Experimental Method for Simulating the Synergistic Effect of Sand Production and Scaling of Fracturing Proppants in Tight Reservoirs

Abstract

The present invention relates to an experimental method for simulating the synergistic effect of sand production and scaling of fracturing proppants in tight reservoirs, including: selecting a core, axially cutting the core after cleaning and drying, and filling the proppant at the cross-section of the core; placing the core in a holder, applying confining pressure to the holder, injecting nitrogen gas into the inlet end of the core, and after the outlet end of the core stably emits gas, raising the pressure of the core to the formation pressure and heating it to the formation temperature; using formation water to displace the core with proppant, measuring the permeability of the core multiple times during the displacement process, recording the change of the pressure difference between the inlet and outlet of the core over time, and simultaneously measuring the ion concentration at the outlet end of the core by an ion chromatograph; slowly reducing the confining pressure and continuing to displace the core with formation water; collecting the proppant in the outlet pipeline and calculating the sand production rate of the proppant. The principle of the present invention is reliable, the operation is simple, the test process is closer to the field working conditions, the test results are intuitive and accurate, and it can provide basic data and theoretical basis for the development of unconventional oil reservoirs.

Description

Technical Field

[0001] The present invention relates to the field of oilfield development experimental methods, and particularly to an experimental method for simulating the synergistic effect of sand production and scaling of fracturing proppants in tight reservoirs. Background Art

[0002] In recent years, sand fracturing has become one of the necessary technical measures for oil and gas fields at home and abroad in the exploration and evaluation of tight oil and gas and reserve increase. At present, the most commonly used in each oil and gas field is hydraulic sand fracturing. During the fracturing process, the sand-carrying fluid carries the proppant into the artificial fractures generated by high pressure and fills and accumulates in the fractures to prevent the fractures from completely closing, so as to improve the conductivity of the near-wellbore zone and thus achieve the purpose of production increase. However, due to the incompatibility between the fracturing fluid and the formation fluid, as well as the changes in bottom-hole pressure drop, temperature, and pH value, etc., the production of underground fluid causes scaling in the oil production system and the reservoir. Downhole scaling generally goes through three stages: the first stage is the combination of ions in water to form salt molecules with very low solubility; the second stage is the orderly arrangement of molecular combination to form crystals; the third stage is the accumulation and precipitation of a large number of crystals to form scale. Under the actual field conditions of the oilfield, when the fracturing residue fluid or the formation fluid returns, the artificial fracture has insufficient compaction force on the proppant, the fluid has too large a drag force on the proppant, the pressure gradient of the sand-filled fracture is too large or the pressure fluctuation causes the proppant in the fracture to be unstable, resulting in the backflow of the proppant. Along with the influence of bottom-hole pressure drop, temperature, and pH value on the high salinity formation water, the backflow of the proppant will provide more possibilities for the formation and accumulation of crystals, shortening the time for scaling ions to go through the second and third stages, thus greatly aggravating the bottom-hole scaling and causing a decline in the productivity of the oil well.

[0003] In recent years, there have been many devices and methods for studying proppant sand production. For the invention patent "An experimental device and method for simulating single-slit backflow after shale gas fracturing" (CN201510716601.7), it simulates the fracture state after shale gas fracturing under formation high-temperature and high-pressure environments, considering factors such as fracture length, fracture width, proppant concentration, sand production volume, sand production critical velocity, proppant embedding degree, and pressure drop during the backflow process on proppant backflow; "A test device and method for simulating proppant backflow" (CN201110050030.X) simulates the influence of different proppant concentrations and closure pressures on proppant backflow and the real process of proppant flowing back to the wellbore from near-wellbore fractures through perforation holes. There are also many devices and methods for studying oilfield scaling. "An oilfield dynamic scaling and scale inhibition evaluation device and evaluation method" (CN202110034699.3) can simulate the scaling conditions under different flow states (laminar flow, turbulent flow, etc.) in the field and can achieve tests under high temperature and high pressure; "A dynamic scaling instrument and its test method" (CN104458689A) injects fluorescent tracers into the mixed fluid and calculates the residence time distribution, dispersion condition, and specific scaling location of the scaling fluid in the pipe according to the fluorescent tracer concentration test. However, based on the actual field situation, for unconventional tight oil and gas reservoirs, underground fluid scaling and proppant sand production do not simply exist independently during the development process, but more often there is a synergistic effect between the two, which is specifically manifested as the mutual wrapping and attachment of proppant and scaling crystals. Therefore, studying an experimental method for simulating the synergistic effect of proppant sand production and scaling in tight reservoirs is of great significance for the development of unconventional oil reservoirs. Summary of the Invention

[0004] The purpose of the present invention is to provide an experimental method for simulating the synergistic effect of proppant sand production and scaling in fractured tight reservoirs. The principle of this method is reliable, the operation is simple, the test process is closer to the field working conditions, the test results are intuitive and accurate, and it can provide basic data and theoretical basis for the development of unconventional oil reservoirs.

[0005] To achieve the above technical objectives, the present invention adopts the following technical solutions.

[0006] The present invention combines core flooding experiments. First, the core is cut, filled with proppant, and then closed to simulate the real situation of proppant existing in the core fractures after reservoir fracturing; then formation water is used to flood the core containing proppant under formation conditions, and by reducing the confining pressure, it simulates that the artificial fracture has insufficient compaction force on the proppant and the fluid has too large a drag force on the proppant, resulting in the instability of the proppant in the fracture, so as to study the synergistic effect mechanism of proppant sand production and scaling.

[0007] The experimental method for simulating the synergistic effect of proppant sand production and scaling in fractured tight reservoirs successively includes the following steps:

[0008] (1) Select appropriate cores, clean them, and then bake them at high temperature to fully remove the moisture contained in the cores;

[0009] (2) Axially cut the cores in step (1) and process the cutting surfaces;

[0010] (3) Fill the cross-section of the cores in step (2) with proppants and suture the cores with heat-resistant and pressure-resistant tapes;

[0011] (4) Heating and pressure increasing process: Place the cores with proppants in a core holder, apply confining pressure to the core holder using a confining pressure pump, inject nitrogen gas into the inlet end of the cores through a displacement pump. After the gas outlet at the outlet end of the cores is stable, increase the pressure of the core holder to the formation pressure and increase the temperature to the formation temperature;

[0012] (5) Scaling process: Wait until the pressure and temperature reach the formation conditions, measure the initial permeability of the cores, displace the cores with proppants using formation water. During the displacement process, measure the permeability of the cores multiple times (every 10 minutes), record the change in the differential pressure between the inlet and outlet of the cores over time, and simultaneously measure the ion concentration at the outlet end of the cores through an ion chromatograph;

[0013] (6) Synergistic action process of proppant sand production and scaling: After the flow rate at the outlet end of the cores is stable, slowly reduce the confining pressure, continue to displace the cores with formation water. During the displacement process, measure the permeability of the cores multiple times, record the change in the differential pressure between the inlet and outlet of the cores over time, and simultaneously measure the ion concentration at the outlet end of the cores through an ion chromatograph;

[0014] (7) Depressurize, collect the proppants in the outlet pipeline, and calculate the proppant sand production rate.

[0015] The process of step (2) is as follows: Use a core cutting machine to cut the cores into two pieces along the central axis of the cores, and use CPS copper oxide to repair the parts of the cores that fall off or are damaged during the cutting process to ensure the integrity of the cores.

[0016] The process of step (3) is as follows: Spread the proppants with a particle size of 70 - 150 mesh evenly on the core cutting surface, then close the cores, and wind the cores with proppants with heat-resistant and pressure-resistant tapes to ensure that the proppants inside the cores will not fall out. The mass of the proppants is M1.

[0017] The process of step (4) is as follows: Place the core containing proppant in a core holder. The inlet end of the core holder is connected to a displacement pump through a nitrogen intermediate container and a formation water intermediate container respectively. The outlet end is connected to a test tube, with a confining pressure pump connected in the middle and pressure gauges connected at both ends. First, apply confining pressure to the core holder through the confining pressure pump, and then inject nitrogen into the core through the displacement pump until there is no obvious increase in particles in the test tube. The test tube contains unstable proppant. Remove the test tube and weigh it. The mass of the unstable proppant is M2. Then, connect the outlet end of the core to a filter and a backpressure valve in sequence. The backpressure valve is connected to a backpressure pump and an ion chromatograph respectively. Open the nitrogen intermediate container, raise the pressure of the core to the formation pressure, synchronously increase the confining pressure and the backpressure. The confining pressure is always greater than the inlet and outlet pressures of the core (ensuring that the proppant in the fracture will not be flushed out of the core by gas during the pressure building process), and at the same time, raise the temperature of the core to the formation temperature.

[0018] The purpose of step (4) is to eliminate the unstable and easily flowing proppant in the core fracture, ensuring that during the pressure building process, the proppant will not migrate due to loosening, resulting in blocking the outlet pipeline and piercing the backpressure valve.

[0019] The process of step (7) is as follows: After the experiment, relieve the pressure, collect the proppant in the outlet pipeline, dry it and weigh it. The mass is M3, and calculate the proppant sand production rate as: M3 / (M1 - M2).

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention combines with displacement experiments to simulate the scaling process under formation conditions, and by slowly reducing the confining pressure, it simulates the insufficient compaction force of the proppant by the artificial fracture and the excessive drag force of the fluid on the proppant, resulting in the instability of the proppant in the fracture, so as to study the synergistic effect of proppant sand production and scaling, which is more in line with the actual on-site conditions. At the same time, analyze the scaling situation through changes in permeability, pressure over time, ion concentration, etc., and the experimental results are more accurate. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of a core with proppant.

[0023] (a) shows winding the core containing proppant with heat-resistant and pressure-resistant tape; (b) is a sectional view.

[0024] Figure 2 It is a schematic diagram of the device used in the present invention to simulate the synergistic effect of proppant sand production and scaling in tight reservoirs.

[0025] In the figure: 1 - high-pressure displacement pump; 2, 7, 8, 11, 15, 18 - three-way valves; 3 - nitrogen intermediate container; 4 - formation water intermediate container; 5, 6, 20 - upper valves of intermediate containers; 9, 12, 16, 19 - pressure gauges; 10 - core holder; 13 - confining pressure pump; 14 - filter; 17 - backpressure valve; 21 - backpressure intermediate container; 22 - backpressure pump; 23 - ion chromatograph; 24 - computer.

[0026] Figure 3 It is the curve of the differential pressure at the inlet and outlet of the core changing with time.

[0027] Figure 4 It is the curve of the calcium ion concentration changing with the cumulative injection volume (PV). Specific implementation manners

[0028] The present invention will be further described below with reference to the accompanying drawings to facilitate the understanding of those skilled in the art of the present technology. However, it should be clear that the present invention is not limited to the scope of the specific implementation manners. For those skilled in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, they are all within the scope of protection.

[0029] Refer to Figure 1 and Figure 2 .

[0030] An experimental method for simulating the synergistic effect of proppant sand production and scaling in a tight reservoir fracturing successively includes the following steps:

[0031] (1) Select a suitable core, and through high-temperature baking, fully remove the water components contained in the cleaned core. After baking at 100°C for 12 h, weigh the core, and then take it out every 2 h for weighing until the mass of the core no longer changes;

[0032] (2) Use a core cutting machine to cut the core into two pieces along the central axis of the core, process the cut section, and use CPS copper oxide to repair the parts of the core that fall off or are damaged during the cutting process to ensure the integrity of the cut core without damage;

[0033] (3) Spread a certain amount of proppant with a particle size of 70 - 150 mesh evenly on the cut end face of the core, then close the core, and use heat-resistant and pressure-resistant tape to wind the core containing the proppant to ensure that the proppant inside the core will not fall out. The mass of the proppant is M1;

[0034] (4) Place the core containing proppant in the core holder 10, and connect the displacement pump 1, nitrogen intermediate container 3, formation water intermediate container 4, core holder 10 and test tube through pipelines. Apply a confining pressure of 3 - 5 Mpa to the core holder 10 through the confining pressure pump 13. Set the pump speed of the displacement pump 1 to 0.05 ml / min, and slowly inject nitrogen into the core. After the mass of the test tube no longer changes, increase the pump speed of the displacement pump 1 to 0.1 ml / min until there is no obvious increase in particles in the outlet test tube. Remove the test tube and weigh it. Record the mass of the unstable proppant as M2. Connect the outlet end of the core to the filter 14, three-way valve 15, and backpressure valve 17 in sequence. The backpressure valve is respectively connected to the backpressure intermediate container 21, backpressure pump 22, ion chromatograph 23, and computer 24. Open the upper valve 5 of the nitrogen intermediate container, close the valve after a moment, open the three-way valve 8 at the inlet end of the core holder 10, and repeat the above operation until the required formation pressure is reached. At the same time, synchronously increase the confining pressure and backpressure. The confining pressure is always 7 - 9 Mpa greater than the inlet and outlet pressures to ensure that the proppant in the fracture will not be flushed out of the core by gas during the pressure building process. Open the oven to heat the device;

[0035] (5) Scaling process: After the pressure and temperature reach the formation conditions and the gas production is stable for 30 min, measure the initial permeability. Then close the upper valve 5 of the nitrogen intermediate container, open the upper valve 6 of the formation water intermediate container, set the pump speed of the displacement pump to 0.01 ml / min, open the outlet three-way valve 15, measure the core permeability multiple times at a certain time interval, and record the change of the inlet and outlet pressures with time. Synchronously measure the ion concentration through the ion chromatograph 23;

[0036] (6) Synergistic process of proppant sand production and scaling: After the temperature rise is stable, pressurize the formation water intermediate container 4 to the formation pressure through the displacement pump 1, and open the upper valve 6 of the intermediate container. Set the pump speed of the displacement pump 1 to 0.01 ml / min, open the ion chromatograph 23, and record the initial ion concentration. After the outlet flow rate is stable, slowly reduce the confining pressure. When there is an obvious pressure difference between the inlet and outlet, stop reducing the confining pressure. Measure the core permeability every 10 minutes, record the change of the inlet and outlet pressures with time, and draw a curve of the pressure difference with time (see Figure 3 ), synchronously measure the ion concentration through the ion chromatograph, and draw a curve of the calcium ion concentration with the cumulative injection volume (see Figure 4 );

[0037] (7) After the experiment is completed, close the three-way valve 8 at the core inlet, reduce the backpressure and confining pressure, clean the outlet pipeline and filter with acetic acid, collect the proppant in the outlet pipeline and dry and weigh it. The mass is M3.

[0038] Table 1 shows the composition table of formation water.

[0039] Table 1 Composition of Formation Water

[0040]

Claims

1. An experimental method for simulating the synergistic effect of proppant sand production and scaling in tight reservoirs, which successively includes the following steps: (1) Select a suitable core, clean it and then bake it at high temperature to fully remove the moisture contained in the core; (2) Axially cut the core in step (1) and treat the cutting surface; (3) Fill the cross-section of the core in step (2) with proppant and suture the core with heat-resistant and pressure-resistant tape; (4) Heating and pressure increasing process: Place the core with proppant in a core holder, use a confining pressure pump to apply confining pressure to the core holder, inject nitrogen into the inlet end of the core through a displacement pump. After the outlet end of the core stably emits gas, raise the pressure of the core holder to the formation pressure and raise the temperature to the formation temperature; (5) Scaling process: When the pressure and temperature reach the formation conditions, measure the initial permeability of the core. Use formation water to displace the core with proppant. During the displacement process, measure the permeability of the core multiple times, record the change of the pressure difference between the inlet and outlet of the core with time, and simultaneously measure the ion concentration at the outlet end of the core through an ion chromatograph; (6) Synergistic effect process of proppant sand production and scaling: After the flow rate at the outlet end of the core is stable, slowly reduce the confining pressure, continue to use formation water to displace the core. During the displacement process, measure the permeability of the core multiple times, record the change of the pressure difference between the inlet and outlet of the core with time, and simultaneously measure the ion concentration at the outlet end of the core through an ion chromatograph; (7) Depressurize, collect the proppant in the outlet pipeline, and calculate the proppant sand production rate.

2. The experimental method for simulating the synergistic effect of sand production and scaling of proppants in tight reservoirs according to claim 1, wherein The process of step (2) is as follows: Use a core cutter to cut the core into two pieces along the central axis of the core. Use CPS copper oxide to repair the parts of the core that fall off or are damaged during the cutting process to ensure the integrity of the core.

3. The experimental method for simulating the synergistic effect of sand production and scaling of proppants in tight reservoirs according to claim 1, characterized in that, The process of step (3) is as follows: Spread the proppant with a particle size of 70 - 150 mesh evenly on the core cutting surface, then close the core, and wind the core with proppant with heat-resistant and pressure-resistant tape to ensure that the proppant in the core will not fall out. The mass of the proppant is M1.

4. The experimental method for simulating the synergistic effect of sand production and scaling of proppants in tight reservoirs as described in claim 3, wherein The process of step (4) is as follows: Place the core with proppant in a core holder. The inlet end of this core holder is respectively connected to a displacement pump through a nitrogen intermediate container and a formation water intermediate container, the outlet end is connected to a test tube, a confining pressure pump is connected in the middle, and pressure gauges are connected at both ends. First, use the confining pressure pump to apply confining pressure to the core holder, and then inject nitrogen into the core through the displacement pump until there is no obvious increase in particles in the test tube. The test tube contains unstable proppant. Remove the test tube and weigh it. The mass of the unstable proppant is M2; Then connect the outlet end of the core to a filter and a backpressure valve in sequence. The backpressure valve is respectively connected to a backpressure pump and an ion chromatograph. Open the nitrogen intermediate container, raise the pressure of the core to the formation pressure, synchronously raise the confining pressure and the backpressure. The confining pressure is always greater than the pressure at the inlet and outlet of the core, and at the same time raise the temperature of the core to the formation temperature.

5. The experimental method for simulating the synergistic effect of sand production and scaling of proppants in tight reservoirs according to claim 4, wherein The process of step (7) is as follows: After the experiment is completed, depressurize, collect the proppant in the outlet pipeline, dry and weigh it, and the mass is M3. Calculate the proppant sand production rate as: M3 / (M1 - M2).

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