Calculation method of ultra-low permeability reservoir water drive displacement boundary

Abstract

The invention relates to an ultra-low permeability reservoir water drive displacement boundary calculation method. The method comprises the steps that S1, the permeability K of a plurality of rock cores is obtained; s2, establishing a conversion model of core nuclear magnetic resonance relaxation time and pore throat radius r; s3, according to the pore throat radius r and the cumulative distribution frequency thereof, calculating the average pore throat radius of the rock core and a variable coefficient Dr; s4, acquiring displacement boundaries of a plurality of rock cores; and S5, establishing a core displacement boundary mathematical model, and calculating the displacement boundary of the ultra-low permeability reservoir water-flooding development through the model. According to the characteristics of the ultra-low permeability reservoir, the displacement boundary model is established by integrating the macroscopic physical property parameters and the microcosmic pore throat characteristic parameters, the displacement boundary of ultra-low permeability reservoir water drive development can be effectively predicted, selection of ultra-low permeability reservoir development modes is effectively guided, and ultra-low permeability reservoir scale benefit development is supported.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development, and specifically relates to a method for calculating the water drive displacement limit of ultra-low permeability reservoirs. Background Technology

[0002] As the development targets of Changqing Oilfield gradually shift from ultra-low permeability Class I reservoirs to ultra-low permeability Class II and III reservoirs with denser reservoir conditions (ultra-low permeability Class I reservoirs have a permeability / average permeability ≥ 0.5 × 10⁻⁶), -3 μm 2 The permeability of ultra-low permeability Class II reservoirs is 0.3~0.5×10⁻⁶. -3 μm 2 The permeability of ultra-low permeability Class III reservoirs is ≤0.3×10⁻⁶. -3 μm 2 The transformation of ultra-low permeability reservoirs has led to challenges such as increased development difficulty, ineffective water injection for displacement, and rapid production decline. The development results also fall short of expectations. Previous methods using porosity-permeability parameters as indicators for displacement boundary assessment are no longer adequate for the current development needs of ultra-low permeability Class II and III reservoirs. Therefore, researching comprehensive reservoir displacement boundary characterization methods and selecting appropriate technical policies are of paramount importance for the development of ultra-low permeability reservoirs.

[0003] Currently, the commonly used method for calculating displacement limits is the waterdrive characteristic curve method. This method evaluates the effectiveness of waterdrive development and predicts displacement limits by analyzing the oil-water relationship curves during the water-injection development process of the reservoir. The waterdrive characteristic curve method classifies reservoir production curves into four types—A, B, C, and D—based on the oil-water relationship, reflecting different reservoir production characteristics. It then uses the relationship between water saturation and recovery rate under the waterdrive curves to create a waterdrive characteristic curve fitting chart, thereby predicting displacement limits. However, this method requires a relatively long production time to fit the waterdrive characteristic curve, resulting in low fitting accuracy if the reservoir's production time is short. For new ultra-low permeability reservoirs, where long-term production data is lacking in the early stages of development, the waterdrive characteristic curve method for predicting displacement limits has significant errors and is no longer sufficient to meet the current needs of ultra-low permeability reservoir development. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the purpose of this invention is to combine the physical properties of ultra-low permeability reservoirs with pore throat characteristic parameters to provide a method for calculating the water drive displacement limit of ultra-low permeability reservoirs, thereby guiding the formulation of development technology policies for ultra-low permeability reservoirs.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for calculating the water drive displacement limit in ultra-low permeability reservoirs includes the following steps: S1. Obtain core samples from multiple wells in the target reservoir, remove residual oil or mud from the core samples, and measure the permeability K of each core sample; S2. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, combined with the high-pressure mercury injection curve of the target reservoir of the core, the conversion model of the relaxation time of the core nuclear magnetic resonance T2 spectrum and the pore throat radius r is obtained. S3. Based on the conversion model of relaxation time and pore throat radius r in the T2 spectrum of the core nuclear magnetic resonance, the T2 spectrum curve of the core is converted into a high-pressure mercury intrusion porosimetry curve, and the average pore throat radius of the core is calculated from the high-pressure mercury intrusion porosimetry curve. and coefficient of variation D r ; S4. Set multiple different experimental flow rates for each core sample and conduct water drive displacement tests by increasing the flow rate from low to high. Record the recovery rate η at stable pressure under different flow rates. When the recovery rate of the core sample no longer increases after increasing the flow rate, the recovery rate is the displacement limit of the core sample. S5. Based on the permeability K obtained in S1 and the coefficient of variation D obtained in S3. r A displacement characteristic index is constructed, and the displacement characteristic index of multiple core samples is fitted with the displacement limit obtained from S4. The regression yields a mathematical model of the core displacement limit, which can be used to calculate the displacement limit for water injection development in ultra-low permeability reservoirs.

[0006] As a further improvement to the above-mentioned technical solution of the present invention, the calculation method further includes: S6. Based on the mathematical model of core displacement limits, combined with the ultimate recovery rate η in oilfield production. kc The displacement characteristic index T at the site was obtained. kc Based on different combinations of permeability, average pore throat radius, and coefficient of variation, a water drive displacement boundary map for ultra-low permeability reservoirs is established, and the development method is selected based on this boundary map.

[0007] In S2, further, the method for obtaining the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state is as follows: prepare a core saturated CaCl2 solution, and then put the core saturated CaCl2 solution into the nuclear magnetic resonance instrument for measurement, thereby obtaining the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state.

[0008] Furthermore, in S2, the method for obtaining the conversion model of core nuclear magnetic resonance relaxation time and pore throat radius r based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state and the high-pressure mercury intrusion curve of the core includes the following steps: S201. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, plot the cumulative distribution frequency curve of nuclear magnetic resonance T2. At the same time, based on the high pressure mercury intrusion curve of the core, plot the cumulative distribution frequency curve of the pore throat radius r. S202. According to the cumulative frequency distribution curve, in the effective saturation range, i.e., S... pcd i Hgmax Inside, take S=S i Interpolating the cumulative frequency distribution curve of the throat radius r, we obtain the cumulative frequency distribution S. i The radius of the throat at that time r i It can simultaneously obtain the same cumulative distribution frequency S i T below 2i and r i The value; Where S i Let r be the arbitrary throat radius i The corresponding cumulative distribution frequency, S Hgmax S represents the cumulative distribution frequency corresponding to the maximum mercury saturation. pcd This represents the cumulative distributed frequency corresponding to the exhaust pressure. S203. The conversion relationship between the core nuclear magnetic resonance relaxation time and the pore throat radius r is as follows: (I) In the formula: ρ2 is the transverse surface relaxation rate, μm·ms -1 ; Fs is the pore shape factor; T2 is the core nuclear magnetic resonance relaxation time; C is the conversion factor; n is the power exponent; S204. By fitting and regressing T2 and r at different cumulative distribution frequencies S of the core, the values ​​of C and n are obtained. The average value of C and n is obtained based on the fitting regression of multiple cores. Thus, the conversion model of the relaxation time of the nuclear magnetic resonance T2 spectrum of the target reservoir and the pore throat radius r can be obtained.

[0009] Specifically, the method for obtaining the conversion relationship between core magnetic resonance relaxation time and pore throat radius r is as follows: a. The relaxation time T2 of the fluid in the pores of a porous medium under an ideal uniform magnetic field can be expressed as:

[0010] (II) In the formula: S is the surface area of ​​a single pore, in μm 2 ; V is the volume of a single pore, in μm. 3 ; b. The relationship between specific surface area and pore size is: (III) In the formula: Fs is the pore shape factor; r is the pore throat radius, μm; ​​c. Substituting equation (Ⅲ) relating specific surface area to aperture into equation (Ⅱ), we obtain equation (Ⅳ). (Ⅳ); d. Due to the complexity of the pore-throat structure in actual reservoirs, the experimental results show that the relaxation time of core nuclear magnetic resonance (NMR) is not linearly related to the pore-throat radius, but rather exhibits a power function relationship. Therefore, the conversion relationship between the relaxation time of the core NMR T2 spectrum and the pore-throat radius r is obtained as follows: (I).

[0011] Furthermore, in S3, the average pore throat radius of the core... and coefficient of variation D r The calculation formula is as follows: (V)

[0012] (VI) In the formula: The average pore throat radius is in μm; r i r i+1 These are the radii of any throat; S Hgi Let the throat radius be r i The corresponding cumulative distribution frequency, % S Hgi+1 Let the throat radius be r i+1 The corresponding cumulative distribution frequency, % ΔS Hgi For the corresponding r i With average pore throat radius Cumulative frequency difference between intervals, % r is the radius of the throat; D r is the coefficient of variation.

[0013] Furthermore, in S5, the formula for calculating the displacement characteristic index is: (VII) In the formula: T is the displacement characteristic index; K represents the penetration rate; The average orifice throat radius; D r is the coefficient of variation.

[0014] Furthermore, in S5, the method for establishing the mathematical model of the core displacement boundary is as follows: By fitting and regressing the displacement characteristic index on the x-axis and the displacement limit on the y-axis using the production function, a mathematical model of the core displacement limit can be obtained.

[0015] Furthermore, the mathematical model for the core displacement limit is as follows: (VIII) In the formula: η is the displacement limit; T is the displacement characteristic index; e is the natural constant, 2.71; a, b, and c represent the fitting parameters.

[0016] Furthermore, in S6, the specific method for establishing a waterflooding and displacement boundary map for ultra-low permeability reservoirs and selecting the development mode based on this boundary map is as follows: combining the ultimate recovery rate η in the field production of ultra-low permeability reservoirs. kc The displacement characteristic index T at the site was obtained. kc Based on the permeability K and average pore throat radius of different reservoirs Coefficient of variation D r The characteristic index T is calculated by combining the characteristics of any combination, where T ≥ T. kc When the characteristic index T < T0 of any combination is less than T0, the corresponding reservoir is suitable for water injection development. kc If the reservoir corresponding to this combination is unsuitable for water injection development, then the calculation results of the characteristic indices should be collected to create a water drive displacement boundary map. Compared with the prior art, the present invention has the following beneficial effects: This invention addresses the characteristics of ultra-low permeability reservoirs by establishing a displacement limit model that integrates macroscopic physical properties and microscopic pore throat characteristics. This model can effectively predict the displacement limit for waterflooding development of ultra-low permeability reservoirs, effectively guide the selection of development methods for ultra-low permeability reservoirs, and support the large-scale and efficient development of ultra-low permeability reservoirs.

[0017] To make the above description of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other design solutions and drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The cumulative frequency distribution curves for nuclear magnetic resonance relaxation time T2 and pore throat radius r are shown. Figure 2The time map of the T2 spectrum of the core nuclear magnetic resonance (C-1) is shown. Figure 3 The cumulative frequency distribution curves (c-1) are for nuclear magnetic resonance relaxation time T2 and pore throat radius r. Figure 4 The fitting curves for the nuclear magnetic resonance relaxation time T2 and the pore throat radius r are shown in (c-1). Figure 5 This is a cross-plot of pressure and recovery rate for core C-1. Figure 6 This is a diagram showing the intersection of core permeability K and displacement boundary; Figure 7 This is a diagram showing the intersection of the core displacement characteristic index T and the displacement boundary; Figure 8 This is a fitting diagram of the displacement characteristic index and displacement limit in mining production.

[0020] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below. Detailed Implementation

[0021] The invention can be further understood in conjunction with the following detailed description of preferred embodiments and included examples. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. If any definition of a specific term disclosed in the prior art differs from any definition provided herein, the definition provided herein shall prevail.

[0022] In one typical embodiment of this application, a method for calculating the water drive displacement limit of ultra-low permeability reservoirs is provided, comprising: S1. Obtain core samples from multiple wells in the target reservoir, remove residual oil or mud from the core samples, and measure the permeability K of each core sample; S2. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, combined with the high-pressure mercury injection curve of the target reservoir of the core, the conversion model of the relaxation time of the core nuclear magnetic resonance T2 spectrum and the pore throat radius r is obtained. S3. Based on the conversion model of relaxation time and pore throat radius r in the T2 spectrum of the core nuclear magnetic resonance, the T2 spectrum curve of the core is converted into a high-pressure mercury intrusion porosimetry curve, and the average pore throat radius of the core is calculated from the high-pressure mercury intrusion porosimetry curve. and coefficient of variation D r ; S4. Set multiple different experimental flow rates for each core sample and conduct water drive displacement tests by increasing the flow rate from low to high. Record the recovery rate η at stable pressure under different flow rates. When the recovery rate of the core sample no longer increases after increasing the flow rate, the recovery rate is the displacement limit of the core sample. S5. Based on the permeability K obtained in S1 and the coefficient of variation D obtained in S3. r A displacement characteristic index is constructed, and the displacement characteristic index of multiple core samples is fitted with the displacement limit obtained from S4. The regression yields a mathematical model of the core displacement limit, which can be used to calculate the displacement limit for water injection development in ultra-low permeability reservoirs.

[0023] As a further improvement to the above-mentioned technical solution of the present invention, the calculation method further includes: S6. Based on the mathematical model of core displacement limits, combined with the ultimate recovery rate η in oilfield production. kc The displacement characteristic index T at the site was obtained. kc Based on different combinations of permeability, average pore throat radius, and coefficient of variation, a water drive displacement boundary map for ultra-low permeability reservoirs is established, and the development method is selected based on this boundary map.

[0024] Furthermore, in S2, the method for obtaining the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state is as follows: prepare a core saturated CaCl2 solution, and then put the core saturated CaCl2 solution into the nuclear magnetic resonance instrument for measurement, thereby obtaining the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state.

[0025] Furthermore, specific methods for obtaining the conversion model of core nuclear magnetic resonance relaxation time and pore throat radius r include: S201. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, plot the cumulative distribution frequency curve of nuclear magnetic resonance T2. At the same time, based on the high pressure mercury intrusion curve of the core, plot the cumulative distribution frequency curve of the pore throat radius r. S202. According to the cumulative frequency distribution curve, in the effective saturation range, i.e., S... pcd i Hgmax Inside, take S=S i Interpolating the cumulative frequency distribution curve of the throat radius r, we obtain the cumulative frequency distribution S. i The radius of the throat at that time r i It can simultaneously obtain the same cumulative distribution frequency S i T below 2i and r i The value; Where S i Let r be the arbitrary throat radius i The corresponding cumulative distribution frequency, S Hgmax ​​S represents the cumulative distribution frequency corresponding to the maximum mercury saturation. pcd This represents the cumulative distributed frequency corresponding to the exhaust pressure. S203. The conversion relationship between the core nuclear magnetic resonance relaxation time and the pore throat radius r is as follows: (I) In the formula: ρ2 is the transverse surface relaxation rate, μm·ms -1 Fs is the pore shape factor; T2 is the core nuclear magnetic resonance relaxation time; C is the conversion coefficient; n is the power exponent. S204. By fitting and regressing T2 and r at different cumulative distribution frequencies S of the core, the values ​​of C and n are obtained. The average value of C and n is obtained based on the fitting regression of multiple cores. Thus, the conversion model of the relaxation time of the nuclear magnetic resonance T2 spectrum of the target reservoir and the pore throat radius r can be obtained.

[0026] Furthermore, in S203, the method for obtaining the conversion relationship between the core nuclear magnetic resonance relaxation time and the pore throat radius r is as follows: a. The relaxation time T2 of the fluid in the pores of a porous medium under an ideal uniform magnetic field can be expressed as: (II) In the formula: S is the surface area of ​​a single pore, in μm 2 V is the volume of a single pore, in μm. 3 ; b. The relationship between specific surface area and pore size is: (III) In the formula: Fs is the pore shape factor; r is the pore throat radius, μm; c. Substituting equation (Ⅲ) relating specific surface area to aperture into equation (Ⅱ), we obtain equation (Ⅳ). (IV) d. Due to the complexity of the pore-throat structure in actual reservoirs, the experimental results show that the relaxation time of core nuclear magnetic resonance (NMR) is not linearly related to the pore-throat radius, but rather exhibits a power function relationship. Therefore, the conversion relationship between the relaxation time of the core NMR T2 spectrum and the pore-throat radius r is obtained as follows: (I).

[0027] Furthermore, in S3, the average pore throat radius of the core... and coefficient of variation D r The calculation formula is as follows: (V) (VI) In the formula: r is the average pore throat radius, in μm.i r i+1 These are the radii of any throat; S Hgi Let the throat radius be r i The corresponding cumulative distribution frequency, %; S Hgi+1 Let the throat radius be r i+1 The corresponding cumulative distribution frequency, %; ΔS Hgi For the corresponding r i With average pore throat radius The cumulative distribution frequency difference between intervals, %; r is the throat radius; D r is the coefficient of variation.

[0028] Furthermore, in S5, the formula for calculating the displacement characteristic index is: (VII) In the formula: T is the displacement characteristic index; K is the penetration rate; D is the average pore throat radius; r is the coefficient of variation.

[0029] Furthermore, in S5, the method for establishing the mathematical model of the core displacement boundary is as follows: By fitting and regressing the displacement characteristic index on the x-axis and the displacement limit on the y-axis using the production function, a mathematical model of the core displacement limit can be obtained.

[0030] Furthermore, the mathematical model for the core displacement limit is as follows: (VIII) In the formula: η is the displacement limit; T is the displacement characteristic index; e is the natural constant, 2.71; a, b, and c represent the fitting parameters.

[0031] Furthermore, in S6, the specific method for establishing a waterflooding and displacement boundary map for ultra-low permeability reservoirs and selecting the development mode based on this boundary map is as follows: combining the ultimate recovery rate η in the field production of ultra-low permeability reservoirs. kc The displacement characteristic index T at the site was obtained. kc Based on the permeability K and average pore throat radius of different reservoirs Coefficient of variation D r The characteristic index T is calculated by combining the characteristics of any combination, where T ≥ T. kc When the characteristic index T < T0 of any combination is less than T0, the corresponding reservoir is suitable for water injection development. kc If the reservoir corresponding to this combination is unsuitable for water injection development, then the calculation results of the characteristic indices should be collected to create a water drive displacement boundary map. In another typical embodiment of this application, a method for calculating the water drive displacement limit of an ultra-low permeability reservoir is mentioned, which includes the following detailed steps: Step 1: Core samples are taken from the ultra-low permeability reservoir. The obtained core samples are washed with oil and dried to remove residual oil or mud. The permeability K of each core sample is then measured. Step 2: Prepare a saturated CaCl2 solution for the core, and then place the saturated CaCl2 solution into a nuclear magnetic resonance instrument for measurement. This will obtain the nuclear magnetic resonance T2 spectrum curve of the core under saturated water conditions. Combined with the high-pressure mercury intrusion curve of the target reservoir in the core, a conversion model of the relaxation time of the core nuclear magnetic resonance T2 spectrum and the pore throat radius r can be obtained. Specifically, the method for obtaining the conversion model between core magnetic resonance relaxation time and pore throat radius r includes: S201. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, plot the cumulative distribution frequency curve of nuclear magnetic resonance T2. At the same time, based on the high pressure mercury intrusion curve of the core, plot the cumulative distribution frequency curve of the pore throat radius r. S202. According to the cumulative frequency distribution curve, in the effective saturation range, i.e., S... pcd i Hgmax Inside, take S=S i Interpolating the cumulative frequency distribution curve of the throat radius r, we obtain the cumulative frequency distribution S. i The radius of the throat at that time r i It can simultaneously obtain the same cumulative distribution frequency S i T below 2i and r i The value; Where S i Let r be the arbitrary throat radius i The corresponding cumulative distribution frequency, S Hgmax S represents the cumulative distribution frequency corresponding to the maximum mercury saturation. pcd This represents the cumulative distributed frequency corresponding to the exhaust pressure. S203. The relaxation time T2 of the fluid in the pores of a porous medium under an ideal uniform magnetic field can be expressed as:

[0032] (II) In the formula: T2 is the core nuclear magnetic resonance relaxation time; ρ2 is the transverse surface relaxation rate, μm·ms -1 S is the surface area of ​​a single pore, in μm. 2 V is the volume of a single pore, in μm. 3 ; The relationship between specific surface area and pore size is as follows: (III) In the formula: Fs is the pore shape factor; r is the pore throat radius, μm; ​​Substituting equation (Ⅲ) relating specific surface area to aperture into equation (Ⅱ), we obtain equation (Ⅳ). (IV) Due to the complexity of pore-throat structures in actual reservoirs, the experimental results show that the relaxation time of core nuclear magnetic resonance (NMR) is not linearly related to the pore-throat radius, but rather exhibits a power function relationship. Therefore, the conversion relationship between the relaxation time of the core NMR T2 spectrum and the pore-throat radius r is obtained as follows: (I) In the formula: ρ2 is the transverse surface relaxation rate, μm·ms -1 Fs is the pore shape factor; T2 is the core nuclear magnetic resonance relaxation time; C is the conversion coefficient; n is the power exponent. S204. By fitting and regressing T2 and r at different cumulative distribution frequencies S of the core, the values ​​of C and n are obtained. The average value of C and n is obtained based on the fitting regression of multiple cores. Thus, the conversion model of the relaxation time of the nuclear magnetic resonance T2 spectrum of the target reservoir and the pore throat radius r can be obtained.

[0033] Step 3: Based on the conversion model of relaxation time and pore throat radius r in the core nuclear magnetic resonance T2 spectrum, convert the core's T2 spectrum curve into a high-pressure mercury intrusion porosimetry curve, and calculate the average pore throat radius of the core using the high-pressure mercury intrusion porosimetry curve. and coefficient of variation D r Specifically, the average pore throat radius of the core. and coefficient of variation D r The calculation formula is as follows: (V) (VI) In the formula: r is the average pore throat radius, in μm. i r i+1 These are the radii of any throat; S Hgi Let r be the radius of the throat. i The corresponding cumulative distribution frequency, %; S Hgi+1 Let r be the radius of the throat. i+1 The corresponding cumulative distribution frequency, %; ΔS Hgi For the corresponding r i With average pore throat radius The cumulative distribution frequency difference between intervals, %; r is the throat radius; D r is the coefficient of variation.

[0034] Step 4: Set multiple different experimental flow rates for each core sample and conduct water drive displacement tests by gradually increasing the flow rate. Record the recovery rate η at stable pressure under different flow rates. When the recovery rate of the core sample no longer increases after increasing the flow rate, this recovery rate is the displacement limit of the core sample. Step 5: Construct a displacement characteristic index based on the permeability K obtained in S1 and the coefficient of variation Dr obtained in S3. Fit the displacement characteristic index of multiple core samples with the displacement limit obtained in S4 and regress to obtain a mathematical model of the core displacement limit. The displacement limit of water injection development in ultra-low permeability reservoirs can be calculated using this model.

[0035] Specifically, the formula for calculating the displacement characteristic index is as follows: (VII) In the formula: T is the displacement characteristic index; K is the penetration rate; D is the average pore throat radius; r is the coefficient of variation.

[0036] Preferably, the method for establishing the mathematical model of the core displacement boundary is as follows: By fitting and regressing the displacement characteristic index on the x-axis and the displacement limit on the y-axis using the production function, a mathematical model of the core displacement limit can be obtained.

[0037] Specifically, the mathematical model for the core displacement limit is as follows: (VIII) In the formula: η is the displacement limit; T is the displacement characteristic index; e is the natural constant, 2.71; a, b, and c represent the fitting parameters.

[0038] Step 6: Based on the mathematical model of core displacement limit and combined with the ultimate recovery rate ηkc in oilfield production, calculate the displacement characteristic index T in the field. kc The characteristic index T is calculated based on combinations of different permeabilities, average pore throat radii, and coefficients of variation. When the characteristic index T ≥ T for any combination... kc When the characteristic index T < T0 of any combination is less than T0, the corresponding reservoir is suitable for water injection development. kc If the reservoir corresponding to this combination is not suitable for water injection development, then the calculation results of the characteristic indices are collected to create a water drive displacement boundary map.

[0039] Taking the ultra-low permeability reservoir of Changqing Oilfield as an example, nuclear magnetic resonance, core displacement and other related experiments are conducted to illustrate the specific technical solution of this invention: (1) Core samples were taken from the Chang 6 and Chang 8 ultra-low permeability reservoirs in the Ordos Basin. Ten representative core samples were selected, and the core samples were washed and dried to remove residual oil or mud. The physical properties of the core samples were obtained through physical property testing, as shown in Table 1. Table 1. Permeability and Permeability Test Results of Different Cores

[0040] (2) The core sample was placed in a nuclear magnetic resonance instrument with CaCl2 saturated solution to obtain the nuclear magnetic resonance T2 spectrum curve of the rock sample under water saturation state (e.g., Figure 2 (as shown) In actual geological formations, the pore structure is complex. Analysis of numerous experimental results reveals that the T2 distribution exhibits a power-law relationship with the pore throat radius r.

[0041] In the formula: ρ2 is the transverse surface relaxation rate, μm·ms -1 Fs is the pore shape factor; T2 is the core nuclear magnetic resonance relaxation time; C is the conversion coefficient; n is the power exponent.

[0042] The conversion between core nuclear magnetic resonance relaxation time T2 and pore throat radius r consists of the following three steps: ① Plot the cumulative frequency distribution curve of the core's nuclear magnetic resonance T2, and the cumulative frequency distribution curve of the corresponding pore throat radius r (e.g., Figure 1 As shown); where S i Let r be the arbitrary throat radius i The corresponding cumulative distribution frequency, S Hgmax S represents the cumulative distribution frequency corresponding to the maximum mercury saturation. pcd This represents the cumulative distributed frequency corresponding to the exhaust pressure.

[0043] ② Within the effective saturation range (S pcd i Hgmax ), take S=S i Interpolating the cumulative frequency distribution curve of the throat radius r, we obtain the cumulative frequency distribution S. i The radius of the throat at that time r i It can simultaneously obtain the same cumulative distribution frequency S i T below 2i and r i The value (e.g.) Figure 3 (As shown).

[0044] ③ Perform power-law fitting on the obtained values ​​of T2 and r (refer to...) Figure 4 The values ​​of C and n for 10 core samples were obtained (as shown in Table 2). Taking the average of the 10 core samples, the value of C was 0.0384 and the value of n was 0.4659. Therefore, the T2 distribution has a power function relationship with the pore throat radius as follows: .

[0045] Table 2 Fitting results for C and n from different core samples

[0046] ​​(3) Based on the pore throat radius r and its cumulative distribution frequency, the average pore throat radius of each core is calculated using equations (V) and (VI). and coefficient of variation D r .

[0047] The average pore throat radius and coefficient of variation of the 10 core samples were calculated (as shown in Table 3): Table 3. Calculation results of average pore throat radius and coefficient of variation for different core samples.

[0048] (4) To simulate the characteristics of actual reservoir waterflooding development, waterflooding displacement test parameters were set according to the similarity criterion. The viscosity of the oil was 2 mPa·s, the viscosity of the water was 2 mPa·s, and the salinity of the water was 40 g / L. After saturating the core with simulated oil, six flow rate points of 0.002, 0.005, 0.006, 0.007, 0.008, and 0.01 mL / min were set to conduct conventional waterflooding displacement tests from low to high speed. The stable pressure difference and recovery rate of the core at different flow rates were monitored, and the intersection curve of pressure difference and recovery rate was plotted. When the recovery rate no longer increases with increasing pressure difference, this recovery rate is the displacement limit of the core (e.g., Figure 5 (As shown).

[0049] Table 4 Calculation results of displacement limits for different core samples

[0050] (5) By intersecting the permeability and displacement limits (refer to the diagram) Figure 6 It was found that the fit between permeability and the displacement limit was generally poor, with a weak correlation. In the ultra-low permeability range, permeability can no longer be used as a characteristic parameter. To improve the fitting effect, a new characteristic parameter for the displacement limit was established by combining permeability, average pore throat radius, and coefficient of variation. The displacement characteristic indices and displacement boundaries of multiple core samples were fitted using linear, logarithmic, power, and growth function methods, respectively. It was found that the growth function method yielded the best fitting result (see reference). Figure 7 Therefore, a growth function is used to fit and regress the data to obtain a mathematical model of the displacement limit of the core sample. The model is as follows:

[0052] (6) Field practice shows that the displacement limit for waterflooding development of ultra-low permeability reservoirs in the Ordos Basin is 0.18. The displacement characteristic index T of the lower limit of waterflooding is determined by the mathematical model of the displacement limit. kc =0.006 (refer to Figure 8 The displacement characteristic index of the core is T>T kcIf the core sample corresponds to a reservoir suitable for water injection development, then water injection development is appropriate; for different permeability K and average pore throat radius... Coefficient of variation D r The combination of these factors is used to calculate the characteristic index T, and the calculation results are collected to create a water drive displacement limit chart (as shown in Table 5). This chart allows for a simple and quick determination of whether a reservoir is suitable for water drive development.

[0053] Table 5. Parameters of Water Drive and Displacement Boundaries in Ultra-Low Permeability Reservoirs of the Ordos Basin

[0054] In summary, based on the characteristics of ultra-low permeability reservoirs, this invention establishes a displacement limit model by integrating macroscopic physical property parameters and microscopic pore throat characteristic parameters. This model can effectively predict the displacement limit for waterflood development of ultra-low permeability reservoirs, effectively guide the selection of development methods for ultra-low permeability reservoirs, and support the large-scale and efficient development of ultra-low permeability reservoirs.

[0055] Those skilled in the art will understand that the above embodiments are specific examples of implementing the present invention, and in practical applications, various changes in form and detail may be made without departing from the spirit and scope of the present invention.

Claims

1. A method for calculating the water drive displacement limit in ultra-low permeability reservoirs, characterized in that, Includes the following steps: S1. Obtain core samples from multiple wells in the target reservoir, remove residual oil or mud from the core samples, and measure the permeability K of each core sample; S2. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, combined with the high-pressure mercury injection curve of the target reservoir of the core, the conversion model of the relaxation time of the core nuclear magnetic resonance T2 spectrum and the pore throat radius r is obtained. S3. Based on the conversion model of relaxation time and pore throat radius r in the T2 spectrum of the core nuclear magnetic resonance, the T2 spectrum curve of the core is converted into a high-pressure mercury intrusion porosimetry curve, and the average pore throat radius of the core is calculated from the high-pressure mercury intrusion porosimetry curve. and coefficient of variation D r ; S4. Set multiple different experimental flow rates for each core sample and conduct water drive displacement tests by increasing the flow rate from low to high. Record the recovery rate η at stable pressure under different flow rates. When the recovery rate of the core sample no longer increases after increasing the flow rate, the recovery rate is the displacement limit of the core sample. S5. Based on the permeability K obtained in S1 and the coefficient of variation D obtained in S3. r A displacement characteristic index is constructed, and the displacement characteristic index of multiple core samples is fitted with the displacement limit obtained from S4. The regression yields a mathematical model of the core displacement limit, which can be used to calculate the displacement limit for water injection development in ultra-low permeability reservoirs.

2. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1, characterized in that, This calculation method also includes: S6. Based on the mathematical model of core displacement limits, combined with the ultimate recovery rate η in oilfield production. kc The displacement characteristic index T at the site was obtained. kc Based on different combinations of permeability, average pore throat radius, and coefficient of variation, a water drive displacement boundary map for ultra-low permeability reservoirs is established, and the development method is selected based on this boundary map.

3. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1, characterized in that, In S2, the method for obtaining the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state is as follows: prepare a core saturated CaCl2 solution, and then put the core saturated CaCl2 solution into the nuclear magnetic resonance instrument for measurement, thereby obtaining the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state.

4. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1, characterized in that, The method for obtaining the conversion model of nuclear magnetic resonance relaxation time and pore throat radius r of the core based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state and the high-pressure mercury intrusion curve of the core includes the following steps: S201. Based on the nuclear magnetic resonance T2 spectrum curve of the core under saturated water state, plot the cumulative distribution frequency curve of nuclear magnetic resonance T2. At the same time, based on the high pressure mercury intrusion curve of the core, plot the cumulative distribution frequency curve of the pore throat radius r. S202. According to the cumulative frequency distribution curve, in the effective saturation range, i.e., S... pcd i Hgmax Inside, take S=S i Interpolating the cumulative frequency distribution curve of the throat radius r, we obtain the cumulative frequency distribution S. i The radius of the throat r at that time i It can simultaneously obtain the same cumulative distribution frequency S i T below 2i and r i The value;​​ Where S i Let r be the arbitrary throat radius i The corresponding cumulative distribution frequency, S Hgmax S represents the cumulative distribution frequency corresponding to the maximum mercury saturation. pcd This represents the cumulative distributed frequency corresponding to the exhaust pressure. S203. The conversion relationship between the core nuclear magnetic resonance relaxation time and the pore throat radius r is as follows: （Ⅰ） In the formula: ρ2 is the transverse surface relaxation rate, μm·ms -1 ; Fs is the pore shape factor; T2 is the core nuclear magnetic resonance relaxation time; C is the conversion factor; n is the power exponent; S204. By fitting and regressing T2 and r at different cumulative distribution frequencies S of the core, the values ​​of C and n are obtained. The average value of C and n is obtained based on the fitting regression of multiple cores. Thus, the conversion model of the relaxation time of the nuclear magnetic resonance T2 spectrum of the target reservoir and the pore throat radius r can be obtained.

5. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1 or 4, characterized in that, In step S203, the method for obtaining the conversion relationship between the core nuclear magnetic resonance relaxation time and the pore throat radius r is as follows: a. The relaxation time T2 of the fluid in the pores of a porous medium under an ideal uniform magnetic field can be expressed as: （Ⅱ） In the formula: S is the surface area of ​​a single pore, in μm 2 ; V is the volume of a single pore, in μm. 3 ; b. The relationship between specific surface area and pore size is: （Ⅲ） In the formula: Fs is the pore shape factor; r is the pore throat radius, μm; c. Substituting Equation III, which relates specific surface area to aperture, into Equation II, we obtain Equation IV. （Ⅳ） d. Due to the complexity of the pore-throat structure in actual reservoirs, the experimental results show that the relaxation time of core nuclear magnetic resonance (NMR) is not linearly related to the pore-throat radius, but rather exhibits a power function relationship. Therefore, the conversion relationship between the relaxation time of the core NMR T2 spectrum and the pore-throat radius r is obtained as follows: （Ⅰ）。 6. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1, characterized in that, In S3, the average pore throat radius of the core The formula for calculating the coefficient of variation Dr is as follows: （Ⅴ） （Ⅵ） In the formula: The average pore throat radius is in μm; r i r i+1 These are the radii of any throat; S Hgi Let the throat radius be r i The corresponding cumulative distribution frequency, % S Hgi+1 Let the throat radius be r i+1 The corresponding cumulative distribution frequency, % ΔS Hgi For the corresponding r i With average pore throat radius Cumulative frequency difference between intervals, % r is the radius of the throat; D r is the coefficient of variation.

7. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1, characterized in that, In S5, the formula for calculating the displacement characteristic index is: （Ⅶ） In the formula: T is the displacement characteristic index; K represents the penetration rate; The average orifice throat radius; D r is the coefficient of variation.

8. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1, characterized in that, In S5, the method for establishing the mathematical model of the core displacement limit is as follows: By fitting and regressing the displacement characteristic index on the x-axis and the displacement limit on the y-axis using the production function, a mathematical model of the core displacement limit can be obtained.

9. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 1 or 7, characterized in that, The mathematical model for the core displacement limit is as follows: （Ⅷ） In the formula: η is the displacement limit; T is the displacement characteristic index; e is the natural constant, 2.71; a, b, and c represent the fitting parameters.

10. The method for calculating the water drive displacement limit of ultra-low permeability reservoirs according to claim 2, characterized in that, In S6, the specific method for establishing a water drive displacement boundary map for ultra-low permeability reservoirs and selecting the development mode based on this boundary map is as follows: combining the ultimate recovery rate η in reservoir field production. kc The displacement characteristic index T at the site was obtained. kc Based on the permeability K and average pore throat radius of different reservoirs Coefficient of variation D r The characteristic index T is calculated by combining the characteristics of any combination, where T ≥ T. kc When the characteristic index T < T0 of any combination is less than T0, the corresponding reservoir is suitable for water injection development. kc If the reservoir corresponding to this combination is not suitable for water injection development, then the calculation results of the characteristic indices are collected to create a water drive displacement boundary map.

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