Gas well productivity evaluation method for fractured tight gas reservoirs and three-dimensional seepage experimental device

Abstract

The present invention relates to the technical field of oil and gas reserve evaluation, and in particular to a method for evaluating the productivity of gas wells in fractured tight gas reservoirs and a three-dimensional seepage experimental device. The method for evaluating the productivity of gas wells in fractured tight gas reservoirs of the present invention comprises the following steps: establishing a physical model based on a three-dimensional seepage mechanism and according to the physical properties of the gas reservoir in the target area and the structure of the target gas well; conducting productivity test experiments on the physical model under different working conditions through a three-dimensional seepage experimental device to obtain the production change law of the gas well under different working conditions; determining the productivity equation of the target gas well; and determining the EUR of the target gas well based on the obtained production change law of the gas well and the production equation of the target gas well. In the present invention, based on the three-dimensional seepage mechanism, different working conditions are adjusted to study the influence of parameters such as different reservoir physical properties and fracture distribution characteristics on the gas well production from a mechanistic perspective, laying the foundation for the productivity evaluation of dual-branch horizontal wells in fractured tight gas reservoirs and the analysis of the production change law.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas reserve evaluation, and in particular to a method for evaluating the productivity of a gas well in a fractured tight gas reservoir and a three-dimensional seepage experimental device. Background Art

[0002] Tight, low-permeability gas reservoirs are widespread, accounting for one-third of total natural gas resources. The development of natural fractures significantly influences reservoir productivity, serving as a key indicator for selecting the sweet spots in tight sandstone gas reservoirs and a crucial factor influencing reservoir stimulation effectiveness. However, due to the complexity of natural fracture development, the impact of fractures on productivity and production remains a challenge.

[0003] In the late 1990s, multilateral well drilling technology saw significant development abroad. It is considered a key technology for the international oil industry in the 21st century and a future trend in drilling technology. Compared with conventional vertical wells, it can significantly improve oil and gas reservoir recovery and reduce the overall cost of oil and gas reservoir development, offering significant economic benefits and broad application prospects. In particular, the successful natural production model of dual-branch horizontal wells in the Changbei Gas Field has provided a new approach for the efficient development of tight, low-permeability gas reservoirs.

[0004] However, current methods for predicting the productivity of bilateral horizontal wells (or well groups) primarily rely on empirical formulas and electrical simulation experiments. The empirical formula method utilizes the principles of fluid dynamics, combined with mathematical analytical methods, to establish a productivity prediction model and analyze the factors influencing productivity. However, a productivity model that considers the impact of varying natural fracture distribution characteristics (length, inclination, filling level, aperture, and density) on bilateral wells (or well groups) has yet to be established. Research on the impact of varying reservoir and fracture characteristics on the production patterns of bilateral horizontal well groups in fractured tight gas reservoirs has yet to be conducted.

[0005] The electrical simulation method primarily applies the principle of hydroelectric similarity, using an electric field to simulate the flow patterns of formation fluids and analyze the factors and patterns that influence the productivity of multi-branch wells. However, this method only performs two-dimensional simulations and does not simulate the impact of horizontal well completion methods on production in three dimensions, limiting its guidance for field applications.

[0006] Therefore, in view of the above problems and current situation, it is necessary to explore new research methods and ideas, carry out research methods on the seepage mechanism and gas well production variation law of double-branch horizontal well groups in fractured tight gas reservoirs, and study the influence of different reservoir physical properties, fracture distribution characteristics and other parameters on gas well production, so as to lay the foundation for the productivity evaluation and production variation law analysis of double-branch horizontal wells in fractured tight gas reservoirs. Summary of the Invention

[0007] The present invention provides a method for evaluating the productivity of a gas well in a fractured tight gas reservoir and a three-dimensional seepage experimental device, which are used to solve at least one of the above technical problems.

[0008] One aspect of the present invention provides a method for evaluating the productivity of a gas well in a fractured tight gas reservoir, comprising the following steps:

[0009] Step 1: Establish a physical model based on the three-dimensional seepage mechanism and the physical properties of the gas reservoir in the target area and the structure of the target gas well;

[0010] Step 2: Conducting productivity test experiments under different working conditions on the physical model using a three-dimensional seepage experimental device to obtain the production variation patterns of the gas wells under different working conditions;

[0011] Step 3: Determine the productivity equation of the target gas well;

[0012] Step 4: Determine the EUR of the target gas well based on the gas well production variation pattern obtained in step 2 and the production capacity equation of the target gas well obtained in step 3.

[0013] In one embodiment, step 1 includes the following sub-steps:

[0014] Step 1.1: Select rock slabs based on the porosity and permeability of the gas reservoir in the target area;

[0015] Step 1.2: constructing a simulated gas well on the rock plate according to the structure of the target gas well;

[0016] Step 1.3: Pressing apart the rock slabs according to the natural fracture distribution characteristics of the gas reservoir in the target area, and splicing and combining multiple rock slabs to form experimental rock samples;

[0017] Step 1.4: The experimental rock sample is dried and then quantitatively saturated with bound water to form the physical model.

[0018] In one embodiment, step 2 includes the following sub-steps:

[0019] Step 2.1: Using the three-dimensional seepage experimental device, the physical model is subjected to depletion-type production under constant net overburden pressure and variable development rate, so as to obtain the production variation pattern of the gas well under depletion-type production under the same fracture conductivity and different development rates;

[0020] Step 2.2: Using the three-dimensional seepage experimental device, the physical model is subjected to depletion-type production under constant net overburden pressure and variable development rate to obtain the production variation pattern of the gas well under depletion-type production at the same development rate and different fracture conductivities.

[0021] In one embodiment, in step 2, before performing the capacity test experiment on the physical model, the step of calibrating the pore volume of the physical model according to the PVT equilibrium state equation is also included;

[0022] Among them, the PVT equilibrium state equation is:

[0023] P S1 ×V S =P S2 ×(V S +V X );

[0024] Where, P S1 is the calibration system pressure, V S is the calibration system volume, P S2 is the pressure of the system to be measured, V X is the volume of the system to be tested.

[0025] In one embodiment, step 3 includes:

[0026] Step 3.1: Divide the gas reservoir fractures in the target area into several micro-element segments;

[0027] Step 3.2: Characterize the radial permeability of each micro-element segment separately;

[0028] Step 3.3: Calculate the radial permeability of the gas reservoir fractures.

[0029] Among them, the radial permeability of the gas reservoir fracture is the sum of the radial permeabilities of each micro-element segment;

[0030] Step 3.4: Derive the productivity equation of the target gas well using the equivalent seepage resistance method.

[0031] In one embodiment, step 3.4 includes:

[0032] Step 3.4.1: Develop a theoretical model of the gas reservoir in the target area based on the following assumptions:

[0033] The gas reservoir is divided into Z1, Z 21 、Z 22 and Z 23 Four seepage zones, including Z1, Z 21 、Z 23 is the pure matrix seepage area, Z 22 It is a high-conductivity fracture development area;

[0034] Step 3.4.2: Based on the equivalent seepage resistance method, calculate the seepage areas Z1 and Z2 respectively. 21 、Z 22 and Z 23 Seepage resistance R1, R 21、R 22 and R 23 ;

[0035] Step 3.4.3: Combine the relationships between the four seepage zones and the seepage paths to calculate the total seepage resistance R of the gas reservoir in the target area:

[0036]

[0037] Where z is the average compression factor; l1 is the seepage area Z 21 The radial length of l2 is the seepage area Z 22 The radial length of r w is the wellbore radius; r e is the seepage radius of the gas reservoir; h is the thickness of the gas reservoir; k m is the matrix permeability; k fa is the permeability of the entire fracture;

[0038] Step 3.4.4: Introduce the productivity equation of multi-branch horizontal wells to obtain the productivity equation of the target gas well.

[0039] In one embodiment, the target gas well is a double-branch horizontal well.

[0040] The productivity equation of the dual-branch horizontal well is:

[0041]

[0042] Where Q h is the daily gas production of the horizontal well; p e is the formation pressure; p wh is the bottom hole flowing pressure; h is the effective thickness of the gas reservoir; r eh is the producing radius of the horizontal gas well; r wh is the horizontal wellbore radius; r ev is the vertical well operating radius; r wv is the wellbore radius of the vertical well; S v is the skin coefficient of vertical well; S h is the skin coefficient of the horizontal gas well; a is the distance from the horizontal well to the bottom of the reservoir; L is the length of the horizontal section of the horizontal well; n is the number of branch wells; U = K h / K v , K h is the horizontal permeability, K v is the vertical permeability.

[0043] Another aspect of the present invention provides a three-dimensional seepage experimental device for implementing the above-mentioned method for evaluating the productivity of a gas well in a fractured tight gas reservoir, comprising:

[0044] a sealed model room for accommodating the physical model;

[0045] an overlying pressure applying system, which is used to apply an overlying pressure to the physical model;

[0046] A gas pressurizing system, which is used to fill the model chamber with gas to saturate the physical model and reach a set formation pressure;

[0047] a heating system for heating the physical model to simulate formation temperature;

[0048] A simulated production system is used to make the gas in the physical model flow out at a set rate, thereby simulating depletion production of the gas well; and

[0049] The data acquisition system is used to detect and collect overburden pressure data, formation pressure data of the physical model and production flow data during the experiment.

[0050] In one embodiment, the overburden pressure applying system comprises: an overburden pressure applying pump and a piston connected to each other,

[0051] The overlying pressure applying pump drives the piston to press the upper surface of the physical model by providing hydraulic pressure, thereby applying overlying pressure to the physical model, and a first valve is provided between the overlying pressure applying pump and the piston.

[0052] In one embodiment, the gas boosting system comprises: a gas source and a booster connected to each other,

[0053] The gas source is connected to the model chamber through the booster to fill the model chamber with gas, and a second valve is provided between the booster and the model chamber.

[0054] Compared with the existing technology, the advantages of the present invention are as follows: In this invention, the conventional long core displacement experimental method is broken. Based on the three-dimensional seepage mechanism and combined with the actual situation of branched horizontal wells in fractured tight gas reservoirs, the exhaustion-type production process of the gas well is most realistically simulated. It not only considers the structure of the target gas well, but also considers the influence of matrix and fracture development characteristics on gas well production. Therefore, it can mechanistically study the influence of different reservoir physical properties, fracture distribution characteristics and other parameters on gas well production, clarify the relationship between different working systems and gas well production, and the variation pattern of production under different production conditions. At the same time, combined with the production capacity equation of the target gas well, the gas production of the actual gas reservoir well group can be determined and the EUR of the target gas well can be calculated, laying the foundation for the production capacity evaluation and production variation analysis of dual-branch horizontal wells in fractured tight gas reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Hereinafter, the present invention will be described in more detail based on embodiments with reference to the accompanying drawings.

[0056] Figure 1This is a flow chart of the method for evaluating the productivity of a gas well in a fractured tight gas reservoir according to the present invention;

[0057] Figure 2 is a schematic diagram of a three-dimensional seepage experimental device in one embodiment of the present invention;

[0058] Figure 3 is a schematic diagram of a material model (crack distribution characteristics) in one embodiment of the present invention;

[0059] Figure 4 is a schematic diagram of pore volume calibration of a physical model in one embodiment of the present invention;

[0060] Figure 5 is a graph showing the relationship between average flow rate and fracture permeability at a constant net overburden pressure in one embodiment of the present invention;

[0061] Figure 6 This is a schematic diagram of the flow rate changing over time in one embodiment of the present invention;

[0062] Figure 7 is a schematic diagram of a three-dimensional seepage theoretical model in one embodiment of the present invention;

[0063] Figure 8 Schematic diagram of spatial distribution of seepage areas in a fractured gas reservoir in one embodiment of the present invention;

[0064] Figure 9 1 is a schematic diagram of a dual-branch well group EUR in one embodiment of the present invention.

[0065] Reference numerals:

[0066] 1-physical model; 2-model chamber; 3-metal pad; 4-first valve; 5-second valve;

[0067] 6-third valve; 7-fourth valve; 8-precision pressure regulating valve; 9-flow controller; 10-calibration container;

[0068] 11-booster; 12-air source; 13-overburden pressure applying pump; 14-differential pressure sensor; 15-flow sensor;

[0069] 16 - heating system; 17 - simulated gas well; 18 - second pressure sensor; 19 - third pressure sensor. DETAILED DESCRIPTION

[0070] The present invention will be further described below with reference to the accompanying drawings.

[0071] like Figure 1 As shown, one aspect of the present invention provides a method for evaluating the productivity of a gas well in a fractured tight gas reservoir, comprising the following steps:

[0072] Step 1: Establish a physical model 1 based on the three-dimensional seepage mechanism and the physical properties of the gas reservoir in the target area and the structure of the target gas well;

[0073] Step 2: Conduct productivity test experiments under different operating modes on the physical model 1 using a three-dimensional seepage experimental device to obtain the production variation patterns of the gas wells under different operating modes;

[0074] Step 3: Determine the productivity equation of the target gas well;

[0075] Step 4: Based on the gas well production variation pattern obtained in step 2 and the target gas well productivity equation obtained in step 3, determine the EUR of the target gas well.

[0076] This study breaks with conventional long-core flooding experimental methods. Based on three-dimensional seepage mechanisms and combined with the actual situation of branched horizontal wells in fractured tight gas reservoirs, it simulates the exhaustion-based production process of gas wells in the most realistic way. This method not only considers the structure of the target gas well, but also the influence of matrix and fracture development characteristics on gas well production. This allows for a mechanistic study of the effects of different reservoir properties, fracture distribution characteristics, and other parameters on gas well production, clarifying the relationship between different operating modes and gas well production, and the variation patterns of production under different production conditions. Furthermore, by combining the production capacity equation of the target gas well, the actual gas production of the gas reservoir well group can be determined and the target gas well's EUR can be calculated, laying the foundation for the production capacity evaluation and production variation analysis of dual-branch horizontal wells in fractured tight gas reservoirs.

[0077] In one embodiment, step 1 includes the following sub-steps:

[0078] Step 1.1: Select rock slabs based on the porosity and permeability of the gas reservoir in the target area;

[0079] Step 1.2: construct a simulated gas well 17 on the rock plate according to the structure of the target gas well;

[0080] Step 1.3: Press the rock slabs apart according to the natural fracture distribution characteristics of the gas reservoir in the target area, and splice and combine multiple rock slabs to form experimental rock samples;

[0081] Step 1.4: Dry the experimental rock sample and quantitatively saturate it with bound water to form physical model 1.

[0082] In one embodiment, step 2 includes the following sub-steps:

[0083] Step 2.1: Using a three-dimensional seepage experimental device, depletion-type production is conducted on physical model 1 under constant net overburden pressure and varying development rates to obtain the production variation pattern of the gas well under depletion-type production at different development rates with the same fracture conductivity.

[0084] Step 2.2: Use a three-dimensional seepage experimental device to conduct depletion-type production on physical model 1 under constant net overburden pressure and variable development rate to obtain the variation pattern of gas well production under depletion-type production at the same development rate but different fracture conductivities.

[0085] In one embodiment, in step 2, before performing the production capacity test experiment on the physical model 1, the step further includes calibrating the pore volume of the physical model 1 according to the PVT equilibrium state equation;

[0086] Among them, the PVT equilibrium state equation is:

[0087] P S1 ×V S =P S2 ×(V S +V X );

[0088] Where, P S1 is the calibration system pressure, V S is the calibration system volume, P S2 is the pressure of the system to be measured, V X is the volume of the system to be tested.

[0089] In one embodiment, step 3 includes:

[0090] Step 3.1: Divide the gas reservoir fractures in the target area into several micro-element segments;

[0091] Step 3.2: Characterize the radial permeability of each micro-element segment separately;

[0092] Step 3.3: Calculate the radial permeability of the gas reservoir fractures.

[0093] Among them, the radial permeability of the gas reservoir fracture is the sum of the radial permeabilities of each micro-element segment;

[0094] Specifically, the radial permeability of gas reservoir fractures is calculated as follows:

[0095]

[0096] Where k fa is the permeability of the entire fracture, mD; k fi is the permeability of the fracture microelement, mD; β is the radial angle of the end point of the fracture microelement, rad.

[0097] Step 3.4: Derive the productivity equation of the target gas well using the equivalent seepage resistance method.

[0098] In one embodiment, step 3.4 includes:

[0099] Step 3.4.1: Develop a theoretical model of the gas reservoir in the target area based on the following assumptions:

[0100] The gas reservoir is divided into Z1, Z 21 、Z 22 and Z 23 Four seepage zones, including Z1, Z 21 、Z 23 is the pure matrix seepage area, Z 22 It is a high-conductivity fracture development area;

[0101] Step 3.4.2: Based on the equivalent seepage resistance method, calculate the seepage areas Z1 and Z2 respectively. 21 、Z 22 and Z 23 Seepage resistance R1, R 21 、R 22 and R 23 ;

[0102] Step 3.4.3: Combine the relationships between the four seepage zones and the seepage paths to calculate the total seepage resistance R of the gas reservoir in the target area:

[0103]

[0104] Where z is the average compression factor; l1 is the seepage area Z 21 The radial length of l2 is the seepage area Z 22 The radial length of r w is the wellbore radius; r e is the seepage radius of the gas reservoir; h is the thickness of the gas reservoir; k m is the matrix permeability; k fa is the permeability of the entire fracture;

[0105] Step 3.4.4: Introduce the productivity equation of multi-branch horizontal wells to obtain the productivity equation of the target gas well.

[0106] In one embodiment, the target gas well is a double-branch horizontal well.

[0107] The productivity equation of a double-branch horizontal well is:

[0108]

[0109] Where Q h is the daily gas production of the horizontal well; p e is the formation pressure; p wh is the bottom hole flowing pressure; h is the effective thickness of the gas reservoir; r eh is the producing radius of the horizontal gas well; r wh is the horizontal wellbore radius; r ev is the vertical well operating radius; r wvis the wellbore radius of the vertical well; S v is the skin coefficient of vertical well; S h is the skin coefficient of the horizontal gas well; a is the distance from the horizontal well to the bottom of the reservoir; L is the length of the horizontal section of the horizontal well; n is the number of branch wells; U = K h / K v , K h is the horizontal permeability, K v is the vertical permeability.

[0110] like Figure 2 As shown in , another aspect of the present invention provides a three-dimensional seepage experimental device for realizing the above-mentioned method for evaluating the productivity of a gas well in a fractured tight gas reservoir, comprising: a sealed model chamber 2 for accommodating a physical model 1; an overburden pressure application system for applying overburden pressure to the physical model 1; a gas pressurization system for filling the model chamber 2 with gas to saturate the physical model 1 and reach a set formation pressure; a heating system 16 for heating the physical model 1 to simulate the formation temperature; a simulated production system for causing the gas in the physical model 1 to flow out at a set speed, thereby simulating the depletion-type production of the gas well 17; and a data acquisition system for detecting and acquiring overburden pressure data, formation pressure data of the physical model 1, and production flow data during the experiment.

[0111] It should be noted that the model chamber 2 is provided with an inlet and an outlet to be connected to the gas pressurization system and the simulated production system respectively, and the inlet and outlet of the model chamber 2 are both connected to the simulated gas well 17 of the physical model 1.

[0112] Specifically, the overlying pressure applying system includes: a connected overlying pressure applying pump 13 and a piston. The overlying pressure applying pump 13 applies overlying pressure to the physical model 1 by providing hydraulic pressure to drive the piston to squeeze the upper surface of the physical model 1, and a first valve 4 is provided between the overlying pressure applying pump 13 and the piston.

[0113] The gas boosting system includes: a connected gas source 12 and a booster 11 . The gas source 12 is connected to the model chamber 2 through the booster 11 to fill the model chamber 2 with gas, and a second valve 5 is provided between the booster 11 and the model chamber 2 .

[0114] The simulated production system includes a third valve 6 and a flow controller 9. The third valve 6 is connected to the outlet of the model chamber 2. The flow controller 9 controls the opening of the third valve 6 to allow the gas in the physical model 1 to flow out at a set rate, thereby simulating the depletion-type production of the gas well 17. Preferably, the third valve 6 is a pressure reducing valve.

[0115] Furthermore, the outlet end of the third valve 6 is connected to a precision pressure regulating valve 8, and adjusting the precision pressure regulating valve 8 can accurately control the output flow, thereby effectively controlling the outlet flow through secondary pressure reduction to ensure the stability of the gas flow rate.

[0116] Preferably, the three-dimensional seepage experimental device further includes a calibration container 10 for calibrating the pore volume of the three-dimensional seepage model. Specifically, the calibration container 10 is disposed between the second valve 5 and the booster 11, and a fourth valve 7 is also disposed between the calibration container 10 and the booster 11.

[0117] Specifically, the data acquisition system includes a first pressure sensor, a second pressure sensor 18, a third pressure sensor 19, a differential pressure sensor 14, a flow sensor 15, etc. The first pressure sensor is used to detect the overburden pressure, the second pressure sensor 18 and the third pressure sensor 19 are used to detect the pressure at the model inlet and model outlet, respectively, the differential pressure sensor 14 is used to detect the pressure difference between the model inlet and model outlet, and the flow sensor 15 is used to detect the gas flow at the model outlet.

[0118] Example 1

[0119] In this embodiment, the physical model 1 (such as Figure 3 The production steps are shown in ).

[0120] (1) According to the physical properties of the gas reservoir in the target area, select natural rocks with similar porosity and permeability or make artificial rocks with similar physical properties and make them into several rock slabs.

[0121] (2) Drill one or two circular through holes in the center of the rock slab as wellbores. Simultaneously, based on the principle of similarity, the length of the dual-branch horizontal well (target gas well) to be studied is scaled to the simulation length. A circular through hole is drilled in the corresponding rock slab according to the pre-set length of the horizontal well section and the angle between the two horizontal well sections to form a branch horizontal well.

[0122] (3) According to the natural fracture distribution characteristics (fracture line density, fracture inclination, and fracture aperture) of the gas reservoir in the target area, the rock slabs are pressed open and spliced ​​together to form experimental rock samples. Among them, all the rock slabs are superimposed together to ensure that the circular through holes pre-drilled in the horizontal well section are interconnected. The processed rock slabs are placed in an oven to dry before use.

[0123] (4) The rock samples used in the experiment are dried and then quantitatively saturated with bound water. A certain amount of formation water is added to the rock slab according to the bound water conditions of the gas reservoir. The rock slab after water addition is sealed and stored to allow it to fully absorb the added water.

[0124] Example 2

[0125] In this embodiment, the pore volume V of the three-dimensional seepage model is specifically described using the PVT equilibrium state equation. X Perform calibration steps.

[0126] The rock sample saturated with quantitative bound water was placed in the model chamber 2 of the three-dimensional seepage experimental device, and then connected to peripheral equipment such as the overburden pressure application pump 13, gas source 12, gas booster 11, heating system 16, pressure reducing valve, flow acquisition device, CH4 concentration sensor, inlet and outlet pressure sensors, and differential pressure sensor.

[0127] Before the capacity test experiment begins, the pore volume V of the physical model 1 is calculated using the PVT equilibrium state equation. X Calibration is performed. The calibration principle is as follows Figure 4 As shown in:

[0128] (1) Start the recording control program, open the first valve 4, start the overburden pressure application pump 13, apply a minimum net overburden pressure to the physical model 1, and control the overburden pressure application pump 13 to advance and retreat to ensure that the three-dimensional seepage model is at this stable net overburden pressure.

[0129] (2) Close the second valve 5, open the fourth valve 7, fill the calibration container 10 with gas at a certain pressure, and then close the fourth valve 7; after the pressure stabilizes, record the pressure of the calibration container 10 as P S1 .

[0130] (3) Close the third valve 6 and open the second valve 5 to allow the gas in the calibration system to enter the system to be measured (physical model 1). After the pressure stabilizes, record the calibration system pressure value as P S2 At this time, the pressure value of the calibration system is equal to the pressure value of the system to be measured (ie, physical model 1).

[0131] (4) The volume V of the calibration system is known S , the volume V of the system to be tested can be calculated according to the PVT equilibrium state equation X , the calculation formula is: P S1 ×V S =P S2 ×(V S +V X ).

[0132] The pore volume V of the entire physical model 1 is obtained by calculation S , and by obtaining the total volume produced during the measurement process, the recovery degree EUR can be calculated.

[0133] Example 3

[0134] This embodiment specifically describes the process of conducting capacity test experiments on the physical model 1 under different working modes using a three-dimensional seepage experimental device.

[0135] like Figure 2 As shown in FIG, the three-dimensional seepage model apparatus includes a sealed model chamber 2, an overburden pressure application system, a gas pressurization system, a heating system 16, a simulated mining system, and a data acquisition system. These systems are all controlled by a computer program.

[0136] Among them, the overburden pressure application system includes a movable piston and an overburden pressure application pump 13. The overburden pressure application pump 13 injects hydraulic pressure to provide overburden pressure to simulate formation stress. Applying different overburden pressures can change the tightness between the rock plates and thus obtain different crack widths.

[0137] Model chamber 2 is a square cavity designed to accommodate physical model 1. An air inlet and an air outlet are located on the upper and lower sides of model chamber 2, respectively. These inlet and outlet ports are connected to simulated gas well 17 of physical model 1. The gas pressurization system is connected to the air inlet of model chamber 2, and the simulated extraction system is connected to the air outlet of model chamber 2.

[0138] In addition, the model chamber 2 is provided with a clamp and a metal spacer 3 for fixing the physical model 1. Since the height of the clamp is fixed, if the physical model 1 cannot fill the clamp, the metal spacer 3 is used to replace the core to fill the height space of the clamp.

[0139] The simulated formation stress reached a maximum of 40 MPa and the maximum formation temperature reached 150°C. The experiments were conducted using nitrogen, CH4 gas, and standard brine at 109°C. Each set of experiments maintained a certain net overburden pressure for depletion-type mining.

[0140] The model was then charged with net overburden pressure, according to the predetermined net overburden pressure for each experimental set. Nitrogen was then introduced to the predetermined formation pressure. A program was started to record the overburden pressure, model inlet pressure, outlet pressure, differential pressure, outlet flow rate, and cumulative gas recovery. The experiment was terminated when the model formation pressure dropped to the predetermined pressure. Depletion-based production was performed under conditions where net overburden pressure varied with development rate (flow rate) and net overburden pressure varied with development rate (flow rate), yielding production at varying conductivities.

[0141] (1) Start the recording control program, open the first valve 4, start the overburden pressure applying pump 13, apply a minimum predetermined overburden pressure to the physical model 1, set a minimum predetermined net overburden pressure value in the program and control the overburden pressure applying pump 13 to ensure that the physical model 1 is at a stable net overburden pressure.

[0142] (2) Close the third valve 6, open the second valve 5, and fill the physical model 1 with natural gas to a predetermined formation pressure. During the filling process, the hydraulic pressure of the overburden pressure pump 13 is controlled by the program to maintain a certain net overburden pressure on the physical model 1.

[0143] (3) After the physical model 1 is fully saturated with gas, the program is started to record the overburden pressure, model inlet pressure, outlet pressure, differential pressure, outlet flow rate and cumulative gas recovery in real time, the precision pressure regulating valve 8 is adjusted, the flow controller 9 is set to control the flow rate, the third valve 6 is opened, and depletion-type mining is carried out; until the formation pressure of the physical model 1 drops to the predetermined exhaust gas pressure, the third valve 6 is closed to stop mining.

[0144] (4) Under the same net overburden pressure, the flow controller 9 is set to change the development rate, and steps (1) to (3) are repeated to perform depletion-type production until the formation pressure in the three-dimensional seepage model drops to the predetermined exhaust gas pressure, and the outlet valve is closed to stop production. By changing the development rate to obtain different production pressure differentials, the depletion-type production output under the same net overburden pressure and different production pressure differentials is obtained.

[0145] (5) Changing the net overburden pressure, calibrate the pore volume of the physical model 1 under different net overburden pressures according to step (2); set the flow controller 9 to make the development rate at each group of different net overburden pressures the same, repeat steps (1) to (3) to perform depletion mining until the formation pressure of the physical model 1 drops to a predetermined exhaust gas pressure, then close the third valve 6 to stop mining. Changing the net overburden pressure can change the width of the cracks between the rock slabs, resulting in different crack conductivities, and obtain the depletion mining output at the same development rate but different crack conductivities.

[0146] (6) Complete all depletion-type production under the conditions of fixed development rate and net overburden pressure, and fixed net overburden pressure and development rate. Based on the data collected by the program, the study of different working modes and gas well production under this model can be obtained. Among them, the working mode includes net overburden pressure and development rate.

[0147] (7) Change the length of the simulated branch well horizontal well, the angle between the two branches, and the distribution characteristics of the natural fractures (fracture line density, fracture inclination, opening, etc.), complete steps (1) to (5), and obtain the relationship between different working conditions and gas well production under different reservoir physical parameters, as well as the change law of production under different development speeds. For example Figure 5 The relationship between average flow rate and fracture permeability at a constant net overburden pressure is shown in , and Figure 6 Schematic diagram of the changing pattern of fixed net output shown in .

[0148] Example 4

[0149] This example specifically illustrates the steps of determining the productivity equation of the target gas well. In this embodiment, the target gas wells are two double-branch horizontal wells (well groups).

[0150] (1) Derivation of the equation for the effect of fractures on gas well production

[0151] On the basis of experiments, a theoretical model is established. Figure 7 shown.

[0152] The basic assumptions are as follows:

[0153] ①The gas reservoir is a circular homogeneous gas reservoir that is closed at the top and bottom.

[0154] ② The fluid in the gas reservoir is incompressible and the flow of the fluid in the gas reservoir is stable.

[0155] ③The gas reservoir has a uniform thickness h.

[0156] ④ According to the pressure sweep range of the branch well, the gas reservoir is divided into four seepage areas Z1, Z2, and Z3 according to the single branch. 21 、Z 22 and Z 23 Four seepage zones, including Z1, Z 21 、Z 23 is the pure matrix seepage area, Z 22 It is a zone where high-conductivity fractures develop.

[0157] In order to describe the gas well productivity of fractured gas reservoirs, it is necessary to characterize the radial permeability of the fracture microelement segment, calculate the radial permeability of the entire fracture segment, and substitute it into the productivity model to calculate the gas well productivity.

[0158] In view of the actual situation that the reservoir fracture morphology is irregular, the fractures are divided into several micro-element segments by fully considering the morphology and location of the gas reservoir fractures in the target area, such as Figure 8 As shown, the permeability of the fracture micro-element segment is characterized and the radial permeability of the entire fracture is obtained. The fracture is divided into n micro-element segments, and the endpoints of all micro-element segments are p1, p2, ..., p n , p n+1 ; The radial angles of the starting points of each microelement segment are β S1 , β S2 ,…,β S(n-1) , β Sn ; The radial angles of the end points of each microelement segment are β E1 , β E2 ,…,β E(n-1) , β En .

[0159] The radial permeability of the entire fracture is the sum of the radial permeabilities of each micro-element of the fracture, that is:

[0160]

[0161] Where: k fa is the permeability of the entire fracture, mD; k fi is the permeability of the fracture microelement, mD; β is the radial angle of the end point of the fracture microelement, rad.

[0162] Fractured gas reservoir productivity model:

[0163] Based on the equivalent seepage resistance method, the seepage areas Z1 and Z 21 、Z 22 and Z 23 Seepage resistance R1, R 21 、R 22 and R 23 , (mPa·s) / m 3 .

[0164] Combined with the mutual relationship of the four seepage areas and the seepage path, the total seepage resistance R of the gas reservoir in the target area is calculated (mPa·s) / m 3 .

[0165]

[0166] Where z is the average compression factor, dimensionless; l1 is the seepage area Z 21 The radial length, m; l2 is the seepage area Z 22 Radial length, m; r w is the wellbore radius, m; r e is the seepage radius of the gas reservoir, m; h is the thickness of the gas reservoir, m; k m is the matrix permeability, mD; k fa is the permeability of the entire fracture, mD.

[0167] By introducing the productivity calculation formula of multi-branch horizontal wells, the productivity equation of a double-branch horizontal well (well group) is derived as follows:

[0168]

[0169] Where Q h is the daily gas production of the horizontal well, 104m3 / d; p e is the formation pressure, MPa; p wh is the bottom hole flowing pressure, MPa; h is the effective thickness of the gas reservoir; r eh is the producing radius of the horizontal gas well; r wh is the horizontal wellbore radius; r ev is the vertical well operating radius; r wv is the wellbore radius of the vertical well; S v is the vertical well skin factor, f; S his the skin factor of the horizontal gas well, f; a is the distance from the horizontal well to the bottom of the reservoir; L is the length of the horizontal section of the horizontal well; n is the number of branch wellbores, (n = 2, 3, 4); U = K h / K v , K h is the horizontal permeability, mD, K v is the vertical permeability, mD.

[0170] (2) Based on the experimental results of the relationship between different working conditions and gas well production under different reservoir physical parameters, and the changing law of gas well production under different development speed conditions, the decline law is evaluated, such as Figure 9 As shown in . Combined with the productivity equation of the double-branch well group in fractured tight gas reservoirs, the initial gas production of the actual gas reservoir well group is determined, and the well group EUR is calculated.

[0171] While the present invention has been described with reference to preferred embodiments, various modifications may be made and equivalent components may be substituted without departing from the scope of the present invention. In particular, the various technical features described in the various embodiments may be combined in any manner, provided no structural conflicts exist. The present invention is not limited to the specific embodiments disclosed herein, but encompasses all technical solutions within the scope of the claims.

Claims

1. A method for evaluating the productivity of a gas well in a fractured tight gas reservoir, characterized in that: The following steps are involved: Step 1: Establish a physical model based on the three-dimensional seepage mechanism and the physical properties of the gas reservoir in the target area and the structure of the target gas well; Step 2: Conducting productivity test experiments under different working conditions on the physical model using a three-dimensional seepage experimental device to obtain the production variation patterns of the gas wells under different working conditions; Step 3: Determine the productivity equation of the target gas well; Step 4: Determine the EUR of the target gas well based on the gas well production variation pattern obtained in step 2 and the production capacity equation of the target gas well obtained in step 3; Step 1 includes the following sub-steps: Step 1.1: Select rock slabs based on the porosity and permeability of the gas reservoir in the target area; Step 1.2: Construct a simulated gas well on the rock slab according to the structure of the target gas well; drill one or two circular through holes in the center of the rock slab as wellbores, scale the length of the target gas well to the simulated length, and drill circular through holes on the corresponding rock slab according to the pre-set length of the horizontal well section and the angle between the two horizontal well sections to form a branch horizontal well; Step 1.3: Pressing apart the rock slabs according to the natural fracture distribution characteristics of the gas reservoir in the target area, and splicing and combining multiple rock slabs to form experimental rock samples; Step 1.4: drying the experimental rock sample and then saturating it with bound water to form the physical model; The three-dimensional seepage experimental device comprises: a sealed model room for accommodating the physical model; an overlying pressure applying system, which is used to apply an overlying pressure to the physical model; A gas pressurizing system for filling the model chamber with gas to saturate the physical model and reach a set formation pressure; a heating system for heating the physical model to simulate formation temperature; A simulated production system is used to make the gas in the physical model flow out at a set rate, thereby simulating depletion production of the gas well; and A data acquisition system, which is used to detect and collect overburden pressure data, formation pressure data of the physical model, and production flow rate data during the experiment; Step 2 includes the following sub-steps: Step 2.1: Using the three-dimensional seepage experimental device, the physical model is subjected to depletion-type production under constant net overburden pressure and variable development rate, so as to obtain the production variation pattern of the gas well under depletion-type production under the same fracture conductivity and different development rates; Step 2.2: Using the three-dimensional seepage experimental device, the physical model is subjected to depletion-type production under constant net overburden pressure and variable development rate to obtain the production variation pattern of the gas well under depletion-type production at the same development rate and different fracture conductivities.

2. The method for evaluating the productivity of a gas well in a fractured tight gas reservoir according to claim 1, wherein: In step 2, before conducting the production capacity test experiment on the physical model, the pore volume of the physical model is calibrated according to the PVT equilibrium state equation; Among them, the PVT equilibrium state equation is: ； Where, To calibrate the system pressure, To calibrate the system volume, is the pressure of the system to be measured, is the volume of the system to be tested.

3. The method for evaluating the productivity of a gas well in a fractured tight gas reservoir according to claim 1, wherein: Step 3 includes: Step 3.1: Divide the gas reservoir fractures in the target area into several micro-element segments; Step 3.2: Characterize the radial permeability of each micro-element segment separately; Step 3.3: Calculate the radial permeability of the gas reservoir fractures. Among them, the radial permeability of the gas reservoir fracture is the sum of the radial permeabilities of each micro-element segment; Step 3.4: Derive the productivity equation of the target gas well using the equivalent seepage resistance method.

4. A method for evaluating the productivity of a gas well in a fractured tight gas reservoir according to claim 3, characterized in that: Step 3.4 includes: Step 3.4.1: Develop a theoretical model of the gas reservoir in the target area based on the following assumptions: The gas reservoir is divided into Z1, Z 21 , Z 22 and Z 23 Four seepage zones, including Z1, Z 21 , Z 23 is the pure matrix seepage area, Z 22 It is a high-conductivity fracture development area; Step 3.4.2: Based on the equivalent seepage resistance method, calculate the seepage areas Z1 and Z2 respectively. 21 , Z 22 and Z 23 Seepage resistance R1, R 21 、R 22 and R 23 ; Step 3.4.3: Calculate the total seepage resistance R of the gas reservoir in the target area by combining the relationship between the four seepage zones and the seepage path: Where, is the average compression factor; is the seepage area Z 21 The radial length of is the seepage area Z 22 The radial length of is the wellbore radius; is the seepage radius of the gas reservoir; is the gas reservoir thickness; is the matrix permeability; is the permeability of the entire fracture; Step 3.4.4: Introduce the productivity equation of multi-branch horizontal wells to obtain the productivity equation of the target gas well.

5. A method for evaluating the productivity of a gas well in a fractured tight gas reservoir according to claim 4, characterized in that: The target gas well is a double-branch horizontal well. The productivity equation of the dual-branch horizontal well is: Where, is the daily gas production of the horizontal well; is the formation pressure; is the bottom hole flowing pressure; is the effective thickness of the gas reservoir; is the producing radius of the horizontal gas well; is the horizontal wellbore radius; is the vertical well operating radius; is the wellbore radius of the vertical well; is the skin coefficient of vertical well; is the skin coefficient of horizontal gas well; is the distance from the horizontal well to the bottom of the reservoir; is the length of the horizontal section of the horizontal well; is the number of branch wellbores; , is the horizontal permeability, is the vertical permeability.

6. The method for evaluating gas well productivity in fractured tight gas reservoirs according to claim 1, wherein: The overburden pressure applying system comprises: an overburden pressure applying pump and a piston connected to each other, The overlying pressure applying pump drives the piston to press the upper surface of the physical model by providing hydraulic pressure, thereby applying overlying pressure to the physical model, and a first valve is provided between the overlying pressure applying pump and the piston.

7. A method for evaluating the productivity of a gas well in a fractured tight gas reservoir according to claim 1 or 6, characterized in that: The gas boosting system includes: a connected gas source and a booster, The gas source is connected to the model chamber through the booster to fill the model chamber with gas, and a second valve is provided between the booster and the model chamber.

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