CN113431537B - Unsteady variable-flow-rate large-scale rock core water flooding gas relative permeability testing method - Google Patents

Abstract

The invention belongs to the technical field of water-driven gas reservoir mining, and is specifically an unsteady-state variable flow rate large-scale core water-driven gas relative permeability testing method, which includes step 1: after taking cores from the research area, measure the porosity of the cored samples Φ and permeability K, combined with the well logging curve of the study area to determine the range of porosity Φ and permeability K, and perform X-diffraction on the cored sample to obtain its mineral composition. The corresponding porosity Φ and For artificial large-scale full-diameter cores with a permeability range of K, put the artificial large-scale full-diameter cores into an oven to dry for 48 hours and then take them out. Measure their diameter D, length L, calculate cross-sectional area A, and porosity Φ in accordance with industry standards. and permeability K, and its dry weight M1 at atmospheric pressure Pa. Its structure is reasonable. It can measure gas-driving water and water-driving gas experiments at the same time by relying on two six-way valves. It is simple to operate, low in cost, and has many functions and high accuracy. high.

Description

A method for testing the relative permeability of water-driven gas in large-scale cores with unsteady variable flow rates

Technical field

The invention relates to the technical field of water-driven gas reservoir mining, and is specifically an unsteady variable flow rate large-scale core water-driven gas relative permeability testing method.

Background technique

Optimizing the world's energy structure requires energy to gradually develop towards low-carbon or even carbon-free energy. As a low-carbon energy source with high thermal efficiency and good environmental benefits, natural gas occupies a pivotal position in the current energy structure. According to the 2020 World Energy Statistical Analysis, natural gas accounted for 24.2% of total energy in the world's oil and gas primary energy consumption structure, a record high. Judging from the situation in our country, the country has elevated the development and utilization of natural gas to a very important position. Therefore, increasing the production of natural gas will contribute to the growing demand for natural gas in our national economy. Domestic water-driven gas reservoirs account for a large proportion of gas reservoirs that have been put into development, and they are mainly distributed in Sichuan gas fields, Nanhai gas fields, etc. Water-driven gas reservoirs do not produce water in the early stages of development. As production pressure decreases, edge water intrusion is the main reason for gas-water two-phase flow in the later stages of water-driven gas reservoir development. Gas-water two-phase seepage theory belongs to multi-phase seepage theory research. Most multi-phase seepage theories mainly focus on oil-water two-phase, oil-gas two-phase and oil-gas-water three-phase seepage. There are relatively few studies on the gas-water two-phase seepage process. For porous loose sandstone water-driven gas reservoirs with strong heterogeneity, the development process is mainly in the form of water-driven gas seepage. The two-phase seepage process during the development of water-driven gas reservoirs is relatively complex. During the development of gas reservoirs, water-driven gas two-phase seepage occurs due to the intrusion of edge water into the gas zone. As the pressure of the mining formation decreases, the volume of closed gas expands in the gas reservoir. Gas-driven water two-phase seepage will also form. Therefore, in the seepage process of water-driven gas reservoirs, there are two seepage modes: water-driven gas and gas-driven water. The seepage processes of gas-driven water and water-driven gas are different, and their experimental measurement methods and relative permeability curves are also different, resulting in large differences in the obtained residual gas saturation. Obviously, the experimental measurement results of relative permeability of gas-driven water commonly used in current mine experiments to obtain the distribution of remaining gas in water-driven gas reservoirs do not match the distribution of remaining gas in actual water-driven gas reservoirs. It is difficult to guide the efficient use of remaining gas in actual water-driven gas reservoirs. Mining.

Existing technologies can obtain relative permeability curves of water and gas. However, due to the special accumulation conditions and production requirements of water-driven gas reservoirs, these methods still have certain shortcomings when applied to water-driven gas reservoirs:

(1) Water-driven gas reservoirs are highly heterogeneous, and the absolute permeability of the reservoir ranges from a few millidarcies to hundreds of millidarcies. The pore throat distribution is relatively complex, and it is difficult and expensive to directly obtain the water gas relative permeability curve through conventional experiments.

(2) Most water-driven gas reservoirs are rich in relatively active edge and bottom water. The intrusion of edge and bottom water makes the seepage law of water-driven gas reservoirs very complicated. The seepage mechanism of conventional gas reservoirs, the calculation formula of the relative permeability of gas and water phases, and the determination of water saturation cannot meet the actual production requirements of water-driven gas reservoirs.

(3) At present, most gas reservoirs use the unsteady-state gas-water drive experimental method to obtain the gas-water relative permeability curve. This method overcomes the shortcomings of the steady-state method, which is more time-consuming. However, due to the high permeability of most water-driven gas reservoirs, The gas-water relative permeability curve obtained by the gas-water flooding experimental method has a wide two-phase co-permeability zone and a low residual gas saturation. If there is a large deviation between the remaining gas distribution calculated using the relative permeability curve obtained by gas-driving water and the actual remaining gas distribution in the gas reservoir, inaccurate prediction will not be conducive to improving the recovery rate of the remaining gas.

(4) During the water invasion process in water-driven gas reservoirs, the particles on the reservoir surface fall off and migrate, causing changes in the relative permeability of gas and water. Current experimental methods cannot measure and calibrate.

(5) The conventional standard core (5cm*2.5cm) contains less gas, and the pressure difference between the inlet and outlet ends causes the gas volume to expand. It is difficult to accurately measure the cumulative gas production in conventional standard cores using the water flooding experimental method. The measured relative permeability curve is inaccurate.

(6) During the experiment, the dead pore volume in the pipeline, holder and metering device has a great impact on the acquisition of relative permeability data of the gas and water phases.

(7) Using other indirect methods to obtain the gas-water two-phase relative permeability, such as production data calculation, capillary pressure calculation, etc., requires a large amount of calculation and has a high cost.

Therefore, accurately obtaining the distribution of remaining gas in water-driven gas reservoirs and measuring the gas-water two-phase seepage law have always been the forefront of theoretical research on seepage in water-driven gas reservoirs, and are also key technologies that need to be solved urgently to improve the recovery rate of water-driven gas reservoirs. In order to better study the development rules of water-driven gas reservoirs, it is necessary to understand the effects of solid particle migration, water invasion speed and other factors on the relative permeability of water and gas phases during the water invasion process. Because the pore range in the conventional standard core (2.5cm*5cm) is too small and the amount of gas contained is small, it is impossible to accurately measure the cumulative gas production. Therefore, this patent uses artificial large-scale rock core (7cm*10cm) with a core porosity of about 20% to ensure that the amount of gas in the core is sufficient to be accurately measured at the outlet. In addition, this patent utilizes the U-shaped tube principle and adopts the drainage gas production method to measure the cumulative gas production and cumulative water production that increase over time at the outlet end. Compare the experimental measurement results with the NMR T2 spectrum results to reduce experimental errors and make the experimental results more consistent with actual gas reservoirs. Finally, in the process of experimental data processing, unsteady JBN fitting is used to fit the data of water gas phase penetration. Due to the compressibility of gas, we must solve for the gas volume increment under the average pressure. When calculating water saturation using large-scale core experiments, dead pore volume is a quantity that cannot be ignored. During the experimental process and data processing, the dead pore volume should be removed artificially to avoid causing a shift in the relative permeability curve.

Contents of the invention

The purpose of this section is to outline some aspects of embodiments of the invention and to briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section, the abstract and the title of the invention to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions cannot be used to limit the scope of the invention.

In view of the problems existing in the existing gas drive water relative permeability testing method, the present invention is proposed.

Therefore, the purpose of the present invention is to provide a method for testing the relative permeability of large-scale rock core water-driven gas at unsteady-state variable flow rates, which can simultaneously measure gas-driven water and water-driven gas experiments using only two six-way valves. The operation is simple. It has low cost, realizes many functions and has high accuracy.

In order to solve the above technical problems, according to one aspect of the present invention, the present invention provides the following technical solutions:

An unsteady variable flow rate large-scale rock core water drive gas relative permeability testing method, which includes the following steps:

Step 1: After taking the core from the study area, measure the porosity Φ and permeability K of the core sample, determine the range of porosity Φ and permeability K based on the well logging curve of the study area, and perform X-diffraction on the core sample to obtain For its mineral composition, artificial large-scale full-diameter cores within the corresponding porosity Φ and permeability K ranges are synthesized based on the mineral composition of the cored samples. The artificial large-scale full-diameter cores are placed in an oven to dry for 48 hours and then taken out. According to the industry Standardly measure its diameter D, length L, calculate cross-sectional area A, porosity Φ and permeability K, weigh its dry weight M1 under atmospheric pressure Pa, and then put the dry weight core sample into a vacuum pump to evacuate and pressurize it to saturation Simulate the formation water KCL solution, weigh the wet weight M2, put the core sample saturated with the KCL solution into the nuclear magnetic resonance T ₂ monitoring system 14, and scan the T ₂ spectrum in the fully saturated state;

Step 2: Load the core after saturated KCL solution into the large-scale full-diameter core holder 6, and connect it according to the connection method in Figure 1. First, conduct a gas-flooding water experiment to establish the irreducible water saturation. The specific method is as follows: The six-way valve on the left end of the core holder connected to the nitrogen bottle 1 is open, and the valve connected to the ISCO fluid injection pump 2 is closed. The valve connected to the drying bottle 10 on the right end of the six-way valve is open, and the valve connected to the graduated glass tube 11 is closed. , at this time, the line shown in Figure 1 is the gas drive water experimental process;

Step 3: Establish irreducible water saturation; use a high-precision confining pressure pump to add confining pressure to the large-scale full-diameter core holder 6 to 15MPa and then close the confining pressure pump valve so that the confining pressure in the core holder 6 remains constant Keep it at 15MPa; open the nitrogen bottle 1, and conduct the gas-displacing water experiment through the pressure reducing valve according to the reference displacement pressure difference calculated by Formula 1; during the experiment, use a stopwatch to measure the accumulated time, and the pressure sensor 4 to measure the displacement during the experiment. Instead of the pressure difference, the electronic balance 9 measures the weight M3 of the absorbed solution in the drying bottle 4, and uses the gas flow measurement device 13 to measure the amount of gas produced; until the weight of the electronic balance 9 no longer changes or the amount of injected gas is greater than 30 times the core After the pore volume is determined, stop the gas supply; weigh the core M3 in the bound water state, and calculate the pore volume V _Φ in the bound water state;

V _Φ =ALΦ-(M3-M1)*μ _w

Step 4: Put the rock core in the bound water state into the nuclear magnetic resonance T2 spectrum monitoring system 14, and measure the distribution of bound water in the core; conduct a water displacement experiment under the action of different capillary numbers on the rock core in the bound water state; in the water During the gas purging experiment, the six-way valve at the left end of the large-scale full-diameter core holder 6 opens the valve connected to the ISCO fluid injection pump 2 and closes the valve connected to the nitrogen bottle 1; while the right end of the large-scale full-diameter core holder 6 is closed Connect the valve of the drying bottle 10, open the valve connected to the graduated glass tube 11; adjust the ISCO fluid injection pump 2 so that the injection flow rate is selected to be 0.5ml/min, 1.5ml/min, 2.5ml/min respectively; use the capillary number calculation formula Ca= (v*μw)/σgw calculate the number of injected capillaries; use a stopwatch to record the cumulative time Δt, use the change in water level on the graduated glass tube 11 to measure the cumulative gas production ΔG that increases over time, and connect it to the six-way valve 14 on the left The pressure sensor 4 reads the displacement pressure difference Δp that changes with time, and uses the electronic balance 9 under the beaker 12 to read the accumulated water production amount ΔW that increases with time;

Among them, according to Darcy's formula and the law of conservation of energy, the water saturation, water phase relative permeability and gas phase relative permeability during water flooding are deduced;

Gas saturation of water driven gas:

Water saturation of water drive gas: S _w = 100-S _g

Relative permeability of water phase:

Gas phase relative permeability:

in: (Due to the compressibility of gas, the gas volume will change during the water drive gas experiment. The △G' obtained here refers to the increment of the gas volume under the average pressure, which is the correction value). The water drive gas is deduced. Experimental water saturation S _o , water phase relative permeability K _rw and gas phase relative permeability Kr _g calculation formula; and the change of gas volume with pressure difference is taken into account in the gas phase relative permeability formula to solve for △G' average pressure The cumulative gas production;

Step 5: During the water flooding experiment, select a suitable time to measure the NMR T ₂ spectrum, and obtain the distribution pattern of the injected water in the core as time increases. Calculate based on the NMR T ₂ spectrum. For residual gas saturation, compare and correct the residual gas saturation value obtained from the nuclear magnetic resonance T2 spectrum and the residual gas saturation value obtained from the water drive gas experiment to obtain the error range of the experiment. Solve the correction coefficient ε in the calculation formula of water gas relative permeability in water flooding experiment.

As a preferred solution for the relative permeability test method of large-scale rock core water drive gas with unsteady-state variable flow rate according to the present invention, the water drive gas experimental device is used in combination with the nuclear magnetic resonance T ₂ spectrum to complete the test. This device It is mainly completed by the following four systems, including energy supply system, experimental test system, experimental measurement system and nuclear magnetic resonance _T2 spectrum monitoring system; the energy supply system includes nitrogen bottle 1, ISCO fluid injection pump 2, and intermediate container 3 to store formation water. , pressure sensor 4 and pressure reducing valve 5; the experimental test system includes a large-scale full-diameter core holder 6, a high-precision confining pressure pump 7, and a pressure gauge 8; the experimental test system includes an electronic balance 9, a drying bottle 10, and a scaled Glass tube 11, beaker 12, gas flow metering device 13; nuclear magnetic resonance _T2 spectrum monitoring system 14; load the water-saturated core into the large-scale full-diameter core holder 6, and use a high-precision confining pressure pump 7 to add confining pressure to 15MPa, turn off the confining pressure pump so that the pressure in the large-scale full-diameter confining pressure pump remains unchanged at 15MPa during the experiment; the left end of the large-scale full-diameter core holder 6 is connected to a six-way valve 14, and one of the six-way valves 14 The port is connected to the nitrogen bottle 1 through the pressure reducing valve 5, and the other port is connected to the ISCO fluid injection pump 2 through the pressure gauge 8 and the intermediate container 3. The simulated formation water in the intermediate container 3 is injected at a constant flow rate or through the ISCO fluid injection pump 2. It is injected into the core holder at a constant pressure; a pressure sensor 4 is also installed on the six-way valve, and the pressure change of the injected gas or liquid can be accurately monitored through the pressure sensor; another feature of the large-scale full-diameter core holder 6 One end is also connected to a six-way valve 14. One port of the six-way valve 14 first passes through a drying bottle (containing anhydrous calcium chloride) 10. The drying bottle is placed on an electronic balance 9, and is metered into the drying bottle through the electronic balance. The amount of water; the dried gas enters the gas flow metering device 13 for gas measurement; this port is designed because the amount of gas is large and the amount of water is small during the gas-driving water experiment; when the gas-driving water experiment is performed, this port is opened, When doing the water drive experiment, this port is closed; the other port of the six-way valve 14 is connected to the graduated glass tube 11 through a pipeline. The graduated glass tube 11 is filled with water and is inserted upside down into the beaker 12 filled with water. middle and fixed; an electronic balance 9 is placed at the lower end of the beaker 12; when the water displacement gas experiment is carried out, the gas volume is less and the water volume is more. This method can accurately measure the trace gas volume.

Compared with the prior art, the beneficial effects of the present invention are:

(1) A water-driven gas relative permeability experimental device is added to the traditional gas-driven water relative permeability experimental device, and only two six-way valves can be used to easily switch between the two sets of experiments. It has simple operation, strong controllability, low cost and accurate measurement.

(2) Use large-size full-diameter cores to conduct water drive gas experiments. The saturated gas volume in the cores is high and easy to measure.

(3) Experimental metering system. Different metering methods are used in the gas-water drive and water-gas drive experiments. In the gas-flooding water experiment, due to the small amount of water and the large amount of gas, the mixed fluid at the outlet end of the core holder first passes through the drying bottle to measure the water, and the dried gas is measured through the gas metering device. In the water drive gas experiment, the gas volume is small and the water volume is large, so the mixed fluid at the outlet end of the core holder first passes through a graduated glass tube to measure the gas, and at the same time, the water volume is measured through an electronic balance.

(4) It is proposed that by changing the injection capillary number Ca, the remaining gas saturation in the water flooding experiment can be effectively changed. Completely opposite to conventional gas or oil reservoir injection production. As the number of injected capillaries Ca is increased, the water-gas relative permeability curve shifts to the left. The shorter the water-free gas production period, the less conducive it is to natural gas production. For water-driven gas reservoirs, this process is equivalent to the faster the edge water intrusion, the shorter the water-free gas production period. The gas well will soon break out of water, which is not conducive to natural gas production, and the remaining gas saturation will be very high. Decreasing the number of injected capillaries, Ca, is just the opposite. The slower the edge water intrusion speed, the longer the water-free gas production period, and the later the water breakthrough time of the gas well, the higher the gas production volume.

(5) Each experimental link of the present invention has corresponding monitoring and verification methods. During the gas-displacement water experiment, nuclear magnetic resonance is used to obtain the gas-water distribution pattern in the bound water state and during the water-displacement gas experiment, the remaining gas changes with the water injection time. The change. After the water drive gas experiment is completed, the nuclear magnetic resonance T ₂ spectrum is used to correct and monitor the gas and water distribution rules of the water drive gas experiment.

(6) The present invention eliminates the influence of dead pore volume and reduces experimental errors during the experiment.

(7) During the experimental process, the present invention takes into account the characteristics of gas volume changing with pressure changes, and uses the gas volume increment ΔG' under the average pressure to participate in the calculation, and reduces the relative permeability of the gas phase due to changes in experimental conditions. error.

(8) The experimental method of the present invention is simple, and can realize experimental research on the accumulation process and development process of water-driven gas reservoirs at the same time. At the same time, the nuclear magnetic resonance T2 spectrum is used for real-time monitoring. The experimental accuracy is high and is less affected by human errors.

Description of drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be described in detail below with reference to the drawings and detailed implementation modes. Obviously, the drawings in the following description are only some embodiments of the present invention. For ordinary people in the art Technical personnel can also obtain other drawings based on these drawings without exerting creative labor. in:

Figure 1 is a diagram of the experimental device for gas-driving water and water-driving gas;

Figure 2 shows the relative permeability curve of water driving gas under the injection flow rate corresponding to different capillary number Ca (in the figure: K _rg (0.5ml/min) is the gas phase relative permeability curve corresponding to the injection flow rate of low Ca, K _rw (0.5ml/min) is the water phase relative permeability curve corresponding to the injection flow rate of low Ca; K _rg (1.5ml/min) is the gas phase relative permeability curve corresponding to the injection flow rate of medium Ca, K _rw (1.5ml/min) ) is the water phase relative permeability curve corresponding to the injection flow rate of medium Ca; K _rg (2.5ml/min) is the gas phase relative permeability curve corresponding to the injection flow rate of high Ca, K _rw (2.5ml/min) is the gas phase relative permeability curve corresponding to the injection flow rate of high Ca Water phase relative permeability curve corresponding to the injection flow rate);

Figure 3 shows the NMR T2 spectrum of the water drive gas experiment.

In the picture; 1. Nitrogen cylinder; 2. ISCO fluid injection pump; 3. Intermediate container; 4. Pressure sensor; 5. Pressure reducing valve; 6. Large-scale full-diameter core holder; 7. High-precision confining pressure pump; 8 , Pressure gauge; 9. Electronic balance; 10. Drying bottle; 11. Glass tube with scale; 12. Beaker; 13. Gas flow measuring device.

Detailed ways

In order to make the above objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described here. Those skilled in the art can do so without departing from the connotation of the present invention. Similar generalizations are made and the present invention is therefore not limited to the specific embodiments disclosed below.

Secondly, the present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention in detail, for the convenience of explanation, the cross-sectional diagrams showing the device structure will be partially enlarged according to the general scale. Moreover, the schematic diagrams are only examples and should not be limited here. protection scope of the present invention. In addition, the three-dimensional dimensions of length, width and depth should be included in actual production.

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

Example 1

An unsteady-state variable capillary number large-scale rock core water drive gas relative permeability testing method. A water drive gas experimental device is added to the traditional gas drive water experimental device to complete the water invasion process of the water drive gas reservoir. , the impact of water invasion speed on natural gas recovery rate and remaining gas distribution. The device (see Figure 1) consists of an energy supply system including a nitrogen bottle 1, an ISCO fluid injection pump 2, an intermediate container 3 to store formation water, a pressure sensor 4 and a pressure reducing valve 5; the experimental test system includes a large-scale full-diameter core clamp Holder 6, high-precision confining pressure pump 7, pressure gauge 8; the experimental test system includes an electronic balance 9, a drying bottle 10, a graduated glass tube 11, a beaker 12, a gas flow measurement device 13; a nuclear magnetic resonance T ₂ spectrum monitoring system 14. The water-saturated core is loaded into the large-scale full-diameter core holder 6, and the high-precision confining pressure pump 7 is used to increase the confining pressure to 15MPa. The confining pressure pump is turned off so that the pressure in the large-scale full-diameter confining pressure pump increases during the experiment. Keep 15MPa unchanged. The left end of the large-scale full-diameter core holder 6 is connected to the six-way valve 14. One port of the six-way valve 14 is connected to the nitrogen bottle 1 through the pressure reducing valve 5, and the other port is connected to the ISCO fluid injection through the pressure gauge 8 and the intermediate container 3. On the pump 2, the simulated formation water in the intermediate container 3 is injected into the core holder at a constant flow or constant pressure through the ISCO fluid injection pump 2; a pressure sensor 4 is also installed on the six-way valve, and the pressure sensor can Accurately monitor the pressure changes of injected gas or injected liquid. The other end of the large-scale full-diameter core holder 6 is also connected to a six-way valve 14. One port of the six-way valve 14 first passes through a drying bottle (containing anhydrous calcium chloride) 10. The drying bottle 10 is placed in the electronic On the balance 9, the amount of water entering the drying bottle is measured through the electronic balance 9; the dried gas enters the gas flow measurement device 13 for gas measurement; this port is designed due to the large amount of gas and small amount of water during the gas drive water experiment. . When performing a gas-driving water experiment, this port is open, and when performing a water-driving gas experiment, this port is closed. The other port of the six-way valve 14 is connected to the graduated glass tube 11 through a pipeline. The graduated glass tube 11 contains water and is inserted upside down into the beaker 12 filled with water and fixed; an electronic balance 9 is placed at the lower end of the beaker 12. When conducting a water drive gas experiment, the gas volume is less and the water volume is larger. This method can accurately measure trace amounts of gas.

Specific steps are as follows:

S1. After taking the core from the research area, measure the porosity Φ=14.88% and permeability K=3.0622mD of the core sample, and perform X-diffraction on the core sample to obtain its mineral composition. According to the mineral composition of the core samples, artificial large-scale full-diameter cores with corresponding porosity Φ = 17.2% and permeability K = 3.0622mD were synthesized. Put the artificial large-scale full-diameter core into the oven to dry for 48 hours and then take it out. According to industry standards, its diameter D = 7cm, length L = 10cm, calculated cross-sectional area A = πD ² /4 = 38.465cm ² , sample volume V = AL = 384.650cm ³ , and its dry weight M1 at atmospheric pressure Pa = 0.1MPa. =874.07g, then put the dry weight core sample into a vacuum pump to vacuum and pressurize the saturated simulated formation water KCL solution, and weigh the wet weight M2 = 926.16g. Put the core sample saturated with the KCL solution into the nuclear magnetic resonance T ₂ monitoring system 14 and scan the T ₂ spectrum in the fully saturated state.

S2. Load the core after the saturated KCL solution into the large-scale full-diameter core holder 6, and connect it according to the connection method in Figure 1. First, a gas-flooding water experiment was performed to establish the irreducible water saturation. The specific method is as follows: the six-way valve at the left end of the core holder connected to the nitrogen bottle 1 is opened, and the valve connected to the ISCO fluid injection pump 2 is closed. The valve connected to the drying bottle 10 at the right end of the six-way valve is opened, and the valve connected to the graduated glass tube is Valve 11 is closed. At this time, the circuit shown in Figure 1 is the gas drive water experimental process.

S3. Establish irreducible water saturation. Use the high-precision confining pressure pump 7 to add confining pressure to the large-scale full-diameter core holder 6 to 15MPa and then close the confining pressure pump valve so that the confining pressure in the core holder 6 remains constant at 15MPa. Open the nitrogen bottle 1, and conduct the gas-displacing water experiment through the pressure reducing valve according to the reference displacement pressure difference calculated by Equation 1. During the experiment, a stopwatch is used to measure the accumulated time, the pressure sensor 4 is used to measure the displacement pressure difference during the experiment, the electronic balance 9 is used to measure the weight M3 of the absorbed solution in the drying bottle 4, and the gas flow measuring device 13 is used to measure the amount of gas produced. Stop the gas supply until the weight of the electronic balance 9 no longer changes or the injected gas amount is greater than 30 times the core pore volume. Weigh the core weight M3 = 885.41g in the bound water state, and calculate the pore volume V _Φ in the bound water state.

Reference displacement pressure difference:

The pore volume occupied by gas in the bound water state: V _Φ =VΦ-(M3-M1)*μ _w =45.91cm ³

S4. Put the core with bound water into the nuclear magnetic resonance T ₂ spectrum monitoring system 14 and measure the distribution of bound water in the core. The cores in the bound water state were then subjected to water displacement experiments under different capillary numbers. In the water flooding experiment, the six-way valve at the left end of the large-scale full-diameter core holder 6 opens the valve connected to the ISCO fluid injection pump 2 and closes the valve connected to the nitrogen bottle 1. The right end of the large-scale full-diameter core holder 6 closes the valve connected to the drying bottle 10 and opens the valve connected to the graduated glass tube 11. Adjust the ISCO fluid injection pump 2 so that the injection flow rate is selected as 0.5ml/min, 1.5ml/min, and 2.5ml/min respectively. Calculate the number of injected capillaries according to the capillary number calculation formula Ca=(v*μ _w )/σ _gw . Use a stopwatch to record the cumulative time Δt, use the change in water level on the graduated glass tube 11 to measure the cumulative gas production ΔG that increases over time, and use the pressure sensor 4 connected to the six-way valve 14 at the left end to read the displacement that changes over time. Pressure difference Δp, use the electronic balance 9 under the beaker 12 to read the cumulative water production ΔW that increases with time. The following formula is used to calculate the relative permeability of unsteady water flooding gas as the capillary number changes:

Based on Darcy's formula and the law of conservation of energy, the present invention derives the water saturation, water phase relative permeability and gas phase relative permeability during the water flooding process.

Gas saturation of water driven gas:

Water saturation of water drive gas: S100-S

Relative permeability of water phase:

Gas phase relative permeability:

in: (Due to the compressibility of gas, the gas volume will change during the water drive experiment. The △G' obtained here refers to the increment of gas volume under the average pressure, which is the correction value)

The present invention derives the calculation formulas for water saturation S _o , water phase relative permeability K _rw and gas phase relative permeability Kr _g in the water driving gas experiment. The change of gas volume with pressure difference is also considered in the gas phase relative permeability formula, and the cumulative gas production under the average pressure of △G' is solved.

S5. During the water flooding experiment, select the appropriate time to measure the NMR T ₂ spectrum, and obtain the distribution pattern of the injected water in the core as time increases. Calculate the residual value based on the NMR T ₂ spectrum. For gas saturation, compare and correct the residual gas saturation value obtained from the nuclear magnetic resonance T2 spectrum and the residual gas saturation value obtained from the water flooding experiment to obtain the error range of the experiment.

Although the present invention has been described above with reference to the embodiments, various modifications may be made and equivalents may be substituted for components thereof without departing from the scope of the invention. In particular, as long as there is no structural conflict, various features in the embodiments disclosed in the present invention can be combined with each other in any way. The description of these combinations is not exhaustive for the purpose of illustration only. In consideration of omitting space and saving resources. Therefore, the present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims (2)

1. A method for testing the relative permeability of large-scale core water drive gas with unsteady variable flow velocity, which is characterized by including the following steps: Step 1: After taking the core from the study area, measure the porosity Φ and permeability K of the core sample, determine the range of porosity Φ and permeability K based on the well logging curve of the study area, and perform X-diffraction on the core sample to obtain For its mineral composition, artificial large-scale full-diameter cores within the corresponding porosity Φ and permeability K ranges are synthesized based on the mineral composition of the cored samples. The artificial large-scale full-diameter cores are placed in an oven to dry for 48 hours and then taken out. According to the industry Standardly measure its diameter D, length L, calculate cross-sectional area A, porosity Φ and permeability K, weigh its dry weight M1 under atmospheric pressure Pa, and then put the dry weight core sample into a vacuum pump to evacuate and pressurize it to saturation Simulate the formation water KCL solution, weigh the wet weight M2, put the core sample after the saturated KCL solution into the nuclear magnetic resonance T ₂ monitoring system, and scan the T ₂ spectrum in the fully saturated state; Step 2: Load the core after saturated KCL solution into a large-scale full-diameter core holder. First, conduct a gas-driving water experiment to establish the irreducible water saturation. The specific method is as follows: The six-way valve at the left end of the core holder is connected to a nitrogen bottle. The valve of (1) is open, while the valve connected to the ISCO fluid injection pump (2) is closed, the six-way valve on the right end of the core holder connected to the drying bottle is opened, and the valve connected to the graduated glass tube (11) is closed. At this time The line shown is the gas drive water experimental process; Step 3: Establish irreducible water saturation; use a high-precision confining pressure pump to add confining pressure to the large-scale full-diameter core holder (6) to 15MPa and then close the confining pressure pump valve to increase the confining pressure in the core holder. Always keep it at 15MPa; open the nitrogen bottle (1), and conduct the gas-displacing water experiment through the pressure reducing valve according to the reference displacement pressure difference calculated by Formula 1; during the experiment, use a stopwatch to measure the accumulated time, and the pressure sensor (4) During the measurement experiment, the displacement pressure difference is measured. The electronic balance (9) measures the weight of the absorption solution in the drying bottle (10), and the gas flow measurement device (13) is used to measure the amount of gas produced; until the weight of the electronic balance (9) is no longer After further changes occur or the injected gas volume is greater than 30 times the core pore volume, stop the gas supply; weigh the core weight M3 in the bound water state, and calculate the pore volume V _Φ occupied by the gas in the bound water state; V _Φ =ALΦ-(M3-M1)*μ _w Among them, ΔP ₁ is the reference displacement pressure difference; Step 4: Put the core in the bound water state into the nuclear magnetic resonance T2 _spectrum monitoring system, and measure the distribution of bound water in the core; conduct water displacement experiments under different capillary numbers on the core in the bound water state; in the water During the gas purging experiment, the six-way valve at the left end of the large-scale full-diameter core holder (6) opens the valve connected to the ISCO fluid injection pump (2) and closes the valve connected to the nitrogen bottle (1); while the large-scale full-diameter core holder Close the six-way valve on the right end of the holder (6) that connects to the drying bottle (10), and open the valve that connects the graduated glass tube (11); adjust the ISCO fluid injection pump (2) so that the injection flow rate is 0.5ml/min. 1.5ml/min, 2.5ml/min; calculate the number of injected capillaries through the capillary number calculation formula Ca = (v*μ _w )/σ _gw ; use a stopwatch to record the accumulation time Δt, and use a graduated glass tube (11) to measure the water level To measure the cumulative gas production ΔG that increases over time, use the pressure sensor (4) connected to the six-way valve (14) at the left end to read the displacement pressure difference Δp that changes over time, and use the electronic balance (9) under the beaker (12) ) Read the cumulative water production ΔW that increases over time; Among them, according to Darcy's formula and the law of conservation of energy, the water saturation, water phase relative permeability and gas phase relative permeability during water flooding are deduced; Gas saturation of water driven gas: Water saturation of water drive gas: S _w = 100-S _g ; Relative permeability of water phase: Gas phase relative permeability: in: Since the gas is compressible, the gas volume will change during the water drive experiment. The ΔG' obtained here refers to the increment of the gas volume under the average pressure, which is the correction value. The water saturation of the water drive experiment is derived. Degree S _w , water phase relative permeability K _rw and gas phase relative permeability K _rg calculation formula; and in the gas phase relative permeability formula, the change of gas volume with displacement pressure difference is considered, and the gas volume under the average pressure of ΔG' is solved increment; Step 5: During the water flooding experiment, select a suitable time to measure the NMR T ₂ spectrum, and obtain the distribution pattern of the injected water in the core as time increases. Calculate based on the NMR T ₂ spectrum. For residual gas saturation, compare and correct the residual gas saturation value obtained from the nuclear magnetic resonance T2 spectrum and the residual gas saturation value obtained from the water drive gas experiment to obtain the error range of the experiment. 2. A kind of unsteady-state variable flow rate large-scale rock core water drive gas relative permeability testing method according to claim 1, characterized in that: it is completed by using a water drive gas experimental device combined with a nuclear magnetic resonance T ₂ spectrum. The device It is mainly completed by the following four systems, including energy supply system, experimental test system, experimental measurement system and nuclear magnetic resonance _T2 spectrum monitoring system; the energy supply system includes nitrogen bottle (1), ISCO fluid injection pump (2), intermediate container ( 3), pressure sensor (4) and pressure reducing valve (5), in which formation water is stored in the intermediate container (3); the experimental test system includes a large-scale full-diameter core holder (6), a high-precision confining pressure pump ( 7), pressure gauge (8); the experimental measurement system includes an electronic balance (9), a drying bottle (10), a graduated glass tube (11), a beaker (12), and a gas flow measurement device (13); the saturated water The core is loaded into the large-scale full-diameter core holder (6), and the high-precision confining pressure pump (7) is used to increase the confining pressure to 15MPa. The confining pressure pump is turned off so that the pressure in the large-scale full-diameter confining pressure pump increases during the experiment. Keep 15MPa unchanged; the left end of the large-scale full-diameter core holder (6) is connected to a six-way valve (14), and one port of the six-way valve (14) is connected to the nitrogen bottle (1) through the pressure reducing valve (5) , the other port is connected to the ISCO fluid injection pump (2) through the pressure gauge (8) and the intermediate container (3), and the simulated formation water in the intermediate container (3) is pumped through the ISCO fluid injection pump (2) at a constant flow rate or It is injected into the core holder at a constant pressure; a pressure sensor (4) is also installed on the six-way valve, and the pressure change of the injected gas or liquid can be accurately monitored through the pressure sensor; the large-scale full-diameter core holder ( The right end of 6) is also connected to a six-way valve (14). One port of the six-way valve (14) first passes through a drying bottle (10) containing anhydrous calcium chloride, and the drying bottle is placed on the electronic balance (9) , measure the amount of water entering the drying bottle through an electronic balance; the dried gas enters the gas flow metering device (13) for gas measurement; this port is designed because the amount of gas is large and the amount of water is small during the gas-driving water experiment; when When performing a gas-driving water experiment, this port is open, and when performing a water-driving experiment, this port is closed; the other port of the six-way valve (14) at the right end of the core holder is connected to the graduated glass tube (11) through a pipeline , the graduated glass tube (11) is filled with water, and is inserted upside down into the beaker (12) filled with water and fixed; an electronic balance (9) is placed at the lower end of the beaker (12); when performing a water displacement experiment, the gas volume is relatively small Small amount of air and large amount of water. This method can accurately measure trace amounts of gas.