Composite acid fracturing method and application thereof

Abstract

The invention belongs to the field of oil-gas field development, and discloses a composite acid fracturing method and application thereof. According to the method, aiming at the problem that an ultra-deep and high-temperature carbonate rock reservoir is difficult to reform at present, through liquid nitrogen cold brittle fracturing, the crack initiation and extension difficulty is reduced, the crack complexity is improved, and the reformation degree of the reservoir is improved. The minimum liquid nitrogen consumption is defined based on the law of conservation of heat, and the cold brittleness effect is guaranteed. On the basis of traditional liquid nitrogen fracturing, the temperature characteristic of the liquid nitrogen fracturing is utilized in an overlapped mode, and the using benefit of liquid nitrogen is improved; and meanwhile, the reduced temperature of the reservoir is beneficial to reducing the acid fracturing reaction rate and improving the transformation effect of the deep part of the reservoir.

Description

Technical Field

[0001] The invention belongs to the field of oil and gas field development, and in particular relates to a composite acid fracturing method and application thereof. Background Art

[0002] Acid fracturing is the primary process for transforming carbonate reservoirs. After years of development, it is now the most mature and effective targeted development process. Current innovations and breakthroughs in acid fracturing focus on improving and optimizing acid fluid performance and the injection process.

[0003] In the invention patent (CN201510341176.8) "A method for acid fracturing carbonate reservoirs with variable hydrochloric acid concentration", high-concentration and low-concentration hydrochloric acid are injected sequentially based on a variant of conventional acid fracturing. The core lies in the injection process of hydrochloric acid of different concentrations.

[0004] In the invention patent (CN201510381559.8) "A high-efficiency acid fracturing method suitable for fracture-cavity carbonate reservoirs", different acid amount calculation methods are proposed for different reservoir types, such as fracture-cavity type and fracture type, in the conventional acid fracturing process, and then the acid fracturing fluid is designed, and finally the pumping program is set and the residual acid return is designed.

[0005] In the invention patent (CN201710078993.8) "Supercritical CO2 Atomization Deep Penetration Acid Fracturing Method", the changes in pressure and temperature during the acid fracturing process are used to transform liquid carbon dioxide into a supercritical state, and then form atomized acid with special acid liquid; the atomized acid formed can achieve a deep-penetrating acid-etched crack system, and then the conventional acid injection process is used to achieve the formation of high-conductivity main cracks, thereby increasing the impact range of the acid liquid.

[0006] In the invention patent (CN201910385857.2) "Carbonate Reservoir Fracture Network Volume Acid Fracturing Method", a near-wellbore fracture network is constructed by pre-placement of slickwater acid solution, and then plant glue is pumped in to complete the subsequent acid diversion transformation, ultimately forming a volume fracture network.

[0007] In the invention patent (CN201910524782.1) "A large-volume composite acid fracturing transformation method for carbonate reservoirs", the acid reaction rate is reduced and the length of acid-etched cracks is increased by multi-stage alternating injection of slick water, gelled acid, and variable viscosity acid, which is beneficial to connecting natural cracks and increasing the volume of reservoir acid fracturing transformation; the wormhole acid system is used to achieve deep penetration of acid in the cracks, forming a longer effective action distance and improving the conductivity; the use of resistance-reducing acid in the later stage can expand the degree of acid corrosion of the reservoir around the wellbore and further improve the conductivity of the reservoir in the near-wellbore area; volumetric acid fracturing of carbonate reservoirs with relatively developed fractures can make full use of the natural fractures of the reservoir, and with the help of the dissolution effect of acid on carbonate minerals, while expanding and extending cracks and caves, it connects natural fractures and increases the conductivity of seepage channels, thereby achieving the purpose of maximizing production increase.

[0008] In the invention patent (CN201910842977.0) "A variable concentration carbon dioxide acid fracturing method", the carbon dioxide injection concentration is changed by controlling the valve opening in series between the acid tank and the liquid tank, thereby improving the transformation effect.

[0009] Currently, no quantitative design method for cold brittle cracking effect is involved in acid fracturing related designs. Liquid nitrogen is mainly used for its property of increasing crack complexity, and its performance is not fully utilized. There is room for further improvement in its application benefits. Summary of the Invention

[0010] This invention addresses the current difficulties in transforming ultra-deep, high-temperature carbonate reservoirs. By using liquid nitrogen cold fracturing, the difficulty of fracture initiation and extension is reduced, fracture complexity is increased, and the degree of reservoir transformation is enhanced. Based on the law of conservation of heat, a minimum liquid nitrogen dosage is defined to ensure the cold fracturing effect. This technology leverages the temperature characteristics of traditional liquid nitrogen fracturing, improving the efficiency of liquid nitrogen use. Furthermore, the reduced reservoir temperature helps reduce the acid fracturing reaction rate, enhancing the transformation effect deep within the reservoir.

[0011] To achieve the above objectives, in a first aspect, the present invention provides a composite acid fracturing method, comprising the following steps:

[0012] S01: After perforating the target well, clean water is used for forward flushing to reduce the wellbore temperature;

[0013] S02: inject working fluid to perform small pressure test;

[0014] S03: After stopping the forward wash, inject liquid nitrogen;

[0015] S04: injecting resistance-reducing acid for acid fracturing;

[0016] S05: Inject gelled acid and perform acid fracturing.

[0017] In a preferred embodiment, in step S01, while performing forward washing, liquid nitrogen surface pump circulation is performed.

[0018] In a preferred embodiment, in step S02, the working fluid is slick water.

[0019] In a preferred embodiment, in step S03, the mass of the injected liquid nitrogen is m N2 ;

[0020] m N2 ≥(c 地层 / c N2 )×((T 地层 -T) / (TT N2 ))×L×h×d×ρ 地层 ;

[0021] Among them, c 地层 represents the specific heat capacity of the target reservoir rock mass, c N2 represents the specific heat capacity of liquid nitrogen, T 地层 represents the original temperature of the target reservoir rock mass, T represents the critical temperature of the target reservoir rock mass at which cold brittleness occurs, and T N2 represents the initial temperature of liquid nitrogen, L represents the total length of the fracture, h represents the reservoir thickness, d represents the distance along the fracture surface where the cold brittle effect occurs, ρ 地层 Indicates the average density of the target reservoir rock mass.

[0022] In a preferred embodiment, in step S03, the liquid nitrogen pumping capacity is ≤ the maximum capacity of the construction pressure limit.

[0023] In a preferred embodiment, in step S03, after the ground circulation is completed, the cement truck is stopped from washing and liquid nitrogen is pumped in.

[0024] In a preferred embodiment, in step S04, when injecting the drag-reducing acid, liquid nitrogen is injected simultaneously, and the liquid nitrogen injection displacement is ≤1m 3 / min.

[0025] In a preferred embodiment, in step S05, when injecting gelled acid, liquid nitrogen is injected simultaneously, and the liquid nitrogen injection displacement is ≤1m 3 / min.

[0026] In a preferred embodiment, in step S04 and step S05, the pumping rate of the friction-reducing acid and the gelling acid is the maximum rate that does not exceed the construction pressure limit.

[0027] In a second aspect, the present invention also provides the application of the aforementioned composite acid fracturing method in carbonate rock acid fracturing construction.

[0028] The specific operation process and principle of the present invention are explained as follows:

[0029] (1) Cooling down the washing cycle

[0030] After perforating the target well, a cement truck is used to perform a forward flushing cycle with clean water to lower the wellbore temperature and reduce subsequent liquid temperature loss. At the same time, liquid nitrogen is circulated by a surface pump.

[0031] (2) Use low viscosity working fluid (such as slippery water) for small pressure test

[0032] On the one hand, the low-pressure test obtains formation parameters, and on the other hand, it provides initial main fractures for the subsequent pumping of liquid nitrogen, thereby increasing the initial heat exchange area between the liquid nitrogen and the formation, improving the heat exchange efficiency, and improving the effect of cold brittle cracking.

[0033] (3) Pump in liquid nitrogen

[0034] After the surface circulation is complete, the cement truck is stopped for washing and liquid nitrogen is pumped in. The purpose of using liquid nitrogen as a pre-fluid is to, on the one hand, leverage its low temperature properties to create a sudden temperature drop within the deep, high-temperature reservoir, thereby inducing cold brittle fractures, and, on the other hand, to construct a complex fracture network. Furthermore, the low temperature effect can slow the etching reaction of subsequent acid fluids once they enter the bottom layer, thereby enhancing the etching effect deep within the reservoir.

[0035] The pumping displacement is the maximum displacement that does not exceed the construction pressure limit.

[0036] Pumping scale design:

[0037] In the early stage, the specific heat capacity c of the target reservoir rock mass can be determined based on experiments and research. 地层 , the specific heat capacity of liquid nitrogen c N2 , the average density of the rock mass ρ 地层 , the density of liquid nitrogen ρ N2 and other parameters.

[0038] Set the original formation temperature T 地层 , initial temperature of liquid nitrogen T N2 , the critical temperature T at which the rock mass undergoes cold brittleness.

[0039] According to the law of conservation of heat, the heat released by the formation is less than the heat absorbed by the injected liquid nitrogen:

[0040] c 地层 ×m 地层 ×(T 地层 -T)≤ c N2 ×m N2 ×(TT N2 ) Formula (1)

[0041] Where m is the mass of rock involved in heat exchange.

[0042] Assuming that the crack is a long straight crack and ignoring the complex crack network conditions, the total crack length is designed to be L.

[0043] Assuming that the cold brittle effect occurs within a range of distance d along the fracture surface, the mass of the rock mass involved in heat exchange is (assuming the reservoir thickness is h and the fracture height is equal to the reservoir thickness):

[0044] m 地层 =L×h×d×ρ 地层 Formula (2)

[0045] Combining equations (1) and (2) we can obtain the inequality about the mass of liquid nitrogen:

[0046] m N2 ≥(c 地层 / c N2 )×((T 地层 -T) / (TT N2 ))×L×h×d×ρ 地层 Formula (3)

[0047] Thus, the minimum scale (mass unit, converted to volume by density) required for cold brittleness under the conditions of a given design seam length can be obtained.

[0048] (4) Pumping in resistance-reducing acid

[0049] After the liquid nitrogen is pumped in, a relatively complex fracture network has been formed in the formation due to the cold brittleness and the characteristics of liquid nitrogen. At this time, low-viscosity drag-reducing acid is pumped in to enter, fill, and acid-etch the complex fracture network that has been formed, while further extending the length of the fractures.

[0050] The pumping volume is the maximum volume that does not exceed the construction pressure limit. During this period, liquid nitrogen can be injected at a low volume (no more than 1m 3 / min), increasing the flowback rate and reducing the residual acid in the seam.

[0051] (5) Pumping in gelled acid

[0052] Pump in gelled acid to etch the fracture surface in the main fracture channel to improve the conductivity of the main fracture. During this period, liquid nitrogen can be injected at a low rate (no more than 1m 3 / min), increasing the flowback rate and reducing the residual acid in the seam.

[0053] The beneficial effects of the present invention are:

[0054] The effects of the present invention are mainly reflected in five aspects: (1) using liquid nitrogen as a pre-fluid to increase the complexity of the fracture; (2) quantifying the minimum liquid nitrogen injection amount when the cold brittle fracture effect occurs through the principle of thermodynamic balance, ensuring the effect of cold brittle fracture and reducing the subsequent construction pressure; (3) the reservoir temperature lowered by liquid nitrogen is conducive to reducing the acid fracturing reaction rate and improving the transformation effect of the deep reservoir; (4) the liquid nitrogen injected in the early stage is conducive to rapid backflow; (5) when the well is opened for production, the liquid nitrogen dissolves in the crude oil, which is conducive to reducing the viscosity of the crude oil and realizing viscosity reduction development. DETAILED DESCRIPTION

[0055] In order to facilitate understanding of the present invention, the present invention is described in more detail below with reference to specific embodiments.

[0056] Example 1

[0057] This embodiment provides a liquid nitrogen composite acid fracturing method for reforming a high-temperature reservoir, and the specific steps are as follows:

[0058] (1) Cooling down the washing cycle

[0059] After perforating the target well, a cement truck is used to perform a forward flushing cycle with clean water to lower the wellbore temperature and reduce subsequent liquid temperature loss. At the same time, liquid nitrogen is circulated by a surface pump.

[0060] (2) Use low viscosity working fluid (such as slippery water) for small pressure test

[0061] On the one hand, the low-pressure test obtains formation parameters, and on the other hand, it provides initial main fractures for the subsequent pumping of liquid nitrogen, thereby increasing the initial heat exchange area between the liquid nitrogen and the formation, improving the heat exchange efficiency, and improving the effect of cold brittle cracking.

[0062] (3) Pump in liquid nitrogen

[0063] After the surface circulation is complete, the cement truck is stopped for washing and liquid nitrogen is pumped in. The purpose of using liquid nitrogen as a pre-fluid is to, on the one hand, leverage its low temperature properties to create a sudden temperature drop within the deep, high-temperature reservoir, thereby inducing cold brittle fractures, and, on the other hand, to construct a complex fracture network. Furthermore, the low temperature effect can slow the etching reaction of subsequent acid fluids once they enter the bottom layer, thereby enhancing the etching effect deep within the reservoir.

[0064] The pumping displacement is the maximum displacement that does not exceed the construction pressure limit.

[0065] Pumping scale design:

[0066] In the early stage, the specific heat capacity c of the target reservoir rock mass can be determined based on experiments and research. 地层 (The specific heat capacity of limestone reservoir is about 0.59 kJ / (kg·K)), the specific heat capacity of liquid nitrogen c N2(1.04kJ / (kg·K)), the average density of the rock mass ρ 地层 (2.8g / cm 3 ), the density of liquid nitrogen ρ N2 (0.808g / cm 3 ) and other parameters.

[0067] Set the original formation temperature T 地层 (150℃), liquid nitrogen initial temperature T N2 (-196℃), the critical temperature T (40℃) at which the target reservoir rock mass experiences cold brittleness.

[0068] According to the law of conservation of heat, the heat released by the formation is less than the heat absorbed by the injected liquid nitrogen:

[0069] c 地层 ×m 地层 ×(T 地层 -T)≤c N2 ×m N2 ×(TT N2 ) Formula (1)

[0070] Where m 地层 is the mass of rock involved in heat exchange.

[0071] Assuming that the crack is a long straight crack and ignoring the complex crack network conditions, the total crack length is designed to be L = 20m.

[0072] Assuming that the cold brittle effect occurs within a range of d = 1m along the fracture surface, the mass of the rock mass involved in heat exchange is (assuming the reservoir thickness is h and the fracture height is equal to the reservoir thickness):

[0073] m 地层 =L×h×d×ρ 地层 Formula (2)

[0074] Combining equations (1) and (2) we can obtain the inequality about the mass of liquid nitrogen:

[0075] m N2 ≥(c 地层 / c N2 )×((T 地层 -T) / (TT N2 ))×L×h×d×ρ 地层 Formula (3)

[0076] Thus, the minimum scale required for cold brittleness under the given design seam length can be obtained (mass unit, converted to volume by density, the minimum liquid nitrogen volume used is about 184m 3 ).

[0077] (4) Pumping in resistance-reducing acid

[0078] After the liquid nitrogen is pumped in, a relatively complex fracture network has been formed in the formation due to the cold brittleness and the characteristics of liquid nitrogen. At this time, low-viscosity drag-reducing acid is pumped in to enter, fill, and acid-etch the complex fracture network that has been formed, while further extending the length of the fractures.

[0079] The pumping volume is the maximum volume that does not exceed the construction pressure limit. During this period, liquid nitrogen can be injected at a low volume (no more than 1m 3 / min), increasing the flowback rate and reducing the residual acid in the seam.

[0080] (5) Pumping in gelled acid

[0081] Pump in gelled acid to etch the fracture surface in the main fracture channel to improve the conductivity of the main fracture. During this period, liquid nitrogen can be injected at a low rate (no more than 1m 3 / min), increasing the flowback rate and reducing the residual acid in the seam.

[0082] Table 1: Calculation parameters in Example 1

[0083] <![CDATA[Specific heat capacity c of the target reservoir rock mass 地层 , kJ / (kg·K)]]> 0.59 <![CDATA[Specific heat capacity c of liquid nitrogen N2 , kJ / (kg·K)]]> 1.04 <![CDATA[Original formation temperature T 地层 , °C]]> 150 <![CDATA[Initial temperature T of liquid nitrogen N2 , °C]]> -196 Critical temperature T for cold brittleness of target reservoir rock mass, ℃ 40 Total length of crack L, m 20 Distance d along the crack surface where the cold brittle effect occurs, m 1 Reservoir thickness h, m 10 <![CDATA[Target reservoir rock mass - limestone density ρ 地层 , g / cm 3 > 2.8 <![CDATA[Liquid nitrogen density ρ N2 , g / cm 3 > 0.808 <![CDATA[Rock mass quality m participating in heat exchange 地层 , kg]]> 560000 <![CDATA[Liquid nitrogen mass m N2 , kg]]> 148077 <![CDATA[Liquid nitrogen volume, m 3 > 183.2636

[0084] Comparative Example 1

[0085] If conventional water-based fracturing fluid is used instead of liquid nitrogen in Example 1 above, the required volume is larger (445 cubic meters) when the crack size is the same as that of liquid nitrogen fracturing. Moreover, after the crack parameters are corrected by the crack-forming ability of the water-based fracturing fluid, the amount of liquid required to achieve cold brittle fracturing is greatly increased (because the water-based fracturing fluid has a stronger crack-forming ability and a larger crack length). Therefore, it is difficult to achieve the cold brittle fracturing effect using water-based liquid.

[0086] Table 2: Calculation parameters of water-based fracturing fluid

[0087]

[0088]

[0089] The temperature of water-based fracturing fluid is not much different from that of the formation, so the cold brittle effect is not obvious. In addition, the amount used is large, and the fracture-forming ability is stronger, which results in a larger amount of water required and a larger fracture length.

[0090] The present invention provides a calculation formula for liquid nitrogen, which can accurately calculate the amount of liquid nitrogen required. In the prior art, the fracturing fluid used either fails to achieve the cold brittle effect or wastes a large amount of fracturing fluid. Therefore, the present invention has a good comprehensive effect.

[0091] It should be noted that the above-described specific embodiments can enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way. Therefore, although the present invention has been described in detail in the embodiments of this specification, it should be understood by those skilled in the art that the present invention can still be modified or replaced by equivalents. In short, all technical solutions and changes that do not depart from the spirit and scope of the present invention should be included in the scope of protection of the patent of this invention.

Claims

1. Composite acid fracturing method, characterized in that: The steps include: S01: After perforating the target well, clean water is used for forward flushing to reduce the wellbore temperature; S02: inject working fluid to perform small pressure test; S03: After stopping the forward wash, inject liquid nitrogen; S04: injecting resistance-reducing acid for acid fracturing; S05: Inject gelled acid and perform acid fracturing.

2. The composite acid fracturing method according to claim 1, wherein: In step S01, while the forward wash is being performed, a liquid nitrogen surface pump circulation is performed.

3. The composite acid fracturing method according to claim 1, wherein: In step S02, the working fluid is slick water.

4. The composite acid fracturing method according to claim 1, wherein: In step S03, the mass of the injected liquid nitrogen is m N2 ; m N2 ≥(c 地层 / c N2 )×((T 地层 -T) / (T-T N2 ))×L×h×d×ρ 地层 ; Among them, c 地层 represents the specific heat capacity of the target reservoir rock mass, c N2 represents the specific heat capacity of liquid nitrogen, T 地层 represents the original temperature of the target reservoir rock mass, T represents the critical temperature of the target reservoir rock mass at which cold brittleness occurs, and T N2 represents the initial temperature of liquid nitrogen, L represents the total length of the fracture, h represents the reservoir thickness, d represents the distance along the fracture surface where the cold brittle effect occurs, ρ 地层 Indicates the average density of the target reservoir rock mass.

5. The composite acid fracturing method according to claim 1, wherein: In step S03, the liquid nitrogen pumping capacity is less than or equal to the maximum capacity of the construction pressure limit.

6. The composite acid fracturing method according to claim 1, wherein: In step S03, after the ground circulation is completed, the cement truck is stopped from washing and liquid nitrogen is pumped in.

7. The composite acid fracturing method according to claim 1, wherein: In step S04, when injecting resistance-reducing acid, liquid nitrogen is injected simultaneously, and the liquid nitrogen injection displacement is ≤1m 3 / min.

8. The composite acid fracturing method according to claim 1, wherein: In step S05, when injecting gelled acid, liquid nitrogen is injected simultaneously, and the liquid nitrogen injection displacement is ≤1m 3 / min.

9. The composite acid fracturing method according to claim 1, wherein: In step S04 and step S05, the pumping rate of the drag-reducing acid and the gelling acid is the maximum rate that does not exceed the construction pressure limit.

10. Use of the composite acid fracturing method according to any one of claims 1 to 9 in carbonate rock acid fracturing construction.

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