An intermittent pumping sampling method for low permeability reservoirs

Abstract

The application relates to an intermittent pumping sampling method for a low-permeability reservoir, which comprises the following steps: calculating the lower limit of the formation mobility of continuous pumping sampling of an ultra-low-permeability reservoir; when the mobility is less than the lower limit of the formation mobility of continuous pumping sampling, intermittent pumping is carried out; the breakthrough volume of the formation fluid is obtained, and the original invasion depth of drilling fluid is obtained through simulation inversion; according to experimental data, the initial invasion speed of mud filtrate under different original invasion depths is obtained, and the upper limit of the continuous invasion speed of mud filtrate in the pumping sampling process is determined; the numerical simulation of the key factors of intermittent pumping is carried out, intermittent pumping operation schemes under different conditions and the total operation time of the corresponding operation schemes are obtained; the optimal pressure recovery ratio is determined; the intermittent pumping operation scheme under the continuous invasion speed of mud filtrate and the optimal pressure recovery ratio and the total operation time of the operation scheme are obtained. The scheme can solve the sampling problem of the ultra-low-permeability reservoir and improve the success rate of sampling.

Description

Technical Field

[0001] The present invention relates to the technical field of reservoir pump sampling, and more particularly to an intermittent pump sampling method for low-permeability reservoirs. Background Art

[0002] Offshore low-permeability reservoirs hold vast reserves of oil and gas, making them a crucial component of establishing an offshore "oil and gas resource supply center." They are crucial for meeting national energy needs and ensuring oil and gas security. However, due to the poor physical properties, complex pore structure, and low contrast of fluid response in low-permeability reservoirs, conventional logging makes it difficult to accurately assess reservoir fluid properties. Currently, the primary and most economical means of determining fluid properties is cable pump sampling, which is crucial for reserve evaluation and operational decision-making during the exploration phase.

[0003] Wireline formation testing is a method used to obtain information about the petrophysical properties and fluids in low-permeability formations. This reduces the number of well tests, shortens drilling cycles, and improves overall efficiency. This technology lowers a logging tool into a fixed position in the reservoir, using pressure differentials to drive formation fluids into the tool, enabling reservoir testing and sampling. This technology is crucial for evaluation during the exploration phase and has become an essential technology for offshore reserve assessment, cost reduction, and efficiency improvement.

[0004] In 2020, Zhao Yuanliang et al. studied the mechanism of pump breakthrough and concluded that porosity and invasion depth are the two main factors affecting the purity of formation fluids during pump sampling operations. However, no pump sampling method has been developed for ultra-low permeability reservoirs. In 2022, Gao Yongde simulated the correlation between porosity, invasion depth, and filtrate contamination rate and pumping time for low-porosity and low-to-ultra-low permeability reservoirs in the Wenchang A Depression of the Pearl River Mouth Basin, effectively guiding probe selection and improving the operational efficiency of wireline formation testing. However, for ultra-low permeability reservoirs with mobility less than 0.03 mD / cP, the pressure drop exceeds the instrument limit, making continuous pumping impossible and the pump sampling success rate very low.

[0005] In summary, in low-permeability and ultra-low-permeability formations, the continuous pumping sampling method has a low success rate and cannot obtain formation fluid samples, which in turn affects the accuracy of decision-making and evaluation. Summary of the Invention

[0006] In order to overcome the problem that the continuous pumping sampling method in the above-mentioned prior art has a low success rate in low permeability and ultra-low permeability formations and cannot obtain formation fluid samples, the present invention provides an intermittent pumping sampling method for low permeability reservoirs, which can improve the success rate of sampling.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: an intermittent pump sampling method for low permeability reservoirs, comprising the following steps:

[0008] Step 1: After determining that the area is an ultra-low permeability reservoir based on regional geological data and reservoir characteristics, calculate the lower limit of the formation mobility for continuous pumping sampling using the testing instrument. When the mobility is less than the lower limit of the formation mobility for continuous pumping sampling, perform intermittent pumping.

[0009] For example: Taking the Yingqiong Basin as the research target area, combined with regional geological data and reservoir characteristics, it is clear that this block is an ultra-low permeability reservoir. Specifically, the permeability of the Ling 3rd Member reservoir in Baodao District is distributed in the range of 0.1~267.67×10 －3 μm 2 The average is 6.02×10 －3 μm 2 The reservoir is generally classified as low to extra-low permeability. A preferred combination of a formation testing instrument pump module and probe module is used. Three common pump modules used in cable sampling operations in the region: the high-pressure pump module (HP), the extra-high-pressure pump module (XHP), and the extra-extra-high-pressure pump module (XXHP). These are combined with three probe modules with different discharge areas: the extra-large diameter probe, the elliptical probe, and the Speedstar probe. This provides nine solutions, offering varying pumping capacities and adaptability ranges. The lower limit of formation mobility for continuous pumping sampling is calculated to determine the mobility below which intermittent pumping is performed, clarifying the scope of the study.

[0010] Calculating the lower limit of formation mobility for continuous pump sampling involves the following steps:

[0011] S1. Determine the initial pump hydraulic pressure: Determine the initial pump hydraulic pressure based on the reservoir characteristics of the target area.

[0012] S2. Determine the pumped fluid flow rate: Obtain the pump displacement based on the initial pump hydraulic pressure, and obtain the pumped fluid flow rate based on the pump displacement. The pump displacement is calculated as follows:

[0013] y=-1.2524·10 -17 ·x 5 +1.1166·10 -13 x 4 -3.0068·10 -10 ·x 3 +1.2524

[0014] 10 -17 ·x 2 -1.2525·10 -17 ·x+1.0218

[0015] Where y is the pump displacement; x is the pump hydraulic pressure;

[0016] The formula for confirming the pumped fluid flow rate is:

[0017]

[0018] Where POFR is the pump fluid flow rate; s is the motor speed; D u is the substitution coefficient.

[0019] S3. Calculate the flow pressure based on the flow rate of the pumped fluid.

[0020] The pressure difference between the formation pressure and the flow pressure is calculated based on the flow rate of the pumped fluid. The flow pressure is calculated based on the pressure difference between the formation pressure and the flow pressure and the formation pressure. The calculation formula for the pressure difference between the formation pressure and the flow pressure is:

[0021]

[0022] Wherein, ΔP is the pressure difference between formation pressure and flow pressure; is the formation mobility; c s is the probe shape factor.

[0023] S4. Calculate the pressure after the pump based on the flow pressure and pump hydraulic pressure. The calculation formula for the pressure after the pump is:

[0024] P m0 =D u ·P h +P l

[0025] Among them, P m0 is the pressure after the pump; P l is the flow pressure; P h For pump hydraulics.

[0026] S5. Calculate the lower limit of the formation mobility of the continuous pumping of the pump module and probe module instrument combination and determine the pumping method.

[0027] Step 2: Based on the data of historical wells in the area in step 1, the breakthrough volume of the formation fluid during the pump sampling process is obtained, and the original invasion depth of the drilling fluid is obtained by simulation inversion based on the pump breakthrough volume and porosity.

[0028] The original drilling fluid invasion depth is calculated based on the pumped breakthrough volume and porosity inversion of conventional wells. The drilling fluid invasion depth in low-permeability reservoirs ranges from 5 to 17 inches, with an average of 11.8 inches.

[0029] Step 3: Based on the experimental data, the initial invasion velocity of the mud filtrate at different original invasion depths is obtained, that is, the upper limit of the continuous invasion velocity of the mud filtrate during the pump sampling process is determined.

[0030] The static mud invasion rate is obtained from mud invasion experiments in low-porosity and permeability sandstone reservoirs without considering the increase in differential pressure. The effective pumping rate is calculated by the total pumping volume and the total operation time.

[0031] Use the different pumping volume breakthrough charts to find the total pumping volume under selected conditions.

[0032] The simulated "intermittent" pumping pressure recovery ratios of the oversized probe are 40%, 50%, 60%, 70%, 80%, and 90%, the intrusion depths are 8in, 15in, and 20in, and the mud contamination rates are 20%, 50%, and 80%, and the total operation time under the corresponding conditions is obtained.

[0033] Step 4: Conduct numerical simulation of key factors of intermittent pumping to obtain intermittent pumping operation plans and the total operation time of the corresponding operation plans under different pressure recovery ratios, different invasion depths, and different continuous invasion speeds.

[0034] When simulating key factors in intermittent pumping, the ultra-large diameter probe was used as the research object. The basic model parameters were input and determined, and a numerical simulation grid was established. The pumping process was numerically simulated under different porosities and invasion depths based on the numerical simulation grid. A breakthrough chart for different pumping breakthrough volumes was established, and the sampleable pumping volume of the ultra-large probe under different porosities and invasion depths was obtained. The basic model parameters include porosity, horizontal permeability, vertical permeability, invasion depth, formation fluid viscosity, formation fluid density, overbalance pressure, water-based mud filtrate viscosity, and water-based mud filtrate density.

[0035] Step 5: Determine the optimal pressure recovery ratio.

[0036] Total operation time increases with the pressure recovery degree. Based on actual data, the pressure recovery ratio is 40% when the instrument failure rate is less than 10%. When the pressure recovery degree is less than 50%, the increase in operation time is smaller, but the low pressure recovery ratio means more frequent pump starts and stops, which places a heavy workload on the hydraulic and electronic equipment. When the pressure recovery degree exceeds 70%, the operation time increases significantly. Therefore, the optimal pressure recovery degree is between 50% and 70%.

[0037] Step 6: Obtain the intermittent pumping operation plan and the total operation time of the operation plan under the drilling invasion depth and the optimal pressure recovery ratio.

[0038] Compared to existing technologies, the present invention offers the following advantages: It utilizes an intermittent pumping sampling method for low-permeability reservoirs, surpassing the continuous pumping capabilities of cable pressure sampling instruments to achieve higher-quality offshore low-permeability reservoir sampling. By innovating the processes and systems of existing equipment and processes, the sampling process is optimized to overcome the limitations of instrument hardware capabilities. This improves sampling accuracy and representativeness. This sampling technology overcomes the original lower limit of sampling fluidity, enabling effective sampling in low-permeability reservoirs below 0.1-0.3 mD / cP. Compared to more advanced traditional tools such as Speedstar probes and dual packers, this optimized sampling technology can save at least 2 million yuan per operation. Furthermore, due to its reduced risk, it can reduce operational uncertainty and potential losses. The optimized sampling technology has been successfully applied to eight low-permeability wells, increasing the sampling success rate from 60% to 90%, playing a significant role in securing low-permeability reserves and improving operational quality and efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a flow chart of an intermittent pump sampling method for low permeability reservoirs of the present invention;

[0040] Figure 2 is a flow chart of the present invention for calculating the upper limit of formation mobility for intermittent pump sampling;

[0041] Figure 3 This is the relationship between the total operation time and the pressure recovery ratio when the penetration depth is 8 inches;

[0042] Figure 4 This is the relationship between the total operation time and the pressure recovery ratio when the penetration depth is 15 inches;

[0043] Figure 5 This is the relationship between the total operation time and the pressure recovery ratio at a penetration depth of 20 inches;

[0044] Figure 6 It is a design of a pump sampling scheme for ultra-large diameter probes in ultra-low permeability reservoirs;

[0045] Figure 7 This is the interpretation result of intermittent pump sampling of low-permeability sandstone in a well in the South China Sea. DETAILED DESCRIPTION

[0046] The drawings are for illustrative purposes only and should not be construed as limiting this patent. To better illustrate the embodiments, some components in the drawings may be omitted, enlarged, or reduced in size, and do not represent actual product dimensions. Those skilled in the art will understand that some well-known structures and their descriptions may be omitted from the drawings. The positional relationships depicted in the drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0047] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "long", "short", etc. indicating the orientation or position relationship, they are based on the orientation or position relationship shown in the drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0048] The technical solution of the present invention is further described in detail below through specific embodiments and in conjunction with the accompanying drawings:

[0049] Example 1

[0050] Example 1 of an intermittent pump sampling method for a low permeability reservoir, as Figure 1 As shown, the following steps:

[0051] Step 1: After determining that the area is an ultra-low permeability reservoir based on regional geological data and reservoir characteristics, calculate the lower limit of the formation mobility for continuous pumping sampling using the test instrument. When the mobility is less than the lower limit of the formation mobility for continuous pumping sampling, perform intermittent pumping;

[0052] Step 2: Based on the data of historical wells in the area in step 1, the breakthrough volume of the formation fluid during the pump sampling process is obtained, and the original invasion depth of the drilling fluid is obtained by simulation and inversion based on the pump breakthrough volume and porosity;

[0053] Step 3: Based on the experimental data, the initial invasion velocity of the mud filtrate at different original invasion depths is obtained, that is, the upper limit of the continuous invasion velocity of the mud filtrate during the pump sampling process is determined;

[0054] Step 4: Conduct numerical simulations of key factors in intermittent pumping to obtain intermittent pumping operation plans and the total operation time for different pressure recovery ratios, different invasion depths, and different sustained invasion speeds;

[0055] Step 5: Determine the optimal pressure recovery ratio;

[0056] Step 6: Obtain the intermittent pumping operation plan and the total operation time of the operation plan under the continuous invasion speed of the mud filtrate and the optimal pressure recovery ratio.

[0057] The beneficial effects of this embodiment are as follows: The present invention adopts an intermittent pumping sampling method for low permeability reservoirs, breaking through the continuous pumping operation capacity of cable pressure sampling instruments to achieve higher-quality offshore low-permeability reservoir sampling. By innovating the processes and systems of existing equipment and processes, the sampling process is optimized to break through the limitations of instrument hardware capabilities. This can improve the accuracy and representativeness of sampling. This sampling technology can break through the original lower limit of sampling fluidity, thereby achieving effective sampling in low-permeability reservoirs below 0.1-0.3mD / cP. Compared with traditional tools such as higher-end Speedstar probes and double packers, the optimized sampling technology can save at least 2 million per operation. At the same time, due to the low risk, operational uncertainty and possible losses can be reduced. The optimized sampling technology has been successfully applied to 8 low-permeability wells, and the sampling success rate has increased from the original 60% to 90%, playing an important role in the implementation of low-permeability reserves and improving the quality and efficiency of operations.

[0058] Example 2

[0059] Example 2 of an intermittent pump sampling method for a low permeability reservoir further defines steps 1 to 5 based on Example 1.

[0060] Specifically, in step 1, if Figure 2 As shown in Figure 2, calculating the upper limit of formation mobility for intermittent pump sampling includes the following steps:

[0061] S1. Determine the initial pump hydraulic pressure: Determine the initial pump hydraulic pressure based on the reservoir characteristics of the target area.

[0062] S2. Determine the pumped fluid flow rate: Obtain the pump displacement based on the initial pump hydraulic pressure, and obtain the pumped fluid flow rate based on the pump displacement. The pump displacement is calculated as follows:

[0063] y=-1.2524·10 -17 ·x 5 +1.1166·10 -13 ·x 4 -3.0068·10 -10 ·x 3 +1.2524

[0064] 10 -17 ·x 2 -1.2525·10 -17 ·x+1.0218

[0065] Where y is the pump displacement; x is the pump hydraulic pressure;

[0066] The formula for confirming the pumped fluid flow rate is:

[0067]

[0068] where POFR is the pump fluid flow rate; s is the motor speed; D u is the displacement coefficient.

[0069] S3. Calculate the flowing pressure according to the pump fluid flow rate.

[0070] Calculate the formation pressure and flowing pressure differential according to the pump fluid flow rate, calculate the flowing pressure according to the formation pressure and the formation pressure and flowing pressure differential, and the calculation formula of the formation pressure and flowing pressure differential is:

[0071]

[0072] where ΔP is the formation pressure and flowing pressure differential; is the formation mobility; c s is the probe shape coefficient.

[0073] S4. Calculate the post-pump pressure according to the flowing pressure and pump pressure, and the calculation formula of the post-pump pressure is:

[0074] P m0 = D u ·P h + P l

[0075] where P m0 is the post-pump pressure; P l is the flowing pressure; and P h is the pump pressure.

[0076] S5. Calculate the lower limit of the formation mobility of the continuous pumping of the combination of the pump module and the probe module instrument and determine the pumping mode.

[0077] Specifically, in step two, the original invasion depth of the drilling fluid is obtained by inverting the pump breakthrough volume and porosity of the conventional well. The drilling fluid invasion depth range of the low permeability reservoir is 5-17in, and the average is 11.8in.

[0078] Specifically, in step three, the continuous invasion speed during pumping is determined according to the original invasion depth of the mud.

[0079] where the mud static invasion rate is obtained by mud invasion experiment in low porosity and permeability sandstone reservoir without considering the increase of the pressure differential. The effective pumping speed is calculated by the total pumping volume and the total operation time.

[0080] According to the different pump breakthrough volume breakthrough chart, the total pumping volume under the selected condition is found.

[0081] The simulation of the "intermittent" pumping pressure recovery ratio of the super-large probe is 40%, 50%, 60%, 70%, 80%, and 90%, the invasion depth is 8in, 15in, and 20in, and the mud contamination rate is 20%, 50%, and 80%. The total operation time under the corresponding conditions is obtained, such as Figure 3-Figure 5 shown.

[0082] Specifically, in step 4, when simulating the key factors of intermittent pumping, Figure 6 As shown in the figure, using an ultra-large diameter probe as the research object, the basic model parameters are input to determine the model, and a numerical simulation grid is established. The pumping process is numerically simulated under different porosity and invasion depth ranges based on the numerical simulation grid. A breakthrough volume chart is established to obtain the sampleable pumping volume of the ultra-large probe under different porosity and invasion depth conditions. The basic model parameters include porosity, horizontal permeability, vertical permeability, invasion depth, formation fluid viscosity, formation fluid density, overbalance pressure, water-based mud filtrate viscosity, and water-based mud filtrate density.

[0083] Specifically, in step five, the total operation time increases with the pressure recovery level. Based on actual data, the pressure recovery ratio is 40% when the instrument failure rate is less than 10%. When the pressure recovery level is less than 50%, the increase in operation time is smaller, but the low pressure recovery ratio means more frequent pump starts and stops, which places a heavy workload on the hydraulic and electronic equipment. When the pressure recovery level exceeds 70%, the operation time increases significantly. Therefore, the optimal pressure recovery level is between 50% and 70%.

[0084] Example 3

[0085] A practical application of an intermittent pumping sampling method for low-permeability reservoirs has been verified and applied in eight exploration wells of the Hainan Branch of China National Offshore Oil Corporation. This method not only lowered the lower limit of wireline formation testing but also obtained effective formation samples.

[0086] In a well logging operation of Hainan Branch of China National Offshore Oil Corporation, the pressure flow rate was less than 0.03mD / cp, and a three-dimensional radial probe was used for pumping operation. Figure 7 As shown in the figure, intermittent pumping was performed, with a cumulative pumping time of 220 minutes, 3 L of fluid pumped, and formation gas samples successfully obtained.

[0087] Obviously, the above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. An intermittent pump sampling method for low permeability reservoirs, characterized in that: The steps include: Step 1: After determining that the area is an ultra-low permeability reservoir based on regional geological data and reservoir characteristics, calculate the lower limit of the formation mobility for continuous pumping sampling using the test instrument. When the mobility is less than the lower limit of the formation mobility for continuous pumping sampling, perform intermittent pumping; Step 2: Based on the data of historical wells in the area in step 1, the breakthrough volume of the formation fluid during the pump sampling process is obtained, and the original invasion depth of the drilling fluid is obtained by simulation and inversion based on the pump breakthrough volume and porosity; Step 3: Based on the experimental data, the initial invasion velocity of the mud filtrate at different original invasion depths is obtained, that is, the upper limit of the continuous invasion velocity of the mud filtrate during the pump sampling process is determined; Step 4: Conduct numerical simulations of key factors in intermittent pumping to obtain intermittent pumping operation plans and the total operation time for different pressure recovery ratios, different invasion depths, and different sustained invasion speeds; Step 5: Determine the optimal pressure recovery ratio; Step 6: Obtain the intermittent pumping operation plan and the total operation time of the operation plan under the continuous invasion speed of the mud filtrate and the optimal pressure recovery ratio.

2. The intermittent pump sampling method for low permeability reservoirs according to claim 1, characterized in that: In step 1, calculating the lower limit of formation mobility for continuous pump sampling includes the following steps: S1. Determine the initial pump hydraulic pressure: Determine the initial pump hydraulic pressure based on the target area reservoir characteristics; S2. Determine the pumped fluid flow rate: Obtaining a pump displacement according to an initial pump hydraulic pressure, and obtaining a pumped fluid flow rate according to the pump displacement; S3. Calculate the flow pressure based on the pumped fluid flow rate; S4. Calculate the pressure after the pump according to the flow pressure and the pump hydraulic pressure; S5. Calculate the lower limit of the formation mobility of the continuous pumping of the pump module and probe module instrument combination and determine the pumping method.

3. The intermittent pump sampling method for low permeability reservoirs according to claim 2, characterized in that: In S2, the calculation formula of the pump displacement is: in, For pump hydraulics; The formula for calculating the pumped fluid flow rate is: in, is the flow rate of the pumped fluid; s is the motor speed; is the substitution coefficient.

4. The intermittent pump sampling method for low permeability reservoirs according to claim 2, characterized in that: In S3, the pressure difference between the formation pressure and the flow pressure is calculated according to the flow rate of the pumped fluid, and the flow pressure is calculated according to the pressure difference between the formation pressure and the flow pressure and the formation pressure. The calculation formula of the pressure difference between the formation pressure and the flow pressure is: in, is the pumping fluid flow rate, is the pressure difference between formation pressure and flow pressure; is the formation mobility; is the probe shape factor.

5. The intermittent pump sampling method for low permeability reservoirs according to claim 2, characterized in that: In the above-mentioned S4, the calculation formula of the pressure after the pump is: in, is the pressure after the pump; is the flow pressure; For pump hydraulics; is the substitution coefficient.

6. The intermittent pump sampling method for low permeability reservoirs according to claim 1, characterized in that: In step 2, the original invasion depth of the drilling fluid is obtained by inversion based on the pumping breakthrough volume and porosity of the conventional well.

7. The intermittent pump sampling method for low permeability reservoirs according to claim 1, characterized in that: In step four, when conducting numerical simulation of key factors of intermittent pumping, the ultra-large diameter probe is taken as the research object, the basic parameters of the model are input and determined, a numerical simulation grid is established, and a pumping breakthrough volume breakthrough map is established based on the numerical simulation grid to obtain the sampleable pumping volume of the ultra-large probe under different porosity and invasion depth conditions.

8. The intermittent pump sampling method for low permeability reservoirs according to claim 7, characterized in that: The basic parameters of the model include porosity, horizontal permeability, vertical permeability, invasion depth, formation fluid viscosity, formation fluid density, overbalance pressure, water-based mud filtrate viscosity and water-based mud filtrate density.

9. The intermittent pump sampling method for low permeability reservoirs according to claim 1, characterized in that: In step five, the optimal pressure recovery ratio is 50% to 70%.

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