A method for determining the volume of free gas space in a reservoir-type gas storage after multiple rounds of injection and production

Abstract

The present invention discloses a method for determining the volume of free gas space after multiple rounds of injection and production in a reservoir-type gas storage, which specifically includes the following steps: S1, conducting multiple rounds of injection and production experiments under different flooding conditions, and obtaining the water saturation (Sw), oil saturation (So) and gas saturation (Sg) in rock samples after multiple rounds of gas-driven oil and water under different initial flooding conditions; S2, drawing the curves of the changes of Sw and So with injection and production rounds under different flooding conditions according to the experimental data, and then fitting the curves; S3, considering the influence of the number of injection and production and the initial reservoir flooding intensity, based on the multiple rounds of injection and production experimental data under different initial flooding conditions, and combining the volumetric method and the mathematical statistical analysis method to establish a method for calculating the volume of free gas space after multiple rounds of injection and production. This method is more targeted for calculating the volume of free gas space when renovating a gas storage reservoir from a depleted oil reservoir developed by water injection, and is simple to operate and convenient to calculate, and can provide analytical data for the storage capacity calculation of a reservoir-type gas storage.

Description

Technical Field

[0001] The invention relates to the technical field of gas storage development, and in particular to a method for determining the volume of free gas space in an oil reservoir type gas storage after multiple rounds of injection and production. Background Art

[0002] The establishment of gas storage is to ensure the country's energy security and seasonal gas supply to cities with high consumption to ensure people's livelihood. The reconstruction of depleted oil reservoirs into gas storage is an important direction in my country's construction of gas storage. Most domestic oil reservoirs have been developed through water injection. They are gradually exhausted after flooding. There is a large amount of water in the reservoir. After the reconstruction of the gas storage, strong injection and strong production and multiple rounds of injection and production will produce crude oil and water, and have a certain impact on the reservoir properties, resulting in changes in the injection and production capacity and storage capacity of the gas storage. At present, it is generally believed that multiple rounds of injection and production will increase the storage capacity of the gas storage, and it is mainly in the first few cycles of injection and production. Other factors considered mainly include injection and production speed, injection pressure, reservoir properties, reservoir properties, etc. However, for oil reservoirs, the flooding conditions of the upper, middle and lower parts of the reservoir are not the same. Generally, the lower part has a high degree of flooding and the upper part has a low degree of flooding. The middle and upper parts are the main target layers for reservoir construction, while the lower part is the potential development layer for reservoir capacity. The free gas space volume is one of the key parameters for calculating the storage capacity of gas storage. The existing determination method is mainly to determine the saturation of residual oil and bound water by determining the phase permeability curve, and then use the formula (1-S rw -S ro ) is used for calculation. Therefore, accurate acquisition of the phase permeability curve is the prerequisite for accurate calculation of the effective pore volume. However, the phase permeability curve is currently mainly studied through one-dimensional small core testing. The test results often have certain errors and are greatly affected by the representativeness of the rock sample. In addition, the use of the phase permeability curve to determine the residual oil saturation (S ro ) and irreducible water saturation (S rw ) means that the values ​​of each layer in the reservoir are assumed to be consistent. However, due to the influence of reservoir heterogeneity, permeability and other physical parameters, the S values ​​of different layers are different. ro and S rw There are some differences, so they should be discussed separately.

[0003] For oil reservoir-type gas storage, the storage capacity potential of different layers is also different to a certain extent, and targeted analysis is required for layers with different flooding degrees. The present invention avoids the problem that the phase permeability curve can only obtain a single average S ro and S rw In order to solve the problem of free gas space volume in reservoir-type gas storage after multiple rounds of injection and production, a method for calculating the free gas space volume of reservoir-type gas storage after multiple rounds of injection and production was formed based on actual experimental test results and the volumetric method. This method can more quickly and conveniently quantitatively analyze the free gas space volume of reservoir-type gas storage and provide reference data for its storage capacity evaluation. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for determining the free gas space volume of a reservoir-type gas storage after multiple rounds of injection and production. This method avoids the problem that the phase permeability curve can only obtain a single average S ro and S rw The problem of different layers of reservoir-type gas storage has certain differences in storage capacity potential, and the problem of different flooding degrees requiring targeted analysis of layers is solved.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: A method for determining the volume of free gas space in a reservoir-type gas storage after multiple rounds of injection and production, specifically comprising the following steps:

[0006] S1. Carry out multiple rounds of injection and production experiments under different water flooding conditions to obtain the water saturation (S) of rock samples after multiple rounds of gas flooding under different initial water flooding conditions. w ), oil saturation (S o ) and gas saturation (S g );

[0007] S2. Draw S under different flooding conditions based on experimental data w , S o Scatter plot with changes in injection and production rounds, and then fit the curve;

[0008] S3. Use the volumetric method to determine the relationship between the volume of the free gas space and the initial water saturation and the number of injections and productions, and establish a free gas space volume calculation equation. The volumetric method calculation formula is:

[0009] V fg =V b ×S g ×η g

[0010] =V b ×η g ×(1-S w -S o )

[0011] Preferably, the rock samples used in the multiple rounds of injection and production experiments are actual cores from the mine, and are arranged in a core holder in a manner of harmonic average permeability.

[0012] Preferably, the fluids used in the multiple rounds of injection and production experiments include brine, crude oil and nitrogen configured based on the salinity of formation water, wherein the easily accessible and relatively stable nitrogen is used to replace methane (natural gas).

[0013] Preferably, the different initial flooding conditions are simulated with different initial water saturations to respectively establish weak flooding, medium-strong flooding and strong flooding conditions.

[0014] Beneficial Effects

[0015] The present invention provides a method for determining the volume of free gas space after multiple rounds of injection and production in an oil reservoir-type gas storage. Compared with the prior art, it has the following beneficial effects: the method for determining the volume of free gas space after multiple rounds of injection and production in an oil reservoir-type gas storage takes into account the influence of the number of injection and production times and the initial reservoir flooding intensity, and is based on multiple rounds of injection and production experimental data under different initial flooding conditions, and is combined with a volumetric method and a mathematical statistical analysis method to establish a method for calculating the volume of free gas space after multiple rounds of injection and production. The method is more targeted at calculating the volume of free gas space in depleted oil reservoir-type gas storage. By conducting multiple rounds of injection and production experiments, then correcting the coefficients in the free gas space volume expression, and finally conducting rapid quantitative analysis on development layers with different water saturations, certain analytical data can be provided for storage capacity evaluation in the construction of oil reservoir-type gas storage. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a scatter diagram showing the change of oil saturation with injection and production rounds under different water flooding conditions of the present invention;

[0017] Figure 2 It is a scatter diagram showing the change of water saturation with injection and production rounds under different water flooding conditions of the present invention;

[0018] Figure 3 It is a fitting diagram of the relationship between the coefficient of the present invention and the initial water saturation;

[0019] Figure 4 It is a graph showing the volume percentage of free gas space after multiple rounds of injection and production at different initial water saturations of the present invention;

[0020] Figure 5 The present invention is a flowchart for implementing the method. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] The present invention provides a technical solution: a method for determining the volume of free gas space in an oil reservoir type gas storage after multiple rounds of injection and production, which specifically comprises the following steps:

[0023] S1. Carry out multiple rounds of injection and production experiments under different water flooding conditions to obtain the water saturation (S) of rock samples after multiple rounds of gas flooding under different initial water flooding conditions. w ), oil saturation (S o ) and gas saturation (S g ); specifically comprising the following steps:

[0024] ① Taking the actual mine core as the target, prepare several experimental rock samples, dry the rock samples and put them into a 50×2.5cm rock sample holder according to the permeability harmonic average method;

[0025] ② Use displacement methods to establish different initial water saturation and oil saturation in the rock sample, and the rock sample is completely saturated with crude oil and water;

[0026] ③ Simulate the gas injection process of the gas storage reservoir, inject nitrogen into the rock sample holder, and increase the pressure to the upper limit pressure of the target gas storage reservoir;

[0027] ④Simulate the gas production process of the gas storage, set a certain production pressure difference to carry out the gas production process, and record the final gas production, water production and oil production;

[0028] ⑤Simulate multiple rounds of injection and production, repeat ③ and ④ processes 6 times, and then calculate S after each injection and production w , S o and S g ;

[0029] S2. Draw S under different flooding conditions based on experimental data w , S o The scatter plot of the changes with injection and production rounds, and then the curve fitting, including the following steps:

[0030] ① Use experimental data to draw the S w , S o Scatter plot of changes with injection and production rounds;

[0031] ②Use logarithmic form to fit S under different flooding conditions w , S o The relationship between the injection and production times is as follows: the coefficient of the non-constant term in the fitting formula is set to A, and the constant term is set to B, as shown in formula (1), where l represents oil (o) or water (w):

[0032] S l =Aln(x)+B (1)

[0033] S3. Use the volumetric method to determine the relationship between the volume of the free gas space and the initial water saturation and the number of injections and productions, and establish a free gas space volume calculation equation. The volumetric method calculation formula is:

[0034]

[0035] Combining formulas (1) and (2), the volume of the free air space is:

[0036]

[0037] Among them, A′ and B′ are determined by formula (4). Under the same initial flooding conditions, the coefficients A and B in the fitting formula between water saturation, oil saturation and injection and production times are mathematically added to obtain new coefficients A′ and B′. Then, the relationship between the new coefficients A′ and B′ and water saturation is established in polynomial form, as shown in formula (5):

[0038] S g =1-S w -S o (4)

[0039] A′=m1S w 2 +m1′S w +m1″ (5)

[0040] B′=m2S w 2 +m2′S w +m2″

[0041] Finally, combining formulas (3) and (5), the expression for the volume of the free air space is (6):

[0042]

[0043] The parameters in the formula are: l—represents oil (o) or water (w); A—is the coefficient of the non-constant term; B—is the constant term; x—is the number of injections and productions; V fg — is the volume of free air space, m 3 ; V b — is the total pore space volume of the reservoir, m 3 ; η g — is the total swept volume coefficient of the gas, f; S g , S o , S w — are gas saturation and oil-water saturation, respectively, %; m1, m2— are the constant term and constant term coefficients in the relationship between coefficients A′ and B′ and water saturation; N is the geological reserves of crude oil, 10 4 t; B oi is the original volume coefficient of crude oil, f; ρ o is the density of ground degassed crude oil, g / cm 3 ; S oi is the original oil saturation, f.

[0044] In the present invention, the rock samples used in the multiple rounds of injection and production experiments are actual cores from the mine, and are arranged in a core holder in a manner of harmonized average permeability.

[0045] In the present invention, the fluids used in the multiple rounds of injection and production experiments include brine, crude oil and nitrogen configured based on the salinity of formation water, wherein the easily accessible and relatively stable nitrogen is used to replace methane (natural gas).

[0046] In the present invention, different initial flooding conditions are simulated with different initial water saturations, and weak flooding, medium-strong flooding and strong flooding conditions are established respectively.

[0047] Meanwhile, the contents not described in detail in this specification belong to the prior art known to those skilled in the art.

[0048] Example Analysis

[0049] For the X reservoir type gas storage, according to the production capacity requirements and the numerical simulation of the typical well group, the upper limit pressure of the gas storage operation is determined to be 35MPa, the lower limit pressure is 20MPa, the lowest bottom hole flow pressure is 16MPa, the production pressure difference is 3-4MPa, the middle temperature of the oil layer is 161.3℃, and the average water saturation is 48%. The experimental fluid includes filtered formation water, crude oil and nitrogen. The viscosity of crude oil is 10.66mPa﹒s and the density is 0.83g / cm 3 The mineralization of the formation water is 5600 mg / L and the density is 1002.49 kg / cm 3 , viscosity is 0.94mPa·s, and viscosity of nitrogen is 0.018mPa·s.

[0050] (1) The scheme of multiple rounds of injection-production experiments is shown in Table 1:

[0051] Table 1 Multi-round injection and production experimental scheme

[0052]

[0053] Taking the actual mine core as the target, several experimental rock samples were prepared. After the rock samples were dried, they were placed in a rock sample holder with a length × width of 50 × 2.5 cm according to the permeability harmonic average method. The rock sample parameters are shown in Table 2.

[0054] Table 2 Core parameters and their order in the long core holder

[0055]

[0056] Then, the multifunctional core displacement equipment was used to carry out multiple rounds of injection and production experiments according to the experimental requirements. The experimental results are shown in Table 3:

[0057] Table 3 Data of multiple injection-production experiments under different flooding conditions

[0058]

[0059] (2) Plot S under different flooding conditions based on experimental data w , So Scatter plot of the changes with injection and production rounds, and then fit the curve ( Figure 1 and Figure 2 ), the parameters A in the oil saturation fitting formula under low water flooding, medium-strong water flooding and strong water flooding conditions were -0.0507, -0.363 and -0.0030, and the parameters B were 0.2764, 0.2337 and 0.1411, respectively; the parameters A in the water saturation fitting formula were -0.1027, -0.0204 and -0.0331, and the parameters B were 0.5313, 0.3068 and 0.2690, respectively.

[0060] (3) The relationship between the free gas space volume and the initial water saturation and the injection and production times is determined by the volumetric method, and the free space volume calculation equation is established. First, the parameters A' and B' are calculated using the parameters A and B fitted in step (2). A' is -0.0838, -0.0567 and -0.1057, respectively, and B' is 0.5454, 0.5405 and 0.6724, respectively. Then the relationship between A' and B' and the initial water saturation is established. The fitting diagram is shown in the figure below. Figure 3 .

[0061] The expression of the free air space volume is formula (7):

[0062]

[0063] The percentage of free gas space volume to total storage space after multiple rounds of injection and production at different initial water saturations in X reservoir-type gas storage is as follows: Figure 4 shown.

[0064] The average initial water saturation of the reservoir is 48%. Then, using formula (7) and the relevant parameters in Table 4, it is calculated that the free gas space volume of the reservoir after 6 injection and production cycles accounts for 57.5%, and the underground reservoir space is 869.14×10 4 m 3 , and the underground reservoir construction space obtained by numerical simulation is 854.69×10 4 m 3 The error between the two is 1.69%, which is relatively small. Therefore, it is believed that the calculation results have a certain degree of credibility, and the final underground storage space is taken as 861.915×10 4 m 3 The upper limit pressure and reservoir temperature parameters are used to calculate the natural gas volume coefficient under the upper limit pressure to be 0.00453, and the ground free gas reservoir capacity is calculated to be 19.03×10 8 m 3 .

[0065] Table 4 Parameters of gas storage space calculated by volume method in Block X

[0066]

[0067] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0068] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the volume of free gas space in a reservoir-type gas storage after multiple rounds of injection and production, characterized in that: The specific steps include: S1. Carry out multiple rounds of injection and production experiments under different water flooding conditions to obtain the water saturation S of the rock samples after multiple rounds of gas flooding under different initial water flooding conditions. w , oil saturation S o and gas saturation S g ; S2. Draw S under different flooding conditions based on experimental data w , S o The scatter plot of the changes with injection and production rounds, and then the curve fitting, including the following steps: ① Use experimental data to draw the S w , S o Scatter plot of changes with injection and production rounds; ②Use logarithmic form to fit S under different flooding conditions w , S o The relationship between the injection and production times is as follows: the coefficient of the non-constant term in the fitting formula is set to A, and the constant term is set to B, as shown in formula (1), where l represents oil o or water w: S l =Aln(x)+B (1); S3. Use the volumetric method to determine the relationship between the volume of the free gas space and the initial water saturation and the number of injections and productions, and establish a free gas space volume calculation equation. The volumetric method calculation formula is: Combining formulas (1) and (2), the volume of the free air space is: Among them, A′ and B′ are determined by formula (4). Under the same initial flooding conditions, the coefficients A and B in the fitting formula between water saturation, oil saturation and injection and production times are mathematically added to obtain new coefficients A′ and B′. Then, the relationship between the new coefficients A′ and B′ and water saturation is established in polynomial form, as shown in formula (5): S g =1-S w -S o (4) Finally, combining formulas (3) and (5), the expression for the volume of the free air space is (6): The parameters in the formula are: l—represents oil o or water w; A—is the coefficient of the non-constant term; B—is the constant term; x—is the number of injections and productions; V fg — is the volume of free air space; V b — is the total pore space volume of the reservoir; η g — is the total swept volume coefficient of the gas; S g , S o , S w — are gas saturation and oil-water saturation respectively; m1, m2— are the non-constant term and constant term coefficients in the relationship between coefficients A′ and B′ and water saturation; N is the geological reserves of crude oil; B oi is the original volume coefficient of crude oil; ρ o S is the density of ground degassed crude oil; oi is the original oil saturation.

2. The method for determining the volume of free gas space in a reservoir-type gas storage after multiple rounds of injection and production according to claim 1, characterized in that: The rock samples used in the multiple rounds of injection and production experiments are actual cores from the mine, and are arranged in a long core holder in a manner of harmonic average permeability.

3. The method for determining the volume of free gas space in a reservoir-type gas storage after multiple rounds of injection and production according to claim 1, characterized in that: The fluids used in the multiple rounds of injection and production experiments include brine, crude oil and nitrogen configured based on the salinity of formation water, among which the easily available and relatively stable nitrogen is used to replace methane.

4. The method for determining the volume of free gas space in a reservoir-type gas storage after multiple rounds of injection and production according to claim 1, characterized in that: The different initial flooding conditions are simulated with different initial water saturations to respectively establish weak flooding, medium-strong flooding and strong flooding conditions.

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