Online acid fracturing method for carbonate reservoir

Abstract

The invention discloses an online acid fracturing method for a carbonate reservoir, which comprises the following steps: based on the characteristics of a target reservoir and an acid fracturing transformation scheme, obtaining the average temperature and acid etching fracture parameters of acid liquids with different viscosities and different concentrations in a fracture in an acid fracturing process, and determining the acid liquid concentration and the viscosity range which meet the requirements; preparing acid liquids with different densifier concentrations, carrying out an indoor rheological experiment according to the average temperature of the cracks, obtaining the viscosities of the acid liquids with different densifier concentrations at different temperatures, and determining the densifier concentrations meeting different viscosity requirements of the acid liquids; calculating the discharge capacity of industrial raw acid, the discharge capacity of clear water, the discharge capacity of a thickening agent and the discharge capacity of other additives corresponding to the pump-injected acid liquids with different concentrations and different viscosities; and on-line acid fracturing construction ground equipment is arranged, acid fracturing construction is conducted, displacement is dynamically adjusted, and on-line acid fracturing is achieved. According to the method, the concentration, viscosity and dosage of the acid liquid in acid fracturing construction can be regulated and controlled in real time, so that safer, more efficient and controllable yield-increasing transformation of the carbonate reservoir is realized.

Description

Technical Field

[0001] The invention relates to the technical fields of petroleum engineering and geothermal exploitation, in particular to an online acid fracturing method for carbonate reservoirs in reservoir transformation technology. Background Art

[0002] In deep carbonate reservoirs, oil and gas are stored in pores, caves, and fractures. There is usually no direct medium between these reservoirs and the wellbore, so oil and gas cannot be produced naturally. The acid fracturing process refers to a production-increasing technology that presses open rocks to form cracks, injects acid to dissolve the crack surface to form a non-uniform crack surface, and keeps the acid-etched cracks incompletely closed under the action of closing stress to form a high-speed channel for oil and gas seepage. Acid fracturing is a technical tool for building and increasing production of deep carbonate oil and gas wells.

[0003] Deep carbonate reservoirs are deeply buried, and acid fracturing fractures connecting dispersed oil and gas reservoirs are an important guarantee for increasing oil and gas well production. Achieving this goal puts forward diverse requirements for the performance of acid fluids: on the one hand, low-viscosity acid fluids are required to activate near-well fractures and cave reservoirs to form complex acid-etched fractures; high-viscosity acid fluids can reduce near-well acid fluid filtration and connect far-well reservoirs; on the other hand, acid fluids are required to have a good retarding effect, maintain a certain acid concentration in the far-well zone, extend the length of acid-etched fractures, and connect far-well reservoirs. Patent ZL202210907710.7 proposes a reverse acid fracturing method that meets the requirements of this type of reservoir transformation, requiring that the acid fluid viscosity and acid injection volume can be dynamically adjusted. However, for deep carbonate reservoirs, the commonly used pre-mixed acid solution mode has the following technical bottlenecks: first, the viscosity of the pre-mixed acid solution is constant, which cannot meet the diverse acid viscosity requirements of different types of reservoirs; second, the concentration of the pre-mixed acid solution is constant, which makes it difficult to meet the diverse acid concentration requirements of special acid fracturing processes; third, the amount of pre-mixed acid solution is constant, which makes it difficult to adjust the acid injection amount in real time according to on-site needs; fourth, the quality of the pre-mixed acid solution is not conducive to long-term maintenance under extreme weather conditions such as high or low temperatures, and its ability to cope with temporary shutdowns for mine acid fracturing is poor. Summary of the invention

[0004] The purpose of the present invention is to provide 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-mixed acid fracturing technology is difficult to meet the diversified requirements of the dispersed reservoirs for acid concentration and viscosity. An online acid fracturing method for carbonate reservoirs is proposed, which realizes the real-time adjustment of acid concentration and viscosity, and provides a new technical means for efficient acid fracturing transformation of carbonate reservoirs. The present invention has reliable principles and is easy to operate. It has broad market application prospects in the field of acid fracturing transformation of deep oil and gas reservoirs and geothermal reservoirs.

[0005] In order to achieve the above technical objectives, the present invention adopts the following technical solutions.

[0006] A carbonate reservoir online acid fracturing method comprises the following steps:

[0007] Step S1: Based on the target reservoir characteristics and the acid fracturing reconstruction plan, the average temperature and acid-etched fracture parameters of the fractures during the acid fracturing process of acids with different concentrations and viscosities are obtained, and the acid concentration and viscosity range that meet the target reservoir acid-etched fracture parameters are determined;

[0008] Step S2: preparing the same acid solution system with different thickener concentrations, and conducting indoor rheological experiments according to 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;

[0009] Step S3: Calculate the displacement of industrial raw acid, clean water, thickener and additive when pumping acid solutions of different concentrations and viscosities according to the acid solution concentration determined in step S1, the thickener concentration determined in step S2, and the wellhead displacement and additive concentration required on site;

[0010] Step S4: Arrange the ground equipment for online acid fracturing construction, perform online acid fracturing construction, and 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.

[0011] Preferably, 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 corrosion fractures formed when acid solutions of different concentrations and viscosities are injected during the acid fracturing process.

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

[0013] Preferably, the indoor rheological test in step S2 is carried out at 170 s -1 The shear rate was kept constant for 90 min to obtain the viscosity of the acid solution under different thickener concentration conditions.

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

[0015] Preferably, in step S3, according to the crack extension pressure P 1 、Wellbore friction P 2 , hydrostatic column pressure P 3 , Wellhead limit pressure P L Determine the preset wellhead displacement Q A , which satisfies the following relationship:

[0016] P M =P 1 +P 2 -P 3 (1)

[0017] P 1 =η 1 ×H(2)

[0018]

[0019] P 3 =ρgH(4)

[0020] Optimize wellhead pressure P M The preset wellhead displacement Q when the conditions shown in formula (5) are met A :

[0021] P M ≤P L (5)

[0022] Where P M is the wellhead pressure, MPa; P 1 is the crack extension pressure, MPa; P 2 is the wellbore friction, MPa; P 3 is the hydrostatic column pressure, MPa; η 1 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 pipe, m; r is the drag reduction rate, %; ρ is the density of the liquid entering the well, kg / m 3 ; g is the acceleration due to gravity; P L is the wellhead limiting pressure, MPa.

[0023] Preferably, in step S3, the industrial raw acid discharge volume Q a , Clean water discharge Q w , Thickener displacement Q n And the additive displacement Q i The calculation method is:

[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] Where Q ais 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 entering the well, g / cm 3 ; C A is the concentration of the acid 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 additive, m 3 / min; C i is the additive content, %.

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

[0030] The present invention has the following advantages: in view of the dispersed reservoir bodies of carbonate reservoirs, the conventional acid fracturing technology of pre-mixed acid solution is difficult to meet the diversified requirements of the viscosity and concentration of the acid solution for communicating dispersed pores, caves, fractures and other reservoir bodies. On the basis of fully considering 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 the properties such as the concentration and viscosity of the acid solution during the acid fracturing construction of carbonate reservoirs, and is also conducive to the dynamic adjustment of the amount of acid injection, thereby realizing the efficient transformation of carbonate reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the method flow of the present invention;

[0032] Figure 2 It is a 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 solution 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 used to limit the present invention.

[0034] The present invention changes the traditional acid-pressing technology mode of pre-mixed acid solution, proposes an integrated online acid-pressing method, and realizes convenient switching of acid solutions of different concentrations and viscosities without stopping the pump or changing the acid solution system. It overcomes the defect that the acid-pressing technology of pre-mixed acid solution is difficult to control the acid solution viscosity, acid solution concentration and acid solution dosage in real time, and provides a safer, more efficient and convenient acid-pressing method.

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

[0036] Step S1: Based on the target reservoir characteristics and the acid fracturing reconstruction plan, the average temperature and acid-etched fracture parameters of the fractures during the acid fracturing process of acids with different concentrations and viscosities are obtained, and the acid concentration and viscosity range that meet the target reservoir acid-etched fracture parameters are 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 system with different thickener concentrations, and carry out indoor rheological experiments according to the average crack temperature determined in step S1 to obtain the viscosity of the acid with different thickener concentrations at different temperatures, and determine the thickener concentration that meets the different viscosity requirements of the acid, wherein the effect of the acid concentration on the viscosity can be ignored.

[0039] Specifically, the same system of 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 Haake rheometer was used to measure the crack thickness at 170s. -1 The shear rate was kept constant for 90 minutes to obtain the viscosity of the acid solution at different thickener concentrations. Since the acid solution concentration has almost no 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 solution viscosity in Table 1, the thickener concentration corresponding to the required acid solution viscosity can be obtained.

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

[0042] In a preferred embodiment, those skilled in the art can calculate the fracture extension pressure P according to the fracture extension pressure P 1 、Wellbore friction P 2 , Hydrostatic column pressure P 3, Wellhead limit pressure P L , determine the preset wellhead displacement Q A , which satisfies the following relationship:

[0043] P M =P 1 +P 2 -P 3 (1)

[0044] P 1 =η 1 ×H(2)

[0045]

[0046] P 3 =ρgH(4)

[0047] Optimize wellhead pressure P M The 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; P 1 is the crack extension pressure, MPa; P 2 is the wellbore friction, MPa; P 3 is the hydrostatic column pressure, MPa; η 1 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] Further, 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 Calculate industrial acid discharge Q a , Thickener displacement Q n , Clean water discharge Q w , and other additives displacement Q i as follows:

[0051]

[0052] Qw =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 entering the well, g / cm 3 ; C A is the concentration of the acid 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 various additives except thickener, m 3 / min; C i is the concentration of additives other than thickener, %.

[0056] Step S4: Arrange the ground equipment for online acid fracturing construction, perform online acid fracturing construction, and 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.

[0057] Specifically, the on-site operators connected the acid storage tank and acid pump, the additive storage tank and integrated skid, the clean water tank and water supply pump to the acid supply skid from three lines respectively, and the remaining devices and pipelines were connected according to Figure 2 Install and place;

[0058] Store industrial raw acid in the acid storage tank and connect it to the acid supply pump; store the prepared liquid additive in the additive storage tank and connect it to the acid supply skid through an integrated skid; connect the clean water tank to the acid supply skid through a water supply pump;

[0059] When preparing the acid solution with the required viscosity, start the acid supply pump, water supply pump and additive pump at the same time with the displacement Q a , Q w , Qn , Q i Pumping industrial raw acid, clean water, thickener and other additives;

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

[0061] The thickener displacement is adjusted in real time according to the acid pressure pumping procedure or the on-site construction response. When the acid is continuously pumped, the acid with different concentrations and viscosities can be switched. For example, when the wellhead pressure increases abnormally, the acid viscosity can be reduced or maintained by reducing the thickener displacement. At the same time, the acid and thickener displacements can be reduced to keep the relationship between the two as shown in formula (8). When the acid concentration needs to be switched, the displacement of the acid supply pump and the water supply pump is changed according to formulas (6) and (7). When the acid displacement or viscosity needs to be increased, the operation is similar.

[0062] Application Examples

[0063] The target layer of Well A in a certain area in the west is a carbonate reservoir with a burial depth of 6987m and a reservoir temperature of 160℃. The reservoir porosity is 3-8.36% and the permeability is 0.8-0.889mD, which is an ultra-deep low-porosity and low-permeability carbonate reservoir. There are few holes and fractures near the wellbore. There is no obvious stress barrier in the middle, and the stress in the upper part is relatively low. Therefore, "multi-stage alternating acid fracturing, increasing the acid injection displacement, and increasing the acid intensity under the premise of controlling the fracture spacing" is adopted to achieve the longitudinal transformation of the entire well section while promoting the fractures to communicate with the fracture body as deep as possible. The reservoir temperature of this well is high. First, a low-viscosity acid with a concentration of 15% is used to dissolve the cracks in the near-wellbore zone to form a complex fracture network, while preventing excessive dissolution of the reservoir by the acid. After the fracture temperature drops, 20% high-viscosity acid and low-viscosity acid are pumped alternately to promote the formation of non-uniform acid-etched cracks, while ensuring that the acid maintains a certain concentration in the far-wellbore zone to communicate with the far-wellbore reservoir. Finally, 15% low-viscosity acid is pumped for closed acidification to expand the cracks in the near-wellbore zone.

[0064] (1) Based on the engineering geological characteristics of the target reservoir, the acid fracturing model is used to obtain the average temperature and fracture geometry in the hydraulic fracture, and the online acid fracturing acid concentration range is 15-24%. The acid viscosity range is shown in Table 1;

[0065] (2) The corresponding relationship between the acid viscosity and the thickener concentration range obtained from the rheological experiment; the thickener concentration is preferably selected according to the viscosity test results. As shown in Table 1, the thickener concentrations are selected to be 1.0% and 3.0%, respectively.

[0066] Table 1 Acid viscosity requirements and preferred thickener concentration

[0067] Acid type Viscosity range, mPa·s Thickener concentration range, % Preferred thickener 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 estimated wellhead displacement of low-viscosity acid and high-viscosity acid is calculated according to the wellhead limit pressure and acid viscosity; and the displacement of industrial raw acid, clean water, thickener and other additives is calculated according to the acid formula shown in Table 2, as shown in Table 3; the concentrations of corrosion inhibitors, iron stabilizers, drainage aids and composite multi-effect agents are always consistent and can be mixed and stored in equal proportions. In addition, the principle of limiting pressure but not limiting drainage is adopted during construction to adjust the displacement in real time.

[0069] Table 2 Acid solution formula

[0070] Acid system Acid Recipe 15% low viscosity acid 15% HCl + 4% corrosion inhibitor + 1% thickener + 1% iron stabilizer + 1% drainage aid + 1% composite multi-effect agent 20% low viscosity acid 20% HCl + 4% corrosion inhibitor + 1% thickener + 1% iron stabilizer + 1% drainage aid + 1% composite multi-effect agent 20% high mucic acid 20% HCl + 4% corrosion inhibitor + 3% thickener + 1% iron stabilizer + 1% drainage aid + 1% composite multi-effect agent

[0071] Table 3 Some pumping procedures and pumping parameters

[0072]

[0073] (4) Acid fracturing construction site equipment and pipeline layout, such as Figure 2 As shown in the figure, industrial raw acid is stored in the acid storage tank and connected to the acid supply skid through the acid pump, clean water is connected to the acid supply skid through the liquid supply pump, and additives are connected to the acid supply skid through the integrated integrated skid; when the acid is pumped into the well, industrial raw acid, clean water, and various liquid thickeners are mixed in the acid supply skid at the same time; then the fracturing truck pumps it into the well. When switching to acids of different concentrations and viscosities, the displacement of industrial raw acid, clean water, and various additives can be adjusted. In addition, a sampling end is installed at the outlet of the acid supply skid, and the mixed acid can be sampled and tested. The average results of the sampling test are shown in Table 4. The online configuration of acid meets the on-site acid fracturing requirements.

[0074] Table 4 Partial test data of sampled acid

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

[0076] According to the operation in step (4), well A is subjected to integrated online multi-stage alternating acid fracturing; compared with the 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 production of acid-fracturing wells is 1.5 times that of similar reservoirs in adjacent wells.

[0077] The present invention is specifically described above through the embodiments. It is necessary to point out that the embodiments are only preferred embodiments of the present invention, and do not limit the present invention in any way, nor are they limited to the forms disclosed herein, and should not be regarded as excluding other embodiments. The modifications and simple changes made by those skilled in the art do not deviate from the technical idea and scope of the present invention, and all belong to the protection scope of the technical solution of the present invention.

Claims

1. A carbonate reservoir online acid fracturing method, comprising the following steps: Step S1: Based on the target reservoir characteristics and the acid fracturing reconstruction plan, the average temperature and acid-etched fracture parameters of the fractures during the acid fracturing process of acids with different concentrations and viscosities are obtained, and the acid concentration and viscosity range that meet the target reservoir acid-etched fracture parameters are determined; Step S2: preparing the same acid solution system with different thickener concentrations, and conducting indoor rheological experiments according to 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 displacement of industrial raw acid, clean water, thickener and additive when pumping acid solutions of different concentrations and viscosities according to the acid solution concentration determined in step S1, the thickener concentration determined in step S2, and the wellhead displacement and additive concentration required on site; Step S4: Arrange the ground equipment for online acid fracturing construction, perform online acid fracturing construction, and 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 acid corrosion fracture parameters 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 170 s -1 The shear rate was kept constant for 90 min to obtain the viscosity of the acid solution under different thickener concentration conditions.

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

5. The method for online acid fracturing of carbonate reservoirs according to claim 1, wherein step S3 further comprises: According to the fracture extension pressure P1, wellbore friction P2, static liquid column pressure P3, wellhead limit 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) Optimize 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 pipe 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 displacement 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) In the formula, 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 entering the well, g / cm 3 ; C A is the concentration of the acid 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 additive, %.

7. The online acid fracturing method for carbonate reservoirs as claimed in 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 performance of the acid solution meets the requirements.

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