Method for evaluating injection capacity of oil layer

Abstract

The invention relates to an evaluation method for oil layer injection capacity, and belongs to the field of oil and gas reservoir exploration and development. The bottom hole pressure at different moments in the construction process of the fractured well is calculated, and the injection pressure difference is obtained according to the bottom hole pressure and the initial oil layer pressure; obtaining daily injection rates at different moments, obtaining an injection index according to the daily injection rates at different moments and the injection pressure difference, obtaining a specific injection index, determining the relation between the injection index and the injection pressure difference, using the relation between the injection index and the injection pressure difference, and taking the bottom hole pressure when the injection index is 0 as bottom hole starting pressure; according to the method, the relation between the daily injection rate and the bottom hole pressure is determined, the relation between the specific injection index and the bottom hole pressure is determined, the injection capacity of the target fractured well is evaluated through at least one of the relation between the daily injection rate and the bottom hole pressure and the relation between the specific injection index and the bottom hole pressure, and the evaluation mode of the method is reliable.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the injection capacity of an oil reservoir, belonging to the field of oil and gas reservoir exploration and development. Background Art

[0002] In the preparation and research of an oil reservoir development plan, the demonstration of the oil reservoir development mode is a very important link. Since the injection medium is cheap and the technology is mature and complete, the water injection development mode is the first development mode to be considered for an oil reservoir. The demonstration of the water injection development mode usually needs to demonstrate parameters such as the injection startup pressure, injection pressure, injection pressure difference, daily injection volume, injection index, and specific injection index of the oil reservoir. Especially when preparing the preliminary development plan for an oil reservoir in the early stage, due to the lack of injection wells and injection data, the analogy method, indoor water flooding experiment and other methods are usually used to demonstrate the water injection development mode. The demonstration by the analogy method means using some relatively important oil reservoir parameters for analogy. Since it is impossible to find an oil reservoir completely identical to the target oil reservoir, in the evaluation process of the water injection capacity of a fractured well, the selected analogous oil reservoir parameters may not be suitable for the target oil reservoir, resulting in insufficient demonstration and thus affecting the accuracy of the demonstration. In the initial stage of oil reservoir evaluation, there is no injection data, and the problem of not being able to specifically understand the injection capacity of the oil reservoir cannot be solved. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for evaluating the injection capacity of an oil reservoir to solve the problem that the injection capacity of an oil reservoir cannot be evaluated without injection data in the early stage of oil reservoir evaluation.

[0004] To achieve the above purpose, the solution of the present invention includes:

[0005] A method for evaluating the injection capacity of an oil reservoir of the present invention includes the following steps:

[0006] Using the casing pressure, static liquid column pressure in the casing, and wellbore friction during the construction of a fractured well, calculate the bottom hole pressure at different times, and obtain the injection pressure difference based on the bottom hole pressure and the initial oil reservoir pressure; and obtain the daily injection volume of the fracturing fluid based on the stage injection volume corresponding to the continuous construction time period; the construction process of the fractured well includes the preflush stage and the fracturing stage, and the casing pressure, static liquid column pressure in the casing, and wellbore friction are data when the displacement is greater than 0 and the pressure is less than the fracture pressure;

[0007] Obtain the corresponding injection index based on the obtained daily injection volume and the injection pressure difference in the corresponding time period, determine the relationship between the obtained injection indices and the corresponding injection pressure differences, and obtain the specific injection index based on the injection index;

[0008] Using the relationship between the injection index and the injection pressure difference, taking the bottom-hole pressure when the injection index is 0 as the bottom-hole starting pressure, determining the relationship between the daily injection volume and the bottom-hole pressure by using the bottom-hole starting pressure, the daily injection volume, and the corresponding bottom-hole pressure, and determining the relationship between the specific injection index and the bottom-hole pressure by using the bottom-hole starting pressure, the specific injection index, and the corresponding bottom-hole pressure;

[0009] Evaluating the injection capacity of the target fractured well by using at least one of the relationship between the daily injection volume and the bottom-hole pressure and the relationship between the specific injection index and the bottom-hole pressure.

[0010] Further, the method further includes: obtaining the wellhead starting pressure according to the bottom-hole starting pressure, obtaining the corresponding wellhead pressure according to the bottom-hole pressure, determining the relationship between the daily injection volume and the wellhead pressure by using the wellhead starting pressure, the daily injection volume, and the corresponding wellhead pressure; determining the relationship between the specific injection index and the wellhead pressure by using the wellhead starting pressure, the specific injection index, and the corresponding wellhead pressure, and evaluating the injection capacity of the target fractured well by using at least one of the relationship between the daily injection volume and the wellhead pressure and the relationship between the specific injection index and the wellhead pressure.

[0011] Further, when the reservoir pressure drops, calculating the bottom-hole starting pressure after the reservoir pressure drops based on the dropped pressure value, and determining the relationship between the daily injection volume and the bottom-hole pressure and the relationship between the specific injection index and the bottom-hole pressure according to the bottom-hole starting pressure under the dropped reservoir pressure.

[0012] Further, when the reservoir pressure drops, obtaining the wellhead starting pressure based on the bottom-hole starting pressure after the reservoir pressure drops, and determining the relationship between the daily injection volume and the wellhead pressure and the relationship between the specific injection index and the wellhead pressure by using the wellhead starting pressure corresponding to different reservoir pressures. Further, the bottom-hole starting pressure after the reservoir pressure drops is:

[0013] P s1 =P s -P i +P 1

[0014] In the formula, P s1 is the bottom-hole starting pressure after the reservoir pressure drops, P s is the bottom-hole starting pressure under the initial reservoir pressure, P i is the initial reservoir pressure, P 1 is the reservoir pressure after the drop.

[0015] Further, the relationship between the injection index and the injection pressure difference is a linear relationship.

[0016] Further, the specific injection index is:

[0017]

[0018] In the formula, j w is the specific injection index, and J w is the injection index, and h is the thickness of the reservoir formation in the fracturing stage.

[0019] Furthermore, the bottom-hole pressure is:

[0020]

[0021] In the formula, P d is the bottom-hole pressure, P c is the casing pressure, P h is the static liquid column pressure in the annulus between the tubing and the casing, P f is the wellbore friction, g is the gravitational constant, and ρ w is the water density, H is the bottom-hole depth of the reservoir formation, and the wellbore friction is the clear water friction.

[0022] Furthermore, the wellhead pressure is:

[0023]

[0024] In the formula, P d is the bottom-hole pressure, P h is the static liquid column pressure in the tubing, P f is the wellbore friction.

[0025] Beneficial effects: The evaluation method for the injection capacity of the reservoir formation in the present invention obtains data when the displacement is greater than 0 and the pressure is less than the fracture pressure, calculates the injection index according to the injection pressure difference, further obtains the specific injection index, establishes the relationship between the injection index and the injection pressure difference at different times, determines the bottom-hole startup pressure by using the relationship between the injection index and the injection pressure difference, determines the relationship between the daily injection volume and the bottom-hole pressure based on the bottom-hole startup pressure, and determines the relationship between the specific injection index and the bottom-hole pressure, so as to realize the evaluation of the injection capacity of the target fractured well. The method of the present invention studies the injection capacity of the reservoir formation without the well test injection of the injection well, evaluates the injection capacity through two relationships, provides a reliable basis for water injection development. The present invention can be popularized and applied to study the injection parameters of other fractured wells in the reservoir formation, and can also be popularized and applied to other fractured wells in the oil reservoir, providing a reliable basis for the water injection development of the oil reservoir. Description of the Drawings

[0026] Figure 1 is the flow chart of the evaluation method for the injection capacity of the reservoir formation in the method embodiment of the present invention;

[0027] Figure 2 is the curve graph of the construction pressure, displacement and time in the method embodiment of the present invention;

[0028] Figure 3 is the curve graph of the injection index and the injection pressure difference in the method embodiment of the present invention;

[0029] Figure 4 It is a schematic diagram of the injection string in Well G10 in the method embodiment of the present invention;

[0030] Figure 5 It is a curve graph showing the relationship between the specific injection index and the bottom hole pressure in Well G10 in the method embodiment of the present invention;

[0031] Figure 6 It is a curve graph showing the relationship between the specific injection index and the wellhead pressure in Well G10 in the method embodiment of the present invention;

[0032] Figure 7 It is a curve graph showing the relationship between the daily injection volume and the wellhead pressure in Well G10 in the method embodiment of the present invention. Detailed implementation manners

[0033] The following further elaborates on the present invention in conjunction with the accompanying drawings.

[0034] Embodiment of the evaluation method for oil reservoir injection capacity:

[0035] As Figure 1 shown, an evaluation method for oil reservoir injection capacity starts from the construction data of a fractured well. During the pressure testing stage and the process of injecting fracturing fluid into the oil reservoir, the injection index and injection pressure difference are calculated based on the construction data. Then, the fitting relationship corresponding to the injection index and injection pressure difference is established based on the starting pressure under different oil reservoir pressures, and the relationships between the bottom hole pressure, wellhead pressure, daily injection volume, and injection index (or specific injection index) at different times are plotted, thereby solving the problem that there is no water injection data in the initial stage of oil reservoir exploration or evaluation and it is impossible to understand the injection capacity of the oil reservoir. At the same time, it provides a basis for demonstrating the early water injection development method of the oil reservoir. The following are the specific implementation manners of this embodiment.

[0036] This embodiment takes Well G10 as an example for specific illustration. The well depth of Well G10 is 4171.0 m, the outer diameter of the production casing is 177.8 mm, the wall thickness is 10.36 mm, the inner diameter is 157.08 mm, the steel grade is P110, the setting depth is 3973.53 m, the interval for well testing is 3733.5 - 3746.9 m, the thickness is 12.6 m / 2 layers, and the measured original bottom hole pressure is 37.1 MPa. The schematic diagram of the fracturing string in Well G10 is as Figure 4 shown. The preflush fluid is 210 m 3 , and the fracture pressure is 69.1 Mpa.

[0037] During the fracturing well construction process, the bottom-hole pressure at different times is calculated using the casing pressure, the static liquid column pressure in the casing, and the wellbore friction. The injection pressure difference is obtained based on the bottom-hole pressure and the initial reservoir pressure. The daily injection volume of the fracturing fluid is obtained based on the stage injection volume corresponding to the continuous construction time period. The fracturing well construction process includes a preflush stage and a fracturing stage. The casing pressure, the static liquid column pressure in the casing, and the wellbore friction are data when the displacement is greater than 0 and the pressure is less than the fracture pressure. Specifically, as Figure 2 shown, first, based on the pressure test data of the fracturing well or the construction data of the preflush stage, the casing pressure, the tubing pressure, and the displacement are determined, and the relationship curves between the tubing pressure, the casing pressure, the displacement, and the construction time are constructed. According to the relationship curves, the data records corresponding to multiple time periods with a displacement greater than zero and a pressure less than the fracture pressure are obtained. The fracture pressure in this embodiment can be set to 69.1 Mpa, and the cumulative liquid volume in the stage before reaching the fracture pressure is 8.31 m 3 .

[0038] According to the casing pressure, the static pressure in the casing, and the wellbore friction pressure in the construction data, the bottom-hole pressure corresponding to different times is calculated respectively. The calculation formula for the bottom-hole pressure is:

[0039]

[0040] In formula (1), P d is the bottom-hole pressure, MPa; P c is the casing pressure (casing pressure), MPa; P h is the static liquid column pressure in the tubing-casing annulus, MPa; P f is the wellbore friction (i.e., the resistance generated by friction when the liquid flows through the wellbore), MPa; g is the gravitational constant, 9.8 N / kg; ρ w is the liquid density, 1.02 g / cm 3 ; H is the vertical depth corresponding to the mid-depth of the reservoir, 3710 m for a depth of 3740 m. The wellbore friction is the friction of clear water.

[0041] Among them, the wellbore friction is different corresponding to different liquids flowing through the wellbore. That is, the friction loss of the slickwater fracturing fluid is calculated as 30% of the clear water friction. The friction loss calculation of clear water adopts the regression formula proposed by Lord, that is:

[0042] P f = 1.385×10 6 ×d -4.8 ×q 1.8 ×H c (2)

[0043] In formula (2), d is the inner diameter of the casing, which can be obtained by measurement, q is the displacement, m 3 / min; H c is the depth of the well section through which the fluid flows.

[0044] Using the bottom-hole pressure at different times and the initial reservoir pressure obtained from the monitoring data, the injection pressure difference ΔP corresponding to the bottom-hole pressure at different times can be calculated. That is, the formula for the injection pressure difference is:

[0045] ΔP = P d - P i (3)

[0046] In formula (3), P i is the initial reservoir pressure. The daily injection volume Q1 can be calculated by multiplying the continuous construction duration of each stage by the corresponding stage injection volume. The daily injection volume is:

[0047]

[0048] In formula (4), Q1 is the daily injection volume, unit: m 3 / d; t i is the continuous construction duration of each stage, unit: s; N i is the stage injection volume corresponding to t i , unit: m 3 ; N 0 is the initial stage injection volume corresponding to the continuous construction duration of each stage, m 3 .

[0049] The basis for determining whether each continuous construction stage is continuous is as follows: the stage injection volume is continuously increasing within the continuous time. If the stage injection volumes corresponding to two consecutive time periods are equal, it indicates that the construction is not continuous during these two consecutive time periods.

[0050] Moreover, the fracturing construction parameters are automatically stored in seconds. To eliminate the large error in the subsequent calculation results caused by inaccurate displacement records, the daily injection volume is calculated using the stage injection volumes within the continuous injection time and the continuous injection duration, in order to obtain an accurate and reliable daily injection volume.

[0051] According to the daily injection volume Q1 of the fracturing fluid at different times and the corresponding injection pressure difference ΔP, the injection index J is calculated w :

[0052]

[0053] In formula (5), J w is the injection index, unit: m 3 / d.Mpa.

[0054] Determine the relationship between the injection index and the injection pressure difference corresponding to different time periods. Specifically, for example Figure 3As shown in the figure, linear curve regression is performed using the injection index at different times and the corresponding injection pressure difference to obtain the linear regression curve relationship. The formula is:

[0055] J w = aΔP + b (6)

[0056] In Equation (6), J w is the injection index, m 3 / d·MPa. The parameters a and b are determined according to the multiple continuous injection times and the corresponding stage injection volumes of this fracturing well. For example, the parameter a in this embodiment is 0.8142 and b is -26.144.

[0057] Taking Well G10 as an example, using the fracturing parameters of Well Guai 10 for calculation, the relationship curve between the injection index J w and the injection pressure difference ΔP is as Figure 3 shown. Before the injection pressure difference of 35.08 MPa and the injection volume of 1.53 m 3 , since the injection test has just started, the pipe string may not be filled with fracturing fluid. At this time, it is in an uncontinuous construction state of pressure test settlement, which is caused by reasons such as liquid compression or gas compression in the oil casing annulus and is not the real injection into the oil layer. When the injection pressure difference is greater than 35.08 MPa, the fracturing fluid starts to be injected into the oil layer. At this time, it is in a continuous injection state. As the injection pressure difference increases, there is a linear relationship between the injection index and the injection pressure difference that is proportional.

[0058] This embodiment may also include calculating the corresponding daily injection volume Q2 according to the linear regression relationship between the injection index and the injection pressure difference and the injection pressure difference:

[0059] Q2 = J w ΔP = (aΔP + b)ΔP (7)

[0060] Using the linear regression relationship between the injection index calculated by Equation (6) and the injection pressure difference and the current thickness of the fracturing section oil layer to calculate the specific injection index. The specific injection index is:

[0061]

[0062] Using the relationship between the injection index and the injection pressure difference, taking the bottom hole pressure when the injection index is 0 as the bottom hole startup pressure, and using the bottom hole startup pressure, the daily injection volume, and the corresponding bottom hole pressure to determine the relationship between the daily injection volume and the bottom hole pressure.

[0063] Specifically, when the initial oil layer pressure is the first set value P i , the injection index is zero. At this time, the bottom hole startup pressure P s is:

[0064]

[0065] In Equation (9), the parameters a and b are the parameters in the linear regression relationship between the injection index and the injection pressure difference.

[0066] Based on the bottom-hole startup pressure P s , and then determine the relationship between the daily injection volume and the bottom-hole pressure according to the daily injection volume and the corresponding bottom-hole pressure. And use the bottom-hole startup pressure, the specific injection index, and the corresponding bottom-hole pressure to determine the relationship between the specific injection index and the bottom-hole pressure, and the relationship is as Figure 5 shown.

[0067] Moreover, the method of this embodiment further includes: obtaining the wellhead startup pressure according to the bottom-hole startup pressure, obtaining the corresponding wellhead pressure according to the bottom-hole pressure, and using the wellhead startup pressure, the daily injection volume at different times, and the corresponding wellhead pressure to determine the relationship between the daily injection volume and the wellhead pressure.

[0068] Where the wellhead pressure is:

[0069]

[0070] In the formula, P d is the corresponding bottom-hole pressure, P h is the static water column pressure in the tubing, and P f is the wellbore friction.

[0071] And the wellhead startup pressure P st is:

[0072]

[0073] During the injection process, the initial reservoir pressure will change, which in turn causes changes in both the bottom-hole startup pressure and the wellhead startup pressure.

[0074] When the reservoir pressure drops, calculate the bottom-hole startup pressure after the reservoir pressure drop based on the dropped pressure value, and determine the relationship between the daily injection volume and the bottom-hole pressure, and the relationship between the specific injection index and the bottom-hole pressure according to the bottom-hole startup pressure under the dropped reservoir pressure. At the same time, obtain the wellhead startup pressure based on the bottom-hole startup pressure after the reservoir pressure drop, and determine the relationship between the daily injection volume and the wellhead pressure, and the relationship between the specific injection index and the wellhead pressure using the wellhead startup pressure corresponding to different reservoir pressures.

[0075] Specifically, when the reservoir pressure increases to the threshold P 1 , for example, when P 1 increases to 40.1 MPa, the corresponding bottom-hole startup pressure is P s1 , and the bottom-hole startup pressure P s1 at this time is:

[0076] P s1 = P s - Pi +P 1 (12)

[0077] The corresponding wellhead startup pressure P st1 is as follows:

[0078]

[0079] When the initial reservoir pressure is the second set value P i at this time, the bottom-hole pressure

[0080] P D = P i + ΔP (14)

[0081] According to the above calculations, when the reservoir pressure is the set threshold value P 1 the wellhead pressure, bottom-hole pressure, injection pressure difference, daily injection volume, and specific injection index are obtained. The calculation results are shown in Table 1 as follows:

[0082] Table 1

[0083] Wellhead pressure Bottom hole pressure Injection pressure differential Daily injection volume Specific injection index (MPa) (MPa) (MPa) <![CDATA[(m 3 / d)]]> <![CDATA[(m 3 / d.MPa.m)]]> 35.86 72.22 32.12 0.3 0.001 38.81 75.10 35.00 82.4 0.187 39.87 76.10 36.00 114.0 0.251 40.94 77.10 37.00 147.3 0.316 42.03 78.10 38.00 182.2 0.381 43.15 79.10 39.00 218.8 0.445 44.29 80.10 40.00 257.0 0.510 45.45 81.10 41.00 296.8 0.574 46.63 82.10 42.00 338.2 0.639 47.85 83.10 43.00 381.3 0.704 49.09 84.10 44.00 426.0 0.768 50.37 85.10 45.00 472.3 0.833

[0084] Based on the specific injection index and bottom-hole pressure obtained from Table 1, the relationship curve between the specific injection index and the bottom-hole pressure is plotted, as Figure 5 shown. The solid line represents the relationship curve between the specific injection index and the bottom-hole pressure when the reservoir pressure is 40.1 Mpa calculated by the method in this embodiment. The cross represents the actual test data points, indicating that the calculation results are consistent with the actual data.

[0085] Based on Table 1, the relationship curve between the specific injection index and the wellhead pressure is plotted. The calculation results are as Figure 6 shown. The solid line represents the relationship curve between the specific injection index and the wellhead pressure when the reservoir pressure is 40.1 Mpa calculated by the method in this embodiment. The cross represents the actual test data points, and the calculation results also indicate consistency with the actual data.

[0086] Based on Table 1, the relationship curve between the daily injection volume and the wellhead pressure is plotted. The calculation results are as Figure 7 shown. The solid line represents the relationship curve between the daily injection volume and the wellhead pressure when the reservoir pressure is 40.1 Mpa calculated by the method in this embodiment. The cross represents the actual test data points, and the calculation results also indicate consistency with the actual data.

[0087] Similarly, when the reservoir pressure is the initial reservoir pressure, the wellhead pressure, bottom-hole pressure, injection pressure difference, daily injection volume, and specific injection index obtained are shown in Table 2,

[0088] Table 2

[0089]

[0090]

[0091] Separate plots are made of the injection index versus bottom-hole pressure as shown in Figure 5 and the injection index versus wellhead pressure as shown in Figure 6 . The relationship curves of daily injection volume versus wellhead pressure are shown in Figure 7 . The calculation results are shown at the position of the dashed line. The dashed line in the figure also represents tubing injection, and the injection string uses external upset tubing and is run to a depth of 3740 m in the middle of the pay zone. The injection water quality is fresh water. Using this invention patent, the pay zone pressure is calculated to be 33.39 Mpa. In the case where there is no injection well for a pilot injection, at least one of the relationships between daily injection volume and bottom-hole pressure, injection index and bottom-hole pressure, daily injection volume and wellhead pressure, and injection index and wellhead pressure is used to evaluate the injection capacity of the target fractured well. To provide a reliable basis for water injection development, the implementation method of this embodiment can be extended to study the injection parameters of other fractured wells in the pay zone, and can also be extended to other fractured wells in other reservoirs to provide a reliable basis for reservoir water injection development.

Claims

1. A method for evaluating the injection capacity of an oil reservoir, characterized in that, it includes the following steps: Using the casing pressure, the static liquid column pressure in the casing, and the wellbore friction during the fracturing well construction process, calculate the bottom-hole pressure at different times, and obtain the injection pressure difference based on the bottom-hole pressure and the initial oil reservoir pressure; and obtain the daily injection volume of the fracturing fluid based on the stage injection volume corresponding to the continuous construction time period; the fracturing well construction process includes a preflush stage and a fracturing stage, and the casing pressure, the static liquid column pressure in the casing, and the wellbore friction are data when the displacement is greater than 0 and the pressure is less than the fracture pressure; Obtain the corresponding injection index based on the obtained daily injection volume and the injection pressure difference in the corresponding time period, determine the relationship between the obtained injection indices and the corresponding injection pressure differences, and obtain the specific injection index based on the injection index; Using the relationship between the injection index and the injection pressure difference, taking the bottom-hole pressure when the injection index is 0 as the bottom-hole startup pressure, determine the relationship between the daily injection volume and the bottom-hole pressure using the bottom-hole startup pressure, the daily injection volume, and the corresponding bottom-hole pressure, and determine the relationship between the specific injection index and the bottom-hole pressure using the bottom-hole startup pressure, the specific injection index, and the corresponding bottom-hole pressure; Evaluate the injection capacity of the target fracturing well using at least one of the relationship between the daily injection volume and the bottom-hole pressure and the relationship between the specific injection index and the bottom-hole pressure.

2. The method for evaluating the injection capacity of an oil reservoir according to claim 1, characterized in that, this method further includes: obtaining the wellhead startup pressure based on the bottom-hole startup pressure, obtaining the corresponding wellhead pressure based on the bottom-hole pressure, and determining the relationship between the daily injection volume and the wellhead pressure using the wellhead startup pressure, the daily injection volume, and the corresponding wellhead pressure; determining the relationship between the specific injection index and the wellhead pressure using the wellhead startup pressure, the specific injection index, and the corresponding wellhead pressure, and evaluating the injection capacity of the target fracturing well using at least one of the relationship between the daily injection volume and the wellhead pressure and the relationship between the specific injection index and the wellhead pressure.

3. The method for evaluating the injection capacity of an oil reservoir according to claim 1, characterized in that, When the oil reservoir pressure drops, calculate the bottom-hole startup pressure after the drop of the oil reservoir pressure based on the dropped pressure value, and determine the relationship between the daily injection volume and the bottom-hole pressure and the relationship between the specific injection index and the bottom-hole pressure according to the bottom-hole startup pressure under the dropped oil reservoir pressure.

4. The method for evaluating the injection capacity of an oil reservoir according to claim 2, characterized in that, When the oil reservoir pressure drops, obtain the wellhead startup pressure based on the bottom-hole startup pressure after the drop of the oil reservoir pressure, and determine the relationship between the daily injection volume and the wellhead pressure and the relationship between the specific injection index and the wellhead pressure using the wellhead startup pressures corresponding to different oil reservoir pressures.

5. The method for evaluating the injection capacity of an oil reservoir according to claim 3 or 4, characterized in that, The bottom-hole startup pressure after the drop of the oil reservoir pressure is: P s1 = P s -P i +P 1 Wherein, P s1 is the bottom-hole starting pressure after the oil reservoir pressure drops, P s is the bottom-hole starting pressure under the initial oil reservoir pressure, P i is the initial oil reservoir pressure, P 1 is the oil reservoir pressure after the drop.

6. The method for evaluating the injection capacity of an oil reservoir according to claim 1, characterized in that, The relationship between the injection index and the injection pressure difference is a linear relationship.

7. The method for evaluating the injection capacity of an oil reservoir according to claim 1 or 2, characterized in that, The specific injection index is: where j w is the specific injection index, J w is the injection index, and h is the thickness of the reservoir layer in the fracturing section.

8. The method for evaluating the injection capacity of an oil reservoir according to claim 1, It is characterized in that the bottom-hole pressure is: Wherein, P d is the bottom hole pressure, P c is the casing pressure, P h is the static liquid column pressure in the annulus between the tubing and the casing, P f is the wellbore friction, g is the gravitational constant, ρ w is the water density, H is the bottom hole depth of the oil reservoir, and the wellbore friction is the clear water friction.

9. According to the evaluation method of the reservoir injection capacity described in claim 2, it is characterized in that the wellhead pressure is: Where P d is the bottom-hole pressure, P h is the static hydrostatic pressure in the tubing, and P f is the wellbore friction.

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