A method for estimating geological reserves of a fracture-cave type oil reservoir

Abstract

This invention belongs to the field of oil and gas reservoir exploration and development technology, specifically relating to a method for estimating the geological reserves of fractured-cavity oil reservoirs. The specific steps include: This invention obtains the abandonment pressure of a target well by analyzing its production data and the relationship between permeability and abandonment pressure coefficient; then, it monitors the cumulative oil production of the target well reaching the abandonment pressure using formation pressure monitoring data; based on the relationship between permeability and natural energy recovery rate (REER) and the permeability of the oil reservoir, it obtains the RER value; and finally, it combines this with the cumulative oil production of the well to dynamically estimate the geological reserves of the target well. This invention is simple to calculate, convenient, and practical. It provides a method for estimating geological reserves when static methods are not feasible, providing a material basis for future development and production capacity construction.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas reservoir exploration and development technology, specifically relating to a method for estimating the geological reserves of fractured karst reservoirs. Background Technology

[0002] Oil and gas reservoir reserves, including geological reserves and recoverable reserves, are crucial national resource figures and a vital basis for oil and gas reservoir development planning. Therefore, determining oil and gas reservoir reserves is a critical task in oil and gas field exploration and development. Methods for determining reserves mainly fall into two categories: static methods and dynamic methods. Static methods utilize static geological parameters of the reservoir, calculating reserves based on the volume of pore space occupied by oil and gas fluids; this is also known as the volumetric method. Dynamic methods utilize dynamic production parameters that change over time, such as pressure, production rate, and cumulative production, to calculate reserves. Examples include the mass balance method, water drive characteristic curve method, production decline method, and predictive model method. Correspondingly, the reserves calculated by static methods are geological reserves, representing the reservoir's accumulated reserves; while the reserves obtained by dynamic methods are dynamic recoverable reserves, representing the size of recoverable reserves under certain technical and economic conditions.

[0003] Carbonate reservoirs are an important component of oil and gas resources, and therefore, in recent years, they have become a crucial area for oil and gas exploration and development in my country. Carbonate reservoirs are highly heterogeneous, with numerous fractures, solution pores, and caverns, making accurate calculation of their geological reserves extremely difficult. Currently, the geological reserves of carbonate reservoirs are primarily calculated using the volumetric method, where the geological reserves of an oil and gas reservoir can be expressed as the product of parameters such as the oil and gas-bearing area, effective thickness, effective porosity, and oil and gas saturation.

[0004] For example, Chinese invention patent CN103293562B discloses a method and equipment for determining the geological reserves of carbonate reservoirs. The method involves collecting electrical imaging logging data and well test data of the carbonate reservoir; dividing the carbonate reservoir into four productive zones based on the electrical imaging logging data; determining the reservoir effectiveness coefficient of each productive zone based on the well test data; determining the geological reserves of each of the four productive zones based on the reservoir effectiveness coefficients; and determining the total geological reserves of the carbonate reservoir based on the geological reserves of the four productive zones. This invention patent uses a static method to calculate the sum of geological reserves in different productive zones based on the reservoir effectiveness coefficients, which has the following problems:

[0005] For severely heterogeneous fractured-cavitary reservoirs in carbonate rocks, the heterogeneity and spatial continuity of the reservoir's spatial distribution vary greatly, limiting the volumetric method for calculating geological reserves based on static data of the surrounding strata. For carbonate reservoirs with highly heterogeneous fracture-cavitary development, parameters such as porosity are difficult to determine, making volumetric methods challenging and inaccurate. Furthermore, when using the volumetric method, reserve parameters (oil and gas area, effective thickness, effective porosity, oil and gas saturation, etc.) are all calculated using flattened... Using the mean value to represent the reservoir does not fully account for the heterogeneity of the reservoir, thus making the accuracy of the reserve calculation insufficient. The occurrence state of oil and gas saturation in porous carbonate media varies greatly, and the different combinations of these reservoir spaces in three-dimensional space make the saturation of fluids within the same reservoir vary greatly. It is not possible to use a single reservoir space as a representative, nor can it be simply calculated as an average value. Therefore, the volumetric method is mainly suitable for relatively homogeneous reservoirs. For carbonate fractured and cavernous reservoirs with extremely high heterogeneity, the volumetric method is obviously not applicable. Summary of the Invention

[0006] The purpose of this invention is to provide a method for estimating the geological reserves of fractured-cavity oil reservoirs, in order to solve the problems of high calculation difficulty and low accuracy of calculation results when using the static method to calculate the geological reserves of oil reservoirs with strong heterogeneity due to fracture and cavity development.

[0007] To address the aforementioned technical problems, this invention provides a method for estimating the geological reserves of fractured-cavity oil reservoirs. The method involves obtaining the well's permeability based on logging or well testing parameters, calculating the well's abandonment pressure coefficient based on the relationship between permeability and the abandonment pressure coefficient and the obtained reservoir permeability, and multiplying the abandoned pressure coefficient by the well's saturation pressure to obtain the well's abandonment pressure. The method also calculates the well's cumulative oil production as the formation pressure decreases from the initial pressure to the abandonment pressure based on the well's elastic production rate. Finally, the method obtains the natural energy recovery rate (RER) based on the relationship between permeability and the RER and the well's reservoir permeability, and combines this with the well's cumulative oil production to calculate the geological reserves of the target well.

[0008] Its beneficial effects are as follows: To solve the problems of high calculation difficulty and low accuracy of calculation results when using the static method to calculate the geological reserves of highly heterogeneous oil reservoirs with fractures and vulnerabilities, this invention obtains the abandonment pressure of the well by analyzing the production data of the target well and the relationship between permeability and abandonment pressure coefficient. Then, by monitoring the cumulative oil production of the target well when it reaches the abandonment pressure through formation pressure monitoring data, the geological reserves of the target well are dynamically estimated. This invention is simple to calculate, convenient and practical. It can provide a method for estimating geological reserves when the requirements of applying the static method for calculating geological reserves are not met, thus providing a material basis for the next step of development deployment and production capacity construction.

[0009] Furthermore, the specific relationship between permeability and waste pressure coefficient is as follows: permeability less than or equal to 1 mD, waste pressure coefficient is 0.95; permeability greater than 1 mD but less than 3 mD, waste pressure coefficient is 0.90; permeability greater than or equal to 3 mD but less than 10 mD, waste pressure coefficient is 0.85; permeability greater than or equal to 10 mD but less than 100 mD, waste pressure coefficient is 0.80; permeability greater than or equal to 100 mD but less than 500 mD, waste pressure coefficient is 0.75; permeability greater than or equal to 500 mD, waste pressure coefficient is 0.70.

[0010] Furthermore, there is a positive correlation between natural energy recovery rate and reservoir permeability. Specifically, the relationship is as follows: for permeability less than or equal to 1 mD, the natural energy recovery rate is 0.04; for permeability greater than 1 mD but less than 3 mD, the natural energy recovery rate is 0.055; for permeability greater than or equal to 3 mD but less than 10 mD, the natural energy recovery rate is 0.07; for permeability greater than or equal to 10 mD but less than 100 mD, the natural energy recovery rate is 0.085; for permeability greater than or equal to 100 mD but less than 500 mD, the natural energy recovery rate is 0.12; and for permeability greater than or equal to 500 mD, the natural energy recovery rate is 0.15.

[0011] Furthermore, the target well elastic yield is calculated based on the initial formation pressure, post-test formation pressure, oil production during the testing phase, and oil production during the production phase. The calculation formula is as follows:

[0012]

[0013] Where β is the elastic yield; P i P1 represents the original formation pressure; P2 represents the formation pressure after trial production; N p1 This refers to the oil production during the trial oil production phase, N p2 This represents the oil production during the trial production phase.

[0014] Furthermore, the specific methods for calculating the cumulative oil production of the target well include: adding the stage oil production calculated based on the elastic production rate when the formation pressure of the target well decreases from the current pressure to the abandonment pressure, to the oil production during the test and production stages of the target well, to obtain the cumulative oil production of the target well. The calculation formula is as follows:

[0015] N p =N p1 +N p2 +N p3

[0016] Where, N p The cumulative oil production of the target well; N p1 This refers to the oil production during the trial oil production phase; N p2 This refers to the oil production during the trial production phase; N p3This represents the stage of oil production when the formation pressure decreases from the current pressure to the abandonment pressure.

[0017] Furthermore, the formula for calculating the stage production of the target well when the formation pressure drops from the current pressure to the abandonment pressure is as follows:

[0018] N p3 =β(P1-P d )

[0019] Where, N p3 β is the stage production rate when the formation pressure decreases from the current pressure to the abandonment pressure; β is the elastic production rate; P1 is the formation pressure after trial production; P d For the pressure of disposal.

[0020] Furthermore, the formula for calculating the geological reserves of the target well is as follows:

[0021]

[0022] Where, N p The cumulative oil production of the target well; N p1 This refers to the oil production during the trial oil production phase; N p2 This refers to the oil production during the trial production phase; N p3 E represents the stage of oil production as the formation pressure decreases from the current pressure to the abandonment pressure. R is the natural energy recovery rate; N is the geological reserves. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method of the present invention;

[0024] Figure 2 This is a graph showing the relationship between formation pressure and stage oil production according to the present invention. Detailed Implementation

[0025] The basic concept of this invention is as follows: Based on the production data of the target well and the analyzed relationship between permeability and abandonment pressure coefficient, the abandonment pressure of the well is obtained. Then, the cumulative oil production of the target well reaching the abandonment pressure is monitored using formation pressure monitoring data. Based on the relationship between permeability and natural energy recovery rate (REER) and the permeability of the oil reservoir in the well, the RER value is obtained. Combined with the cumulative oil production of the well, the geological reserves of the target well are dynamically estimated. Based on this concept, a method for estimating the geological reserves of fractured-cavity reservoirs can be realized.

[0026] The present invention will now be described in detail with reference to the accompanying drawings and method embodiments.

[0027] Method Implementation Examples:

[0028] The present invention provides a method for estimating the geological reserves of fractured-cavity oil reservoirs, the flowchart of which is shown below. Figure 1 As shown, the specific implementation steps include:

[0029] Step 1: Obtain data on the original formation pressure, formation pressure after production, saturation pressure, oil production during the testing phase, and oil production during the production phase of the target well based on the oil testing and production testing.

[0030] Step two: Based on the original formation pressure, post-test formation pressure, and production rates during the test and production phases of the target well obtained in Step one, calculate the elastic production rate of the target well, and calculate the elastic production phase abandonment pressure of the target well based on the saturation pressure. The specific calculation formulas are as follows:

[0031]

[0032] P d =αP b

[0033] In the formula, β is the elastic yield, 10 4 t / MPα; P i P1 represents the original formation pressure (MPa); P2 represents the formation pressure after trial production (MPa); N p1 This refers to the oil production during the trial oil production phase, 10 4 t; N p2 This refers to the oil production during the trial production phase, 10 4 t;

[0034] In the formula, p d The pressure at which oil is discharged during the elastic production stage is measured in MPa; p b α is the saturation pressure, in MPa; α is the waste pressure coefficient, a decimal.

[0035] Step 3: Based on the formation pressure obtained in Step 1 after the trial production of the target well, combined with the elastic production rate of the target well obtained in Step 2, and the abandonment pressure during the elastic production stage, calculate the oil production rate when the formation pressure drops to the abandonment pressure value. The calculation formula is as follows:

[0036] N p3 =β(P1-P d )

[0037] In the formula, N p3 This refers to the oil production rate when the formation pressure drops to the exhaust pressure, 10 4 t;

[0038] Step four: Based on the oil production during the trial production and production phases obtained in Step one, and combined with the oil production during the phase when the formation pressure drops to the abandonment pressure calculated in Step three, the cumulative oil production during the elastic production phase is calculated. The calculation formula is as follows:

[0039] N p =Np1 +N p2 +N p3

[0040] In the formula, N p This represents the cumulative oil production during the flexible oil production phase, 10 4 t;

[0041] Step 5: Based on the cumulative oil production during the elastic oil production stage obtained in Step 4, and combined with the natural energy recovery rate, the geological reserves are estimated. The specific calculation formula is as follows:

[0042]

[0043] In the formula, E R is the natural energy recovery rate; N is the geological reserves.

[0044] The geological reserves of a target well were estimated using the above-mentioned method for estimating reservoir geological reserves. The specific implementation process is as follows:

[0045] The cumulative mud loss of a certain target well (M6) was 2202m. 3 Well M6 has a depth of 3958.01m, a drill string venting depth of 3.26m, and a leakage velocity of 15.0m / s². 3 / h, leakage rate 95m 3 No backflow of drilling mud at the wellhead outlet; drilling tool emptying unknown at a well depth of 3975.55m, leakage rate 30m / s. 3 / h, leakage 2107m 3 No backflow of drilling mud was observed at the wellhead outlet. Drill string was vented at depths of 3975.55-3978.55m, with an overflow flame height of 8-10m. Open-hole testing was conducted at depths of 3957.74-3978.55m, with a 6mm nozzle producing 211m³ / day of oil. 3 And natural gas 10.7×10 4 m 3 Thirteen days later, the oil production during the trial production phase was 0.076678 × 10⁻⁶. 4 t and natural gas 48.62×10 4 m 3 .

[0046] 1) The initial formation pressure P measured during the initial oil testing of well M6 was... i =48.67MPa, oil production N during the trial oil production stage p1 =0.076678×10 4 t, oil production N during the trial production phase as of April 26, 2023 p2 =1.06330×10 4 t corresponds to a formation pressure P1 = 38.0 MPa after the trial production. Substitute the obtained data from well M6 into the formula. In the study, the elastic yield β of well M6 was obtained as 0.10684 × 10⁻⁶. 4 t / MPa;

[0047] According to statistical analysis of developed reservoirs, the abandoned pressure coefficient is related to reservoir properties. The lower the permeability, the greater the seepage resistance and the larger the abandoned pressure coefficient; the greater the permeability, the smaller the seepage resistance and the smaller the abandoned pressure coefficient.

[0048] Furthermore, when the permeability is less than or equal to 1 mD, the abandonment pressure coefficient is 0.95; when the permeability is greater than 1 mD but less than 3 mD, the abandonment pressure coefficient is 0.90; when the permeability is greater than or equal to 3 mD but less than 10 mD, the abandonment pressure coefficient is 0.85; when the permeability is greater than or equal to 10 mD but less than 100 mD, the abandonment pressure coefficient is 0.80; when the permeability is greater than or equal to 100 mD but less than 500 mD, the abandonment pressure coefficient is 0.75; when the permeability is greater than or equal to 500 mD, the abandonment pressure coefficient is 0.70. Based on the data from well M6, the analysis shows that its oil layer permeability is relatively high. Drilling into a karst cave and draining the oil layer would result in a permeability of at least several darches. Therefore, the abandonment pressure coefficient is set to α = 0.70, combined with the obtained saturation pressure P of well M6. b =34.266MPa, substitute into formula P d =αP b In the calculation, the abandonment pressure P during the elastic production stage of well M6 was obtained. d =23.99MPa.

[0049] 2) The abandoned pressure P during the elastic oil production stage of well M6 d =23.99 MPa, elastic yield β = 0.10684 × 10 4 Substituting t / MPa and the formation pressure P1 = 38.0MPa after the trial production into the formula N p3 =β(P1-P d In the calculation, the stage oil production N of well M6 when the formation pressure drops to the abandonment pressure was obtained. p3 =1.49682×10 4 t.

[0050] 3) Calculate the stage oil production N of well M6 when the formation pressure drops to the abandonment pressure. p3 =1.49682×10 4 t, and the oil production N collected during the testing phase of well M6. p1 =0.076678×10 4 t and oil production N during the trial production stage p2 =1.06330×10 4 Substituting t into formula N p =N p1 +N p2 +Np3 In the calculation, the cumulative oil production N during the elastic oil production stage of well M6 was obtained. p =2.6368×10 4 t; From this, the relationship between formation pressure and stage oil production can be obtained, such as Figure 2 As shown, the horizontal axis represents formation pressure, and the vertical axis represents stage oil production. At the initial formation pressure of 48.67 MPa, the oil production was 0 t; at the current pressure of 38.0 MPa, the cumulative oil production is 0.076678 × 10⁻⁶ t, representing the production during the trial production stage. 4 t and oil production during the trial production stage: 1.06330 × 10 4 The sum of t; when the formation pressure drops to the elastic stage abandonment pressure of 23.99 MPa, the cumulative oil production during the elastic oil production stage is 2.6368 × 10 4 t; the absolute value of the slope, i.e., the elastic yield (β), is 0.10684 × 10⁻⁶. 4 t / MPa.

[0051] According to statistical analysis of developed reservoirs, the natural energy recovery rate (RER) is related to reservoir physical properties, and there is a positive correlation between RER and reservoir permeability. Specifically, the RER is 0.04 for permeability less than or equal to 1 mD, 0.055 for permeability greater than or equal to 1 mD but less than 3 mD, 0.07 for permeability greater than or equal to 3 mD but less than 10 mD, 0.085 for permeability greater than or equal to 100 mD but less than 500 mD, 0.12 for permeability greater than or equal to 500 mD, and 0.15 for permeability greater than or equal to 500 mD.

[0052] Based on data from well M6, analysis revealed that its oil layer permeability was high. Drilling through a karst cave and emptying the oil layer resulted in a permeability of at least several Darcy's. Therefore, the natural energy recovery rate was set at E. R =0.15, combined with N p =2.6368×10 4 t and substitute it into the formula In the calculation, the geological reserves were found to be N = 17.58 × 10⁻⁶. 4 The geological reserves estimation results are shown in Table 1 below:

[0053] Table 1

[0054]

[0055] In order to accurately determine geological reserves, the elastic yield calculation requires a sufficiently long trial production time and a certain degree of decrease in formation pressure. The greater the decrease in formation pressure, the more accurate the calculated elastic yield and the more reliable the estimated geological reserves. Conversely, the estimated geological reserves will have a larger error.

[0056] This invention provides a dynamic method for estimating geological reserves using production data and formation pressure monitoring data. Unlike conventional dynamic methods (water drive characteristic curve method, decreasing method, well test method), this invention is simple to calculate, convenient and practical. When the requirements for calculating geological reserves using static methods are not met, it can provide a method for estimating geological reserves, providing a material basis for the next step of development deployment and production capacity construction.

Claims

1. A method for estimating the geological reserves of fractured-cavity type oil reservoirs, characterized in that, The permeability of the target well is obtained based on the logging or well test parameters. The abandonment pressure coefficient is then calculated based on the relationship between permeability and the abandonment pressure coefficient, along with the obtained reservoir permeability. This abandonment pressure is obtained by multiplying the abandonment pressure coefficient by the well's saturation pressure. The relationship between permeability and the abandonment pressure coefficient is that the higher the permeability, the lower the abandonment pressure coefficient. The cumulative oil production of the well is calculated based on the well's elastic production rate as the formation pressure decreases from the initial pressure to the abandonment pressure. Finally, the value of the natural energy recovery rate is obtained based on the relationship between permeability and the natural energy recovery rate, along with the reservoir permeability. Combined with the well's cumulative oil production, the geological reserves of the target well are calculated. The relationship between permeability and natural energy recovery rate is positively correlated.

2. The method for estimating the geological reserves of fractured-cavity oil reservoirs according to claim 1, characterized in that, The specific relationship between permeability and waste pressure coefficient is as follows: permeability less than or equal to 1 mD, waste pressure coefficient is 0.95; permeability greater than 1 mD but less than 3 mD, waste pressure coefficient is 0.90; permeability greater than or equal to 3 mD but less than 10 mD, waste pressure coefficient is 0.85; permeability greater than or equal to 10 mD but less than 100 mD, waste pressure coefficient is 0.80; permeability greater than or equal to 100 mD but less than 500 mD, waste pressure coefficient is 0.75; permeability greater than or equal to 500 mD, waste pressure coefficient is 0.

70.

3. The method for estimating the geological reserves of fractured-cavity oil reservoirs according to claim 1, characterized in that, The specific relationship between natural energy recovery rate and reservoir permeability is as follows: permeability ≤ 1 mD, natural energy recovery rate is 0.04; permeability > 1 mD < 3 mD, natural energy recovery rate is 0.055; permeability ≥ 3 mD < 10 mD, natural energy recovery rate is 0.07; permeability ≥ 10 mD < 100 mD, natural energy recovery rate is 0.085; permeability ≥ 100 mD < 500 mD, natural energy recovery rate is 0.12; permeability ≥ 500 mD, natural energy recovery rate is 0.

15.

4. The method for estimating the geological reserves of fractured-cavity oil reservoirs according to claim 1, characterized in that, The target well's elastic production rate is calculated based on the initial formation pressure, post-test formation pressure, oil production during the test phase, and oil production during the production phase. The calculation formula is as follows: in, β For elastic yield; P i This represents the original formation pressure; P 1 represents the formation pressure after the trial production; N p1 This refers to the oil production during the trial oil production phase. N p2 This represents the oil production during the trial production phase.

5. The method for estimating the geological reserves of fractured-cavity oil reservoirs according to claim 1, characterized in that, The specific method for calculating the cumulative oil production of a target well includes: adding the oil production during the period when the formation pressure of the target well drops from the current pressure to the abandonment pressure (calculated based on the elastic production rate) to the oil production during the testing and production phases of the target well to obtain the cumulative oil production of the target well. The calculation formula is as follows: in, N p The cumulative oil production of the target well; N p1 This refers to the oil production during the trial oil production phase. N p2 This represents the oil production during the trial production phase. N p3 This represents the stage of oil production when the formation pressure decreases from the current pressure to the abandonment pressure.

6. The method for estimating the geological reserves of fractured-cavity oil reservoirs according to claim 5, characterized in that, The formula for calculating the stage production of the target well when the formation pressure drops from the current pressure to the abandonment pressure is as follows: in, N p3 This refers to the stage of oil production when the formation pressure decreases from the current pressure to the abandonment pressure. β For elastic yield; P 1 represents the formation pressure after the trial production; P d For discarded pressure.

7. The method for estimating the geological reserves of fractured-cavity oil reservoirs according to claim 1, characterized in that, The formula for calculating the geological reserves of the target well is: in, N p The cumulative oil production of the target well; N p1 This refers to the oil production during the trial oil production phase. N p2 This represents the oil production during the trial production phase. N p3 This refers to the stage of oil production when the formation pressure decreases from the current pressure to the abandonment pressure. E R Natural energy recovery rate; N Geological reserves.

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