A method for determining production pressure difference under water lock damage conditions

Abstract

The present invention provides a method for determining the production pressure difference under water lock damage conditions. The method takes a rock sample in a production well as a sample, and obtains the water lock damage critical discharge radius of the rock sample under a certain pressure difference by centrifuging the water-saturated rock sample at different rotation speeds. The method compares the actual maximum immersion depth of operation with the water lock damage critical discharge radius to determine whether the production pressure difference corresponding to the current water lock damage critical discharge radius can be used as the production pressure difference in the actual operation process, thereby providing more objective guidance for on-site production.

Description

Technical Field

[0001] The invention relates to a method for determining a production pressure difference, in particular to a method for determining a production pressure difference under water lock damage conditions, and belongs to the technical field of oil and gas production engineering. Background Art

[0002] During the operation of oil and gas wells, water-based well fluid invades the reservoir under the action of positive pressure difference. Due to the capillary force of rock pore throats, water phase is easily retained, which increases water saturation and causes temporary or permanent reduction in reservoir oil / gas permeability. This phenomenon is called water lock damage. Water lock damage is mainly affected by factors such as low reservoir permeability, hydrophilic rock, low original water saturation, water phase invasion of well fluid at a large depth, and insufficient production pressure difference. Since low-permeability and ultra-low-permeability sandstone reservoirs often have the risk of water lock damage, when water phase invasion occurs, water lock damage will occur if the driving force of the oil and gas reservoir is insufficient, affecting the production capacity of a single well.

[0003] At present, in-depth research has been conducted on the evaluation methods of water lock damage to tight reservoirs at home and abroad, and good results have been achieved: in terms of water saturation establishment, the main methods include: displacement method, centrifugation method, nuclear magnetic resonance method, self-imbibition method, etc., which can establish different water saturations of cores; in terms of permeability measurement methods, there are pulse attenuation method, nuclear magnetic permeability measurement method, back pressure gas measurement method, etc., which can accurately measure the permeability under different water saturation conditions; in terms of analyzing the degree of water lock damage, a variety of empirical formulas or coefficient prediction methods have been created and combined with laboratory evaluation data to accurately evaluate the degree of water lock damage in reservoirs.

[0004] However, the above methods are limited to predicting the water lock damage risk of the reservoir. The relationship between the actual production of a single well and water lock damage has not yet been clarified. The evaluation of water lock damage is only based on theoretical research and experiments on reservoir cores, and is not closely integrated with field production. For example, the Kuche piedmont wells in the Tarim Oilfield are high-risk reservoirs for water lock damage, but most wells do not suffer from water lock damage in the early stages of production. As the development deepens, some wells gradually experience water lock damage; some medium and low permeability reservoirs often cause more serious water lock damage due to low formation pressure coefficients and high density of well fluid.

[0005] Therefore, there is an urgent need for a method to comprehensively assess whether water lock damage occurs in the reservoir through engineering production factors (water phase invasion and production pressure difference). Summary of the invention

[0006] The present invention provides a method for determining production pressure difference under water lock damage conditions, which can find out the area where water lock damage occurs and determine the minimum production pressure difference when water lock damage occurs after water phase intrusion, thereby guiding on-site production.

[0007] The present invention provides a method for determining a production pressure difference under water lock damage conditions, comprising the following steps:

[0008] S1: The water-saturated rock sample is centrifuged N times in sequence to obtain the water saturation of the rock sample at N different rotational speeds and the permeability at N different rotational speeds, and a water saturation-permeability curve of the rock sample is established, wherein the rotational speeds of the N centrifugal treatments are different from each other, and the rotational speed of the Nth centrifugal treatment is greater than the rotational speed of the (N-1)th centrifugal treatment, and N≥2;

[0009] S2: obtaining the water lock damage rate of the rock sample at N different rotation speeds according to the water saturation-permeability curve and the original water saturation of the rock sample;

[0010] S3: obtaining the centrifugal pressure gradient of the rock sample at N different rotational speeds according to the length of the rock sample and the N different rotational speeds;

[0011] S4: establishing a water lock damage rate-pressure gradient curve of the rock sample according to the water lock damage rate of the rock sample at N different rotation speeds and the pressure gradient at N different rotation speeds;

[0012] S5: Obtaining a critical water lock damage pressure gradient according to the water lock damage rate-pressure gradient curve;

[0013] S6: establishing a time-pressure curve of the target well in the first pressure recovery stage according to the unstable well test curve of the target well;

[0014] S7: establishing a drainage radius-pressure curve of the target well according to formula a and the time-pressure curve;

[0015] S8: obtaining a drainage radius-pressure gradient curve of the target well according to the drainage radius-pressure curve of the target well;

[0016] S9: obtaining a critical leakage radius of water lock damage in the first pressure recovery stage according to the critical water lock damage pressure gradient and the leakage radius-pressure gradient curve;

[0017] S10: simulating the time-pressure curve of the target well at M different pressure recovery stages and obtaining the critical discharge radius of water lock damage at M different pressure recovery stages, M≥1;

[0018] S11: determining the production pressure difference according to the critical discharge radius of water lock damage in the first pressure recovery stage, the critical discharge radius of water lock damage in M ​​different pressure recovery stages, and the actual maximum immersion depth of the operation;

[0019]

[0020] In formula a, r j is the discharge radius (detection radius), m; K0 is the permeability at the original water saturation, ×10 -3 μm 2 ;t j is the time value in the first pressure recovery stage, h; is the effective porosity of the rock sample, %; μ is the fluid viscosity, mPa·s; C t is the total compression coefficient of the formation, MPa -1 .

[0021] The rock sample of the present invention is specifically selected from the rock of the reservoir in the production oil well or gas well.

[0022] In S1, the rock sample is firstly subjected to a water saturation operation, and then the water saturated rock sample is subjected to centrifugal treatment using a centrifugal device. The centrifugal treatment of the present invention refers to taking a water saturated rock sample as the initial centrifugal object and sequentially performing N centrifugations at different speeds, and the speed of each centrifugation is greater than the speed of the previous centrifugation, and the object of each centrifugation is the rock sample after the previous centrifugation.

[0023] It can be understood that in specific operations, after the rock sample is initially saturated with water, it can be subjected to a first centrifugal treatment at a first speed for a constant time, and then its water saturation and permeability after centrifugation at the first speed are detected; then, the rock sample after the first centrifugal treatment is subjected to a second centrifugal treatment at a second speed different from the first speed for a constant time, and then its water saturation and permeability after centrifugation at the second speed are detected, and the speed is continuously increased until N centrifugal treatments with different speeds are completed. After N centrifugal treatments, a water saturation-permeability curve of the rock sample is established based on the water saturation and permeability detected after each centrifugal treatment. Optionally, N≥5.

[0024] In one embodiment, after the rock sample is dried and vacuumed, it can be placed in a high-pressure deionized water environment for a period of time, such as about 10 hours, to saturate it. Furthermore, in order to ensure that the rock sample is fully saturated, the pressurized rock sample can also be immersed in deionized water for at least 24 hours.

[0025] Furthermore, before drying and saturating the rock sample, the rock sample can be subjected to a staged heating treatment, wherein the target temperature of the staged heating treatment is 550°C. The staged heating treatment means that during the heating process, the rock sample is kept at an intermediate temperature for a period of time and then continues to be heated until the target temperature is reached. This staged heating treatment can convert water-sensitive minerals such as illite / montmorillonite mixed layer and montmorillonite in the rock sample into illite, thereby achieving the purpose of eliminating water-sensitive minerals.

[0026] When establishing the water saturation-permeability curve, the coordinate axis is established with the water saturation of the rock sample as the horizontal axis and the permeability of the rock sample as the vertical axis. Then, the water saturation and permeability of the rock sample after centrifugation at different speeds are used as coordinates to obtain N points. The water saturation-permeability curve and its equation of the rock sample are obtained through correlation analysis and fitting. When fitting, it is necessary to ensure that the correlation coefficient is not less than 0.9.

[0027] The present invention does not limit the detection method of the water saturation and permeability of the rock sample, and the detection can be carried out using the commonly used methods in the art.

[0028] For example, the water saturation of the rock sample after each centrifugation treatment can be obtained according to formula b.

[0029]

[0030] S wi is the water saturation of rock samples at different centrifugal speeds, %; m i is the mass of the rock sample measured at different centrifugal speeds, g; m0 is the dry weight of the rock sample, g; ρ w is the saturated fluid density (using deionized water as the saturated fluid, the value is 1g / cm 3 ), g / cm 3 ; V p is the pore volume of the rock sample, cm 3 .

[0031] In S2, the original water saturation of the rock sample can be obtained according to the logging results of the production oil well or gas well. Specifically, if the logging results show that the gas saturation of the reservoir where the rock sample is located is w, then the original water saturation of the rock sample is (1-w).

[0032] According to the original water saturation of the rock sample and the water saturation-permeability curve of the rock sample, the water lock damage rate of the rock sample at N different rotation speeds is obtained.

[0033] Among them, the water lock damage rate is used to evaluate the damage to the rock sample caused by the water retained inside the rock sample under the centrifugal action of a certain rotation speed. The higher the water lock damage rate, the easier it is for the rock sample to form water lock damage.

[0034] In one embodiment, S2 may include:

[0035] Obtaining the original permeability of the rock sample according to the water saturation-permeability curve and the original water saturation of the rock sample;

[0036] According to the permeability at the original water saturation, the permeability at N different rotation speeds and formula c, the water lock damage rate of the rock sample at N different rotation speeds is obtained;

[0037]

[0038] In formula c, D wi is the water lock damage rate, %; K0 is the original permeability, ×10 -3 μm 2 ; K i is the permeability of the rock sample under different water saturation conditions, ×10 -3 μm 2 .

[0039] In a specific embodiment, the original water saturation (1-w) of the rock sample is substituted into the water saturation-permeability curve (or the equation of the curve) of the rock sample in S1 to obtain the original permeability K0 of the rock sample. Then, the original permeability K0 and the permeabilities K obtained at N different rotation speeds in S1 are i Substituting into formula c, we can obtain the water lock damage rate D of the rock sample at N different rotation speeds: wi .

[0040] During the centrifugal treatment, since the rock sample itself has a certain length, one end and the other end of the rock sample will be subjected to different centrifugal forces, that is, when the centrifugal treatment is performed at each rotation speed, there is a centrifugal pressure gradient at both ends of the core. Specifically in the present invention, the rotation speed and length can be used to obtain the centrifugal pressure gradient of the rock sample under the current rotation speed. The present invention does not impose any restrictions on the order of steps S3 and S2 for obtaining the centrifugal pressure gradient of the rock sample, which can be performed simultaneously or one after another. It can be understood that the core has a centrifugal pressure gradient at each rotation speed, that is, N centrifugal pressure gradients are obtained.

[0041] In S4, a water lock damage rate-pressure difference gradient curve of the rock sample is established according to the water lock damage rates at N different rotation speeds obtained in S2 and the pressure difference gradients at N different rotation speeds obtained in S3.

[0042] Specifically, when establishing the water lock damage rate-pressure gradient curve, the coordinate axis is established with the pressure gradient of the rock sample as the horizontal axis and the water lock damage rate of the rock sample as the vertical axis, and then N points are obtained with the pressure gradient and water lock damage rate of the rock sample after centrifugation at different speeds each time as coordinates, and the water lock damage rate-pressure gradient curve and its equation of the rock sample are obtained through correlation analysis and fitting. When fitting, it is necessary to ensure that the correlation coefficient is not less than 0.9.

[0043] The present invention does not limit the method for obtaining the centrifugal pressure gradient of the rock sample. Figure 1It is a schematic diagram of the force applied to the rock sample during the centrifugal treatment process. In one embodiment, the pressure difference gradient of the rock sample after each centrifugation at different speeds n can be obtained by the following method. Specifically, the centrifugal force at the end of the rock sample at N different speeds is obtained according to formula d1; the centrifugal force at the front end of the rock sample at N different speeds is obtained according to formula d2; and N centrifugal pressure difference gradients of the rock sample at N different speeds are obtained according to formula e;

[0044] P co =1.097×10 -9 ΔρLRn i 2 Formula d1

[0045] P ci =1.097×10 -9 ΔρL(RL)n i 2 Formula d2

[0046]

[0047] Among them, P co is the centrifugal force at the end of the rock sample at different rotation speeds, MPa; P ci is the centrifugal force at the front end of the rock sample at different centrifugal speeds, MPa; Δρ is the density difference between the two phase fluids, g / cm 3 ; L is the length of the rock sample, cm; R is the centrifugal rotation radius, cm; n i is the speed, r / min; G ci is the centrifugal pressure gradient of the rock sample, MPa / cm.

[0048] In S5, the critical water lock damage pressure gradient can be obtained through the water lock damage rate-pressure gradient curve. Specifically, the critical water lock damage pressure gradient corresponding to the critical water lock damage rate can be obtained by substituting a determined critical water lock damage rate into the water lock damage rate-pressure gradient curve (or its equation).

[0049] In one embodiment, the critical water lock damage rate of 5% can be determined through the water lock damage evaluation index (see Table 1), and then the critical water lock damage pressure gradient can be obtained.

[0050] Table 1

[0051]

[0052]

[0053] In S6, a time-pressure curve of the target well in the first pressure recovery stage is established according to the unstable well test curve of the target well.

[0054] Specifically, a curve of the pressure recovery stage is selected from the test curve, and a time-pressure curve for recovery to the target pressure is established with the initial pressure as the starting point. This curve is the time-pressure curve of the first pressure recovery stage.

[0055] In addition to establishing the time-pressure curve of the target well in the first pressure recovery stage based on the unstable well test curve of the target well, the time-pressure curve of the target well in the first pressure recovery stage can also be established based on the unstable well test curves of the adjacent wells or adjacent block wells of the target well.

[0056] In S6, since the pressure test data in the pressure recovery stage is more stable, the data in the pressure recovery stage is used to determine the production pressure difference to avoid water lock damage, which can overcome the impact of unstable production capacity caused by multiphase flow in low-permeability condensate gas reservoirs and reduce the impact of skin coefficient and reservoir perfection on pressure testing.

[0057] In S7, a drainage radius-pressure curve of the target well is established according to formula a and the time-pressure curve of the target well in the first pressure recovery stage obtained in S6.

[0058] Specifically, different moments (i.e., t j ) into formula a to obtain the discharge radius r at this moment j ;

[0059] Then according to the discharge radius r at that moment j The drainage radius-pressure curve of the target well is established based on the pressure corresponding to the time-pressure curve in the first pressure recovery stage at that moment.

[0060] Specifically, when establishing the discharge radius-pressure curve of the target well, the coordinate axis is established with the discharge radius as the horizontal axis and the pressure as the vertical axis, and then the discharge radius and pressure of the rock sample at a certain moment are used as coordinates to obtain Q points (Q ≥ 2), and the discharge radius-pressure curve and its equation are obtained through correlation analysis and fitting. When fitting, it is necessary to ensure that the correlation coefficient is not less than 0.9.

[0061] In S8, the first-order derivative processing can be performed on the drainage radius-pressure gradient curve of the target well to obtain the drainage radius-pressure gradient curve of the target well. That is, by taking the derivative of the drainage radius-pressure gradient curve (i.e., the slope of the drainage radius-pressure gradient curve), the drainage radius r at different distances from the wellbore can be obtained. i The pressure gradient G i The discharge radius-pressure gradient curve of the target well is fitted.

[0062]

[0063] In formula f, Gj is the pressure gradient under different discharge radius, MPa / cm; P j is the pressure at different leakage radii (or at different times in the first pressure recovery stage), MPa.

[0064] In S9, the critical water lock damage pressure gradient obtained in S5 is substituted into the leakage radius-pressure gradient curve obtained in S8, thereby obtaining the water lock damage critical leakage radius in the first pressure recovery stage.

[0065] According to the critical leakage radius of water lock damage, the risk area of ​​water lock damage can be determined. Specifically, in the leakage radius-pressure curve of the target well, when the current leakage radius is less than the critical leakage radius of water lock damage, since the pressure gradient is greater than the critical water lock damage pressure gradient at this time, no water lock damage occurs in this area; when the current leakage radius is greater than the critical leakage radius of water lock damage, since the pressure gradient is less than the critical water lock damage pressure gradient at this time, if water phase intrusion occurs in this area, water lock damage will occur.

[0066] In S10, assuming that a production system with a fixed bottom hole flow pressure is adopted, the software can be used to simulate the time-pressure curve under the conditions of other M different pressure recovery stages (i.e., the pressure difference between the initial pressure and the target pressure is different), and according to S8 and S9, the critical leakage radius of water lock damage in each stage of the M different pressure recovery stages can be obtained.

[0067] In S11, simulation can be performed with relevant parameters (maximum displacement of construction, maximum pump pressure of construction, injection volume of fracturing fluid, and viscosity of fracturing fluid, etc.) as the object to obtain the maximum immersion depth of actual operation, that is, the maximum invasion depth of fracturing fluid. Then, the maximum immersion depth is compared with the critical discharge radius of water lock damage in the first pressure recovery stage and the critical discharge radius of water lock damage in M ​​different pressure recovery stages to determine the reasonable production pressure difference in actual operation, thereby avoiding the occurrence of water lock damage.

[0068] Specifically, if the critical leakage radius of water lock damage in the first pressure recovery stage and one of the critical leakage radius of water lock damage in M ​​different pressure recovery stages is greater than the maximum immersion depth, then water lock damage will not occur under the production pressure difference conditions corresponding to the critical leakage radius of water lock damage, otherwise water lock damage will occur.

[0069] Furthermore, if the actual maximum intrusion depth of the operation is less than the critical discharge radius of the water lock damage in the first pressure recovery stage, the pressure difference in the first pressure recovery stage is the safe production pressure difference;

[0070] If the actual maximum intrusion depth of operation is less than the critical discharge radii of water lock damage in M1 different pressure recovery stages among the critical discharge radii of water lock damage in M ​​different pressure recovery stages, then the M1 different pressure differences in the M1 different pressure recovery stages are the safe production pressure differences, M1≤M, M≥2. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 It is a schematic diagram of the force on the rock sample during the centrifugal treatment process;

[0072] Figure 2 The water saturation-permeability curve of the rock sample in the embodiment of the present invention;

[0073] Figure 3 This is a well logging interpretation result diagram of the well to which the rock sample belongs in the embodiment of the present invention;

[0074] Figure 4 The water lock damage rate-pressure gradient curve of the rock sample in the embodiment of the present invention;

[0075] Figure 5 is an unstable well test curve of the target well in an embodiment of the present invention;

[0076] Figure 6 is a time-pressure curve of the target well in the first pressure recovery stage in an embodiment of the present invention;

[0077] Figure 7 is a drainage radius-pressure curve of the target well in the first pressure recovery stage in an embodiment of the present invention;

[0078] Figure 8 is a discharge radius-pressure gradient curve of a target well in an embodiment of the present invention;

[0079] Fig. 9 A water lock damage area determination diagram in the first pressure recovery stage in an embodiment of the present invention;

[0080] Fig.10 1 is a discharge radius-pressure gradient curve under different production pressure differences in an embodiment of the present invention. DETAILED DESCRIPTION

[0081] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0082] The method for determining the production pressure difference of this embodiment includes the following steps:

[0083] 1. Establish the water saturation-permeability curve of rock samples

[0084] The rock samples (related parameters are shown in Table 2) were subjected to high-temperature passivation treatment at 550 °C, with the temperature increasing step by step with a gradient of 100 °C, and each gradient was maintained for 2 h until the heating treatment at 550 °C was completed for 2 h.

[0085] After drying the rock sample, evacuate it for more than 4 hours (the vacuum degree is not less than -0.08MPa), measure the dry weight of the core m0=45.0121g, set 25MPa high pressure as the saturation pressure, saturate the rock sample with deionized water and control the pressurization time to 8h, then soak the rock sample in deionized water for more than 24h to ensure that the rock sample is fully saturated.

[0086] The saturated rock sample was placed in a centrifuge and the centrifugal speed was set to centrifuge the rock sample for 50 min. After centrifugation, the rock sample was weighed quickly. i , and calculate the water saturation S according to formula b wi , and determine the permeability K under the water saturation condition i , as shown in Table 3.

[0087] After each centrifugation, the centrifugal speed was increased to centrifuge the rock sample after the last centrifugation again until the water-saturated core was centrifuged at 0 r / min (only saturation without centrifugation), 1000 r / min, 3000 r / min, 6000 r / min, 7000 r / min, and 8000 r / min in sequence.

[0088] According to the water saturation and permeability at each rotation speed in Table 3, the water saturation-permeability curve of the rock sample is fitted. Fitting method: According to the similarity fitting principle and the interval requirements of parameters, the power function fitting is adopted, and the water saturation interval is: 0% ≤ S wi ≤100%, permeability interval is 0μm 2 <K i ≤K, where K = 0.024658 × 10 -3 μm 2 , correlation index R 2 =0.9686.

[0089] Figure 2 is the water saturation-permeability curve of the rock sample in the embodiment of the present invention. The equation of the water saturation-permeability curve is as follows:

[0090] K i =240.75S wi -3.519 Formula 1

[0091] pass Figure 2It can be found that with the increase of water saturation, the permeability drops sharply, reflecting that the water locking effect of this rock sample is very obvious.

[0092] Table 2

[0093]

[0094] Table 3

[0095]

[0096] 2. Obtain the water lock damage rate of rock samples at different rotation speeds

[0097] Figure 3 This is a well logging interpretation result diagram of the well to which the rock sample belongs in the embodiment of the present invention. Since the well from which the rock sample was obtained did not carry out closed coring and formation MDT testing, according to Figure 3 It can be seen that the gas saturation S at the well depth of 6795.5m of the rock sample is g0 is 70%, so the initial water saturation of the rock sample is S w0 =30%.

[0098] Substituting the original water saturation of the rock sample 30% into the above formula 1, the original permeability of the rock sample K0 = 0.001526 × 10 -3 μm 2 .

[0099] The original permeability K0, the permeability K0 at each speed in Table 3 i Substituting into formula c, the water lock damage rate D of the rock sample at each rotation speed is calculated wi , as shown in Table 4.

[0100] Table 4

[0101]

[0102] 3. Establish the water lock damage rate-pressure gradient curve of rock samples

[0103] According to formula e1 (the combination of formula d1, d2, and e), the centrifugal pressure gradient G under different speed conditions is calculated. ci The results are shown in Table 5; where the length of the rock sample is L = 3.85 cm, Δρ is the density difference between the two phase fluids (deionized water and air), ≈ 1 g / cm 3 ;

[0104] G ci =1.097×10 -9 Ln i 2 Formula e1

[0105] According to the water lock damage rate of the rock samples at different rotation speeds in Table 4 and the centrifugal pressure gradient of the rock samples at different rotation speeds in Table 5, the water lock damage rate-pressure gradient curve of the rock samples was fitted and established. Fitting method: According to the similarity fitting principle and the interval requirements of the parameters, linear fitting was adopted, and the correlation index R 2 =0.9825. Figure 4 The water lock damage rate-pressure gradient curve of the rock sample in the embodiment of the present invention is as follows:

[0106] G ci =-0.0003D wi +0.0593 Formula 2

[0107] According to the water lock damage evaluation index (shown in Table 1), the critical water lock damage rate is defined as 5%, and the corresponding centrifugal pressure gradient value is the critical water lock damage pressure gradient G w , calculated according to formula 2, G w =0.058MPa / cm.

[0108] Table 5

[0109]

[0110] 4. Establish the time-pressure curve of the target well in the first pressure recovery stage

[0111] Figure 5 The time t in the first pressure recovery stage (C→D section) is established. j (j represents different time, i.e. 0~0.45h) and pressure P j relationship curve. Figure 6 is the time-pressure curve of the target well in the first pressure recovery stage in the embodiment of the present invention; wherein, t j When the pressure is 0, P0 is Figure 5 The pressure value of point C in the figure is P0 = 102.798 MPa; the pressure is restored to the formation pressure P r The time is Figure 5 The pressure value at point D, that is, P r =120.953MPa, the difference between the pressure values ​​corresponding to points C and D is the production pressure difference ΔP=P r -P0=18.155MPa.

[0112] 5. Establish the drainage radius-pressure curve of the target well

[0113] The different times t in the first pressure recovery stage j Substituting into formula a, the discharge radius r corresponding to different times is calculated. j, the results are shown in Table 6.

[0114] Using the discharge radius r corresponding to different times in Table 6 j as well as Figure 6 The pressure corresponding to different time in the target well is used to establish the discharge radius-pressure curve of the target well; Figure 7 1 is a drainage radius-pressure curve of the target well in the first pressure recovery stage in an embodiment of the present invention.

[0115] Table 6

[0116]

[0117] 6. Obtain the drainage radius-pressure gradient curve of the target well

[0118] right Figure 7 By taking the derivative of the discharge radius-pressure curve (i.e. the slope of the curve), we can obtain the discharge radius r at different distances from the wellbore. j The pressure gradient G j Relationship curve. Figure 8 It is the drainage radius-pressure gradient curve of the target well in the embodiment of the present invention. Fitting method: According to the similarity fitting principle, exponential fitting is adopted, and the correlation index R 2 =0.9573.

[0119] The equation of the discharge radius-pressure gradient curve is as follows:

[0120]

[0121] 7. Identify water lock damage risk areas

[0122] The critical water lock damage pressure gradient G w =0.058MPa / cm Substituting into formula 3, the critical discharge radius r of water lock damage in the first pressure recovery stage can be calculated. w =0.625m, the risk area of ​​water lock damage can be determined. Fig. 9 FIG. 1 is a diagram for determining the water lock damage area in the first pressure recovery stage in an embodiment of the present invention. Fig. 9 It can be seen that when the discharge radius r j Less than r w When the pressure gradient G j Greater than the critical water lock damage pressure gradient G w , so no water lock damage occurs in this area; when the discharge radius r j Greater than r w When the pressure gradient G j Less than the critical water lock damage pressure gradient G w If water intrusion occurs in this area, water lock damage will occur.

[0123] 8. Analysis of the impact of production pressure difference on water lock damage risk areas

[0124] When depletion mining is adopted, the formation pressure P r As the production continues to decline, assuming that the bottom hole flow pressure remains unchanged (i.e., P0 = 102.80 MPa), the COMSOL finite element software is used to simulate the production pressure of a single well for 20 years, and the pressure difference ΔP can be obtained. k (k represents the mining time simulated by the software, i.e. 1 to 20 years) discharge radius-pressure curve under the condition; repeating steps 6 and 7, different production pressure difference conditions ΔP k Critical discharge radius r of water lock damage under wk ( Fig.10 , as shown in Table 7), the results are shown in Fig.10 And Table 7, Fig.10 1 is a discharge radius-pressure gradient curve under different production pressure differences in an embodiment of the present invention.

[0125] according to Fig.10 It can be seen from Table 7 that when the production pressure difference decreases, the critical discharge radius of water lock damage moves toward the wellbore, and the water lock damage area gradually expands. When the production pressure difference is 1.4MPa after 20 years of mining, the critical discharge radius of water lock damage is 0.039m.

[0126] Table 7

[0127]

[0128] 8. Determine the maximum water phase intrusion distance

[0129] Matlab software was used to simulate the maximum distance that the fracturing fluid invaded the core matrix during fracturing construction in a single well: During this fracturing construction, the maximum displacement was 4.1m 3 / min, maximum construction pump pressure 107.7MPa, injection volume 150m 3 , the viscosity of the fracturing fluid is 150mPa·s. The maximum penetration depth of the fracturing fluid in this actual operation is calculated as r d =0.252m.

[0130] 9. Determination of production pressure difference

[0131] For the critical discharge radius of water lock damage and the actual maximum operating depth r in Table 7 d =0.252m for comparison. When the production pressure difference is greater than 5.7MPa, no water lock damage will occur when the well is opened after fracturing construction. When the production pressure difference drops to 3.0MPa, moderate to strong water lock damage will occur when the well is opened after fracturing construction.

[0132] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining production pressure difference under water lock damage conditions, characterized in that: The following steps are involved: The water-saturated rock sample is centrifuged N times in sequence to obtain the water saturation of the rock sample at N different rotational speeds and the permeability at N different rotational speeds, and a water saturation-permeability curve of the rock sample is established, wherein the rotational speeds of the N centrifugal treatments are different from each other, and the rotational speed of the Nth centrifugal treatment is greater than the rotational speed of the N-1th centrifugal treatment, and N≥2; According to the water saturation-permeability curve and the original water saturation of the rock sample, the original permeability of the rock sample is obtained; according to the original permeability, the permeabilities at N different rotation speeds and formula c, the water lock damage rate of the rock sample at N different rotation speeds is obtained; In formula c, D wi is the water lock damage rate, %; K0 is the original permeability, ×10 -3 μm 2 ; K i is the permeability of the rock sample under different water saturation conditions, ×10 -3 μm 2 ; According to the length of the rock sample and N different rotational speeds, a centrifugal pressure gradient of the rock sample at N different rotational speeds is obtained; According to the water lock damage rate of the rock sample at N different rotation speeds and the pressure difference gradient at N different rotation speeds, a water lock damage rate-pressure difference gradient curve of the rock sample is established; Taking 5% as the critical water lock damage rate, the critical water lock damage pressure gradient is obtained according to the water lock damage rate-pressure gradient curve; According to the unstable well test curve of the target well, establishing a time-pressure curve of the target well in the first pressure recovery stage; According to formula a and the time-pressure curve, a drainage radius-pressure curve of the target well is established; Performing first-order derivative processing on the drainage radius-pressure curve of the target well to obtain the drainage radius-pressure gradient curve of the target well; According to the critical water lock damage pressure gradient and the leakage radius-pressure gradient curve, a water lock damage critical leakage radius in the first pressure recovery stage is obtained; Simulating the time-pressure curve of the target well at M different pressure recovery stages and obtaining the critical discharge radius of water lock damage at M different pressure recovery stages, M≥1; Determine the production pressure difference according to the critical discharge radius of water lock damage in the first pressure recovery stage, the critical discharge radius of water lock damage in M ​​different pressure recovery stages, and the actual maximum immersion depth of operation; In formula a, r j is the discharge radius, m; K0 is the permeability at the original water saturation, ×10 -3 μm 2 ; t j is the time value in the first pressure recovery stage, h; is the effective porosity of the rock sample, %; μ is the fluid viscosity, mPa·s; C t is the total compression coefficient of the formation, MPa -1 .

2. The method for determining the production pressure difference according to claim 1, characterized in that: The water-saturated rock sample is centrifuged N times in sequence to obtain the water saturation of the rock sample at N different rotation speeds and the permeability at N different rotation speeds, and a water saturation-permeability curve of the rock sample is established, including: After each centrifugal treatment is completed, the water saturation of the rock sample at the current rotation speed is obtained according to formula b, and N water saturations of the rock sample at N different rotation speeds are obtained; After each centrifugal treatment is completed, the permeability of the rock sample after the centrifugal treatment at the current rotation speed is detected to obtain N permeabilities of the rock sample at N different rotation speeds; Establishing the water saturation-permeability curve according to the N water saturations and the N permeabilities; S wi is the water saturation of rock samples at different centrifugal speeds, %; m i is the mass of the rock sample measured at different centrifugal speeds, g; m0 is the dry weight of the rock sample, g; ρ w is the saturated fluid density, g / cm 3 ; V p is the pore volume of the rock sample, cm 3 .

3. The method for determining the production pressure difference according to claim 1 or 2, characterized in that: The step of obtaining the centrifugal pressure gradient of the rock sample at N different rotational speeds according to the length of the rock sample and N different rotational speeds comprises: According to formula d1, the centrifugal force at the end of the rock sample at N different rotation speeds is obtained; According to formula d2, the centrifugal force at the front end of the rock sample at N different rotation speeds is obtained; According to formula e, N centrifugal pressure gradients of the rock sample at N different rotation speeds are obtained; P co =1.097×10 -9 ΔρLRn i 2 Formula d1 P ci = 1.097×10 -9 ΔρL(R - L)n i 2 Equation d2 Among them, P co is the centrifugal force at the end of the rock sample at different rotation speeds, MPa; P ci is the centrifugal force at the front end of the rock sample at different centrifugal speeds, MPa; Δρ is the density difference between the two phase fluids, g / cm 3 ; L is the length of the rock sample, cm; R is the centrifugal rotation radius, cm; n i is the speed, r / min; G ci is the centrifugal pressure gradient of the rock sample, MPa / cm.

4. The method for determining the production pressure difference according to claim 1, characterized in that: The method of establishing the drainage radius-pressure curve of the target well according to formula a and the time-pressure curve includes: According to formula a, the corresponding leakage radius at different times in the first pressure recovery stage is obtained; A drainage radius-pressure curve of the target well is established according to the time-pressure curve and the drainage radius corresponding to different moments in the first pressure recovery stage.

5. The method for determining the production pressure difference according to claim 1, characterized in that: The determining of the production pressure difference according to the critical discharge radius of water lock damage in the first pressure recovery stage, the critical discharge radius of water lock damage in M ​​different pressure recovery stages, and the actual maximum immersion depth of operation includes: If the actual maximum intrusion depth of the operation is less than the critical discharge radius of the water lock damage in the first pressure recovery stage, the pressure difference in the first pressure recovery stage is the safe production pressure difference; If the actual maximum intrusion depth of the operation is less than the critical discharge radii of water lock damage in M1 different pressure recovery stages among the critical discharge radii of water lock damage in the M different pressure recovery stages, then the M1 different pressure differences in the M1 different pressure recovery stages are the safe production pressure differences, M1≤M, M≥2.

6. The method for determining the production pressure difference according to claim 2, characterized in that: N≥5。 7. The method for determining the production pressure difference according to claim 2, characterized in that: Before the rock sample is saturated with water, the rock sample is subjected to a staged temperature increase treatment, wherein the target temperature of the staged temperature increase treatment is 550°C.

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