Method for determining single well production radius of low-permeability gas reservoir

Abstract

The invention relates to a low-permeability gas reservoir single well production radius determination method. The method comprises the following steps: acquiring gas reservoir basic information data and gas well core data; a gas flow rate and a pressure gradient are obtained through a low-permeability gas well core starting pressure experiment test; taking the pressure gradient as a transverse axis and the gas flow rate as a longitudinal axis, making a scatter relation graph of the pressure gradient and the gas flow rate, performing power function fitting on scatter points, making a tangent line of a point corresponding to the minimum flow rate in the scatter relation graph, enabling the tangent line to intersect with the transverse axis, and setting an intersection point value of the tangent line and the transverse axis as a real starting pressure gradient value of the rock sample; and establishing a low-permeability gas reservoir single well use radius calculation formula based on the real starting pressure gradient value. And the real starting pressure gradient value is used for calculating the single-well production radius of the low-permeability gas reservoir, so that the method has better accuracy compared with the method for calculating the quasi-starting pressure gradient commonly used in the industry.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas development, and more specifically, to a method for determining the single-well active radius of a low-permeability gas reservoir. Background Technique

[0002] With the rapid economic development of our country and the improvement of people's living standards, in order to promote sustainable development and improve the living environment at the same time, vigorously developing and utilizing natural gas, a clean and high-calorific value energy source, has become the focus of resource exploration and development in our country. In recent years, the conventional natural gas resources have been unable to meet the domestic market demand, and unconventional natural gas has gradually become the focus of exploration and development. Among them, low-permeability gas reservoirs, as one of the largest-scale unconventional natural gases currently developed, have become an important area for increasing reserves and production of natural gas in our country. Therefore, how to reasonably develop such gas reservoirs is particularly important.

[0003] During the pre-project research of gas reservoir development, the size of the single-well active radius will directly affect the design of development well locations, and thus affect the early investment and development effect of the gas reservoir. Low-permeability gas reservoirs have the characteristics of poor physical properties and small active radius. Therefore, accurately predicting the active radius of low-permeability gas wells can help determine reasonable well patterns and well spacings to meet the development requirements of low-permeability reserves with the minimum investment. At present, the methods for determining the single-well active radius of low-permeability gas reservoirs are mainly divided into empirical methods, dynamic reserves and other methods. The empirical method mainly analyzes the well patterns and well spacings of similar gas reservoirs and the pressure changes in the gas reservoir during the development process based on the development understanding of gas reservoirs with similar physical properties, and estimates the single-well active radius of low-permeability gas reservoirs. This method has a large uncertainty and is greatly affected by human factors; the dynamic reserves method includes methods such as material balance method, production decline method, and unstable well testing. The dynamic reserves method mainly analyzes the dynamic reserves based on the production laws of developed wells and the changes in flowing pressure or static pressure of gas wells, and combines geological understanding to delineate the active radius of gas wells. This method is more suitable for development adjustment and boundary exploration of produced blocks and has little guidance for undeveloped new areas.

[0004] To sum up, for the determination of the single-well active radius of undeveloped low-permeability gas reservoirs, both of the above two methods have certain limitations and cannot propose a good single-well calculation method according to the characteristics of low-permeability gas reservoirs in this area. Summary of the Invention

[0005] In order to overcome the problem in the above-mentioned prior art that the single-well active radius cannot be determined according to the characteristics of low-permeability gas reservoirs in this area, the present invention provides a method for determining the single-well active radius of a low-permeability gas reservoir, which uses the obtained true starting pressure gradient to replace the commonly used pseudo-starting pressure gradient in the industry, and combines the boundary pressure gradient theory to determine the single-well active radius of a low-permeability gas reservoir.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a method for determining the single-well active radius of a low-permeability gas reservoir, comprising the following steps:

[0007] Step 1: Obtain the basic information data of the gas reservoir and the core data of the gas well;

[0008] Step 2: Test the gas flow rate and pressure gradient through the starting pressure experiment of the core of the low-permeability gas well;

[0009] Step 3: Take the pressure gradient as the horizontal axis and the gas flow rate as the vertical axis, make a scatter plot of the pressure gradient and the gas flow rate, perform power function fitting on the scatter points, and then make a tangent line at the point corresponding to the minimum flow rate in the scatter plot so that it intersects the horizontal axis, and the intersection value with the horizontal axis is defined as the true starting pressure gradient value of the rock sample;

[0010] Step 4: Establish a calculation formula for the single-well active radius of the low-permeability gas reservoir based on the true starting pressure gradient value.

[0011] In the above technical solution, by processing the relevant data of the gas flow rate and pressure gradient obtained through the starting pressure experiment of the core of the low-permeability gas well in Step 3, the true starting pressure gradient value of the rock sample can be obtained. In Step 4, calculating the single-well active radius of the low-permeability gas reservoir with the true starting pressure gradient value and the data obtained in Step 1 can have better accuracy compared with the calculation using the pseudo starting pressure gradient commonly used in the industry.

[0012] Preferably, in Step 4, based on the edge pressure gradient being greater than or equal to its starting pressure gradient, the calculation formula for the single-well active radius of the low-permeability gas reservoir is derived as follows:

[0013]

[0014] In the formula, ψ e is the original formation pseudo-pressure, with the unit of MPa 2 / cp; ψ w is the flowing bottom-hole pseudo-pressure, with the unit of MPa 2 / cp; R e is the active radius, with the unit of m; R w is the wellbore radius, with the unit of m; p e is the original formation pressure, with the unit of MPa; μ is the natural gas viscosity, with the unit of mPa·s; Z is the natural gas deviation factor, dimensionless; λ is the true starting pressure gradient, with the unit of MPa / m.

[0015] The derivation process is as follows:

[0016] When a perfect well at the center of a homogeneous circular and isopachous formation produces at a constant production rate, the differential equation of gas steady-state seepage can be expressed as

[0017]

[0018] This equation is a second-order ordinary differential equation, and its general solution is:

[0019] ψ = C1 + lnr + C2

[0020] Substitute into the inner boundary condition (at the wellbore) and the outer boundary condition:

[0021]

[0022] The specific values of C1 and C2 can be obtained:

[0023]

[0024] Furthermore, the expression of the formation pressure (pseudo - pressure) ψ is obtained as

[0025]

[0026] Therefore, the pressure gradient at any point in the formation is

[0027]

[0028] Substitute the pressure ψ = ψ e , and the radius r = R e , then the pressure gradient at the supply edge can be obtained. In the development of an actual gas reservoir, as the bottom - hole flowing pressure decreases, its supply edge continuously expands. When the bottom - hole pressure drops to a certain level and cannot meet the well - head gas - transmission pressure, it is the minimum bottom - hole flowing pressure. At this time, the maximum supply radius and the corresponding edge pressure gradient can be obtained. According to the calculation law of the starting pressure of the gas, the edge pressure gradient must be greater than or equal to its starting pressure gradient, that is:

[0029]

[0030] Preferably, in step one, data of the pressure under the original gas - reservoir conditions, the temperature under the original gas - reservoir conditions, and the overburden pressure of the formation under the original gas - reservoir conditions are obtained by logging at the production site of the oil and gas field.

[0031] Preferably, in step one, data of the porosity, permeability, irreducible water saturation, water type of the formation water in the gas reservoir, the concentration of each ion, and the total salinity are obtained through experiments on the cores of the exploration wells.

[0032] Preferably, in step two, the temperature, pressure, and overburden pressure of the experiment are consistent with the formation conditions; the type, saturation, and salinity of the irreducible water are consistent with the actual situation of the formation water.

[0033] Preferably, in step two, the specific steps of the starting - pressure experiment test for the cores of low - permeability gas wells are as follows:

[0034] S2.1: Number the representative cores of the production intervals of the low - permeability gas wells;

[0035] S2.2: Select one of the cored samples with a number for drying;

[0036] S2.3: Vacuum saturate the cored sample to simulate formation water;

[0037] S2.4: Place the cored sample into the holder of the breakthrough pressure tester;

[0038] S2.5: Raise the temperature and pressure to the preset working temperature and working pressure;

[0039] S2.6: Establish irreducible water;

[0040] S2.7: Start the breakthrough pressure tester, and measure the gas flow rate and permeability at different pressure differences under constant pressure gas drive according to the decreasing pressure difference. During this period, record the inlet pressure, outlet pressure, time, and gas production;

[0041] S2.8: Calculate the gas flow rate at the corresponding pressure difference through the corresponding time and gas production at different pressure differences, and then calculate the pressure gradient between different pressure differences and the rock sample, and record all of them;

[0042] S2.9: Reduce the temperature and pressure, remove the cored sample in the holder and replace it with another cored sample with a number, and repeat steps S2.2 - S2.8 until all cored samples with numbers have completed the experiment.

[0043] Preferably, in step S2.7, the inlet pressure and outlet pressure are measured by a high - pressure differential pressure sensor. The working pressure of the high - pressure differential pressure sensor is: 0 - 69 MPa, the working pressure difference is: 0 - 1.5 MPa, and the accuracy is: ±0.0002 MPa.

[0044] Preferably, in step S2.7, an automatic gas meter is used for gas metering. The metering accuracy is ±0.01 ml, and the working flow rate is: 0 - 1500 ml / min.

[0045] Preferably, the rated working pressure of the breakthrough pressure tester is: 0 - 137 MPa, and the rated working temperature is: room temperature - 240 °C.

[0046] Compared with the prior art, the beneficial effect of the present invention is that by processing the relevant data of gas flow rate and pressure gradient obtained from the startup pressure experiment test of the cored sample of a low - permeability gas well, the true startup pressure gradient value of the rock sample can be obtained and used for calculating the single - well active radius of a low - permeability gas reservoir, which has better accuracy compared with the pseudo - startup pressure gradient commonly used in the industry for calculation. Description of the Drawings

[0047] Figure 1 is a flow chart of a method for determining the single - well active radius of a low - permeability gas reservoir of the present invention;

[0048] Figure 2 It is a graph showing the relationship between flow rate and pressure gradient;

[0049] Figure 3 It is a graph showing the relationship between pressure gradient and gas flow rate taking a certain offshore low-permeability gas field as an example. Specific implementation manners

[0050] The accompanying drawings are only for illustrative purposes and should not be construed as limiting the present patent; for better illustration of this embodiment, some components in the accompanying drawings will be omitted, enlarged or reduced, which does not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the accompanying drawings may be omitted. The positional relationships described in the accompanying drawings are only for illustrative purposes and should not be construed as limiting the present patent.

[0051] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "long", "short", etc. indicating the orientation or positional relationship, they are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the accompanying drawings are only for illustrative purposes and should not be construed as limiting the present patent. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0052] The technical solutions of the present invention will be further specifically described below through specific embodiments and in conjunction with the accompanying drawings:

[0053] Embodiment 1

[0054] As Figure 1 shown, an embodiment of a method for determining the single-well development radius of a low-permeability gas reservoir includes the following steps:

[0055] Step 1: Obtain the basic information data of the gas reservoir and the core data of the gas well;

[0056] Step 2: Test the gas flow rate and pressure gradient through the core startup pressure experiment of the low-permeability gas well;

[0057] Step 3: Take the pressure gradient as the horizontal axis and the gas flow rate as the vertical axis, make a scatter plot of the pressure gradient and the gas flow rate, perform a power function fitting on the scatter points, and then make a tangent line through the point corresponding to the minimum flow rate in the scatter plot, so that it intersects with the horizontal axis, and the intersection value with the horizontal axis is defined as the true startup pressure gradient value of the rock sample;

[0058] Step 4: Based on the true starting pressure gradient value and the edge pressure gradient must be greater than or equal to its starting pressure gradient, establish the calculation formula for the single-well production radius of low-permeability gas reservoirs.

[0059] The essence of the starting pressure gradient is the starting pressure gradient required for the fluid to start flowing in throats of different sizes, which is controlled by the size of the effective seepage throats. However, there is heterogeneity in the throat sizes in the reservoir, so there is a low-velocity non-Darcy seepage phenomenon in low-permeability media, that is, as Figure 2 shown, the flow rate and the pressure gradient do not show a linear relationship. The curve segment shape of non-Darcy flow reflects the characteristics of pore structure and solid-liquid interaction. Limited by the measurement accuracy of the instrument, Figure 2 point A (also known as the true starting pressure gradient) in Figure 2 cannot be directly measured. In the industry, point B (also known as the pseudo starting pressure gradient) in

[0060] is commonly used to replace point A as the experimental measurement result for calculation. Because there is a difference between the pseudo starting pressure gradient and the true starting pressure gradient, the accuracy of calculating the single-well production radius of low-permeability gas reservoirs using the pseudo starting pressure gradient is insufficient.

[0061] In this embodiment, in step 3, by processing the relevant data of gas flow rate and pressure gradient obtained from the starting pressure experiment test of the low-permeability gas well core, the true starting pressure gradient value of the core sample can be obtained, which can better fit the actual situation. In step 4, using the true starting pressure gradient value and the data obtained in step 1 to calculate the single-well production radius of low-permeability gas reservoirs can have better accuracy compared with the commonly used pseudo starting pressure gradient in the industry.

[0062] Taking a certain offshore low-permeability gas field as an example, representative experimental core samples are selected. The physical properties of the core samples are shown in Table 1, and the experimental conditions are shown in Table 2. According to the experimental design of the present invention, the gas flow rate of each experimental core sample under different pressure differences can be obtained, and then the relationship diagram between the pressure gradient and the gas flow rate can be made (see Figure 3 ), and the true starting pressure gradient of the core sample can be obtained (see Table 3).

[0063] Table 1 Physical property parameters of experimental core samples

[0064]

[0065]

[0066] Table 2 Experimental conditions

[0067]

[0068] Table 3 Experimental Results

[0069]

[0070] According to the theoretical calculation formula for the single - well production range of low - permeability gas wells, combined with the experimental results of the starting pressure, the drainage radius under various physical property / starting pressure gradient conditions can be obtained. The calculation results and parameters are shown in Table 4.

[0071] Table 4 Calculation Results Table of the Drainage Radius of the Gas Field

[0072]

[0073] Example 2

[0074] Example 2 of a method for determining the single - well drainage radius of a low - permeability gas reservoir. Based on Example 1, the difference from Example 1 is that

[0075] In step one, data on the pressure under the original gas reservoir conditions, the temperature under the original gas reservoir conditions, and the overburden pressure of the formation under the original gas reservoir conditions are obtained by logging the production site of the oil and gas field; data on the porosity, permeability, irreducible water saturation, water type of the formation water in the gas reservoir, the concentration of each ion, and the total salinity are obtained through experiments on the core of the exploration well.

[0076] In step four, the calculation formula for the single - well drainage radius of the low - permeability gas reservoir is specifically as follows:

[0077]

[0078] In the formula, ψ e is the original pseudo - pressure of the formation, with the unit of MPa 2 / cp; ψ w is the flowing bottom - hole pseudo - pressure, with the unit of MPa 2 / cp; R e is the drainage radius, with the unit of m; R w is the wellbore radius, with the unit of m; p e is the original formation pressure, with the unit of MPa; μ is the viscosity of natural gas, with the unit of mPa·s; Z is the gas deviation factor, dimensionless; λ is the true starting pressure gradient, with the unit of MPa / m.

[0079] The derivation process is as follows:

[0080] When a perfect well at the center of a homogeneous circular and isopachous formation produces at a constant rate, the differential equation of gas steady - state seepage can be expressed

[0081]

[0082] This equation is a second-order ordinary differential equation, and its general solution is:

[0083] ψ = C1 + lnr + C2

[0084] Substitute the inner boundary condition (at the wellbore) and the outer boundary condition:

[0085]

[0086] The specific values of C1 and C2 can be obtained:

[0087]

[0088] Furthermore, the expression of the formation pressure (pseudo-pressure) ψ is obtained as

[0089]

[0090] Therefore, the pressure gradient at any point in the formation is

[0091]

[0092] Substitute the pressure ψ = ψ e , and the radius r = R e , and the pressure gradient at the supply edge can be obtained. In the development of an actual gas reservoir, as the bottom-hole flowing pressure decreases, its supply edge continuously expands. When the bottom-hole pressure drops to a certain level and cannot meet the wellhead gas transmission pressure, it is the minimum bottom-hole flowing pressure. At this time, the maximum supply radius and the corresponding edge pressure gradient can be obtained. According to the calculation law of the starting pressure of the gas, the edge pressure gradient must be greater than or equal to its starting pressure gradient, that is:

[0093]

[0094] Example 3

[0095] Example 3 of a method for determining the single-well production radius of a low-permeability gas reservoir, based on Example 1 or Example 2, is different in that in step two, the temperature, pressure, and overburden pressure of the experiment are consistent with the formation conditions; the type, saturation, and salinity of the irreducible water are consistent with the actual situation of the formation water.

[0096] Preferably, in step two, the steps of the starting pressure experiment test of the core of the low-permeability gas well are specifically as follows:

[0097] S2.1: Number the representative cores of the production horizon of the low-permeability gas well;

[0098] S2.2: Select one of the numbered cores for drying;

[0099] S2.3: Vacuum saturate the core and simulate the formation water;

[0100] S2.4: Place the core into the holder of the breakthrough pressure instrument;

[0101] S2.5: Raise the temperature and pressure to the preset working temperature and working pressure;

[0102] S2.6: Establish irreducible water;

[0103] S2.7: Start the breakthrough pressure instrument, and measure the gas flow rate and permeability at different pressure differences under constant-pressure gas drive according to the decreasing pressure difference. During this period, record the inlet pressure, outlet pressure, time, and gas production volume;

[0104] S2.8: Calculate the gas flow rate at the corresponding pressure difference through the corresponding time and gas production volume at different pressure differences, and then calculate the pressure gradient of different pressure differences and the rock sample, and record all of them;

[0105] S2.9: Lower the temperature and pressure, remove the core in the holder, and replace it with a core of another number. Repeat steps S2.2 - S2.8 until all numbered cores have completed the experiment.

[0106] Preferably, in step S2.7, the inlet pressure and outlet pressure are measured by a high-pressure differential pressure sensor. The working pressure of the high-pressure differential pressure sensor is: 0 - 69 MPa, the working differential pressure is: 0 - 1.5 MPa, and the accuracy is: ±0.0002 MPa.

[0107] Preferably, in step S2.7, an automatic gas meter is used for gas metering. The metering accuracy is ±0.01 ml, and the working flow rate is: 0 - 1500 ml / min.

[0108] Preferably, the rated working pressure of the breakthrough pressure instrument is: 0 - 137 MPa, and the rated working temperature is: room temperature - 240 °C; in step S2.5, the corresponding working temperature and working pressure are provided by placing the breakthrough pressure instrument into an intermediate container.

[0109] The remaining working principles and features of this embodiment are the same as those of Embodiment 1 or Embodiment 2.

[0110] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limiting the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims

1. A method for determining the producing radius of a single well in a low permeability gas reservoir, characterized in that: The steps include: Step 1: Obtain basic information data of gas reservoirs and gas well core data; Step 2: obtain the gas flow rate and pressure gradient through the low permeability gas well core start-up pressure test; Step 3: With the pressure gradient as the horizontal axis and the gas flow rate as the vertical axis, a scatter plot of the pressure gradient and the gas flow rate is made, and a power function is fitted to the scatter points. Then, a tangent line is made through the point corresponding to the minimum flow rate in the scatter plot, and the tangent line is made to intersect with the horizontal axis. The intersection value with the horizontal axis is defined as the true starting pressure gradient value of the rock sample. Step 4: Establish a calculation formula for the single well producing radius of a low permeability gas reservoir based on the true starting pressure gradient value.

2. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 1, characterized in that: In step 4, based on the fact that the edge pressure gradient must be greater than or equal to its starting pressure gradient, the calculation formula for the producing radius of a single well in a low permeability gas reservoir is derived.

3. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 2, characterized in that: In step 4, the calculation formula for the producing radius of a single well in a low permeability gas reservoir is as follows: In the formula, ψ e is the original pseudo-pressure of the formation, in MPa 2 / cp;ψ w is the bottom hole flow pseudo pressure, in MPa 2 / cp;R e is the radius of action, in m; R w is the wellbore radius, in m; p e is the original formation pressure, in MPa; μ is the viscosity of natural gas, in mPa·s; Z is the natural gas deviation factor, dimensionless; λ is the true starting pressure gradient, in MPa / m.

4. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 1, characterized in that: In step one, data on pressure under original gas reservoir conditions, temperature under original gas reservoir conditions, and overburden pressure of formations under original gas reservoir conditions are obtained by logging wells at the oil and gas field production site.

5. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 4, characterized in that: In step one, data on the porosity, permeability, irreducible water saturation, water type of formation water in the gas reservoir, concentration of various ions and total mineralization of the exploration well core are obtained through experiments.

6. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 1, characterized in that: In step 2, the experimental temperature, pressure and overburden pressure are consistent with the formation conditions; the type, saturation and mineralization of bound water are consistent with the actual conditions of formation water.

7. A method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 6, characterized in that: In step 2, the steps of the low permeability gas well core start-up pressure test are as follows: S2.1: Number the representative cores of the production horizons of low permeability gas wells; S2.2: Select a numbered core and dry it; S2.3: Vacuum and saturate the core to simulate formation water; S2.4: Place the core into the holder of the breakthrough press; S2.5: Raise the temperature and pressure to the preset working temperature and working pressure; S2.6: Establish bound water; S2.7: Start the breakthrough pressure instrument, and measure the gas flow rate and permeability under different pressure differences by constant pressure gas drive according to the pressure difference change from large to small, and record the inlet pressure, outlet pressure, time and gas production during the process; S2.8: Calculate the gas flow rate under the corresponding pressure difference by the corresponding time and gas production under different pressure differences, and then calculate the pressure gradient of the different pressure differences and the rock sample, and record them all; S2.9: Reduce the temperature and pressure, remove the core in the holder and replace it with another numbered core, and repeat steps S2.2-S2.8 until all numbered cores have completed the experiment.

8. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 7, characterized in that: In step S2.7, the inlet pressure and the outlet pressure are measured by a high-pressure differential pressure sensor. The working pressure of the high-pressure differential pressure sensor is 0 to 69 MPa, the working pressure difference is 0 to 1.5 MPa, and the accuracy is ±0.0002 MPa.

9. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 7, characterized in that: In step S2.7, an automatic gas meter is used to measure the gas, with a measurement accuracy of ±0.01 ml and a working flow rate of 0 to 1500 ml / min.

10. The method for determining the producing radius of a single well in a low permeability gas reservoir according to claim 7, characterized in that: The rated working pressure of the breakthrough pressure gauge is 0 to 137 MPa, and the rated working temperature is room temperature to 240° C. In step S2.5, the corresponding working temperature and working pressure are provided by placing the breakthrough pressure gauge in an intermediate container.

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