A method for experimentally determining the timing of acid fracturing flowback in carbonate rocks

Abstract

The present invention discloses a method for experimentally determining the timing of acid fracturing acid liquid backflow in carbonate rocks, which comprises the following steps: S1, based on the injection time, displacement and formation fracture temperature before acid injection during acid fracturing construction, carry out indoor acid etching physical simulation experiment, and monitor the temperature in the fracture at the end of acid injection; S2, taking the temperature monitored by S1 as the initial condition, calculate the formation fracture temperature recovery value under different pump stop times; S3, retain the acid liquid in S1 in the fracture for the pump stop time, synchronously heat the acid etched fracture to the temperature recovery value, and backflow the acid liquid in the fracture; S4, test the conductivity of the acid etched fracture of the rock sample under different pump stop times, and determine the optimal pump stop time and the timing of opening the well for drainage. The present invention takes into account the influence of the temperature recovery in the formation fracture on the conductivity of the acid etched fracture after the acid fracturing pump is stopped, and experimentally determines the timing of acid liquid backflow, overcomes the limitation that the existing backflow timing is determined only by engineering experience, and improves the effect of acid fracturing transformation.

Description

Technical Field

[0001] The invention relates to the field of petroleum engineering, in particular to a method for experimentally determining the flowback time of carbonate rock acid fracturing acid liquid during the acid fracturing production increase process of carbonate rock oil and gas reservoirs. Background Art

[0002] Acid fracturing (acid fracturing) is an important technical measure for the commissioning and production increase of carbonate oil and gas wells. It forms grooved or uneven high-permeability channels through chemical reactions between acid and artificial (natural) fracture wall rocks, thereby improving the seepage conditions of gas wells and increasing the production of oil and gas wells. However, for dense carbonate rocks, when large-scale acid is used to enter the formation, if the well is opened and flowed back too late after the acid fracturing pump is stopped, a large amount of acid will be retained, which will increase the damage to the reservoir pores and acid fracturing fractures and reduce the acid fracturing effect; if the well is opened and flowed back too early, the acid will not fully dissolve the reservoir rocks and the acid has not been consumed. At this time, the flowback leads to low utilization efficiency of the acid. At the same time, the high-concentration acid flowback will also aggravate the corrosion of the pipe column and affect the safety of the pipe column. Therefore, determining the timing of acid return after acid fracturing is crucial to improving the acid fracturing effect and increasing the efficiency of acid utilization. However, most mines currently rely on engineers to determine the acid return time after acid fracturing based on experience, which lacks a certain scientific basis. In actual mines, the problem of acid return time being too early or too late often occurs.

[0003] Therefore, exploring a method to quantitatively determine the timing of acid fracturing acid flowback is of great practical significance for improving the acid fracturing transformation effect and acid utilization efficiency. Summary of the invention

[0004] The purpose of the present invention is to provide a method for experimentally determining the flowback time of carbonate rock acid fracturing acid solution in order to solve the problem that the existing carbonate rock reservoir acid fracturing lacks a method for determining the flowback time, resulting in unsatisfactory acid fracturing transformation effect.

[0005] The present invention provides a method for experimentally determining the flowback time of carbonate rock acid fracturing acid solution, which comprises the following steps in sequence:

[0006] S1, based on the injection time t of acid fracturing construction in , displacement Q and formation fracture temperature T1 before acid injection, carry out indoor acid etching physical simulation experiment, and monitor the temperature T2 in the fracture when acid injection is completed;

[0007] S2, taking the temperature T2 monitored by S1 as the initial condition, calculate different pump stop times t sh Lower formation fracture temperature recovery value T re ;

[0008] S3, the acid in S1 is retained in the crack and the pump is stopped for a period of time t sh , and simultaneously heat the acid-etched crack to the temperature recovery value T re, acid in the backflow fractures;

[0009] S4. Test different pump stop times t sh The conductivity of the acid-etched fractures of rock samples under the influence of the above conditions can be used to determine the optimal time to stop the pump and the time to open the well for drainage.

[0010] Preferably, step S1 specifically includes the following sub-steps:

[0011] S11, based on formation temperature T e and injection displacement Q, and the applicant's prior authorized patent CN201911326643.4, a numerical calculation method for wellbore temperature field during thickened acid acidizing, is used to calculate the formation fracture temperature T1 before acid injection;

[0012] S12, with the temperature T1 calculated by S11, according to the method of experimentally determining the acid etching conductivity of rough fractures in carbonate oil and gas reservoirs in the prior patent 201810443453X of the applicant, the injection displacement Q is converted into the experimental injection displacement q, and the acid etching simulation experiment is carried out, and the injection time is t in After the acid injection is completed, the temperature T2 in the rock sample fracture is monitored and obtained.

[0013] Preferably, step S2 specifically includes the following steps:

[0014] After stopping the pump, the temperature in the acid-etched fracture gradually returns to the reservoir level. sh After that, the temperature field in the crack is obtained by the following formula:

[0015]

[0016]

[0017] Where, T re is the formation fracture temperature recovery value, K; t sh is the pump stop time, s; λ is the effective heat transfer coefficient, W / (m·K); ρC is the weighted value of the product of density and specific heat capacity; y is the distance perpendicular to the crack, m; y max is the farthest boundary affected by the acid cooling effect in the formation perpendicular to the fracture, m; T e is the formation temperature, K; T2 is the temperature in the rock sample fracture monitored after the acid injection is completed, K. Formula (1) and formula (2) are solved numerically using finite difference, and the pump stop time t can be obtained sh The temperature in the crack is T re .

[0018] Preferably, step S3 specifically includes the following steps:

[0019] S31, keep the acid solution in the acid-etched cracks, the retention time is the pump stop time tsh , and heated to the temperature T determined by S2 calculation within the retention time re ;

[0020] S32, when the acid retention is over, the acid inlet and the acid outlet in the acid-etched fracture are interchanged, and the acid in the fracture is discharged back by reverse displacement;

[0021] S33, carry out different pump stop times t according to S31 and S32 sh ,1,t sh ,2,…t sh,i Acid etching simulation experiment under .

[0022] Preferably, step S4 specifically includes the following steps:

[0023] S41, using the method of determining the acid etching conductivity of rough fractures in carbonate oil and gas reservoirs according to the applicant's prior patent 201810443453X, testing S33 different pump stop times t sh,i Fracture conductivity Fc of acid-etched rock samples under reservoir closure pressure i ;

[0024] S42, based on the test results of S41, the optimal pump stop time t opt The flow conductivity Fc opt Need to meet:

[0025] Fc opt =max(Fc1,Fc2,...Fc i ) (3)

[0026] In the formula, F ci The pump stop time t sh，i The flow conductivity at the time of opt is the optimal pump stop time t opt The flow conductivity is Dc·cm. The flowback timing is to stop the pump t opt Then open the well and return the water immediately.

[0027] Compared with the prior art, the present invention is beneficial in that: the present invention adopts an indoor experimental method, takes into account the influence of temperature recovery in formation fractures on the dissolution reaction of acid-retained rock fractures after the acid fracturing pump is stopped, and further proposes a method for experimentally determining the timing of acid fracturing acid return in carbonate rocks from the perspective of conductivity, which overcomes the limitation of the existing return timing relying solely on engineering experience and improves the acid fracturing transformation effect.

[0028] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 : This is the temperature distribution curve at the crack entrance during the acid fracturing process in step S1 of the embodiment.

[0030] Figure 2 It is the conductivity of the acid-etched fracture under different pump-off times in the embodiment. DETAILED DESCRIPTION

[0031] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0032] A method for experimentally determining the timing of acid fracturing acid flowback in carbonate rocks, the specific process is as follows:

[0033] S1, based on the injection time t of acid fracturing construction in , displacement Q and formation fracture temperature T1 before acid injection, carry out indoor acid etching physical simulation experiment, and monitor the temperature T2 in the fracture when acid injection is completed. The specific steps include:

[0034] S11. Based on the formation temperature T of well X in the research area e is 148℃, the target layer is buried at a depth of 5392m, and the acid injection time is t in The injection displacement Q is 4.0m 3 / min, considering the wellbore liquid accumulation of 22.4m 3 Influence and heat exchange effect between injection fluid and wellbore, according to the calculation method and steps described in patent CN201911326643.4, the temperature distribution curve at the entrance of the fracture during acid fracturing is obtained (such as Figure 1 shown), by Figure 1 It can be seen that after 6 minutes, all the liquid in the wellbore entered the formation fractures, and the temperature T1 of the subsequent acid fluid when it contacted the formation fractures was 126°C;

[0035] S12, the temperature T1 calculated by S11 is 126°C, according to the method of the applicant's prior patent 201810443453X to experimentally determine the conductivity of rough fractures in carbonate oil and gas reservoirs by acid etching, the injection displacement Q is 4.0m 3 / min is converted into the experimental injection displacement q of 380mL / min. The acid etching simulation experiment is carried out using the acid system and rock plate in the research area. The injection time is t in The time for acid injection was 40 minutes. After the acid injection was completed, the temperature T2 in the rock sample fracture was monitored to be 55°C.

[0036] S2, taking the temperature T2 monitored by S1 as the initial condition, calculate different pump stop times t according to formulas (1) and (2) sh Temperature recovery value T of formation acid fracturing fracture (0.5, 1, 2, 3 hours)re They were 64°C, 83°C, 94°C and 99°C respectively (Table 1).

[0037] S3, the acid in S1 is retained in the crack and the pump is stopped for a period of time t sh , and simultaneously heat the acid-etched crack to the temperature recovery value T re , to return the acid in the fracture, the specific steps include:

[0038] S31, keep the acid solution in the acid etching device, the rock sample in the device is numbered 1#, and the retention time is the pump stop time t sh (0.5h), and within the residence time of 0.5h, the temperature of the acid etching device is heated from 55°C to the temperature T determined by S2 calculation re (64°C);

[0039] S32, when the acid retention ends, the acid inlet and the acid outlet in the acid etching are interchanged, and a reverse displacement method is used to simulate the acid flowback process in the fracture, so as to completely drive out the acid in the acid etching fracture;

[0040] S33. Similarly, carry out acid etching simulation experiments with pump stop times of 1h, 2h, and 3h according to steps S31 and S32. The specific pump stop times and temperature recovery values ​​are shown in Table 1.

[0041] Table 1 Crack temperature recovery values ​​at different pump stop times

[0042] Slab No. 1# 2# 3# 4# <![CDATA[Pump stop time t sh , h]]> 0.5 1 2 3 <![CDATA[Crack temperature recovery value T re , °C]]> 64 83 94 99

[0043] S4. Test different pump stop times t sh The conductivity of the acid-etched fractures of the rock samples under the condition of the above conditions can be determined to determine the best pump stop time and the best time to open the well for drainage. The specific steps are as follows:

[0044] S41. According to the method for experimentally determining the conductivity of rough fractures in carbonate oil and gas reservoirs in accordance with patent 201810443453X, the fracture conductivity of acid-etched rock samples at different pump-off times in S33 was tested under the reservoir closure pressure. c , and the relationship curve between the conductivity of the acid-etched fracture and the pump stop time was drawn (e.g. Figure 2 shown);

[0045] S42, by Figure 2 It is known that when the pump is stopped for a period of time t sh When the pump is stopped for 2 hours, the flow capacity is the largest, so the optimal pump stop time is t opt The best time for flowback is 2 hours after the acid pressure pump is stopped.

[0046] Well X was flowed back after stopping the pump for 2 hours after acid fracturing as determined by the technical invention, while the adjacent well adopted the work area experience method, that is, stopping the pump for 1.5 hours to flow back. However, the test production of this well after acid fracturing was 35% higher than that of the adjacent well, which confirmed the effectiveness of the technology of the present invention.

[0047] In summary, the present invention adopts an indoor experimental method to consider the effect of temperature recovery in formation fractures on the acid etching effect of acid retained fractures after the acid fracturing pump is stopped, and further proposes a method for experimentally determining the timing of acid fracturing acid return in carbonate rocks from the perspective of conductivity, which overcomes the limitation of the existing return timing based solely on engineering experience and improves the acid fracturing transformation effect.

[0048] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for experimentally determining the timing of acid fracturing acid flowback in carbonate rocks, characterized in that: The steps include: S1, based on the injection time t of acid fracturing construction in , displacement Q and formation fracture temperature T1 before acid injection, carry out indoor acid etching physical simulation experiment, and monitor the temperature T2 in the fracture when acid injection is completed; S2, taking the temperature T2 monitored by S1 as the initial condition, calculate different pump stop times t sh Lower formation fracture temperature recovery value T re ; S3, the acid in S1 is retained in the crack and the pump is stopped for a period of time t sh , and simultaneously heat the acid-etched crack to the temperature recovery value T re , acid in the backflow fractures; S4. Test different pump stop times t sh The conductivity of the acid-etched fractures of rock samples under the influence of the above conditions can be used to determine the optimal pump stop time and the timing of opening the well for drainage.

2. A method for experimentally determining the timing of carbonate rock acid fracturing acid solution flowback as claimed in claim 1, characterized in that: The step S1 includes the following sub-steps: S11, based on formation temperature T e and injection displacement Q, a numerical calculation method for the wellbore temperature field during thickened acid acidizing is used to calculate the formation fracture temperature T1 before acid injection; S12, with the fracture temperature T1 calculated by S11, according to the experimental method for determining the acid etching conductivity of rough fractures in carbonate oil and gas reservoirs, the injection displacement Q is converted into the experimental injection displacement q, and the acid etching simulation experiment is carried out, and the injection time is t in After the acid injection is completed, the temperature T2 in the rock sample fracture is monitored and obtained.

3. A method for experimentally determining the timing of carbonate rock acid fracturing acid solution flowback as claimed in claim 1, characterized in that: The step S2 comprises: After stopping the pump, the temperature in the acid-etched fracture gradually returns to the reservoir temperature level. sh After that, the temperature field in the crack is obtained by the following formula: Where, T re is the formation fracture temperature recovery value, K; t sh is the pump stop time, s; λ is the effective heat transfer coefficient, W / (m·K); ρC is the weighted value of the product of density and specific heat capacity; y is the distance perpendicular to the crack, m; y max is the farthest boundary affected by the acid cooling effect in the formation perpendicular to the fracture, m; T e is the formation temperature, K; T2 is the temperature in the rock sample fracture monitored after the acid injection is completed, K. Formula (1) and formula (2) are solved numerically using finite difference, and the pump stop time t can be obtained sh The temperature in the crack is T re .

4. A method for experimentally determining the timing of carbonate rock acid fracturing acid solution flowback as claimed in claim 1, characterized in that: The step S3 comprises: S31, keep the acid solution in the acid-etched cracks, the retention time is the pump stop time t sh , and heated to the temperature T determined by S2 calculation within the retention time re ; S32, when the acid retention is over, the acid inlet and the acid outlet in the acid-etched fracture are interchanged, and the acid in the fracture is discharged back by reverse displacement; S33, carry out different pump stop times t according to S31 and S32 sh,1 , t sh,2 ,…t sh,i Acid etching crack simulation experiment under .

5. The method for experimentally determining the timing of carbonate rock acid fracturing acid solution flowback according to claim 1, characterized in that: The step S4 comprises: S41, using the experimental method to determine the conductivity of rough fracture acid etching in carbonate oil and gas reservoirs, testing S33 different pump stop time t sh,i Fracture conductivity Fc of acid-etched rock samples under reservoir closure pressure i ; S42, based on the test results of S41, the optimal pump stop time t opt The flow conductivity Fc opt Need to meet: Fc opt =max(Fc1,Fc2,...Fc i ) (3) In the formula, F ci is the pump stop time t sh，i The flow conductivity at the time of opt is the optimal pump stop time t opt The flow conductivity is Dc·cm. The flowback timing is to stop the pump t opt Then open the well and return the water immediately.

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