Tight oil refracturing process method for improving oil-water replacement efficiency

Abstract

The invention discloses a tight oil refracturing process method for improving oil-water replacement efficiency. The method comprises the following steps: designing a surfactant system which has the advantages of hydrophilic wetting, moderate oil washing efficiency and complete demulsification; the preposed fracturing fluid with the characteristic of low viscosity performance is designed; pre-fracturing fluid containing a surfactant is injected into a target layer in a slug mode; according to the characteristics that the low-viscosity fracturing fluid is dispersed in stress, small in ground stress interference and easier to filter off the fracture bandwidth, the low-viscosity pre-fracturing fluid with the preset amount is injected in a slug mode, a surface active agent system with special performance is added into the low-viscosity pre-fracturing fluid, and then the sand-carrying fluid containing acid is pumped; the purposes of increasing the bandwidth direction wave and area of the crack, delaying oil washing and dredging an oil flow channel are achieved, the proportion of crude oil near a wellbore pushed to the far-well end along with pumping of fracturing fluid can be reduced, the oil-water replacement efficiency is improved, crude oil is extracted more easily, and therefore the problems that reconstruction in the bandwidth direction of the re-fracturing crack is insufficient, and the re-fracturing effect is poor are solved. And the yield increase of the old well is realized.

Description

Technical Field

[0001] The present invention belongs to the field of fracturing in oil production technology, and more specifically, particularly relates to a method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement. Background Art

[0002] The matrix permeability of tight oil reservoirs in the Songliao Basin is low (less than 2.0×10-3um2), the pore throat radius is small (0.063 - 1.104μm), natural fractures are not well developed, and the horizontal two-way stress difference is between 4 - 6MPa. Currently, such reservoirs are mostly developed by a well pattern of 600m×300m, natural energy + large-scale volume fracturing of vertical wells to form vertical fractures. The initial single-well output is 3 - 6 tons, achieving good initial development results. However, the production decline is rapid, with an annual decline rate exceeding 30%, and the predicted recovery factor in 10 years is only 3.56%, seriously affecting the development effect, which has become the main problem faced in the development of such reservoirs.

[0003] Currently, repeated fracturing is used to increase the production of old wells, but there are two problems: one is that for tight reservoirs with underdeveloped natural fractures and a horizontal two-way stress difference greater than 3MPa, it is difficult to form a complex fracture network through large-scale volume fracturing, resulting in insufficient transformation in the fracture bandwidth direction; the other is that currently, an oil-displacing agent mainly composed of surfactant is added during repeated fracturing. The oil washing speed is fast, and the washed-out oil is pushed to the far-well end with the injection of fracturing fluid, making it difficult to produce. This is manifested as a relatively high initial water cut after repeated fracturing, a large amount of ineffective liquid production, and a low oil replacement rate of fracturing fluid. The above two problems seriously affect the stimulation effect of repeated fracturing of old wells. Therefore, we need to provide a method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement. By using the characteristics of low-viscosity fracturing fluid with stress dispersion, less interference from in-situ stress, and easier filtration loss to the fracture bandwidth, through first slug-injecting a preset amount of low-viscosity pre-fracturing fluid and adding a surfactant system with special properties, and then pumping a sand-carrying fluid containing acid, the purpose of increasing the swept area in the fracture bandwidth direction, delaying oil washing, and dredging the oil flow channel can be achieved. It can reduce the proportion of crude oil near the wellbore being pushed to the far-well end with the injection of fracturing fluid, improve the efficiency of oil-water displacement, and make it easier to produce crude oil, thereby solving the technical problems of insufficient transformation in the fracture bandwidth direction after repeated fracturing and high initial water cut after fracturing, and realizing the stimulation of old wells.

[0005] To achieve the above purpose, the present invention provides the following technical solution: A method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement, comprising the following steps:

[0006] Design a surfactant system with hydrophilic wetting, moderate oil washing efficiency, and complete demulsification;

[0007] Design a pre - fracturing fluid with low - viscosity performance characteristics;

[0008] Inject the surfactant - containing pre - fracturing fluid into the target interval in a slug - type manner at a displacement higher than the formation fracture pressure;

[0009] After injecting the surfactant - containing pre - fracturing fluid into the target interval in a slug - type manner, inject a sand - carrying fluid containing acid solution into the target interval;

[0010] After the fracturing operation is completed, shut - in the well. The shut - in time is designed to open the well for production after sufficient oil - water displacement.

[0011] Preferably, the designed surfactant system has hydrophilic wetting, moderate oil - washing efficiency, and complete demulsification, including:

[0012] It is required that the wetting angle is less than 22°, so that the surfactant adsorbs on the surface of reservoir rocks. After the surfactant adsorbs on the surface of reservoir rocks, the rocks change from oil - wet to water - wet, which helps to strip the remaining oil on the rock surface;

[0013] It is required that the oil - washing efficiency is 40 - 60%, the oil - washing time is 4 - 6 days, the oil - washing capacity and oil - washing speed are moderate, and the oil is washed sufficiently after the fracturing fluid is injected;

[0014] It is required that the demulsification rate is greater than or equal to 90%, so that the surfactant is completely demulsified after oil - washing, enabling more crude oil to flow back to the wellbore in the form of single - phase flow, reducing the flow resistance of crude oil, and facilitating drainage.

[0015] Preferably, the pre - fracturing fluid with low - viscosity performance characteristics includes:

[0016] Low viscosity: The apparent viscosity of the pre - fracturing fluid is less than 10 mPa·s.

[0017] Preferably, injecting the surfactant - containing pre - fracturing fluid into the target interval in a slug - type manner at a displacement higher than the formation fracture pressure includes:

[0018] It is injected in 1 - 6 slugs. The first slug injects a low - viscosity pre - fracturing fluid without surfactant, which only provides a pressure - maintaining barrier without washing oil;

[0019] The 2nd - 6th slugs inject a low - viscosity pre - fracturing fluid containing 0.1 - 1.0% surfactant, and the surfactant is injected in an increasing - concentration manner.

[0020] Preferably, the specific calculation method for the dosage of the pre - fracturing fluid is:

[0021] V 前置 =(1.2 - 2.0)×(M 采油 / ρ 油 +M 采水 / ρ 水 );

[0022] Among them, V 前置 is the volume of the preflush fluid, with the unit of m 3 ; M 采油 is the mass of the oil produced from the target layer, with the unit of t; M 采水 is the mass of the produced water from the target layer, with the unit of t; ρ 油 is the density of crude oil, with the unit of g / cm 3 ; ρ 水 is the density of the produced water, with the unit of g / cm 3 .

[0023] Preferably, after the preflush fluid containing surfactant is injected into the target layer in a slug manner, a sand-carrying fluid containing acid solution is injected into the target layer, including:

[0024] Acid solution: weak acids such as citric acid, glacial acetic acid, and phosphoric acid with a mass concentration of 0.05 - 0.5%, but not limited to these weak acids.

[0025] Preferably, the specific calculation method for the dosage of the sand-carrying fluid is:

[0026] V 携砂 =(10 - 15)×H 射开 / 0.125;

[0027] Among them, V 携砂 is the volume of the sand-carrying fluid, with the unit of m 3 ; H 射开 is the perforated thickness of the target layer.

[0028] Preferably, after the fracturing construction is completed, the well is shut in, and the shut-in time is designed to open the well for production after sufficient oil-water displacement, including:

[0029] The shut-in time is designed to open the well for production after sufficient oil-water displacement.

[0030] Preferably, the calculation method for the shut-in time is that when it is the nth day of shut-in, P n -P n-1 <0.5 MPa, and continue to shut in for 4 days, that is, the total shut-in time is n + 4 days; among them, P n is the wellhead pressure on the nth day, with the unit of MPa.

[0031] The technical effects and advantages of the present invention: A tight oil refracturing process method for improving the oil-water displacement efficiency provided by the present invention has the following advantages compared with the prior art:

[0032] The present invention utilizes the characteristics of low-viscosity fracturing fluid, such as stress dispersion, little interference from in-situ stress, and easier filtration loss to the fracture bandwidth. By first injecting a preset amount of low-viscosity pre-fracturing fluid in a slug pattern and adding a surfactant system with special properties thereto, and then pumping a sand-carrying fluid containing acid, the object of improving the swept area in the fracture bandwidth direction, delaying oil washing, and dredging the oil flow channel can be achieved. The proportion of crude oil near the wellbore being pushed to the far well end by the pumping of the fracturing fluid can be reduced, the oil-water displacement efficiency can be improved, and it is easier to produce crude oil, thereby solving the technical problems of insufficient transformation in the fracture bandwidth direction during refracturing and high water cut in the initial stage after fracturing, and realizing production increase in old wells.

[0033] Other features and advantages of the present invention will be described in the following specification, and part of them will be obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained through the structures pointed out in the specification and the drawings. Brief Description of the Drawings

[0034] Figure 1 It is the water cut curve of the produced fluid in the indoor core huff and puff experiment of the present invention;

[0035] Figure 2 It is the process flow chart of the refracturing process method of the present invention. Detailed Embodiments

[0036] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the following further details the present invention in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0037] The present invention provides a Figure 1-2 dense oil refracturing process method for improving oil-water displacement efficiency as shown, including the following steps:

[0038] Design a surfactant system with hydrophilic wettability, moderate oil washing efficiency, and complete demulsification;

[0039] Design a pre-fracturing fluid with the performance characteristic of low viscosity;

[0040] Inject the surfactant-containing pre-fracturing fluid into the target interval in a slug pattern at a displacement higher than the formation fracture pressure;

[0041] After injecting the surfactant-containing pre-fracturing fluid into the target interval in a slug pattern, inject a sand-carrying fluid containing acid solution into the target interval;

[0042] After the fracturing construction is completed, shut in the well, and the shut-in time is designed to open the well for production after sufficient oil-water displacement.

[0043] The described design has a surfactant system with hydrophilic wetting, moderate oil washing efficiency, and complete demulsification, including:

[0044] It is required that the wetting angle is less than 22°, so that the surfactant adsorbs on the surface of the reservoir rock. After the surfactant adsorbs on the surface of the reservoir rock, the rock changes from oil-wetting to hydrophilic wetting, which helps to strip the remaining oil on the rock surface;

[0045] It is required that the oil washing efficiency is 40-60%, the oil washing time is 4-6 days, the oil washing capacity and oil washing speed are moderate, and the oil is washed sufficiently after the fracturing fluid is injected;

[0046] Specifically, after the fracturing fluid is injected, the oil is washed sufficiently to achieve the purpose of delaying oil washing. It can not only wash out the remaining oil in the reservoir, but also prevent the crude oil from being pushed to the far wellbore end by the fracturing fluid due to too fast oil washing, resulting in the problem of difficult production. The fracturing fluid mainly plays the role of fracturing the formation and creating fractures with a certain geometric size during the fracturing process, and at the same time quickly filters into the reservoir pores to displace the crude oil;

[0047] It is required that the demulsification rate is greater than or equal to 90%, so that the surfactant is fully demulsified after oil washing, enabling more crude oil to flow back to the wellbore in the form of single-phase flow, reducing the flow resistance of the crude oil, and facilitating drainage. The specific performance and effect differences are shown in the table respectively;

[0048] Surfactant performance comparison table

[0049]

[0050] The described design has a pre-fracturing fluid with low viscosity performance characteristics, including:

[0051] Low viscosity: The apparent viscosity of the pre-fracturing fluid is less than 10 mPa·s,

[0052] Specifically, the viscosity of the pre-fracturing fluid is less than 10 mPa·s. This low-viscosity pre-fracturing fluid is more likely to enter the reservoir matrix with a pore throat radius of 0.1-0.01 m, is less affected by in-situ stress interference, and is more likely to filter loss in the direction of the fracture bandwidth, which can improve the swept area of the fracturing fluid in the direction of the fracture bandwidth;

[0053] The pre-fracturing fluid containing surfactant is injected into the target interval in a slug manner at a displacement higher than the formation fracture pressure, including:

[0054] It is injected in 1-6 slugs in total. The first slug injects a low-viscosity pre-fracturing fluid without surfactant, which only provides a pressure-holding barrier without washing oil;

[0055] The 2nd-6th slugs inject a low-viscosity pre-fracturing fluid containing 0.1-1.0% surfactant, and the surfactant is injected in an increasing concentration manner;

[0056] Specifically, the purpose is to reduce the problem that the washed-out crude oil is pushed to the far end of the wellbore, making it difficult to produce the crude oil.

[0057] The specific calculation method for the dosage of the preflush fluid is as follows:

[0058] V 前置 =(1.2 - 2.0)×(M 采油 / ρ 油 +M 采水 / ρ 水 );

[0059] Wherein, V 前置 is the volume of the preflush fluid, with the unit of m 3 ; M 采油 is the mass of the produced oil in the target layer, with the unit of t; M 采水 is the mass of the produced water in the target layer, with the unit of t; ρ 油 is the density of the crude oil, with the unit of g / cm 3 ; ρ 水 is the density of the produced water, with the unit of g / cm 3 ;

[0060] After the slug injection of the preflush fluid containing surfactant into the target layer, a proppant-carrying fluid containing acid solution is injected into the target layer, including:

[0061] Acid solution: weak acids such as citric acid, glacial acetic acid, phosphoric acid, etc. with a mass concentration of 0.05 - 0.5%, but not limited to these weak acids;

[0062] Specifically, adding the acid solution is to dredge the oil flow channels in the formation, making the washed-out crude oil flow back to the wellbore more easily. The proppant-carrying fluid is the fluid used in the fracturing process, and its main function is to carry the proppant into the formation to enhance the supporting effect of the proppant in the fracture, thereby improving the permeability of the oil and gas reservoir and increasing the oil and gas production efficiency. The proppant-carrying fluid should have good sand suspension performance and filtrate loss performance to ensure that the proppant remains suspended in the fluid and reduce the loss of the fluid in the formation;

[0063] The specific calculation method for the dosage of the proppant-carrying fluid is as follows:

[0064] V 携砂 =(10 - 15)×H 射开 / 0.125;

[0065] Wherein, V 携砂 is the volume of the proppant-carrying fluid, with the unit of m 3 ; H 射开 is the perforated thickness of the target layer;

[0066] After the fracturing construction is completed, shut-in the well. The shut-in time is designed to open the well for production after sufficient oil-water displacement, including:

[0067] The shut-in time is designed to start production after sufficient oil-water displacement;

[0068] The calculation method of the shut-in time is that when it is the nth day of shut-in, P n -P n-1 <0.5 MPa, then continue to shut in for another 4 days, that is, the total shut-in time is n + 4 days; where P n is the wellhead pressure on the nth day, with the unit of MPa;

[0069] Working principle: Taking Well A in a certain oilfield as an example, the oil washing efficiency of the surfactant system is 42.4 - 50.3%, the wetting angle is 20°, and the demulsification rate is about 94%; the apparent viscosity of the preflush fluid is 3 - 5 mPa·s; injecting the preflush fluid of 9083 m 3 in three slugs into the target layer at a displacement of 3 - 4 m 3 , injecting 2290 m 3 of the preflush fluid without surfactant in the first slug, injecting 4525 m 3 of the preflush fluid with low-concentration surfactant in the second slug, and injecting 2268 m 3 of the preflush fluid with high-concentration surfactant in the third slug; injecting 1271 m 3 of the sand-carrying fluid containing 0.2% glacial acetic acid into the target layer, and implementing in-situ temporary plugging and sand-carrying construction during this process; after the fracturing construction, shut in for 17 days and then start production;

[0070] After refracturing, Well A produced oil when 10.6% of the fracturing fluid was backflowed. Compared with the fracturing process before optimization, the ineffective liquid production was reduced by 2665 t; the water cut decreased faster. The initial daily oil production after fracturing was 6.8 t, and the water cut was 28.7%. The daily oil production maintained above 5.0 t 5 months after fracturing. It is expected that the stage recovery degree can be increased by 1.24 percentage points, indicating that the refracturing process has a higher oil-water displacement efficiency and has good effects of increasing production and stabilizing production.

[0071] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement, characterized in that, it comprises the following steps: Design a surfactant system with hydrophilic wetting, moderate oil washing efficiency, and complete demulsification; Design a pre-fracturing fluid with low viscosity performance characteristics; Inject the surfactant-containing pre-fracturing fluid into the target interval in a slug manner at a displacement higher than the formation fracture pressure; After injecting the surfactant-containing pre-fracturing fluid into the target interval in a slug manner, inject a proppant-carrying fluid containing acid solution into the target interval; After the fracturing construction is completed, shut in the well, and the shut-in time is designed to open the well for production after sufficient oil-water displacement.

2. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 1, characterized in that: The design of the surfactant system with hydrophilic wetting, moderate oil washing efficiency, and complete demulsification includes: The wetting angle is required to be less than 22°, so that the surfactant adsorbs on the surface of the reservoir rock. After the surfactant adsorbs on the surface of the reservoir rock, the rock changes from oil-wetting to hydrophilic wetting, which helps to strip the remaining oil on the rock surface; The oil washing efficiency is required to be 40-60%, the oil washing time is 4-6 days, the oil washing capacity and oil washing speed are moderate, and the oil is washed sufficiently after the injection of the fracturing fluid; The demulsification rate is required to be greater than or equal to 90%, so that the surfactant is completely demulsified after oil washing, so that more crude oil flows back to the wellbore in the form of single-phase flow, reducing the flow resistance of crude oil and facilitating drainage.

3. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 1, characterized in that: The design of the pre-fracturing fluid with low viscosity performance characteristics includes: Low viscosity: The apparent viscosity of the pre-fracturing fluid is less than 10 mPa·s.

4. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 1, characterized in that: The injection of the surfactant-containing pre-fracturing fluid into the target interval in a slug manner at a displacement higher than the formation fracture pressure includes: A total of 1-6 slugs are injected. The first slug first injects a low-viscosity pre-fracturing fluid without surfactant, which only provides a pressure-holding barrier without washing oil; The 2-6th slugs inject a low-viscosity pre-fracturing fluid containing 0.1-1.0% surfactant, and the surfactant is injected in an increasing concentration manner.

5. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 4, characterized in that: The specific calculation method for the dosage of the pre-fracturing fluid is: V 前置 = (1.2 to 2.0) × (M 采油 / ρ 油 + M 采水 / ρ 水 ); Among them, V 前置 is the volume of the pre-fracturing fluid, with the unit of m 3 ; M 采油 is the mass of the produced oil in the target layer, with the unit of t; M 采水 is the mass of the produced water in the target layer, with the unit of t; ρ 油 is the density of crude oil, with the unit of g / cm 3 ; ρ 水 is the density of the produced water, with the unit of g / cm 3 .

6. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 1, characterized in that: After injecting the surfactant-containing pre-fracturing fluid into the target interval in a slug manner, injecting a proppant-carrying fluid containing acid solution into the target interval includes: Acid solution: Weak acids such as citric acid, glacial acetic acid, and phosphoric acid with a mass concentration of 0.05-0.5%, but not limited to these weak acids.

7. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 6, characterized in that: The specific calculation method for the dosage of the proppant-carrying fluid is: V 携砂 = (10 to 15) × H 射开 / 0.125; Among them, V 携砂 is the volume of the sand-carrying fluid, with the unit of m 3 ; H 射开 is the perforated thickness of the target formation.

8. The method for repeated fracturing of tight oil to improve the efficiency of oil-water displacement according to claim 1, characterized in that: After the fracturing construction is completed, shut in the well, and the shut-in time is designed to open the well for production after sufficient oil-water displacement, including: The shut-in time is designed to open the well for production after sufficient oil-water displacement.

9. A tight oil refracturing process method for improving the efficiency of oil-water displacement according to claim 8, characterized in that: The calculation method of the shut-in well time is that when it is the nth day of shut-in, P n - P n-1 <0.5 MPa, continue to shut in for another 4 days, that is, the total shut-in well time is n + 4 days; where P n is the wellhead pressure on the nth day, and the unit is MPa.

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