A method for evaluating variable differential pressure blocking performance of drilling fluid by simulating wellbore fluctuating pressure

Abstract

The application discloses a method for evaluating the pressure differential blocking performance of drilling fluid under simulated wellbore fluctuating pressure, which comprises the following steps: selecting a threaded cup cover type permeability blocking instrument as a test device; injecting a drilling fluid to be tested; increasing the temperature to a target temperature; performing a variable pressure cycle test; performing a filtration experiment with distilled water; obtaining the average thickness and diameter of a mud cake, and calculating the area of the mud cake, the absorbance of the filtrate; observing the mud cake and a ceramic disc; and evaluating the blocking performance of the drilling fluid to be tested; and the method has the following effects: the total experimental time is about 3 hours, the equipment and its accessories are easy to purchase in China at present, and the application in a well site can be realized if a nitrogen cylinder is provided on site, so the method is a fast evaluation method. The method simulates the formation of a mud cake of drilling fluid under wellbore fluctuating pressure, analyzes the damage degree of solid phase and liquid phase of the drilling fluid to a reservoir, and proposes an optimization suggestion for the oil retention performance of the drilling fluid, so that the method has important guiding significance for improving the protection performance of the drilling fluid to an oil and gas layer.

Description

Technical Field

[0001] This invention belongs to the field of drilling fluid technology in oil and gas development, and in particular relates to a method for evaluating the performance of drilling fluid differential pressure plugging by simulating wellbore pressure fluctuations. Background Technology

[0002] Research on sandstone reservoir protection technology revolves around "bridging and sealing" and "fuzzy sealing," focusing on the formation of an "ideal" tight shielding zone (i.e., inner and outer mud cakes) under static conditions, and evaluating the degree of solid and liquid phase intrusion into the reservoir.

[0003] However, drilling mud cake forms under dynamic conditions. Pressure changes caused by drilling fluid viscosity and shear, pressure fluctuations caused by connecting single drill pipes, and tripping operations all affect mud cake quality, and consequently, reservoir protection effectiveness. Therefore, focusing solely on static "tightness" is insufficient; greater attention should be paid to the formation of high-quality mud cake under dynamic conditions.

[0004] Currently, the most common methods for evaluating the damage level of sandstone reservoirs are core flow experiments and visual medium-pressure sand bed filtration loss evaluation experiments. However, these methods have many shortcomings in terms of data timeliness, accuracy, comparability, and the rationality of the simulated conditions.

[0005] The core flow test was conducted according to the standard SY / T 6540-2002, "Indoor Evaluation Method for Damage to Oil Reservoirs by Drilling Fluid and Completion Fluid." Regarding the experimental media, the core samples involved varied, including "formation cores," "outcrop cores," and "artificial cores." The saturated fluids used also varied, including "formation water," "simulated formation water," and "standard brine." Furthermore, the displacement fluids varied in oil quality selection. These differences mean that the experimental results can only be used as a comparison before and after the implementation of drilling fluid reservoir protection technology, and cannot be used as an evaluation index for the reservoir protection performance of a particular type of drilling fluid. Regarding the experimental simulation, the experiment only simulated the contamination of the core under drilling fluid circulation conditions, without simulating pressure fluctuations. All other experimental processes were completed under static conditions, resulting in insufficient simulation of the drilling fluid's state in the wellbore. In terms of experimental time, the experiment was too long and lacked timeliness, thus losing its evaluative significance. The visible medium-pressure sand bed filtration loss evaluation experiment is a commonly used method for evaluating drilling fluid plugging performance, and this method is used in multiple standards. However, in terms of experimental conditions, on the one hand, the medium pressure of 0.69MPa is significantly different from the actual drilling pressure difference, and on the other hand, the constant pressure does not simulate the effects of pressure fluctuations. In terms of experimental medium, quartz sand is not a standard experimental medium, and its particle size distribution and particle shape will cause poor experimental repeatability.

[0006] In order to accurately and quickly evaluate the performance of drilling fluids in sandstone reservoirs and guide the selection and optimization of subsequent oil protection technologies, it is essential to establish a new dynamic evaluation method. Summary of the Invention

[0007] To address the aforementioned problems, the present invention aims to provide a method for evaluating the performance of drilling fluid differential pressure plugging by simulating wellbore pressure fluctuations.

[0008] To achieve the above objectives, the drilling fluid differential pressure plugging performance evaluation method based on simulated wellbore pressure fluctuations provided by the present invention includes the following steps performed in sequence:

[0009] 1) Select a threaded cup cap type permeability plugging instrument as the test equipment, and use a ceramic disc as the filter medium;

[0010] 2) Inject the drilling fluid to be tested into the threaded cup cap type permeability plugging device and install the ceramic disc;

[0011] 3) Raise the temperature of the threaded cup cap type permeation plugging instrument to the target temperature;

[0012] 4) Use a hydraulic pump to pressurize the threaded cup cap type permeable plugging instrument. The pressurization time should be controlled within 1 minute until the first target pressure P1 is reached and held for time t1. Then quickly reduce the pressure to the second target pressure P2 and hold for time t2. After that, increase the pressure again to the third target pressure P3 and hold for time t3. This completes one pressure change cycle test. Collect the filtrate and measure its quantity as the filtrate loss.

[0013] 5) Repeat step 4) to perform N transformer cycle tests;

[0014] 6) Cool down the threaded cup cap type permeability plugging instrument, then remove the ceramic disc and the mud cake adhering to the ceramic disc as a whole, and pour out the drilling fluid;

[0015] 7) Inject distilled water into the threaded cup cap type permeability sealing instrument, then put the removed ceramic disc and mud cake back into the instrument. After heating to the target temperature, conduct a filtration test at a pressure difference Δp of 0.7 MPa for 30 minutes. Measure the volume of distilled water lost per unit time, q, every 2 minutes (in cm³). 3 / s;

[0016] 8) After the filtration loss experiment, remove the ceramic plate and the mud cake and separate them. Use a ruler to measure the thickness and diameter of the mud cake, obtain the average thickness H and average diameter, and calculate the area A of the mud cake. At the same time, select a 30mm cuvette and use a visible light spectrophotometer at a wavelength of 680nm to test the absorbance of the filtrate retained in steps 4) to 5).

[0017] 9) Observe whether there are obvious depressions on the surface of the mud cake. If there are obvious depressions, it indicates that there are weak links in the pressure resistance of the drilling fluid to be tested during the pressure change process, and the adhesion needs to be improved. Observe the thickness of the mud cake. If there is an obvious thick layer visible to the eye, and the appearance is jelly-like and swaying, it indicates that the particle size distribution of the drilling fluid to be tested needs to be optimized.

[0018] 10) Use tools to cut open the ceramic disc and use a magnifying glass of 20x or higher to observe whether the solid phase of the drilling fluid to be tested has bridging in the ceramic disc. If so, it indicates that the ceramic disc has been invaded by the drilling fluid to be tested. Then, use the volume of distilled water lost per unit time q, the average thickness of the mud cake H, the viscosity of distilled water at the target temperature μ, the area of ​​the mud cake A and the experimental pressure difference Δp obtained in steps 7) and 8) to calculate the mud cake permeability K to determine the degree of invasion.

[0019] 11) If the filtrate loss and absorbance of the filtrate increase sharply during a certain pressure swing cycle, it indicates that the cake was broken during that pressure swing cycle.

[0020] 12) Combine the filtrates obtained from N pressure-switching cycles to obtain a mixed filtrate, and measure the absorbance and total filtrate loss of the mixed filtrate. The higher the absorbance of the mixed filtrate, the higher the solid content of the drilling fluid, indicating that the drilling fluid solid phase invades the reservoir and causes greater damage to the reservoir. The greater the total filtrate loss, the more the drilling fluid liquid phase invades the reservoir. If a sensitive reservoir is encountered, the probability of water-sensitive, salt-sensitive, and alkali-sensitive conditions will increase. If a low-permeability or ultra-low-permeability reservoir is encountered, the probability of water-locking damage will increase.

[0021] In step 1), the filter medium is a ceramic disc produced by Fann, which has pore sizes of 2.5μm, 5μm, 10μm and 20μm.

[0022] In step 3), the target temperature is set based on the bottom hole temperature. If there is no specific bottom hole temperature, the geothermal gradient is used for calculation.

[0023] In step 4), the first target pressure P1 is the simulated bottom hole pressure, and its value is the hydrostatic pressure P in the wellbore. 液 With formation pore pressure P P The difference;

[0024] The second target pressure P2 is based on the wellbore hydrostatic pressure P. 液 Formation pore pressure P P and the drilling and pumping pressure P SW1 The result is calculated, with units of MPa, and its expression is:

[0025] P2=P 液 -P P -P SW1 (1);

[0026] The third target pressure P3 is based on the wellbore hydrostatic pressure P. 液 Formation pore pressure P P and drilling excitation pressure P SW2 The result is calculated, with units of MPa, and its expression is:

[0027] P3=P 液 -P P +P SW2 (2);

[0028] The units for the times t1 to t3 are all in minutes. They are determined based on the drilling time of a single drill pipe, the start-up time of connecting a single drill pipe, and the lowering time after connecting a single drill pipe. Based on the drilling conditions of most reservoir sections, the relationship between the three times t1 to t3 is as follows:

[0029] t1:t2:t3=16:3:4 (3);

[0030] In step 5), the number of cyclic tests N is set to 4. If a single well may encounter many complex situations, the number of cyclic tests N can be appropriately increased.

[0031] In step 10), the expression for calculating the cake permeability K using the distilled water loss volume q per unit time, the average cake thickness H, the distilled water viscosity μ at the target temperature, the cake area A, and the experimental pressure difference Δp obtained in steps 7) and 8) is as follows:

[0032] K=qHμ / (AΔp) (4;

[0033] Mud cake permeability K is used to characterize the density of the mud cake. When the mud cake permeability K is less than or equal to 10... -6 When D is a Darcy unit, it indicates that the mud cake has good density; if the mud cake permeability K is higher than 10... -6 For option D, a sealing agent needs to be added to improve the density of the mud cake.

[0034] The drilling fluid differential pressure plugging performance evaluation method based on simulated wellbore fluctuation pressure provided by this invention has the following advantages: the entire experiment takes approximately 3 hours; the equipment and accessories are readily available in China; and if nitrogen cylinders are provided on-site, it can be applied at the well site, making it a rapid evaluation method. This method simulates the formation of mud cake by drilling fluid under wellbore fluctuation pressure and analyzes the degree of damage to the reservoir caused by the solid and liquid phases of the drilling fluid. It also proposes optimization suggestions for the oil and gas reservoir protection performance of drilling fluid, which has important guiding significance for improving the oil and gas reservoir protection performance of drilling fluid. Attached Figure Description

[0035] Figure 1 The image shows a mud cake obtained in an embodiment of the present invention. Detailed Implementation

[0036] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0037] The drilling fluid differential pressure plugging performance evaluation method based on simulated wellbore pressure fluctuations provided by this invention includes the following steps performed in sequence:

[0038] 1) A threaded cup cap type permeability plugging instrument was selected as the test equipment. The filter medium used was a ceramic disc produced by Fann, which has pore sizes of 2.5μm, 5μm, 10μm and 20μm.

[0039] 2) Inject the drilling fluid to be tested into the threaded cup cap type permeability plugging device and install the ceramic disc;

[0040] 3) Raise the temperature of the threaded cup cap type permeability plugging instrument to the target temperature, which is set based on the bottom hole temperature. If the bottom hole temperature is not clearly defined, use the geothermal gradient for calculation.

[0041] 4) Use a hydraulic pump to pressurize the threaded cup cap type permeability plugging device, controlling the pressurization time within 1 minute, until the first target pressure P1 is reached and maintained for a time t1; the first target pressure P1 is the simulated bottom hole pressure, and its value is the wellbore hydrostatic pressure P. 液 With formation pore pressure P P The difference; then rapidly reduce the pressure to the second target pressure P2 and hold for time t2; the second target pressure P2 is based on the wellbore hydrostatic pressure P 液 Formation pore pressure P P and the drilling and pumping pressure P SW1 The result is calculated, with units of MPa, and its expression is:

[0042] P2=P 液 -P P -P SW1 (1);

[0043] The pressure is then increased again to the third target pressure P3 and held for t3, thus completing one pressure-changing cycle test. The filtrate is collected and its quantity is measured as the filtrate loss. The third target pressure P3 is based on the wellbore hydrostatic pressure P... 液 Formation pore pressure P P and drilling excitation pressure P SW2 The result is calculated, with units of MPa, and its expression is:

[0044] P3=P 液 -P P +P SW2 (2);

[0045] The units for the times t1 to t3 are all in minutes. They are determined based on the drilling time of a single drill pipe, the start-up time of connecting a single drill pipe, and the lowering time after connecting a single drill pipe. Based on the drilling conditions of most reservoir sections, the relationship between the three times t1 to t3 is generally as follows:

[0046] t1:t2:t3=16:3:4 (3);

[0047] 5) Repeat step 4) to perform N cycles of pressure change test. The number of cycles N is generally set to 4. If a single well may encounter more complex situations, the number of cycles N can be increased appropriately.

[0048] 6) Cool down the threaded cup cap type permeability plugging instrument, then remove the ceramic disc and the mud cake adhering to the ceramic disc as a whole, and pour out the drilling fluid;

[0049] 7) Inject distilled water into the threaded cup cap type permeability sealing instrument, then put the removed ceramic disc and mud cake back into the instrument. After heating to the target temperature, conduct a filtration test at a pressure difference Δp of 0.7 MPa for 30 minutes. Measure the volume of distilled water lost per unit time, q (in cm³), every 2 minutes. 3 / s);

[0050] 8) After the filtration loss experiment, remove the ceramic plate and the mud cake and separate them. Use a ruler to measure the thickness and diameter of the mud cake, obtain the average thickness H and average diameter, and calculate the area A of the mud cake. At the same time, select a 30mm cuvette and use a visible light spectrophotometer at a wavelength of 680nm to test the absorbance of the filtrate retained in steps 4) to 5).

[0051] 9) Observe whether there are obvious depressions on the surface of the mud cake. If there are obvious depressions, it indicates that there are weak links in the pressure resistance of the drilling fluid to be tested during the pressure change process, and the adhesion needs to be improved. Observe the thickness of the mud cake. If there is an obvious thick layer visible to the eye, and the appearance is jelly-like and swaying, it indicates that the particle size distribution of the drilling fluid to be tested needs to be optimized.

[0052] 10) Use tools to cut open the ceramic disc and use a magnifying glass of 20x or higher to observe whether the solid phase of the drilling fluid to be tested has bridging in the ceramic disc. If so, it indicates that the ceramic disc has been invaded by the drilling fluid to be tested. Then, use the volume of distilled water lost per unit time q, the average thickness of the mud cake H, the viscosity of distilled water at the target temperature μ, the area of ​​the mud cake A and the experimental pressure difference Δp obtained in steps 7) and 8) to calculate the mud cake permeability K to determine the degree of invasion.

[0053] The expression for the permeability K of the mud cake is:

[0054] K=qHμ / (AΔp) (4;

[0055] Mud cake permeability K is used to characterize the density of the mud cake. Generally, when the mud cake permeability K is less than or equal to 10... - 6 When D is a Darcy unit, it indicates that the mud cake has good density; if the mud cake permeability K is higher than 10... -6 D requires the addition of a sealing agent to improve the density of the mud cake;

[0056] 11) If the filtrate loss and absorbance of the filtrate increase sharply during a certain pressure swing cycle, it indicates that the cake was broken during that pressure swing cycle.

[0057] 12) Combine the filtrates obtained from N pressure-switching cycles to obtain a mixed filtrate, and measure the absorbance and total filtrate loss of the mixed filtrate. The higher the absorbance of the mixed filtrate, the higher the solid content of the drilling fluid, indicating that the drilling fluid solid phase invades the reservoir and causes greater damage to the reservoir. The greater the total filtrate loss, the more the drilling fluid liquid phase invades the reservoir. If a sensitive reservoir is encountered, the probability of water-sensitive, salt-sensitive, and alkali-sensitive conditions will increase. If a low-permeability or ultra-low-permeability reservoir is encountered, the probability of water-locking damage will increase.

[0058] Example:

[0059] A well has a measured depth of 2500m and a reservoir drilling fluid density of 1.25g / cm³. 3 The formation pore pressure was 27 MPa, the tripping pressure was 4 MPa, the down-the-hole energizing pressure was 3.5 MPa, and the drilling time for a single drill pipe was 32 minutes. The drilling fluid differential pressure plugging performance was evaluated using the method of this invention.

[0060] (1) Determination of experimental parameters:

[0061] ① Target temperature: Based on a ground temperature gradient of 3℃ / 100m and a surface temperature of 20℃, the target temperature is 95℃;

[0062] ② Target pressure and pressurization time are shown in Table 1:

[0063] Table 1. Target Pressure and Pressurization Time

[0064]

[0065] (2) Experimental procedure

[0066] Install the threaded cup cap type permeation plugging device according to the relevant provisions of GB / T 16783.1-2014. After reaching the target temperature, use a hydraulic pump to pressurize to the first target pressure P1 of 4.25 MPa. Open the upper valve stem and start timing at the same time. After stabilizing the pressure for 16 minutes, quickly reduce the pressure to the second target pressure P2 of 0.25 MPa and hold for 3 minutes. Then increase the pressure to the third target pressure P3 of 7.75 MPa and hold for 4 minutes to complete the first pressure change cycle test. Collect the filtrate during this pressure change cycle and measure its quantity as the filtrate loss.

[0067] Repeat the pressure swing cycle test three times as described above, and keep the filtrate from each pressure swing cycle test and measure its quantity as the filtrate loss.

[0068] Cool down the threaded cup cap type permeability plugging device, then remove the ceramic disc and the mud cake adhering to the ceramic disc as a whole, and pour out the drilling fluid;

[0069] Distilled water was injected into the threaded cup cap type permeability sealing instrument. The removed ceramic disc and mud cake were then placed back into the instrument. After heating to the target temperature, a filtration experiment was conducted under an experimental pressure difference Δp of 0.7 MPa for 30 minutes. The filtration volume q (in cm³) of distilled water per unit time was measured every 2 minutes. 3 / s);

[0070] After the filtration experiment, the ceramic plate and the mud cake were removed and separated. The thickness and diameter of the mud cake were measured with a ruler to obtain the average thickness H and average diameter, and the area of ​​the mud cake A was calculated. At the same time, a 30 mm cuvette was selected and a visible light spectrophotometer was used to test the absorbance of the above-collected filtrate at a wavelength of 680 nm.

[0071] Using tools, the ceramic disc is cut open, and a magnifying glass of 20x or more is used to observe whether the solid phase of the drilling fluid to be tested is bridging in the ceramic disc. Then, the filtration volume of distilled water per unit time q, the average thickness of the mud cake H, the viscosity of distilled water at the target temperature μ, the area of ​​the mud cake A and the experimental pressure difference Δp obtained above are used to calculate the mud cake permeability K by formula (4) to determine the degree of invasion.

[0072] The filtrate loss and absorbance of the filtrate are shown in Table 2.

[0073] Table 2. Filtration loss and absorbance of filtrate

[0074]

[0075] According to the test and analysis requirements, observe the mud cake, test the mud cake permeability, observe the ceramic disc, observe the filtrate, read the filtrate loss, and test the filtrate absorbance.

[0076] ① Description of the state of the mud cake, ceramic disc, and filtrate

[0077] A. Mud cake: The mud cake formed by the drilling fluid is about 0.3 cm thick, with a thick, frozen layer on the surface and a depression about 1 cm in diameter. Figure 1 As shown;

[0078] B. Ceramic Plate: The entire ceramic plate was submerged;

[0079] C. Filtrate: The filtrate from the first pressure swing cycle was turbid; the filtrate from the second pressure swing cycle was slightly clear; the filtrate from the third pressure swing cycle was slightly turbid; and the filtrate from the fourth pressure swing cycle was slightly clear.

[0080] ② Evaluation Conclusion

[0081] A. The quality of the mud cake formed by the drilling fluid in this well under variable pressure is very poor. If the pressure in this well fluctuates frequently, obvious weak zones will appear in the mud cake on the well wall.

[0082] B. The mud cake formed by the drilling fluid in this well has high permeability, indicating that its density is poor and it cannot form a good seal.

[0083] C. The absorbance of the drilling fluid filtrate in this well is relatively high, and the quality of the mud cake deteriorates sharply during the third pressure-changing cycle, and the absorbance of the filtrate increases again. This indicates that due to pressure changes, the penetration depth and amount of drilling fluid solid relative to the reservoir have increased significantly.

[0084] D. The drilling fluid loss in this well is high, and the loss surges during the third pressure-changing cycle, indicating that the quality of the mud cake has deteriorated and a large amount of filtrate has penetrated the mud cake, which foreshadows increased damage to the reservoir.

Claims

1. A method for evaluating the performance of drilling fluid differential pressure plugging under simulated wellbore pressure fluctuations, characterized in that: The drilling fluid differential pressure plugging performance evaluation method based on simulated wellbore pressure fluctuations includes the following steps performed in sequence: 1) Select a threaded cup cap type permeability plugging instrument as the test equipment, and use a ceramic disc as the filter medium; 2) Inject the drilling fluid to be tested into the threaded cup cap type permeability plugging device and install the ceramic disc; 3) Raise the temperature of the threaded cup cap type permeation plugging instrument to the target temperature; 4) Use a hydraulic pump to pressurize the threaded cup cap type permeable plugging instrument. The pressurization time should be controlled within 1 minute until the first target pressure P1 is reached and held for time t1. Then quickly reduce the pressure to the second target pressure P2 and hold for time t2. After that, increase the pressure again to the third target pressure P3 and hold for time t3. This completes one pressure change cycle test. Collect the filtrate and measure its quantity as the filtrate loss. 5) Repeat step 4) to perform N transformer cycle tests; 6) Cool down the threaded cup cap type permeability plugging instrument, then remove the ceramic disc and the mud cake adhering to the ceramic disc as a whole, and pour out the drilling fluid; 7) Inject distilled water into the threaded cup cap type permeability sealing instrument, then put the removed ceramic disc and mud cake back into the instrument. After heating to the target temperature, conduct a filtration test at a pressure difference Δp of 0.7 MPa for 30 minutes. Measure the volume of distilled water lost per unit time, q, every 2 minutes (in cm³). 3 / s; 8) After the filtration loss experiment, remove the ceramic plate and the mud cake and separate them. Use a ruler to measure the thickness and diameter of the mud cake, obtain the average thickness H and average diameter, and calculate the area A of the mud cake. At the same time, select a 30mm cuvette and use a visible light spectrophotometer at a wavelength of 680nm to test the absorbance of the filtrate retained in steps 4) to 5). 9) Observe whether there are obvious depressions on the surface of the mud cake. If there are obvious depressions, it indicates that there are weak links in the pressure resistance of the drilling fluid to be tested during the pressure change process, and the adhesion needs to be improved. Observe the thickness of the mud cake. If there is an obvious thick layer visible to the eye, and the appearance is jelly-like and swaying, it indicates that the particle size distribution of the drilling fluid to be tested needs to be optimized. 10) Use tools to cut open the ceramic disc and use a magnifying glass of 20x or higher to observe whether the solid phase of the drilling fluid to be tested has bridging in the ceramic disc. If so, it indicates that the ceramic disc has been invaded by the drilling fluid to be tested. Then, use the volume of distilled water lost per unit time q, the average thickness of the mud cake H, the viscosity of distilled water at the target temperature μ, the area of ​​the mud cake A and the experimental pressure difference Δp obtained in steps 7) and 8) to calculate the mud cake permeability K to determine the degree of invasion. 11) If the filtrate loss and absorbance of the filtrate increase sharply during a certain pressure swing cycle, it indicates that the cake was broken during that pressure swing cycle. 12) Combine the filtrates obtained from N pressure-switching cycles to obtain a mixed filtrate, and measure the absorbance and total filtrate loss of the mixed filtrate. The higher the absorbance of the mixed filtrate, the higher the solid content of the drilling fluid, indicating that the drilling fluid solid phase invades the reservoir and causes greater damage to the reservoir. The greater the total filtrate loss, the more the drilling fluid liquid phase invades the reservoir. If a sensitive reservoir is encountered, the probability of water-sensitive, salt-sensitive, and alkali-sensitive conditions will increase. If a low-permeability or ultra-low-permeability reservoir is encountered, the probability of water-locking damage will increase.

2. The method for evaluating the performance of drilling fluid differential pressure plugging based on simulated wellbore pressure fluctuations according to claim 1, characterized in that: In step 1), the filter medium is a ceramic disc produced by Fann, which has pore sizes of 2.5μm, 5μm, 10μm and 20μm.

3. The method for evaluating the performance of drilling fluid differential pressure plugging based on simulated wellbore pressure fluctuations according to claim 1, characterized in that: In step 3), the target temperature is set based on the bottom hole temperature. If there is no specific bottom hole temperature, the geothermal gradient is used for calculation.

4. The method for evaluating the performance of drilling fluid differential pressure plugging based on simulated wellbore pressure fluctuations according to claim 1, characterized in that: In step 4), the first target pressure P1 is the simulated bottom hole pressure, and its value is the hydrostatic pressure P in the wellbore. 液 With formation pore pressure P P The difference; The second target pressure P2 is based on the wellbore hydrostatic pressure P. 液 Formation pore pressure P P and the drilling and pumping pressure P SW1 The result is calculated, with units of MPa, and its expression is: P2=P 液 -P P -P SW1 (1); The third target pressure P3 is based on the wellbore hydrostatic pressure P. 液 Formation pore pressure P P and drilling excitation pressure P SW2 The result is calculated, with units of MPa, and its expression is: P3=P 液 -P P +P SW2 (2); The units for the times t1 to t3 are all in minutes. They are determined based on the drilling time of a single drill pipe, the start-up time of connecting a single drill pipe, and the lowering time after connecting a single drill pipe. Based on the drilling conditions of most reservoir sections, the relationship between the three times t1 to t3 is as follows: t1:t2:t3=16:3:4 (3).

5. The method for evaluating the performance of drilling fluid differential pressure plugging based on simulated wellbore pressure fluctuations according to claim 1, characterized in that: In step 5), the number of cyclic tests N is set to 4. If a single well may encounter many complex situations, the number of cyclic tests N can be appropriately increased.

6. The method for evaluating the performance of drilling fluid differential pressure plugging based on simulated wellbore pressure fluctuations according to claim 1, characterized in that: In step 10), the expression for calculating the cake permeability K using the distilled water loss volume q per unit time, the average cake thickness H, the distilled water viscosity μ at the target temperature, the cake area A, and the experimental pressure difference Δp obtained in steps 7) and 8) is as follows: K=qHμ / (AΔp) (4); Mud cake permeability K is used to characterize the density of the mud cake. When the mud cake permeability K is less than or equal to 10... -6 When D is a Darcy unit, it indicates that the mud cake has good density; if the mud cake permeability K is higher than 10... -6 For option D, a sealing agent needs to be added to improve the density of the mud cake.

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