An online acid fracturing method for carbonate reservoirs

Abstract

The application discloses an online acid fracturing method for a carbonate reservoir, and comprises the following steps: based on the characteristics of a target reservoir and an acid fracturing reconstruction scheme, average temperatures of acid liquids with different viscosities and different concentrations in a fracture, and acid-etched fracture parameters in an acid fracturing process are obtained, and acid liquid concentration and viscosity ranges meeting the requirements are determined; acid liquids with different thickening agent concentrations are configured, indoor rheological experiments are carried out according to the average temperatures of the fractures, the viscosities of the acid liquids with different thickening agent concentrations at different temperatures are obtained, and the thickening agent concentrations meeting the different viscosity requirements of the acid liquids are determined; the industrial original acid discharge, the clean water discharge, the thickening agent discharge and the discharge of other additives corresponding to the acid liquids with different concentrations and different viscosities are calculated; ground equipment for the online acid fracturing construction is arranged, the acid fracturing construction is carried out, the discharges are dynamically adjusted, and the online acid fracturing is realized. The application can real-time regulate and control the acid liquid concentration, the viscosity, the dosage in the acid fracturing construction, so that the yield reconstruction of the carbonate reservoir is safer, more efficient and controllable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of petroleum engineering and geothermal exploitation, in particular to an online acid fracturing method for carbonate reservoirs. BACKGROUND

[0002] In deep carbonate reservoirs, oil and gas are stored in pore, cave and fracture reservoirs. These reservoirs are usually not directly connected with the wellbore, and oil and gas cannot be naturally produced. Acid fracturing technology is a stimulation technology that forms fractures by pressure, and forms non-uniform fracture surfaces by injecting acid to dissolve the fracture surfaces. Under the action of closure stress, the acid-etched fractures remain partially closed, forming a high-speed channel for oil and gas seepage. Acid fracturing is a powerful tool for building and stimulating production in deep carbonate oil and gas wells.

[0003] Deep carbonate reservoirs are buried deep, and acid fracturing is an important guarantee for the stimulation of oil and gas wells by connecting dispersed oil and gas reservoirs. To achieve this goal, the performance of acid fluid is required to be diversified: on the one hand, low-viscosity acid fluid can activate near-well fracture and cave reservoirs to form complex acid-etched fractures; high-viscosity acid fluid can reduce the filtration of near-well acid fluid to connect far-well reservoirs; on the other hand, the acid fluid requires good retardation effect to maintain a certain acid concentration in the far-well zone, extend the length of the acid-etched fractures, and connect the far-well reservoirs. Patent ZL202210907710.7 proposes an inverse sequence acid fracturing method that meets the requirements of such reservoir reconstruction, and requires the viscosity and injection amount of the acid fluid to be dynamically adjusted. However, for deep carbonate reservoirs, the commonly used pre-allocated acid fluid mode has the following technical bottlenecks: first, the pre-allocated acid fluid viscosity is constant, which cannot meet the diversified needs of acid fluid viscosity for connecting different types of reservoirs; second, the pre-allocated acid fluid concentration is constant, which cannot meet the diversified needs of acid fluid concentration for special acid fracturing processes; third, the amount of pre-allocated acid fluid is constant, which cannot be adjusted in real time according to the needs of the field; fourth, the quality of pre-allocated acid fluid is not conducive to long-term maintenance under extreme weather conditions such as high temperature or low temperature, and the ability to respond to temporary shutdown of field acid fracturing is poor. SUMMARY

[0004] The present application provides an online acid fracturing method for carbonate reservoirs. In view of the deep burial, high temperature and dispersed reservoir characteristics of carbonate reservoirs, the pre-allocated acid fracturing technology cannot meet the diversified needs of acid concentration and viscosity for connecting dispersed reservoirs. An online acid fracturing method for carbonate reservoirs is proposed, which realizes real-time adjustment of acid concentration and viscosity, and provides a new technical means for efficient acid fracturing reconstruction of carbonate reservoirs. The principle of the present application is reliable and easy to operate, and has a wide market application prospect in the field of acid fracturing reconstruction of deep oil and gas reservoirs and geothermal reservoirs.

[0005] To achieve the above technical purpose, the present application adopts the following technical scheme.

[0006] An online acid fracturing method for carbonate reservoirs, comprising the following steps:

[0007] Step S1: based on the target reservoir characteristics and the acid fracturing scheme, the average temperature of the fracture in the acid fracturing process of different concentrations and different viscosity acid fluids, the acid-etched fracture parameters are obtained, and the acid fluid concentration and viscosity range meeting the acid-etched fracture parameters of the target reservoir are determined;

[0008] Step S2: the same acid fluid system with different thickening agent concentrations is configured, the indoor rheological experiment is carried out according to the average temperature of the fracture determined in step S1, the viscosity of the acid fluid with different thickening agent concentrations at different temperatures is obtained, and the thickening agent concentration meeting the different viscosity requirements of the acid fluid is determined;

[0009] Step S3: according to the acid fluid concentration determined in step S1, the thickening agent concentration determined in step S2, and the required wellhead discharge of the field, the discharge of the industrial original acid, water, thickening agent and additive corresponding to the pumping of the acid fluid with different concentrations and different viscosities is calculated;

[0010] Step S4: the on-line acid fracturing ground equipment is arranged, the on-line acid fracturing is carried out, the discharge of the acid supply pump, the water supply pump and the additive pump is dynamically adjusted according to the calculation results in step S3, and the on-line acid fracturing is realized.

[0011] Preferably, in step S1, the acid fracturing numerical simulation method is used to obtain the average temperature T of the acid fracturing fracture and the acid-etched fracture parameters formed when different concentrations and different viscosities of acid fluid are injected in the acid fracturing process.

[0012] Preferably, the acid-etched fracture parameters include the length L, the width w and the height h of the acid-etched fracture.

[0013] Preferably, in step S2, the indoor rheological experiment is constant shearing at a shear rate of 170 s -1 for 90 min to obtain the viscosity of the acid fluid under different thickening agent concentrations.

[0014] Preferably, in step S2, the thickening agent concentration range corresponding to the low viscosity acid is 1.0-1.5%, and the thickening agent concentration range corresponding to the high viscosity acid is 1.5-3.0%.

[0015] Preferably, in step S3, according to the fracture extension pressure P1, the wellbore friction P2, the static liquid column pressure P3 and the wellhead limiting pressure P L , the preset wellhead discharge Q A is determined, which meets the following relationship:

[0016] P M =P1+P2-P3(1)

[0017] P1=η1×H(2)

[0018]

[0019] P3 = pgH (4)

[0020] The wellhead pressure P M The preset wellhead discharge Q A :

[0021] P M ≤ P L (5)

[0022] In the formula, P M is the wellhead pressure, MPa; P1 is the fracture extension pressure, MPa; P2 is the wellbore friction, MPa; P3 is the static liquid column pressure, MPa; η1 is the fracture extension pressure gradient, MPa / m; H is the well depth, m; d is the pipe string diameter, m; Q A is the preset wellhead discharge, m 3 / min; L is the pipe string length, m; r is the resistance reduction rate, %; ρ is the well fluid density, kg / m 3 ; g is the gravitational acceleration; and P L is the wellhead limiting pressure, MPa.

[0023] In the preferred embodiment of the present application, the industrial raw acid discharge Q a , the fresh water discharge Q w , the thickening agent discharge Q n and the additive discharge Q i in step S3 are calculated by the following methods:

[0024]

[0025] Q w = Q A - Q a (7)

[0026] Q n = Q A C n (8)

[0027] Q i = Q A C i (9)

[0028] In the formula, Q a is the industrial raw acid discharge, m 3 / min; Q A is the preset wellhead discharge, m 3 / min; ρ A is the density of the well acid, g / cm 3 ; C A is the concentration of the well acid, %; and ρ a is the density of the industrial raw acid, g / cm3 ; C a is the concentration of industrial raw acid, %; Q w is the clean water discharge, m 3 / min;Q n is the thickener displacement, m 3 / min; C n is the thickener concentration, %; Q i is the additive displacement, m 3 / min; C i is the additive content, %.

[0029] Preferably, the present invention includes step S4 further comprising the step of: sampling and testing the acid solution at a sampling port of the ground pipeline to determine whether the concentration and viscosity of the acid solution meet the requirements.

[0030] The present invention has the following advantages: in view of the dispersed reservoir bodies in carbonate reservoirs, the conventional acid fracturing technology using pre-mixed acid solution is difficult to meet the diverse demands for acid solution viscosity and concentration in communicating dispersed pores, holes, fractures and other reservoir bodies. Based on full consideration of the simplicity of acid solution preparation and pumping process, an online acid fracturing method for carbonate reservoirs is proposed, which is conducive to the real-time regulation of acid solution concentration, viscosity and other properties during the acid fracturing construction of carbonate reservoirs, and is also conducive to the dynamic adjustment of the acid injection amount, thereby realizing efficient transformation of carbonate reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the method flow of the present invention;

[0032] Figure 2 Schematic diagram of the layout of ground pipeline equipment for the online acid fracturing process in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is further described in detail below 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 intended to limit the present invention.

[0034] This invention changes the traditional acid fracturing technology model of pre-mixed acid solution and proposes an integrated online acid fracturing method. This method allows for convenient switching between acids of different concentrations and viscosities without stopping the pump or changing the acid system. This overcomes the drawback of pre-mixed acid fracturing technology, which makes it difficult to control the acid viscosity, concentration, and dosage in real time, and provides a safer, more efficient, and more convenient acid fracturing method.

[0035] The present invention provides the following technical solutions: Figure 1 As shown, an online acid fracturing method for carbonate reservoirs includes the following steps:

[0036] Step S1: Based on the target reservoir characteristics and the acid fracturing scheme, the average temperature and acid-etched fracture parameters of the fractures during the acid fracturing process with acids of different concentrations and viscosities are obtained, and the acid concentration and viscosity range that meet the target reservoir acid-etched fracture parameters is determined;

[0037] Specifically, those skilled in the art can use a self-developed acid fracturing simulation model or mature commercial software to simulate and obtain the average temperature of the cracks during the acid fracturing process and the length L, width w, height h and other parameters of the acid-etched cracks when injecting acid solutions of different concentrations and viscosities.

[0038] Step S2: Prepare the same acid solution system with different thickener concentrations. Based on the average crack temperature determined in step S1, conduct indoor rheological experiments to obtain the viscosity of the acid solutions with different thickener concentrations at different temperatures, and determine the thickener concentration that meets the different viscosity requirements of the acid solution. The effect of the acid solution concentration on the viscosity can be ignored.

[0039] Specifically, the same acid solution was prepared, and different concentrations of thickener were added to the acid solution. Under the condition of the average crack temperature T determined by S1, the Hake rheometer was used to measure the crack thickness at 170s. -1 The viscosity of the acid solution at different thickener concentrations was determined by shearing at a constant shear rate for 90 minutes. Since the acid concentration has little effect on the thickener, the same thickener concentration-viscosity standard can be used for acids of different concentrations.

[0040] According to the relationship between the thickener concentration and the corresponding acid viscosity in Table 1, the thickener concentration corresponding to the required acid viscosity can be obtained.

[0041] Step S3: Calculate the discharge rates of industrial raw acid, clean water, thickener, and additives when pumping acid solutions of different concentrations and viscosities based on the acid solution concentration determined in step S1, the thickener concentration determined in step S2, and the wellhead discharge rate and additive concentration required on site;

[0042] In a preferred embodiment, those skilled in the art can calculate the fracture extension pressure P1, wellbore friction P2, hydrostatic column pressure P3, wellhead limit pressure P L , determine the preset wellhead displacement Q A , which satisfies the following relationship:

[0043] P M =P1+P2-P3(1)

[0044] P1=η1×H(2)

[0045]

[0046] P3=ρgH(4)

[0047] Optimized wellhead pressure P MThe preset wellhead displacement Q when the conditions shown in formula (5) are met A :

[0048] P M ≤P L (5)

[0049] Where, P M is the wellhead pressure, MPa; P1 is the fracture extension pressure, MPa; P2 is the wellbore friction, MPa; P3 is the hydrostatic column pressure, MPa; η1 is the fracture extension pressure gradient, MPa / m; H is the well depth, m; d is the string diameter, m; Q A is the preset wellhead displacement, m 3 / min; L is the length of the column, m; r is the drag reduction rate, %; ρ is the liquid density, kg / m 3 ; g is the acceleration due to gravity; P L is the wellhead limiting pressure, MPa.

[0050] Furthermore, according to the obtained preset wellhead displacement Q A , Acid concentration in well C a , thickener concentration C n , and the concentration of various additives except thickener C i Calculation of industrial raw acid discharge Q a , thickener displacement Q n , clean water discharge Q w , and other additives displacement Q i as follows:

[0051]

[0052] Q w =Q A -Q a (7)

[0053] Q n =Q A C n (8)

[0054] Q i =Q A C i (9)

[0055] Where Q a is the industrial acid discharge, m 3 / min;Q A is the preset wellhead displacement, m 3 / min;ρ A is the density of the acid fluid entering the well, g / cm 3 ; C A is the concentration of the acid fluid entering the well, %; ρ ais the density of the industrial raw acid, g / cm 3 ; C a is the concentration of the industrial raw acid, %; Q w is the water discharge, m 3 / min; Q n is the thickener discharge, m 3 / min; C n is the thickener concentration, %; Q i is the discharge of various additives other than the thickener, m 3 / min; C i is the concentration of the additives other than the thickener, %.

[0056] Step S4: arranging the online acid fracturing ground equipment, performing online acid fracturing, dynamically adjusting the discharges of the acid supply pump, the water supply pump and the additive pump according to the calculation results in step S3, and realizing online acid fracturing.

[0057] Specifically, the field operator connects the acid liquid storage tank and the acid liquid pump, the additive storage tank and the integrated pry, the water tank and the water supply pump to the acid supply pry from three lines respectively, and arranges the remaining devices and pipelines according to the Figure 2 ;

[0058] stores the industrial raw acid in the acid storage tank and connects the acid supply pump; stores the prepared liquid additives in the additive storage tank and connects the acid supply pry through the integrated pry; connects the water tank to the acid supply pry through the water supply pump;

[0059] When the acid liquid with the required viscosity is configured, the acid supply pump, the water supply pump and the additive pump are started simultaneously to pump the industrial raw acid, water, the thickener and other additives at discharges Q a , Q w , Q n , Q i respectively;

[0060] After the acid liquid, water, the thickener and other additives are mixed in the acid supply pry, an acid liquid sample is collected from the sampling port for detection to ensure that the acid liquid concentration and viscosity meet the requirements;

[0061] According to the acid fracturing pumping program or the real-time response of the field construction, the thickener discharge is adjusted, and in the case of continuous pumping of the acid liquid, different concentrations and viscosities of the acid liquid are switched, for example, when the wellhead pressure abnormally increases, the acid liquid viscosity can be reduced by reducing the thickener discharge or the acid liquid viscosity is maintained, and at the same time the acid liquid and thickener discharges are reduced to maintain the relationship as shown in formula (8); when the acid liquid concentration needs to be switched, the acid supply pump and the water supply pump discharges are changed according to formulas (6) and (7); when the acid liquid discharge or viscosity needs to be increased, the operation is the same.

[0062] Application Example

[0063] The target layer of Well A in a certain area in the west is a carbonate reservoir, with a reservoir depth of 6987 m and a reservoir temperature of 160℃. The reservoir has a porosity of 3-8.36% and a permeability of 0.8-0.889 mD, belonging to a super-deep, low-porosity and low-permeability carbonate reservoir. There are few fractures near the wellbore. There is no obvious stress barrier in the middle, and the upper stress is low. Therefore, the use of "multi-stage alternating acid fracturing, increasing the acid injection rate, and increasing the acid strength under the premise of controlling the fracture spacing" can realize the longitudinal reconstruction of the whole well section and promote the communication of fractures to the deep fracture-cave body as much as possible. The reservoir temperature of the well is high. First, use 15% low-viscosity acid to dissolve the fractures near the wellbore, form a complex fracture network, and at the same time avoid excessive dissolution of the reservoir by the acid. After the fracture temperature decreases, pump 20% high-viscosity acid and low-viscosity acid alternately to promote the formation of non-uniform acid-etched fractures, while ensuring that the acid still maintains a certain concentration in the far-well zone, which can communicate the far-well reservoir. Finally, pump 15% low-viscosity acid for closed acidizing to expand the fractures near the wellbore.

[0064] (1) Based on the engineering geological characteristics of the target reservoir, the average temperature and fracture geometry in the hydraulic fracture were obtained by using the acid fracturing model, and the acid concentration range of the online acid fracturing was 15-24%, and the acid viscosity range was as shown in Table 1;

[0065] (2) According to the corresponding relationship between the acid viscosity and the thickening agent concentration range obtained from the rheological experiment, the thickening agent concentration was optimized according to the viscosity test results, as shown in Table 1, and the thickening agent concentrations of 1.0% and 3.0% were selected.

[0066] Table 1 Acid viscosity requirements and optimized thickening agent concentrations

[0067] Acid type Viscosity range, mPa-s Viscosifier concentration range, % Preferred viscosifier concentration, % Viscosity test value, mPa-s High viscosity acid 27-60 1.5-3.0 3.0 60 Low viscosity acid 10-27 1.0-1.5 1.0 10

[0068] (3) According to the tubing combination and the fracture extension pressure gradient, the low-viscosity acid and high-viscosity acid predicted wellhead discharge were calculated according to the wellhead pressure limit and the acid viscosity; and the industrial raw acid, water, thickening agent and other additive discharge were calculated according to the acid formulation shown in Table 2, as shown in Table 3; among them, the corrosion inhibitor, iron stabilizer, cleanup agent and composite multi-effect agent concentration is always consistent, which can be mixed and stored in equal proportions. In addition, the principle of limiting pressure without limiting discharge is adopted to adjust the discharge in real time during construction.

[0069] Table 2 Acid formulation

[0070] Acid system Acid formulation 15% low viscosity acid 15% HC1 + 4% corrosion inhibitor + 1% viscosifier + 1% iron stabilizer + 1% cleanup additive + 1% composite multi-function additive 20% low viscosity acid 20% HC1 + 4% corrosion inhibitor + 1% viscosifier + 1% iron stabilizer + 1% cleanup additive + 1% composite multi-function additive 20% high viscosity acid 20% HC1 + 4% corrosion inhibitor + 3% viscosifier + 1% iron stabilizer + 1% cleanup additive + 1% composite multi-function additive

[0071] Table 3 Part of the pumping program and pumping parameters

[0072]

[0073] (4) The construction site equipment and pipeline layout are as follows Figure 2As shown, the industrial raw acid is stored in the acid storage tank, connected to the acid supply pry through the acid liquid pump, the water is connected to the acid supply pry through the liquid supply pump, and the additives are connected to the acid supply pry through the integrated pry; when the pump injects acid liquid into the well, the industrial raw acid, water and various liquid thickening agents are mixed in the acid supply pry at the same time; and then pumped into the well by the fracturing truck. When switching different concentrations and different viscosity acid liquids, the displacement of industrial raw acid, water and various additives can be adjusted. In addition, the outlet end of the acid supply pry is provided with a sampling end, which can sample and test the mixed acid liquid. The average results of the sampling and testing are shown in Table 4, and the online configuration acid liquid meets the field acid fracturing requirements.

[0074] Table 4: Partial detection data of sampled acid liquid

[0075] Acid Average acid viscosity, mPa-s Average acid concentration, % 15% low viscosity acid 10 15 20% low viscosity acid 10 20 20% high viscosity acid 20% high viscosity acid 60 20

[0076] According to the operation in step (4), the integrated online multi-stage alternating acid fracturing is carried out on well A; compared with adjacent wells, the average construction pressure of well A is reduced by 10-12 MPa, and the construction displacement is increased by 0.2-1.2 m 3 / min; the yield of the acid fracturing well is 1.5 times that of the adjacent well with the same reservoir.

[0077] The above describes the present application through examples, and it is necessary to point out that the examples are only preferred examples of the present application, and do not limit the present application, nor are limited to the forms disclosed herein, and should not be regarded as excluding other examples. The modifications and simple changes made by the person skilled in the art without departing from the technical idea and scope of the present application are all within the protection scope of the technical solution of the present application.

Claims

1. A method for online acid fracturing of a carbonate reservoir, comprising the following steps: Step S1: Based on the target reservoir characteristics and the acid fracturing scheme, the average temperature and acid-etched fracture parameters of the fractures during the acid fracturing process with acids of different concentrations and viscosities are obtained, and the acid concentration and viscosity range that meet the target reservoir acid-etched fracture parameters is determined; Step S2: preparing the same acid solution system with different thickener concentrations, and conducting indoor rheological experiments based on the average crack temperature determined in step S1 to obtain the viscosity of the acid solutions with different thickener concentrations at different temperatures, and determine the thickener concentration that meets the different viscosity requirements of the acid solution; Step S3: Calculate the discharge rates of industrial raw acid, clean water, thickener, and additives when pumping acid solutions of different concentrations and viscosities based on the acid solution concentration determined in step S1, the thickener concentration determined in step S2, and the wellhead discharge rate and additive concentration required on site; Step S4: Arrange the ground equipment for online acid fracturing construction, connect the storage devices for industrial raw acid, clean water, thickener and additives to the acid supply skid, and carry out online acid fracturing construction. Dynamically adjust the displacement of the acid supply pump, water supply pump and additive pump according to the calculation results in step S3 to achieve online acid fracturing.

2. The online acid fracturing method for carbonate reservoirs according to claim 1, wherein in step S1, an acid fracturing numerical simulation method is used to obtain the average temperature T of the acid fracturing fractures and the parameters of the acid-etched fractures formed when acid solutions of different concentrations and viscosities are injected during the acid fracturing process.

3. The online acid fracturing method for carbonate reservoirs according to claim 1, wherein the indoor rheological test in step S2 is carried out at a temperature of 170 s. -1 The viscosity of the acid solution was obtained under different thickener concentrations by constant shearing rate for 90 min.

4. The online acid fracturing method for carbonate reservoirs according to claim 1, wherein in step S2, the thickener concentration range for the low-viscosity acid is 1.0-1.5%, and the thickener concentration range for the high-viscosity acid is 1.5-3.0%.

5. The method for online acid fracturing of a carbonate reservoir according to claim 1, wherein step S3 further comprises: According to the fracture extension pressure P1, wellbore friction P2, hydrostatic column pressure P3, wellhead limiting pressure P L Determine the preset wellhead displacement Q for acid fracturing A , which satisfies the following relationship: P M =P1+P2-P3 (1) P1=η1×H (2) P3=ρgH (4) Optimized wellhead pressure P M The preset wellhead displacement Q when the conditions shown in formula (5) are met A : P M ≤P L (5) Where, P M is the wellhead pressure, MPa; P1 is the fracture extension pressure, MPa; P2 is the wellbore friction, MPa; P3 is the hydrostatic column pressure, MPa; η1 is the fracture extension pressure gradient, MPa / m; H is the well depth, m; d is the string diameter, m; Q A is the preset wellhead displacement, m 3 / min; L is the length of the column, m; r is the drag reduction rate, %; ρ is the liquid density, kg / m 3 ; g is the acceleration due to gravity; P L is the wellhead limiting pressure, MPa.

6. The online acid fracturing method for carbonate reservoirs according to claim 1, wherein the industrial raw acid discharge volume Q in step S3 is a , clean water discharge Q w , thickener displacement Q n And the additive displacement Q i The calculation method is: Q w =Q A -Q a (7) Q n =Q A C n (8) Q i =Q A C i (9) Where Q a is the industrial acid discharge, m 3 / min;Q A is the preset wellhead displacement, m 3 / min;ρ A is the density of the acid fluid entering the well, g / cm 3 ; C A is the concentration of the acid fluid entering the well, %; ρ a is the density of industrial raw acid, g / cm 3 ; C a is the concentration of industrial raw acid, %; Q w is the clean water discharge, m 3 / min;Q n is the thickener displacement, m 3 / min; C n is the thickener concentration, %; Q i is the displacement of the additive, m 3 / min; C i is the concentration of the additive, %.

7. The online acid fracturing method for carbonate reservoirs according to claim 1, wherein step S4 further comprises the step of sampling and testing the acid solution at a sampling port of a surface pipeline to determine whether the acid solution performance meets the requirements.

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