Small-pressure testing method based on backflow and flowback

Abstract

The invention discloses a small-pressure testing method based on backflow and flowback, which belongs to the technical field of oil-gas field development and comprises the following steps: S1, calculating the maximum injection displacement and injection rate of fracturing fluid for a specific reservoir of a well site, and determining the pumping time; s2, fracturing fluid is injected, injection data are recorded, the well is closed temporarily after injection of the fracturing fluid is completed, constant-speed flow-back is conducted immediately, and flow-back data are recorded; and S3, performing quality evaluation and correction on the data collected in the step S2, calculating closing pressure, reservoir pressure and a reservoir productivity index, and completing a small pressure test. By adopting a backflow and flowback mode, detection can be completed within several hours, the process that a well needs to be shut in for a long time to wait for pressure drop in a traditional small-pressure test is avoided, and the test efficiency is remarkably improved; errors caused by high estimation of the oil well productivity index and long-time waiting can be avoided, the accuracy and reliability of the test result are improved, optimization of a volume fracturing construction scheme is facilitated, and the efficiency of oil-gas field development is improved.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development, and in particular to a low-pressure testing method based on reflux. Background Art

[0002] In the field of petroleum engineering, unconventional oil and gas has the characteristics of low porosity, low permeability, and strong reservoir heterogeneity. Its development usually requires the use of volume fracturing technology to transform the reservoir before it can be realized. Before volume fracturing construction, a small pressure test is required to obtain information such as the fracture extension pressure, closure pressure, reservoir pressure, number of effective fluid inlet holes, fracture bending friction, fluid efficiency, fluid loss coefficient, and permeability.

[0003] However, the traditional low-pressure test uses the injection / shut-in method to record the curve of pressure drop over time after shutting in the well. The pressure drop is mainly caused by the filtration of fracturing fluid. In unconventional reservoirs, the filtration of fracturing fluid is slow, resulting in a long pressure drop process, usually several days, weeks or even months. This causes large errors in traditional low-pressure tests and the inability to quickly obtain information such as reservoir parameters, which delays the construction progress.

[0004] Therefore, how to provide an efficient and accurate low-pressure testing method is an urgent problem to be solved by those skilled in the art. Summary of the invention

[0005] The purpose of the present invention is to provide a low-pressure testing method based on reflux flowback to solve the problems in the background technology.

[0006] To achieve the above object, the present invention provides a low-pressure testing method based on reflux flowback, comprising the following steps:

[0007] S1. Calculate the maximum injection displacement and injection volume of the fracturing fluid for a specific reservoir in the well site and determine the pumping time;

[0008] S2, injecting the fracturing fluid according to the result obtained in S1 and recording the injection data, shutting down the well for a short time after completing the injection of the fracturing fluid, immediately performing a constant flowback, and recording the flowback data;

[0009] S3: Perform quality assessment and correction on the data collected in S2, calculate the closure pressure, reservoir pressure and reservoir productivity index based on the corrected data, and complete the low-pressure test.

[0010] Preferably, in S1, the calculation formula for the maximum injection displacement is:

[0011]

[0012] Where k is the reservoir permeability, h is the reservoir thickness, and p fis the formation fracture pressure, p i is the original formation pressure of the reservoir, μ is the fluid viscosity, β is the fluid volume coefficient, r e is the discharge radius, r w is the wellbore radius, and S is the skin coefficient.

[0013] Preferably, in S1, the calculation formula for the injection amount is:

[0014] Q inj =(0.06Ap net Kt / μ)+2x f w f H;

[0015] Where A is the crack surface area, p net is the net pressure, K is the reservoir permeability, t is the time, x f is the crack half length, H is the crack height, w f is the hydraulic fracture width during fracturing, w f The calculation formula is:

[0016] w f =πp net H(1-v 2 ) / 2000E;

[0017] Where v is the Poisson's ratio of the formation rock, and E is the Young's modulus of the formation rock.

[0018] Preferably, in S2, the constant flow rate is 2%-5% of the maximum injection displacement calculated in S1.

[0019] Preferably, in said S2, the injection data includes injection pressure and injection flow rate;

[0020] The flowback data includes the flowback volume, flowback pressure and flowback rate.

[0021] Preferably, in S3, the quality assessment process specifically includes: performing data integrity and continuity assessment, identifying and removing abnormal data;

[0022] This process needs to ensure that all data points (injection pressure, injection rate, flowback pressure, flowback rate) are collected, and at the same time check the continuity of the data, without obvious errors or jumps, and remove the outliers in the data after reasonable analysis to ensure that they do not affect subsequent calculations;

[0023] The specific correction process is as follows: During the flowback stage, the fluid velocity in the fracture decreases, the fluid velocity in the fracture is measured, and the flow pressure is corrected according to the fluid velocity in the fracture to prevent the velocity measured on the ground from being too high, which will cause the oil well productivity index to be estimated too high. The friction near the wellbore and the perforation hole is estimated based on the difference in flow pressure to determine whether re-perforation is needed;

[0024] Under low backflow rate conditions, the effect of near-wellbore friction on the estimation of minimum horizontal stress is negligible; however, in the backflow stage, as the fracture begins to close, the friction of the fluid in the fracture gradually increases, causing the flow velocity in the fracture to drop rapidly and gradually deviate from the rate measured on the ground. At this time, the wellbore unloading effect dominates, delaying the decline of the flow velocity in the fracture, resulting in a higher flow velocity measured on the ground and an overestimation of the oil well productivity index. Therefore, it is necessary to correct the flow pressure at this time by measuring the fluid flow velocity in the fracture and estimate the friction near the wellbore and perforation holes based on the difference.

[0025] Preferably, in S3, the calculation process of the closing pressure is: using the incremental flowback method, the closing pressure is estimated according to the curve relationship between the stable flowback pressure and the flowback liquid volume increment in the flowback data.

[0026] Preferably, in S3, the calculation process of the reservoir pressure is:

[0027] 1) Determine the fluid flow state during the reflux process using the double logarithmic curves of the normalized pressure RNP of the water production and the derivative RNP′ of the normalized pressure of the water production and the material balance time tc;

[0028] 2) The pressure at the point where the control flow just begins to appear near the boundary of the fracture area in the double logarithmic curve is selected as the estimated value of the reservoir pressure.

[0029] Preferably, the calculation formula of the water production normalized pressure RNP is:

[0030]

[0031] Where P ref is the bottom hole pressure at the start of backflow, P wf (t) is the bottom hole pressure corresponding to time t, q w (t) is the water production rate corresponding to time t;

[0032] The calculation formula of the derivative of the normalized pressure of water production RNP′ is:

[0033]

[0034] In the formula, ln(t c ) is the material equilibrium time t c The natural logarithm of

[0035] Material balance time tc The calculation formula is:

[0036]

[0037] In the formula, Q w (t) is the cumulative water production corresponding to time t.

[0038] Preferably, in S3, the calculation process of the reservoir productivity index is:

[0039] Define the initial data of the near-fracture area, perform characteristic curve analysis on the data of the boundary control flow stage, and draw a functional relationship curve between RNP and standard cumulative water production;

[0040] The functional relationship between RNP and standard cumulative water production is expressed as:

[0041]

[0042] In the formula, c t is the overall compressibility of the system, p i-FBR is the initial pressure near the crack area, m FMB is the slope of the curve, b FMB is the y-intercept of the curve;

[0043] Initial water content near the crack area IWIP FBR It is expressed as:

[0044]

[0045] J FMB =b FMB ;

[0046] In the formula, J FMB is the oil well productivity index.

[0047] Therefore, the low-pressure testing method based on reflux flowback of the present invention has the following beneficial effects:

[0048] (1) By adopting the injection / reflux return flow method, the pressure drop and return flow rate curves in the return stage are recorded. The test can be completed in just a few hours, avoiding the long-term well shutdown and waiting for pressure drop in traditional low-pressure tests, and significantly improving the test efficiency.

[0049] (2) Through data quality assessment, flow pressure correction, estimation of friction near the wellbore and perforation holes, and a short testing process, errors caused by overestimation of the well productivity index and long waiting times can be avoided, thereby improving the accuracy and reliability of the test results, helping to optimize the volume fracturing construction plan and improve the efficiency of oil and gas field development.

[0050] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a schematic diagram of a flow chart of an embodiment of the present invention;

[0052] Figure 2 is a curve showing the relationship between the stable pressure and the reflux volume increment according to an embodiment of the present invention;

[0053] Figure 3 is a double logarithmic curve of an embodiment of the present invention;

[0054] Figure 4 4 is a functional relationship curve between RNP and standard cumulative water production according to an embodiment of the present invention. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0056] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0057] Example

[0058] Before conducting the test, ensure that all necessary equipment and tools are ready, including pump trucks, fracturing fluid tanks, high-pressure manifolds, pressure gauges, flow meters, etc.; clean the well site to ensure there is enough space for equipment layout and operation; finally, conduct a safety check to ensure the safety of the entire test process.

[0059] like Figure 1 As shown, the present invention provides a low-pressure test method based on reflux flowback, comprising the following steps:

[0060] S1. Calculate the maximum injection displacement and injection volume of the fracturing fluid for the specific reservoir in the well site and determine the pumping time, specifically:

[0061] 1) Based on the radial Darcy flow formula, the maximum injection displacement under specific reservoir conditions can be derived. The calculation formula for the maximum injection displacement is:

[0062]

[0063] Where k is the reservoir permeability, h is the reservoir thickness, and p f is the formation fracture pressure, p i is the original formation pressure of the reservoir, μ is the fluid viscosity, β is the fluid volume coefficient, r e is the discharge radius, rw is the wellbore radius, and S is the skin coefficient.

[0064] 2) The injection volume is calculated according to the volume equation.

[0065] The volume balance equation is:

[0066] Q inj =Q L +Q F ;

[0067] Among them, Q L is the filtration loss, Q F is the volume of fluid in the fracture, Q inj is the injection volume;

[0068] ①Q L The calculation formula is:

[0069] Q L =0.06A pnet Kt / μ;

[0070] In the formula, p net is the net pressure, K is the reservoir permeability, t is the time, A is the fracture surface area, and the calculation formula of A is:

[0071] A=2x f H;

[0072] In the formula, x f is the crack half-length, H is the crack height;

[0073] ②Q L The calculation formula is:

[0074] Q F The calculation formula is:

[0075] Q F =w f A;

[0076] In the formula, w f is the hydraulic fracture width during fracturing, w f The calculation formula is:

[0077] w f =πp net H(1-v 2 ) / 2000E;

[0078] In the formula, v is the Poisson's ratio of the formation rock, and E is the Young's modulus of the formation rock. This formula can characterize the rock deformation and hydraulic fracture width during fracturing.

[0079] but:

[0080] Qinj =(0.06Ap net Kt / μ)+2x f w f H.

[0081] Given the crack height H and crack half length x f Under the conditions, the injection volume can be estimated, and then the pumping time can be determined by the maximum injection displacement.

[0082] S2, start the fracturing truck, inject the fracturing fluid according to the result obtained in S1 and record the injection data. After the formation fracture pressure is reached, the pressure drops suddenly, and the injection is continued for a period of time to complete the injection of the fracturing fluid. Then, the pump is turned off and the well is temporarily shut down, and a constant speed flowback is immediately performed, and the flowback data is recorded;

[0083] The constant flow rate is 2%-5% of the maximum injection displacement calculated by S1; the injection data includes the injection pressure and the injection flow rate; the flowback data includes the flowback liquid volume, the flowback pressure and the flowback rate.

[0084] S3: Perform quality assessment and correction on the data collected in S2, calculate the closure pressure, reservoir pressure and reservoir productivity index based on the corrected data, and complete the small pressure test, specifically:

[0085] 1) The quality assessment process is as follows: conduct data integrity and continuity assessment to ensure that all injection data and flowback data are collected, check the continuity of the data to ensure that there are no obvious omissions or jumps, identify and remove abnormal data after reasonable analysis, and ensure that it does not affect subsequent calculations;

[0086] 2) The calibration process is as follows:

[0087] Under low return flow rate conditions, the effect of near-wellbore friction on the estimation of minimum horizontal stress can be ignored; however, in the return flow stage, as the fracture begins to close, the friction of the fluid in the fracture gradually increases, causing the flow velocity in the fracture to drop rapidly and gradually deviate from the velocity measured on the ground. At this time, the wellbore unloading effect dominates, delaying the decline of the flow velocity in the fracture, resulting in a higher velocity measured on the ground, which causes the estimated oil well productivity index to be too high. Therefore, it is necessary to correct the flow pressure based on the fluid flow velocity in the fracture to prevent the measured flow velocity on the ground from being too high, which causes the estimated oil well productivity index to be too high. The friction near the wellbore and the perforation holes is estimated based on the difference in flow pressure to determine whether re-perforation is needed.

[0088] 3) The calculation process of closing pressure is:

[0089] Closure pressure is an inherent property of the reservoir and is not affected by the backflow velocity and flow rate. The incremental backflow method does not need to accurately control the backflow rate. Instead, it measures the increment of the backflow volume each time and records the stable backflow pressure of the backflow pipe at this time. The closure pressure is estimated based on the curve relationship between the stable backflow pressure and the backflow volume increment in the backflow data, such as Figure 2 shown.

[0090] 4) The calculation process of reservoir pressure is:

[0091] ① Using the normalized pressure RNP of water production and the derivative RNP′ of the normalized pressure of water production and the material balance time t c The double logarithmic curve is used to determine the fluid flow state during the reflux process;

[0092] Among them, the calculation formula of water production normalization pressure RNP is:

[0093]

[0094] Where P ref is the bottom hole pressure at the start of backflow, P wf (t) is the bottom hole pressure corresponding to time t, q w (t) is the water production rate corresponding to time t;

[0095] The calculation formula of the derivative of the normalized pressure of water production RNP′ is:

[0096]

[0097] In the formula, ln(t c ) is the material equilibrium time t c The natural logarithm of

[0098] Material balance time t c The calculation formula is:

[0099]

[0100] In the formula, Q w (t) is the cumulative water production corresponding to time t.

[0101] ② If Figure 3 As shown in the figure, the pressure at the point where the control flow just begins to appear near the fracture boundary in the double logarithmic curve (the starting point of FR6) is selected as the estimated value of the reservoir pressure. This point indicates that the dominant effect of the near-fracture area begins to subside, and the reservoir pressure begins to dominate the flow of the fluid. At the same time, the double logarithmic curve can also determine the closing pressure, that is, the starting point of FR4, indicating that the fracture is mechanically closed.

[0102] 5) If Figure 4 As shown in Figure 2, the calculation process of the reservoir productivity index is:

[0103] The characteristic curve analysis is performed on the data of the boundary control flow stage to define the initial data of the near-fracture area, including the original pressure, compressibility and water compressibility of the near-fracture area, and the RNP and standard cumulative water production are plotted. The functional relationship curve is expressed as:

[0104]

[0105] Among them, c t is the overall compressibility of the system, p i-FBR is the initial pressure near the crack area, m FMB is the slope of the curve, the x-intercept of the curve It is the initial water content IWIP in the area near the crack FBR , y-intercept b FMB , which is the oil well productivity index J FMB An estimated value of

[0106] The overall compressibility of the system t The calculation formula is:

[0107] C t =C w +C FBR ;

[0108] In the formula, C w is the compressibility of water, C FBR It is the compressibility of the formation in the near-fracture area.

[0109] In addition, the flowback data can also be used to calculate parameters such as formation pressure, fluid efficiency, fluid loss coefficient, permeability, etc.

[0110] Therefore, the present invention provides a low-pressure test method based on reflux flowback, which adopts the injection / reflux reflux flowback mode, records the change curve of the pressure drop and the flowback volume in the reflux stage, and can complete the test in just a few hours, thus avoiding the long-term process of shutting down the well and waiting for the pressure drop in the traditional low-pressure test, and significantly improving the test efficiency;

[0111] Through data quality assessment, flow pressure correction, estimation of friction near the wellbore and perforation holes, and a short testing process, errors caused by overestimation of the oil well productivity index and long waiting times can be avoided, the accuracy and reliability of the test results can be improved, which will help optimize the volume fracturing construction plan and improve the efficiency of oil and gas field development.

[0112] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A low-pressure test method based on reflux, characterized in that: The following steps are involved: S1. Calculate the maximum injection displacement and injection volume of the fracturing fluid for a specific reservoir in the well site, and determine the pumping time; S2, injecting the fracturing fluid according to the result obtained in S1 and recording the injection data, shutting down the well for a short time after completing the injection of the fracturing fluid, immediately performing a constant flowback, and recording the flowback data; S3: Perform quality assessment and correction on the data collected in S2, calculate the closure pressure, reservoir pressure and reservoir productivity index based on the corrected data, and complete the low-pressure test.

2. A low-pressure test method based on reflux according to claim 1, characterized in that: In S1, the calculation formula for the maximum injection displacement is: Where k is the reservoir permeability, h is the reservoir thickness, and p f is the formation fracture pressure, p i is the original formation pressure of the reservoir, μ is the fluid viscosity, β is the fluid volume coefficient, r e is the discharge radius, r w is the wellbore radius, and S is the skin coefficient.

3. A low-pressure test method based on reflux according to claim 2, characterized in that: In S1, the calculation formula for the injection amount is: Q inj =(0.06Ap net Kt / μ)+2x f w f H; Where A is the crack surface area, p net is the net pressure, K is the reservoir permeability, t is the time, x f is the crack half length, H is the crack height, w f is the hydraulic fracture width during fracturing, w f The calculation formula is: w f =πp net H(1-v 2 ) / 2000E; Where v is the Poisson's ratio of the formation rock, and E is the Young's modulus of the formation rock.

4. A low-pressure test method based on reflux according to claim 1, characterized in that: In S2, the constant flow rate is 2%-5% of the maximum injection displacement calculated in S1.

5. A low-pressure test method based on reflux according to claim 1, characterized in that: In said S2, the injection data includes injection pressure and injection flow rate; The flowback data includes the flowback volume, flowback pressure and flowback rate.

6. A low-pressure test method based on reflux according to claim 1, characterized in that: In S3, the quality assessment process specifically includes: performing data integrity and continuity assessment, identifying and removing abnormal data; The correction process is as follows: during the flowback stage, the fluid velocity in the fracture decreases, the fluid velocity in the fracture is measured, the flow pressure is corrected based on the fluid velocity in the fracture, the friction near the wellbore and the perforation hole is estimated based on the difference in flow pressure, and it is determined whether re-perforation is needed.

7. A low-pressure test method based on reflux according to claim 5, characterized in that: In S3, the calculation process of the closing pressure is: using the incremental flowback method, the closing pressure is estimated according to the curve relationship between the stable flowback pressure and the flowback liquid volume increment in the flowback data.

8. The low-pressure testing method based on reflux according to claim 1, characterized in that: In S3, the calculation process of the reservoir pressure is: 1) Determine the fluid flow state during the reflux process using the double logarithmic curves of the normalized pressure of water production and the derivative of the normalized pressure of water production and the material balance time; 2) The pressure at the point where the control flow just begins to appear near the boundary of the fracture area in the double logarithmic curve is selected as the estimated value of the reservoir pressure.

9. A low-pressure test method based on reflux according to claim 8, characterized in that: The calculation formula of the water production normalization pressure is: Where RNP is the normalized pressure of water production, P ref is the bottom hole pressure at the start of backflow, P wf (t) is the bottom hole pressure corresponding to time t, q w (t) is the water production rate corresponding to time t; The formula for calculating the derivative of the normalized pressure of water production is: Where RNP′ is the derivative of the normalized pressure of water production, ln(t c ) is the material equilibrium time t c The natural logarithm of The calculation formula of material balance time is: Where, t c is the material equilibrium time, Q w (t) is the cumulative water production corresponding to time t.

10. A low-pressure test method based on reflux according to claim 9, characterized in that: In S3, the calculation process of the reservoir productivity index is: Define the initial data of the near-fracture area, perform characteristic curve analysis on the data of the boundary control flow stage, and draw a functional relationship curve between the normalized pressure of water production and the standard cumulative water production; The functional relationship between the normalized water production pressure and the standard cumulative water production is expressed as: In the formula, c t is the overall compressibility of the system, p i-FBR is the initial pressure near the crack area, m FMB is the slope of the curve, b FMB is the y-intercept of the curve; Initial water content near the crack area IWIP FBR It is expressed as: J FMB =b FMB ; In the formula, J FMB is the oil well productivity index.

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