A phase state detection method for liquid co2 phase change fracturing process

Abstract

The present application relates to the field of CO2 fracturing, in particular to a phase state determination method for liquid CO2 phase change fracturing process, first, a static temperature and pressure sensor is used to measure the pressure and temperature curve of CO2 in the CO2 fracturing device during the filling process, to determine the phase state of CO2 before the heater is triggered; then a dynamic sensor is used to measure the pressure time history curve of CO2 after the heater is triggered and CO2 breaks through the shear piece and is released into the air; a water bath heating method is used to determine that the temperature and pressure of CO2 in the supercritical state have a linear relationship; according to the linear relationship, the existing CO2 temperature and density data table under different pressure conditions of the Span & Wagner equation is fitted to obtain the temperature and pressure linear relationship formula of different types of CO2 fracturing devices corresponding to the density, to obtain the temperature time history curve corresponding to the dynamic pressure time history curve, to obtain the phase state of CO2 after the heater is triggered, and finally to obtain the complete CO2 phase state change path in the CO2 fracturing process, the present application has the advantages of accurate calculation of CO2 phase state, wide application, etc.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of CO2 fracturing, in particular to a phase state detection method for a liquid CO2 phase change fracturing process. BACKGROUND

[0002] In the past 10 years, the application of liquid CO2 phase change fracturing gas control technology has developed rapidly. Domestic scholars have conducted a large number of field application explorations in high-gas and outburst coal mines in different mining areas in the country, and have achieved good application results. However, the phase state of the liquid CO2 fracturing process has not been thoroughly studied. The phase state change rule of the fracturing process is the theoretical basis for studying the mechanism of liquid CO2 phase change fracturing coal and rock and outburst prevention, and has great guiding significance for the optimization and field implementation of the technology.

[0003] The CO2 phase change fracturing process is divided into two stages before and after the rupture of the shear piece. The first stage: liquid CO2 absorbs heat and changes into a supercritical state, and the pressure in the liquid storage pipe rapidly rises. The second stage: when the pressure inside the liquid storage pipe reaches the rupture pressure of the shear piece, the CO2 inside the liquid storage pipe is rapidly released. The phase change process is completed within tens of milliseconds from start to finish. The temperature and pressure of this process have the characteristics of high frequency and fast decay. Because CO2 exists in different phase states under different pressure and temperature conditions, it is difficult to accurately obtain the temperature and pressure through existing testing techniques, and then determine the phase state of the fracturing process. On the one hand, the existing temperature sensor has a low sampling frequency and cannot capture high-frequency temperature change data. On the other hand, a high pressure sampling frequency can accurately obtain data, resulting in the defect that the temperature and pressure sampling data test results are out of sync, and thus the cognitive of the CO2 phase change fracturing process phase change mechanism is not unified among domestic and foreign researchers. SUMMARY

[0004] The purpose of the present application is to overcome the shortcomings of the prior art and provide a method for determining the phase state of a liquid CO2 phase change fracturing process. The present application is achieved by the following technical solutions, and the steps are as follows:

[0005] (1) A static temperature and pressure sensor is used to measure the pressure, temperature and time P-T-t curve of CO2 inside the CO2 fracturing device during the filling process, and to determine the phase state of CO2 before the heater is activated;

[0006] (2) A dynamic sensor is used to measure the pressure time history P0-t curve of CO2 after the heater is activated and CO2 breaks through the shear piece and is released into the air;

[0007] (3) The CO2 fracturing device is placed in a water bath heating device with adjustable water temperature, and a static temperature and pressure sensor is used to measure the pressure, temperature and time P1-T1-t curve of CO2 inside the CO2 fracturing device at different water bath temperatures;

[0008] (4) fitting the P1-T1-t curve of the water bath heating test in step (3) to obtain the P1-T1-t curve of the temperature and pressure of CO2 in the CO2 fracturing device under supercritical state;

[0009] (5) according to the P1-T1-t curve obtained in step (4), the temperature and pressure curve corresponding to the density of different types of CO2 fracturing devices is fitted by using the Span & Wagner equation and the existing CO2 temperature and density data table under different pressure conditions;

[0010] (6) according to the P1-T1-t curve obtained in step (4), the temperature T0-t curve corresponding to the P0-t curve of the dynamic test in step (2) is calculated, and the P0-T0-t curve of the dynamic calculation fitting is obtained;

[0011] (7) according to the static test P-T-t curve and the dynamic calculation fitting P0-T0-t curve, the phase change curve of the liquid CO2 fracturing process is completed on the standard temperature-pressure phase diagram.

[0012] Further, in step three, (a) setting the sensor, setting the DDI temperature and pressure sensor sampling frequency to 1 Hz, and the sampling time to 48 h;

[0013] (b) installing the sensor, placing the sensor in the liquid storage pipe;

[0014] (c) assembly, assembling the CO2 fracturing device;

[0015] (d) filling liquid CO2;

[0016] (e) standing, standing the CO2 fracturing device for 3-5 h;

[0017] (f) water bath heating, placing the CO2 fracturing device into the water bath heating device, connecting the power supply, starting heating, the lower limit temperature of the heating device is 342K, and the upper limit temperature is 343K, and the water bath heating is kept for 48 h;

[0018] (g) stop water bath heating, after 48 h, cut off the power supply, stop heating, and after the water temperature decreases to the indoor temperature, take out the CO2 fracturing device from the water bath heating device;

[0019] (h) opening the valve, discharging the liquid CO2 in the liquid storage pipe;

[0020] (i) disassembling the fracturing device: taking out the sensor;

[0021] (j) Select the test data at 304K above the data points for analysis, the formula is: P=0.60789T+166.0659, the correlation coefficient R2=0.997.

[0022] Further, the static P-T-t curve is measured by a wireless storage sensor placed in the CO2 fracturing device.

[0023] Further, the dynamic P0-t curve is measured by a wireless high-frequency pressure resistance type sensor placed in the CO2 fracturing device.

[0024] Further, the temperature range of the water bath heating device is 293-373K, and the temperature control error is less than 1K.

[0025] By adopting the above scheme, the application has the following beneficial effects:

[0026] (1) The wireless storage sensor is used to directly measure the temperature and pressure inside the CO2 fracturing device when the heater is not activated, and the P-T-t curve is obtained, which also avoids the lead and storage problems of the conventional sensor, and reduces the test error and workload.

[0027] (2) The wireless high-frequency pressure resistance type sensor is placed inside the CO2 fracturing device, and the high-frequency P0-t curve of the entire CO2 fracturing process after the heater is activated is directly collected without damaging the integrity of the CO2 fracturing device.

[0028] (3) The water bath heating method can determine that the liquid CO2 inside the CO2 fracturing device enters the supercritical state, and the pressure and temperature have a linear relationship.

[0029] (4) Using the Span&Wagner equation and the verification result of the water bath heating, the T0-t corresponding to the P0-t can be directly data-fitted, which makes up for the insufficient high-frequency sampling of the existing temperature sensor for the CO2 fracturing process, and further obtains the P-T-t and P0-T0-t curves of the CO2 fracturing device during the charging process and the fracturing process, and completes the phase state change path of the liquid CO2 fracturing process on the standard temperature-pressure phase diagram.

[0030] (5) Using this method, the P-T-t and P0-T0-t curves of different types of CO2 fracturing devices can be obtained, and the phase state change rule of the CO2 fracturing process can be determined, which provides reliable parameters for revealing the CO2 fracturing coal and rock mechanism and provides reliable theoretical support for engineering application. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The phase state detection method flow chart for the CO2 fracturing process;

[0032] Figure 2CO2 temperature pressure device schematic for measuring CO2 temperature and pressure in the CO2 fracturing vessel during the filling process and after a period of time at rest;

[0033] Figure 3 CO2 pressure time curve device schematic for measuring the pressure time curve after the start of the heater to the rupture of the shear pin;

[0034] Figure 4 CO2 temperature pressure time curve for the filling process and after a period of time at rest;

[0035] Figure 5 CO2 pressure time curve measured for Example 1 after the start of the heater to the rupture of the shear pin;

[0036] Figure 6 Water bath heating device schematic;

[0037] Figure 7 Measured water bath heating temperature, pressure, time P1-T1-t curve;

[0038] Figure 8 Fitted water bath heating temperature pressure P0-T0-t curve;

[0039] Figure 9 Density and pressure curve for supercritical CO2 at different pressures fitted from the Span & Wagner data table;

[0040] Figure 10 CO2 pressure time curve measured for Example 2 after the start of the heater to the rupture of the shear pin.

[0041] Wherein: 1 - inflation head, 2 - heater, 3 - liquid storage tube, 4 - DDI sensor, 5 - liquid CO2, 6 - shear pin, 7 - release head, 8 - detonator, 9 - wire, 10 - Omega sensor, 11 - welded flange, 12 - fixed flange, 13 - bolt, 14 - threading hole, 15 - heating rod, 16 - reaction cylinder. DETAILED DESCRIPTION

[0042] In the description of the present application, it also needs to be explained that, unless otherwise explicitly specified and limited, the terms "arrange", "mount", "connect", "connect" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0043] Clearly, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts fall within the protection scope of the present application.

[0044] As shown in FIG. 1, a phase state determination method of a liquid CO2 phase change fracturing process comprises the following steps: Figure 1

[0045] (1) A static temperature and pressure sensor is used to measure the pressure, temperature and time P-T-t curve of CO2 in the CO2 fracturing device during the filling process, so as to determine the phase state of CO2 before the heater is excited;

[0046] (2) A dynamic sensor is used to measure the pressure time history P0-t curve of CO2 after the heater is excited and the CO2 breaks through the shear piece and is released into the air;

[0047] (3) The CO2 fracturing device is placed in a water bath heating device with adjustable water temperature, and a static temperature and pressure sensor is used to measure the pressure, temperature and time P1-T1-t curve of CO2 in the CO2 fracturing device at different water bath temperatures;

[0048] (4) The P1-T1-t curve of the water bath heating test in step (3) is fitted to obtain the P1-T1-t curve of the temperature and pressure of CO2 in the CO2 fracturing device in a supercritical state;

[0049] (5) According to the P1-T1-t curve obtained in step (4), the temperature and pressure curve corresponding to the density of different types of CO2 fracturing devices is fitted by using the existing CO2 temperature and density data table under different pressure conditions in the Span & Wagner equation;

[0050] (6) According to the P1-T1-t curve obtained in step (4), the temperature T0-t curve corresponding to the P0-t curve of the dynamic test in step (2) is calculated to obtain the dynamically calculated and fitted P0-T0-t curve;

[0051] (7) According to the static test P-T-t curve and the dynamically calculated and fitted P0-T0-t curve, the phase state change curve of the liquid CO2 fracturing process is completed on a standard temperature-pressure phase state diagram.

[0052] Embodiment 1

[0053] As shown in FIG. 1, a phase state determination method of a liquid CO2 phase change fracturing process comprises the following steps: Figure 2 ​As shown, in order to determine the CO2 temperature and pressure change curve inside the CO2 fracturing device before the initiator is started, first place the DDI-T storage pressure sensor 4 in the liquid storage tube 3, DDI-T is a self-storage, piezoresistive sensor, pressure range is 0~10Kpsi, accuracy is ± 0.05% F, error is < 3psi per year, temperature range is 273~423K, accuracy is ± 1K, maximum sampling frequency is 1Hz, then assemble the CO2 fracturing device, and fill liquid CO25, after reaching the rated filling amount, stand for a period of time, and the P-T-t curve is obtained Figure 4 , the pressure in the liquid storage tube after standing is 6.89~8.23MPa, and the temperature is 284~290.2K, according to the physical properties of CO2, it is determined that the phase state of CO2 in the CO2 fracturing device under this state is liquid.

[0054] Next, start measuring the CO2 pressure change curve inside the CO2 fracturing device after the initiator is started. The CO2 fracturing tube type used in this embodiment is C74, the length is 1.8m, the CO25 filling amount is 2.2Kg, and the rated rupture pressure of the shear piece is 120MPa. Since the rupture process of the shear piece occurs in a few milliseconds, the sampling frequency of the temperature sensor is in seconds, so a high-frequency piezoresistive sensor is used for pressure testing. In this embodiment, an Omega temperature and pressure sensor 10 is used for pressure testing. It is a pulse high-frequency storage pressure sensor developed by Omega Well Logging Company, USA. The pressure testing range is 0~30 Kpsi, the accuracy is ± 0.05% F, and the error is < 3psi per year. The temperature testing range is 273~423K, the accuracy is ± 1K, and the maximum sampling frequency is 115200Hz. First, set the sampling frequency of the Omega sensor to 57.6KHz, the sampling time is 4.5s, the trigger pressure is 4349psi, and the sampling frequency is executed every 60s. When the pressure reaches the trigger pressure, high-frequency sampling is executed, otherwise low-frequency sampling is executed. First, place the Omega sensor 10 in the liquid storage tube 3, then assemble and fix the CO2 fracturing device, fill CO2, then initiate the initiator 8 through the lead wire 9, collect data, and the test schematic diagram is shown in Figure 3 ; After measurement, the P0-t curve is obtained Figure 5 , it should be noted that the pressure unit displayed by the software is psi, and after conversion, the peak pressure of the shear piece with a rupture pressure of 120MPa is 106.7MPa.

[0055] The water bath heating test steps are as follows:

[0056] a. Set the sampling frequency of the DDI temperature and pressure sensor 4 to 1HZ, and the sampling time is 48h;

[0057] b. Place the DDI temperature and pressure sensor 4 in the liquid storage tube 3;

[0058] c. Assemble the CO2 fracturing device and fill it with liquid CO25;

[0059] d. Allow the CO2 fracturing device to stand for 3-5 hours;

[0060] e. Water bath heating: Place the CO2 fracturing device into the water bath heating reaction cylinder 16, and start the heating rod 15 through the wire 9 to start heating. The temperature range of the reaction cylinder 16 is 293~373K, and the stable control error is 1K. Control the temperature of the reaction cylinder 16 to 343K so that the CO2 inside the CO2 fracturing device is in a supercritical state. Maintain water bath heating for 48 hours.

[0061] f. Stop water bath heating: After 48 hours, cut off the power supply and stop heating the heating rod 15. After the water temperature in the reaction cylinder 16 drops to the room temperature, remove the CO2 cracker from the reaction cylinder (16).

[0062] g. Remove the DDI temperature and pressure sensor (4) and process the data.

[0063] The water bath heating experimental setup is shown below. Figure 6 The reaction vessel 16 contains a heating rod 15, and both ends of the reaction vessel 16 are equipped with welded flanges 11. The welded flanges 11 are connected to the fixed flanges 12 by bolts 13. A sealing gasket is provided between the flanges to ensure that the water inside the reaction vessel does not leak. The fixed flange is also equipped with a wire hole 14. The temperature and pressure time history curves obtained from the water bath heating are shown below. Figure 6 ,from Figure 7 It can be seen that the temperature-pressure time history curves of CO2 after reaching supercriticality exhibit sinusoidal fluctuations, with a positive correlation between temperature and pressure. The entire experimental process lasted 48 hours. Numerical fitting revealed a linear relationship between temperature and pressure for CO2 in the supercritical state. Figure 8 ,according to Figure 8 The obtained linear relationship was linearly fitted to the data table provided by Span & Wagner, such as... Figure 9 As shown, R 2 All values ​​were above 0.99, indicating a strong correlation. Further analysis of density, pressure, and temperature parameters was performed using the fitting formula to obtain temperature-pressure data fitting formulas for different densities. Based on the dimensions of the C74 fracturing tube and the CO2 filling amount, the density of CO2 in the fracturing tube was calculated to be 890 kg / m³. 3 ~910Kg / m 3 Between these values, the temperature-pressure fitting formula for CO2 at a density of 890 kg / m³ is:

[0064] P = 0.61207 × T - 158.5586

[0065] The correlation coefficient is 0.99913.

[0066] The temperature-pressure fitting formula of CO2 at 910 Kg / m3 density is:

[0067] P = 0.58054 x T - 151.8186

[0068] The correlation coefficient is 0.99904,

[0069] Wherein: P is pressure (MPa); T is temperature (K).

[0070] According to the fitting formula, the peak pressure 106.7 MPa corresponds to the peak temperature of 433.38 K ~ 445.31 K, and according to the physical properties of CO2, it can be determined that the CO2 in the liquid storage pipe under this state is in a supercritical state.

[0071] Combined with the static test P-T-t curve and the dynamic calculation P0-T0-t curve, the phase state change path diagram of the CO2 fracturing device during the fracturing process is completed.

[0072] Example 2

[0073] The same as example 1, the difference is that when measuring the state of CO2 after starting the detonator, a shear piece with a breaking pressure of 185 MPa is used, and after measuring according to the method described in example 1, the pressure curve is Figure 10 According to the calculation method of the application, the fracturing peak pressure of the CO2 fracturing device with a breaking pressure of 185 MPa is 211 MPa, and the corresponding peak temperature is 603.78 K ~ 624.97 K, and at this time the CO2 in the liquid storage pipe is supercritical.

[0074] The complete curve of liquid CO2 fracturing process is composed of two parts, namely before excitation and excitation process. The supercritical is released instantaneously during the excitation process, and the gas is released to the free domain subsequently, which consists of two stages.

[0075] The phase state change curve before excitation is obtained in the following way, Figure 4 The temperature is 284~290.2 K, the pressure is 6.89~8.23 MPa, the sampling frequency is 1 Hz, and the phase state change curve before excitation is obtained by fitting the curve after sampling,

[0076] The change of parameters is according to the change of real temperature and pressure of CO2 during the filling process; Figure 4 The rising part in front of the curve and the falling part behind the curve are the temperature and pressure change values of CO2 during the filling and release process, Figure 4 The middle part of the curve is the temperature and pressure value of CO2 in the fracturing device at room temperature, and the state of CO2 before fracturing work is Figure 4 The middle part of the curve.

[0077] The supercritical phase state change curve is obtained in the following way, for example Figure 7 The temperature is controlled at 343K, the water bath is heated for 48 hours, the detection frequency is 1HZ, the detection temperature-pressure point value is obtained, the temperature value is inquired from the Span & Wagner table, the temperature-density point value is obtained, the temperature-pressure point value under different densities (CO2 fracturing device specifications) is obtained by combination, and the supercritical phase state change curve is fitted.

[0078] The excitation process phase state change curve is obtained in the following way, the pressure test is performed by using the Omega temperature-pressure sensor 10, the trigger pressure is 4349psi, the sampling frequency is 57.6KHz, the sampling time length is 4.5s, and the excitation process phase state change curve is fitted.

[0079] The connection between the pre-excitation curve and the supercritical curve is that, according to the static test P-T-t curve and the dynamic calculation and fitting P0-T0-t curve, the liquid CO2 fracturing process phase state change curve is completed on the standard temperature-pressure phase state diagram.

[0080] The connection between the supercritical curve and the excitation process curve is that, according to the linear relationship between the temperature and the pressure obtained in step (5), the temperature T0-t curve corresponding to the P0-t curve of the dynamic test in step two is calculated, and the dynamic calculation and fitting P0-T0-t curve is obtained.

[0081] Finally, it should be noted that: the above only for the preferred embodiments of the present application, and not for limiting the present application, although the present application is described in detail with reference to the foregoing embodiments, for the person skilled in the art, it still can modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part of the technical features, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application, should be included in the protection scope of the present application.

Claims

1. A method for detecting the phase of a liquid CO2 phase transition fracturing process, characterized in that, The method comprises the following steps: (1) using a static temperature and pressure sensor to measure the pressure, temperature and time P-T-t curve of CO2 in the CO2 fracturing device during the filling process, and determining the phase state of CO2 before the heater is triggered; (2) using a dynamic sensor to measure the pressure time history P0-t curve of CO2 after the heater is triggered and CO2 breaks through the shear piece and is released into air; (3) placing the CO2 fracturing device in a water bath heating device with adjustable water temperature, and using a static temperature and pressure sensor to measure the pressure, temperature and time P1-T1-t curve of CO2 in the CO2 fracturing device at different water bath temperatures; (4) fitting the P1-T1-t curve of the water bath heating test in step (3) to obtain the P1-T1-t curve of the temperature and pressure of CO2 in the CO2 fracturing device in a supercritical state; (5) according to the P1-T1-t curve obtained in step (4), using the CO2 temperature and density data table under different pressure conditions in the Span & Wagner equation, and interpolating and fitting the temperature and pressure curve corresponding to the density of different types of CO2 fracturing devices; (6) according to the curve obtained in step (5), calculating the temperature T0-t curve corresponding to the P0-t curve of the dynamic test in step (2) to obtain the dynamically calculated and fitted P0-T0-t curve; (7) according to the static test P-T-t curve and the dynamically calculated and fitted P0-T0-t curve, completing the liquid CO2 fracturing process phase change curve on a standard temperature-pressure phase state diagram, The liquid CO2 fracturing process phase change curve is composed of two parts, before and during triggering. The phase change curve before triggering is obtained in the following way: the temperature is 284-290.2K, the pressure is 6.89-8.23MPa, the sampling frequency is 1Hz, the curve is fitted after sampling, the phase change curve before triggering is obtained, and the change of parameters is based on the change of the real temperature and pressure value of CO2 during the filling process; the rising part in front of the curve and the falling part at the back are the temperature and pressure change values of CO2 during the filling and release processes, and the middle part of the curve is the temperature and pressure value of CO2 in the fracturing device at room temperature. The state of CO2 before fracturing is the middle part of the curve; The supercritical phase change curve is obtained in the following way: the temperature is controlled at 343K, the water bath is heated for 48h, the detection frequency is 1Hz, the temperature-pressure point value is detected, the temperature-density point value is obtained by querying the Span & Wagner table according to the temperature value, and the temperature-pressure point value under different densities is obtained, and the supercritical phase change curve is fitted; The phase change curve during triggering is obtained in the following way: the pressure is tested by using an Omega temperature and pressure sensor, the trigger pressure is 4349psi, the sampling frequency is 57.6KHz, the sampling time is 4.5s, and the phase change curve during triggering is fitted. The connection of the pre-activation curve and the supercritical curve is achieved according to the static test P-T-t curve and the dynamic calculation P0-T0-t curve on a standard temperature-pressure phase diagram. The connection of the supercritical curve and the activation curve is achieved according to the linear relationship of the temperature and pressure obtained in step (5), the P0-t curve corresponding to the temperature T0-t curve of the dynamic test in step two is calculated, and the dynamic calculation P0-T0-t curve is obtained.

2. The phase state detection method of the liquid CO2 phase change fracturing process according to claim 1, wherein in step three, the water bath heating step is: (a) setting the sensor, setting the DDI temperature and pressure sensor sampling frequency to 1 Hz, and setting the sampling time length to 48 h; (b) installing the sensor, placing the sensor in the liquid storage pipe; (c) assembling, assembling the CO2 fracturing device; (d) filling liquid CO2; (e) standing, standing the CO2 fracturing device for 3-5 h; (f) water bath heating, placing the CO2 fracturing device in the water bath heating device, connecting the power supply, starting heating, the lower limit temperature of the heating device is 342 K, the upper limit temperature is 343 K, and the water bath heating is maintained for 48 h; (g) stopping water bath heating, after 48 h, cutting off the power supply, stopping heating, and taking out the CO2 fracturing device from the water bath heating device after the water temperature decreases to the indoor temperature; (h) opening the valve, discharging the liquid CO2 in the liquid storage pipe; (i) disassembling the fracturing device: taking out the sensor; (j) selecting the data points above 304 K for analysis, and the fitting formula is: P=0.60789×T+166.0659, and the correlation coefficient R2=0.

997.

3. The method of claim 1, wherein: The static P-T-t curve is measured by a wireless storage sensor placed in the CO2 fracturing device.

4. The method of claim 1, wherein: The dynamic P0-t curve is measured by a wireless high-frequency resistance type sensor placed in the CO2 fracturing device.

5. The method of claim 1, wherein: The temperature range of the water bath heating device is 293-373 K, and the temperature control error is less than 1 K.

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