Immersion Index Method for Comprehensive Evaluation of Liquid Phase Damage in Fractures of Oil and Gas Reservoirs

Abstract

The present invention discloses an immersion index method for comprehensive evaluation of liquid phase damage in oil and gas reservoir fractures. Cores of the oil and gas reservoir to be evaluated are prepared, and water phase spontaneous imbibition experiments, water phase displacement experiments, and stress sensitivity evaluation experiments are respectively carried out on the cores. Each parameter is calculated, and then the water phase trapping damage rate and immersion index of the cores are calculated based on the parameters obtained from the above calculations. The present invention takes into account the time effect of water phase diffusion, considers the permeability damage caused by the reduction of rock strength due to water-rock reaction, determines the evaluation index for water-rock reaction damage, makes up for the deficiencies of the evaluation methods for liquid phase damage in oil and gas reservoirs, can achieve more accurate prediction of the water invasion damage effect of oil and gas reservoirs and the stimulation operation effect of reservoirs, and has certain significance for indoor experimental research and field application.

Description

Technical Field

[0001] The present invention relates to the field of reservoir protection for oil and gas reservoirs, and particularly to an immersion index method for comprehensive evaluation of liquid phase damage in fractures of oil and gas layers. Background Art

[0002] In the current situation of continuous growth in global oil and gas demand and continuous decline in conventional oil and gas production, tight oil and gas reservoirs with great potential have gradually become a new highlight area in global oil exploration. Tight oil and gas reservoirs usually exhibit characteristics such as low porosity, low permeability, prominent capillary phenomena, and poor fluidity. Exploitation highly relies on large-scale hydraulic fracturing. Only by establishing good seepage paths through hydraulic fracturing and improving the overall mass transfer capacity can industrial oil and gas flows be formed. During the fracturing transformation process, a large amount of liquid will enter the reservoir and stay in the reservoir. In addition, with the continuous reduction of well spacing and the increasing frequency of fracturing communication, liquid will also stay in the reservoir for a long time.

[0003] After the liquid enters the reservoir, due to the action of surface tension, a water film that cannot be back-produced will form on the fracture wall surface, reducing the gas seepage capacity. At the same time, it will trigger the hydration swelling of clay minerals, causing changes in pore structure and structural strength. Specifically, as time goes by, the rock strength decreases, the proppant embedding deepens, and the fracture width becomes narrower. Currently, the water sensitivity index is usually used to evaluate the permeability damage caused by clay swelling. However, the water sensitivity index mainly evaluates the influence of clay swelling on the permeability of matrix cores and cannot evaluate the fracture seepage capacity. Usually, the stress sensitivity index is used to evaluate the influence of fracture width on permeability. However, in the evaluation process of stress sensitivity, it is considered that the strength of the rock is constant. In fact, as the water-rock reaction progresses, the strength of the rock fracture surface will continuously decrease. Therefore, there are often certain deviations in the damage prediction through the stress sensitivity coefficient.

[0004] In summary, there are generally water phase trapping damage and water-rock reaction damage during the exploitation of tight oil and gas reservoirs. At the same time, there is no evaluation method for fracture water-rock reaction damage. To solve the above problems, the immersion index method for comprehensive evaluation of liquid phase damage in fractures of oil and gas layers is proposed, which has certain significance for indoor experimental research and field application. Summary of the Invention

[0005] The purpose of the present invention is to provide an immersion index method for comprehensive evaluation of liquid phase damage in fractures of oil and gas layers. This method measures the water phase self-imbibition rate of core samples and the change in rock stress sensitivity coefficient, and formulates the immersion index as an indicator, so as to more accurately predict the water invasion damage effect of oil and gas layers and the stimulation operation effect of reservoirs. To achieve the above purpose, the present invention provides the following technical solutions:

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] The immersion index method for comprehensive evaluation of liquid phase damage in fractures of oil and gas layers includes the following steps:

[0008] Step 1: Select the core of the reservoir matrix block, conduct an end-face spontaneous imbibition experiment, and record the wetting height h after a certain time t 1 ;

[0009] Step 2: Select the core of the same horizon, polish the core until its diameter is smaller than the difference between the inner diameter of the gripper rubber sleeve and the proppant particle size. After polishing, use the Brazilian splitting method to create artificial fractures; after laying proppants in the fractures, fix them with heat-shrinkable tubes, and record the core length L and diameter D

[0010] Step 3: Place the fractured core into the gripper, set the confining pressure and injection pressure, and test the initial permeability K 0 ;

[0011] Step 4: Keep the confining pressure unchanged, displace the core with distilled water. After liquid emerges from the outlet end of the core, replace it with nitrogen displacement. After the mass of the desiccant at the end stabilizes, test the permeability K 1 , calculate the degree of aqueous phase trapping damage S WPT , and the calculation formula is

[0012] Step 5: Change the confining pressure, test the permeability under different effective stresses respectively, and calculate the stress sensitivity coefficient S 1 , and the calculation formula is

[0013] Where: K i is the core permeability corresponding to the effective stress σ i , in mD; K * is the core permeability under the initial effective stress σ * ;

[0014] Step 6: Replace with distilled water displacement. Start timing after liquid emerges from the end. The displacement time t 2 , and the calculation formula is

[0015] Where: D is the core diameter, in mm; h is the wetting height of the matrix block core, in mm; t 1 is the spontaneous imbibition time, in h;

[0016] Step 7: After the liquid phase displacement is completed, replace it with nitrogen displacement. After the mass of the desiccant at the end stabilizes, test the permeability under different effective stresses, and calculate the stress sensitivity coefficient S 2 ;

[0017] Step 8: Calculate the leaching index M, and the calculation formula is

[0018] Where: S is the stress sensitivity coefficient; t 2 is the liquid displacement time, in h; M is the leaching index, in h-1 。

[0019] The beneficial effects of the present invention are as follows:

[0020] (1) It makes up for the deficiencies in the field of experimental evaluation of comprehensive damage of fracture liquid phase, and is of great significance for predicting reservoir stimulation effects, production dynamics, water invasion damage degree, fracture dynamic width, etc.;

[0021] (2) By evaluating the degree of water phase trapping and water-rock reaction damage in the target reservoir, it can provide a basis for targeted treatment of reservoir damage;

[0022] (3) By carrying out comprehensive evaluation of fracture liquid phase damage in oil and gas reservoirs, it can provide a basis for optimizing fracturing fluids and proppants. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the self-aspiration device.

[0024] Figure 2 It is a schematic diagram of the core displacement device.

[0025] Reference numerals in the figure: 1, computer; 2, balance; 3, nitrogen gas source; 4, gas source switch; 5, pressure gauge; 6, core holder; 7, pressure gauge; 8, confining pressure pump; 9, confining pressure switch; 10, desiccant; 11, soap bubble flowmeter. Detailed Embodiments

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0027] Refer to Figure 1-2 , for the leaching index method of comprehensive evaluation of fracture liquid phase damage in oil and gas reservoirs, outcrop cores of the Longmaxi Formation were selected to carry out comprehensive evaluation of fracture liquid phase damage. The specific operation steps are as follows:

[0028] Step 1: Select core LY-1 and hang it above the liquid container, only keeping the lower end surface in contact with the liquid surface. Record the wetting height of the core after 10 h, which is 16.3 mm;

[0029] Step 2: Select outcrop core LY-2 of the same horizon, polish the core so that it can fit the size of the holder after sand filling, and then use the Brazilian splitting method to create artificial fractures; after laying 20-30 mesh ceramsite in the fractures, close the fractures, pour out the excess ceramsite, and fix it with heat shrinkable tubes. Record the core length L as 53.4 mm and the diameter D as 23.8 mm;

[0030] Step 3: Place LY-2 in the gripper, set the confining pressure to 10 MPa, the injection pressure to 0.4 MPa, and the measured permeability K0 is 1578.02 mD.

[0031] Step 4: Displace the core with distilled water. After liquid emerges from the outlet end of the core, replace the displacement with nitrogen. After the mass of the desiccant at the end stabilizes, measure the permeability K 1-1 which is 1108.37 mD, and calculate the degree of aqueous phase trapping damage S WPT which is 29%;

[0032] Step 5: Change the confining pressure to 20 MPa and measure the permeability K 1-2 which is 969.82 mD, and calculate the stress sensitivity coefficient S 1 which is 0.14;

[0033] Step 6: Change the confining pressure to 10 MPa, replace the displacement with distilled water, start timing after liquid emerges from the outlet end of the core, and calculate the displacement time t to be 7.3 h.

[0034] Step 7: After the displacement is completed, replace the displacement with nitrogen. After the mass of the desiccant at the end stabilizes, measure K 2-1 which is 1001.12 mD. Change the confining pressure to 20 MPa and measure the permeability K 2-2 which is 689.65 mD, and calculate the stress sensitivity coefficient S 2 which is 0.38;

[0035] Step 8: Calculate the leaching index M to be 3.3%.

[0036] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered within the protection scope of the present invention.

Claims

1. Immersion Index Method for Comprehensive Evaluation of Liquid Phase Damage in Oil and Gas Reservoir Fractures It is characterized in that: Step 1: Select the core of the reservoir matrix block, conduct an end-face spontaneous imbibition experiment, and record the wetting height h after a certain time t 1 ; Step 2: Select cores of the same horizon, grind the cores until the diameter is smaller than the difference between the inner diameter of the gripper rubber sleeve and the proppant particle size. After grinding, use the Brazilian splitting method to create artificial fractures; after laying proppants in the fractures, fix them with heat shrinkable tubes, and record the core length L and diameter D; Step 3: Place the fractured core into the holder, set the confining pressure and injection pressure, and test the initial permeability K 0 ; Step 4: Keep the confining pressure constant. Displace the core with distilled water. After liquid comes out from the outlet end of the core, replace the displacement fluid with nitrogen. After the mass of the desiccant at the end becomes stable, measure the permeability K 1 , and calculate the degree of aqueous phase trapping damage S WPT . The calculation formula is Step 5: Change the confining pressure, test the permeability under different effective stresses respectively, and calculate the stress sensitivity coefficient S 1 , and the calculation formula is Where: K i is the core permeability under the corresponding effective stress σ i , in mD; K * is the core permeability under the initial effective stress σ * ; Step 6, replace with distilled water for displacement. Start timing after the liquid comes out at the end. The displacement time is t 2 , and the calculation formula is Where: D is the core diameter, in mm; h is the wetting height of the matrix core, in mm; t 1 is the spontaneous imbibition time, in h; Step 7: After the liquid-phase displacement is completed, replace it with nitrogen displacement. After the mass of the desiccant at the end stabilizes, measure the permeability under different effective stresses and calculate the stress sensitivity coefficient S 2 ; Step 8, calculate the leaching index M, and the calculation formula is Where: S is the stress sensitivity coefficient; t 2 is the liquid displacement time, h; M is the leaching index, h -1 .

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