Fracture dynamic conductivity calculation method based on production decline

Abstract

The present application relates to a kind of based on yield decline fracture dynamic conductivity calculation method.Using indoor physical simulation method, relative permeability experiment, mercury injection experiment, permeability stress sensitivity experiment and threshold pressure gradient experiment are carried out, according to the relative permeability characteristics of oil phase and the threshold pressure gradient characteristics of different permeability, the producing range of fractured oil production well during depletion development is calculated;Using the producing range of fractured oil production well, the reserves of crude oil and formation water in the producing range are calculated, and then the pressure difference change of fractured oil production well is calculated;And using the calculated pressure difference change of fractured oil production well, the fracture dynamic conductivity of fractured oil production well under different yield decline conditions is calculated.Using this method can be used for offshore low permeability sandstone reservoir fracture development plan design stage to calculate fracture dynamic conductivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of offshore oil and gas field development engineering, in particular to a method for calculating fracture dynamic conductivity suitable for the fracturing development plan design stage of offshore low-permeability sandstone reservoirs. BACKGROUND

[0002] Fracturing is an effective stimulation measure for improving single-well productivity of low-permeability reservoirs. In the fracturing development plan design, fracture conductivity is an important factor affecting the initial productivity of fracturing production wells. At present, the determination of fracturing fracture conductivity is mainly through rock mechanics fracturing simulation, but this method cannot consider the decrease of conductivity caused by the decrease of formation pressure due to production. Therefore, how to calculate the dynamic change of conductivity after fracturing is crucial for the accurate prediction of the indicators of the fracturing development plan of offshore low-permeability reservoirs. SUMMARY

[0003] In view of the above problems, the purpose of the present application is to calculate the fracture dynamic conductivity of fracturing production wells of offshore low-permeability sandstone reservoirs by using laboratory experiments and reservoir engineering methods.

[0004] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0005] A method for calculating the dynamic conductivity of fracturing fractures based on production decline, comprising:

[0006] By using the method of indoor physical simulation, relative permeability experiment, mercury injection experiment, permeability stress sensitivity experiment and starting pressure gradient experiment are carried out to determine the oil phase relative permeability characteristics under different water cut conditions and the starting pressure gradient characteristics of different permeability, and the producing range of fracturing production wells during depletion development is calculated according to the oil phase relative permeability characteristics and the starting pressure gradient characteristics of different permeability;

[0007] The producing range of fracturing production wells is used to calculate the reserves of crude oil and formation water in the producing range, and the production pressure difference under different cumulative production conditions considering the pressure drop in the producing range is calculated, and then the pressure difference change of fracturing production wells is calculated; and

[0008] The calculated pressure difference change of fracturing production wells is used to calculate the production pressure difference at different time steps according to the bottom hole flowing pressure measured by fracturing production wells or the bottom hole flowing pressure converted from the wellhead pressure during the self-flowing stage, and the fracture dynamic conductivity of fracturing production wells under different production decline conditions is calculated according to the production decline and the production pressure difference decline.

[0009] The present application has the following advantages due to the adoption of the above technical solutions:

[0010] The present method can be used to calculate the fracture dynamic conductivity at the fracturing development plan design stage of offshore low-permeability sandstone reservoirs.

[0011] The present application provides a quantitative, operable technical method and implementation steps. BRIEF DESCRIPTION OF DRAWINGS

[0012] Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The accompanying drawings are intended to further assist in understanding the present application, and are not intended to limit the present application. Throughout the drawings, like reference numerals will be used to designate like components. In the drawings:

[0013] Figure 1 is a schematic diagram of a relative permeability and water cut relationship curve;

[0014] Figure 2 is a schematic diagram of a threshold pressure gradient relationship curve;

[0015] Figure 3 is a schematic diagram of a mercury injection curve;

[0016] Figure 4 is a schematic diagram of a permeability stress sensitivity curve; and

[0017] Figure 5 is a schematic diagram of a conductivity calculation curve. DETAILED DESCRIPTION

[0018] Exemplary embodiments of the present application will be described in greater detail below with reference to the accompanying drawings. Although exemplary embodiments of the present application are shown in the drawings, it is understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that this application will be thorough and complete, and will fully convey the scope of the application to those skilled in the art.

[0019] According to some embodiments of the present application, a fracture dynamic conductivity calculation method based on production decline is provided, and through matching analysis of production decline and pressure drop amplitude of a fractured well, dynamic variation characteristics of fracture conductivity are obtained.

[0020] 1. Calculate the producing range of a fractured oil production well during depletion development

[0021] By using a laboratory physical simulation method, relative permeability experiments, mercury injection experiments, permeability stress sensitivity experiments, and threshold pressure gradient experiments are carried out to determine the oil phase relative permeability characteristics under different water cut conditions and the threshold pressure gradient characteristics of different permeabilities. The capillary force conversion formula is used to convert the mercury gas capillary force into oil-water capillary force. According to the results of the permeability stress sensitivity experiment, the permeability stress sensitivity coefficient a is fitted. The producing range of the fractured oil production well is calculated by using the fractured directional well production formula.

[0022] 2. Calculate the pressure differential change of the fractured oil production well

[0023] The PVT experiment of crude oil is carried out by using the method of indoor physical simulation. The JEFRI full window mercury-free high pressure PVT analyzer is cleaned and vacuumed at the formation temperature, then a certain amount of formation crude oil sample is kept in a single phase into the PVT analyzer, and the sample is pressurized to the formation pressure for more than 4 hours at constant temperature, and is fully stirred and stabilized. Above the saturation pressure, the step-by-step pressure reduction method is used, and each step is reduced by 1-3 MPa. Below the saturation pressure, the step-by-step expansion volume method is used, and each step is expanded by 1-20 cm 3 . After each step of pressure reduction and expansion, the sample is fully stirred and stabilized, and the pressure and sample volume are read. The expansion is stopped when the original sample amount is about 2 times. The parameters such as rock compressibility, crude oil compressibility and crude oil volume coefficient are determined. According to the principle of material balance, the reserves of crude oil and formation water in the producing range are calculated by using the producing range obtained in step 1, and then the production pressure differential under different cumulative production conditions considering the pressure drop in the producing range is calculated.

[0024] 3. Calculate the fracture dynamic conductivity of the fractured oil production well

[0025] According to the production dynamic of the fractured oil production well, the production decline characteristics are analyzed, and the reason for the production decline is determined as the increase of water cut or the decrease of liquid production. According to the cumulative oil production and water production of each time step, the average formation pressure drop in the producing range caused by different cumulative production is calculated by using step 2. The production pressure differential of different time steps is calculated according to the bottom hole flowing pressure measured by the fractured oil production well or the bottom hole flowing pressure converted from the wellhead pressure in the flowing production stage. According to the production decline and the production pressure differential, the fracture conductivity under different production decline conditions is calculated.

[0026] The proppant in the fracture is affected by the overburden effective stress, and the conductivity of the fracture will decrease with the decrease of the formation pressure around the oil production well. This will lead to the decrease of the oil production index of the fractured oil production well, thus causing the further decrease of the production on the basis of the production decrease caused by the decrease of the pressure differential. Therefore, by matching the production decline and the pressure differential decrease of the fractured well, the dynamic change characteristics of the fracture conductivity are obtained.

[0027] According to some embodiments of the present application, the fracture dynamic conductivity of the fractured oil production well in the offshore low permeability sandstone reservoir is calculated by using the indoor experiment and reservoir engineering method, including the following steps:

[0028] Step 1. Calculate the producing range of the fractured oil production well in the depletion development

[0029] The relative permeability experiment, the mercury injection experiment, the permeability stress sensitivity experiment and the threshold pressure gradient experiment are carried out by using the indoor physical simulation method, the oil phase relative permeability characteristics under different water content conditions are determined, and the threshold pressure gradient characteristics of different permeability are determined. Figure 1 As shown in Figure 2 .

[0030] The mercury gas capillary force is converted into the oil-water capillary force by using formula (1), as shown in Figure 3 .

[0031]

[0032] p cow — oil-water capillary force;

[0033] p cHg — mercury gas capillary force.

[0034] According to the permeability stress sensitivity experiment results, the permeability stress sensitivity coefficient α is fitted, as shown in Figure 4 .

[0035] The producing range of the fracturing oil well is calculated by formula (2).

[0036]

[0037] In the formula:

[0038] q o — daily oil production, m 3 / d;

[0039] K — permeability, mD;

[0040] α — stress sensitivity coefficient, dimensionless;

[0041] p i — original formation pressure, MPa;

[0042] p e — current formation pressure, MPa;

[0043] K ro — oil phase relative permeability, dimensionless;

[0044] h — effective thickness, m;

[0045] p wf — bottom hole flowing pressure, MPa;

[0046] G — threshold pressure gradient, MPa / m;

[0047] r e — producing range, m;

[0048] μ o ——Crude oil viscosity, mPa·s;

[0049] B o ——Volume coefficient of crude oil, m 3 / m 3 ;

[0050] L——half length of crack, m;

[0051] S——skin coefficient, dimensionless.

[0052] Step 2. Calculate the pressure difference change of the fractured oil well

[0053] Using indoor physical simulation methods, we conducted crude oil PVT experiments and rock compressibility experiments to clarify parameters such as the rock compressibility, crude oil compressibility, and crude oil volume coefficient. Based on the material balance principle, we calculated the production pressure difference under different cumulative production conditions, taking into account the pressure drop within the producing range, using Equation (3).

[0054]

[0055] Where: Δp——production pressure difference, MPa;

[0056] Q o ——Cumulative oil production, m 3 ;

[0057] Q w ——Cumulative water production, m 3 ;

[0058] B w ——Formation water volume coefficient, m 3 / m 3 ;

[0059] N o ——Geological reserves of crude oil within the utilization range, m 3 ,

[0060]

[0061] N w ——Geological reserves of formation water within the utilization range, m 3 ,

[0062]

[0063] C r ——Rock compression coefficient, MPa -1 ;

[0064] C o ——Compression coefficient of crude oil, MPa -1 ;

[0065] S o — oil saturation, decimal;

[0066] C w — formation water compressibility, MPa -1 ;

[0067] S w — water saturation, decimal;

[0068] Φ — porosity, decimal.

[0069] Step 3. Calculate the fracture dynamic conductivity of the fractured oil production well

[0070] According to the production performance of the fractured oil production well, the production decline characteristics are analyzed to determine the reason for the production decline, i.e. the increase of water cut or the decrease of liquid production. According to the cumulative oil production and water production of each time step, the average formation pressure drop in the producing range caused by different cumulative production is calculated by step 2. According to the bottom hole flowing pressure measured by the fractured oil production well or the bottom hole flowing pressure converted from the wellhead pressure in the flowing production stage, the production pressure difference of different time steps is calculated. According to the production decline and the production pressure difference decline, as shown in formula (4), the fracture conductivity under different production decline conditions is calculated: Figure 5

[0071]

[0072] In the formula:

[0073] F c — dynamic conductivity, D·cm;

[0074] F ci — initial conductivity, D·cm;

[0075] q oi — initial deliverability, m 3 / d;

[0076] Δp i — initial production pressure difference.

[0077] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and are not limited thereto. Although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions described in the foregoing examples can be modified, or some technical features can be replaced by equivalent features. Such modifications or replacements do not change the essence of the corresponding technical solutions, and do not deviate from the spirit and scope of the technical solutions of the embodiments of the present application.​

Claims

1. A method for calculating the dynamic conductivity of hydraulic fractures based on production decline, characterized in that: include: Using indoor physical simulation methods, we conducted relative permeability experiments, mercury injection experiments, permeability stress sensitivity experiments, and start-up pressure gradient experiments to clarify the oil phase relative permeability characteristics under different water cut conditions and the start-up pressure gradient characteristics of different permeabilities. Based on these characteristics, we calculated the operating range of fractured oil wells during depletion development. The crude oil PVT experiment was carried out using the indoor physical simulation method. The JEFRI full-window mercury-free high-pressure PVT analyzer was cleaned and evacuated at formation temperature. Then a certain amount of formation crude oil sample was transferred to the PVT analyzer in a single phase. The sample was kept at a constant temperature for more than 4 hours, pressurized to the formation pressure, and fully stirred to stabilize. Above the saturation pressure, the pressure was reduced by 1 to 3 MPa per stage according to the step-by-step pressure reduction method. Below the saturation pressure, the volume was expanded by 1 to 20 cm per stage according to the step-by-step volume expansion method. 3 After each stage of decompression and expansion, the sample is fully stirred and stabilized, and the pressure and sample volume are read. The sample is expanded until it is about twice the original volume, and the rock compressibility coefficient, crude oil compressibility coefficient and crude oil volume coefficient are determined. Utilizing the producing range of the fractured oil production well, calculating the crude oil and formation water reserves within the producing range, calculating the production pressure difference after considering the pressure drop within the producing range under different cumulative production conditions, and then calculating the pressure difference change of the fractured oil production well; and Utilizing the pressure differential change of the fractured oil production well, the production pressure differential at different time steps is calculated based on the bottom hole flowing pressure measured in the fractured oil production well or the bottom hole flowing pressure converted from the wellhead pressure during the flowing phase, and the dynamic fracture conductivity of the fractured oil production well under different production decline conditions is calculated based on the production decline and the production pressure differential decline. The calculation of the producing range of the fractured oil well during depletion development includes: using a capillary force conversion formula to convert the mercury gas capillary force into the oil-water capillary force, fitting the permeability stress sensitivity coefficient based on the permeability stress sensitivity test results, and calculating the producing range of the fractured oil well using the fracture directional well production formula.

2. The method for calculating the dynamic conductivity of hydraulic fractures based on production decline according to claim 1, characterized in that: The mercury gas capillary force is converted into the oil-water capillary force using formula (1): (1) p cow ——Oil-water capillary force; p cHg ——Capillary force of mercury gas.

3. The method for calculating the dynamic conductivity of hydraulic fractures based on production decline according to claim 1, characterized in that: The production pressure difference after considering the pressure drop within the producing range under different cumulative production conditions is calculated using formula (3): (3) Where: ∆ p ——production pressure difference; p e ——Current formation pressure; Q o —Cumulative oil production; B o ——Volume coefficient of crude oil; Q w ——Cumulative water production; B w ——formation water volume coefficient; N o ——Crude oil geological reserves within the scope of utilization ； N w - Geological reserves of formation water within the utilization range; C r ——rock compressibility coefficient; C o ——compressibility coefficient of crude oil; S o ——Oil saturation; C w ——compressibility coefficient of formation water; S w ——water saturation; p wf ——Bottomhole flow pressure.

4. The method for calculating the dynamic conductivity of hydraulic fractures based on production decline according to claim 3, characterized in that: Calculate the crude oil geological reserves No within the producing range according to formula (3-1): (3-1) Where: Φ - porosity; h——effective thickness.

5. The method for calculating the dynamic conductivity of hydraulic fractures based on production decline according to claim 3, characterized in that: Calculate the formation water geological reserves Nw within the utilization range according to formula (3-2): (3-2) Where: Φ - porosity; h——effective thickness.

6. The method for calculating the dynamic conductivity of hydraulic fractures based on production decline according to claim 1, characterized in that: The dynamic conductivity of fractures in fractured oil wells under different production decline conditions is calculated according to formula (4): (4) Where: F c ——dynamic flow conduction capacity; F ci —initial flow conductivity; q o - production capacity; q oi — initial production capacity; ∆p i ——Initial production pressure difference; ∆p ——Production pressure difference.

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