals .

Contents of the invention

The invention provides an experimental method for monitoring the expansion process of hydraulic cracks through electromagnetic signals, and monitors the electromagnetic signals generated by ion discharge at the tip of the cracks through an electromagnetic signal probe, so as to monitor the generation and expansion process of hydraulic cracks.

For this reason, the technical scheme adopted in the present invention is:

The present invention provides a kind of indoor experimental method of detecting hydraulic fracture expansion process by electromagnetic signal, the method comprises the following steps: (a) making cylindrical artificial test piece or natural test piece, prefabricating wellbore at the middle position of the axis of test piece, Place the prepared test piece in the Nacl solution for saturation treatment; (b) after the test piece is saturated, paste the electromagnetic signal monitoring probe on the surface of the test piece; (c) place the test piece in a common triaxial experiment Apply confining pressure on the rack, use clean water as the fracturing fluid, turn on the fracturing fluid injection pump to inject the fracturing fluid into the specimen at a constant rate; (d) turn on the electromagnetic signal detector while injecting the fracturing fluid, and record the electromagnetic signal Intensity, to monitor the crack propagation process.

Further, in step (a), the artificial test piece is a cement test piece, when making the cement test piece, Nacl or Kcl is added to the cement ash

Further, in step (a), for the test piece, for the artificial test piece, the wellbore is prefabricated into the cement when making the cement test piece; for the natural test piece, first drill a hole in the middle of the natural test piece, and then Fix the wellbore in the borehole with planting glue;

Further, in step (a), the artificial specimen or natural specimen is a homogeneous, isotropic specimen;

Further, in step (a), the cylindrical test piece can also be a cuboid test piece;

Further, in step (a), the Nacl solution can also be a Kcl solution;

Further, in step (a), the conductivity of the saturated test piece is measured, until the conductivity of the test piece remains constant, the test piece does not meet the experimental requirements, and no saturation treatment is performed on it;

Further, in step (b), the surface of the test piece is covered with a heat-shrinkable tube, and the electromagnetic signal probe is pasted on the outer surface of the heat-shrinkable tube;

Further, in step (b), the number of the electromagnetic signal probes is different according to different types of test pieces;

Further, in step (c), the three-axis test frame can be a true three-axis test frame;

Further, in step (c), a red dye is added to the clear water fracturing fluid;

Further, in step (c), the constant injection rate, the injection rate value is 2mL/min;

Further, in step (c), the constant rate can be different values;

Further, in step (c), the pressure of the injection fracturing fluid is monitored and recorded while the fracturing fluid injection pump is started;

Further, in step (d), the expansion process of the monitoring fracture is combined with the injection pressure curve to judge whether the hydraulic fracture meets the experimental requirements;

Compared with prior art, the beneficial effect of the present invention is:

Under laboratory conditions, the present invention assumes that the fracture is uniformly expanded in an isotropic medium, and the electromagnetic signal intensity is a function of the fluid pressure profile of the fracturing fluid inside the fracture, and explains the generation mechanism of the electromagnetic signal in the process of hydraulic fracture expansion. A method for monitoring hydraulic fracture propagation is presented. On-site microseismic monitoring of the expansion of hydraulic fractures, combined with downhole tools and surface inclinometer results for comprehensive analysis, the measured fracture shape often exceeds the actual fracture shape, most of the monitoring results do not match the actual results, and the indoor acoustics Launching is also unable to accurately monitor the shape of hydraulic fractures under triaxial pressure conditions, especially the hydraulic fractures caused by tensile damage.

The present invention overcomes the shortcomings of the failure to accurately monitor hydraulic cracks through acoustic emission in previous fracturing experiments, provides an experimental method for monitoring the expansion process of hydraulic cracks through electromagnetic signals, and explores the mechanism and mechanism of electromagnetic signals in the process of hydraulic crack expansion. The distribution of the electromagnetic field guides the site to monitor the hydraulic fracturing process through electromagnetic signals.

Description of drawings

The following drawings are only intended to illustrate and explain the present invention schematically, and do not limit the scope of the present invention. in,

Fig. 1 is a distribution diagram of the electromagnetic field amplitude at the tip of a hydraulic fracture in an embodiment of the present invention;

Fig. 2 is the schematic diagram of the cylindrical test piece of the embodiment of the present invention;

Fig. 3 is a cross-sectional schematic diagram of the arrangement of electromagnetic signal probes in the embodiment described in Fig. 2;

Fig. 4 is a distribution diagram of the measured electromagnetic field amplitude in the embodiment described in Fig. 2 .

Explanation of reference numbers:

1. Wellbore 2. Cylindrical test piece 3. Electromagnetic signal probe

detailed description

In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation manners of the present invention will now be described with reference to the accompanying drawings.

Example 1:

Fig. 1 is the distribution diagram of the electromagnetic field amplitude at the tip of a hydraulic fracture in an embodiment of the present invention; Fig. 2 is a schematic diagram of a cylindrical test piece in an embodiment of the present invention; Fig. 3 is a schematic diagram of a cross-section of an electromagnetic signal probe in the embodiment described in Fig. 2; Fig. 4 is a diagram 2. The measured electromagnetic field amplitude distribution diagram of the embodiment.

As shown in Fig. 1, the dotted line indicates the moment when the specimen breaks and produces hydraulic cracks. At this time, the amplitude of the electromagnetic signal at the tip of the hydraulic crack is the largest, and the amplitude of the electromagnetic signal at the front and rear of the tip of the hydraulic crack is smaller; as shown in Fig. 2 , the experimental specimen of this embodiment is a cylindrical specimen; as shown in Figure 3, a schematic diagram of the distribution of electromagnetic signal probes of a cylindrical specimen, 8 electromagnetic signal probes are in the same plane and distributed at equal intervals; as shown in Figure 4 , Fig. 4 is the electromagnetic signal amplitude distribution diagram monitored by the embodiment of Fig. 2, the electromagnetic signal amplitude data curves of 3 probes, which include two EH and EG probes with larger electromagnetic signal amplitudes and an electromagnetic signal amplitude Smallest EC probe.

In this embodiment, the test piece is a cylindrical cement test piece, and the number of electromagnetic signal probes is 8.

Please refer to Fig. 2, Fig. 3 and Fig. 4 to illustrate the workflow of the present invention.

Firstly, a cylindrical test piece 2 with a diameter of 200mm and a height of 500mm is made, and the shaft 1 is placed in the middle of the test piece while prefabricating the cylindrical test piece. After the cylindrical test piece 2 is placed for about 15 days, the test piece is placed in Nacl solution or Kcl solution for saturation treatment. Then put the heat-shrinkable tube on the outside of the cylindrical test piece, paste 8 electromagnetic signal probes 3 in the middle of the side of the cylindrical test piece, the gaps of the electromagnetic signal probes are distributed at equal intervals, and are on the same plane. After the electromagnetic signal probes are pasted, place The electromagnetic signal probe is connected to the electromagnetic signal acquisition device through the signal transmission line. Place the prepared cylindrical test piece in the triaxial testing machine, connect the fracturing fluid pipeline to the wellbore 1, apply confining pressure to the cylindrical test piece 2, and turn on the fracturing fluid when the confining pressure meets the experimental requirements. The injection pump switch pumps fracturing fluid into the wellbore, and at the same time, the electromagnetic signal monitoring equipment and fracturing pressure acquisition system are turned on, and the electromagnetic signal and the fracturing fluid injection pressure change curve with time are recorded. By observing the pressure curve and the electromagnetic signal amplitude curve, it is judged whether the experiment is stopped. When the pressure curve reaches the peak and remains unchanged, and the electromagnetic signal amplitude is small and does not change, stop injecting fracturing fluid and the experiment ends.

Embodiment 2: An experimental method for monitoring the process of hydraulic fracture propagation through electromagnetic signals, which is the same as Embodiment 1, except that the experimental specimen is a natural rock specimen.

Embodiment 3: An experimental method for monitoring the process of hydraulic fracture propagation through electromagnetic signals, which is the same as Embodiment 1, except that the number of electromagnetic signal probes is 6.

Embodiment 4: An experimental method for monitoring the process of hydraulic fracture propagation through electromagnetic signals, which is the same as Embodiment 1, except that the cylindrical test piece is a cuboid test piece.

The above descriptions are only illustrative specific implementations of the present invention, and are not intended to limit the scope of the present invention. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principle of the present invention shall fall within the protection scope of the present invention.

Claims (7)

1. An experimental method for monitoring hydraulic fracture propagation by means of electromagnetic signals, the method comprising the steps of:

(a) manufacturing a cylindrical artificial test piece or a natural test piece, and placing the prepared test piece in a NaCl solution for saturation treatment; (b) after the test piece is saturated, adhering an electromagnetic signal monitoring probe on the surface of the test piece; (c) placing the test piece on a common triaxial experiment frame, applying confining pressure, fracturing by using clear water as fracturing fluid, and injecting the fracturing fluid into the test piece at a constant speed by starting a fracturing fluid injection pump; (d) and (4) starting the electromagnetic signal detection equipment at the same time of fracturing.

2. The laboratory test method for monitoring hydraulic fracture propagation through electromagnetic signals according to claim 1, wherein in step (a), the test piece can be placed in a Kcl solution for saturation treatment.

3. The indoor experimental method for monitoring the hydraulic fracture propagation through the electromagnetic signal as claimed in claim 2, wherein the test piece is indicated to meet the experimental requirements by monitoring the conductivity of the test piece until the conductivity of the test piece is not changed any more.

4. The experimental method for monitoring hydraulic fracture propagation through electromagnetic signals as claimed in claim 1, wherein in the step (a), the artificial test piece is a cement test piece, and Nacl or Kcl is added into cement ash.

5. The experimental method for monitoring hydraulic fracture propagation through electromagnetic signals as claimed in claim 1, wherein in step (a), the cylindrical test piece may also be a rectangular parallelepiped test piece.

6. The experimental method for monitoring hydraulic fracture propagation through electromagnetic signals as claimed in claim 1, wherein in the step (b), the number of the electromagnetic signal probes is different according to different types of test pieces.

7. The experimental method for monitoring hydraulic fracture propagation through electromagnetic signals as claimed in claim 1, wherein in step (c), said constant injection rate, the injection rate value can be set to different values.