A method for predicting water saturation of gas storage considering evaporation and salt precipitation of formation water

Abstract

The invention relates to the technical field of dynamic analysis of underground gas storage, and in particular to a method for predicting water saturation of a gas storage considering evaporation and salting out of formation water: by establishing a saturated water vapor content map in natural gas and calculating the drying and salting out range of a storage space around a well during the gas storage stage, calculating the average formation pressure of the gas storage at different stages through a gas production and peak regulation scheme of the gas storage, and calculating the formation pressure at different radii of the gas storage through the daily production of gas wells of the gas storage, calculating the water saturation at different radii of the gas storage through the formation pressure at different radii of the gas storage, repeating the above steps to calculate the water saturation of the gas storage at different operation cycles, adopting the above method to quantitatively study the water saturation at different reservoir positions after evaporation and salting out of formation water in the gas storage, so as to correctly understand the distribution characteristics and change laws of water saturation of the gas storage at different stages, and provide a basis for carrying out gas storage capacity accounting and gas well injection and production operation parameter evaluation.

Description

Technical Field

[0001] The invention relates to the technical field of dynamic analysis of underground gas storage reservoirs, and in particular to a method for predicting water saturation of gas storage reservoirs taking into account evaporation and salting out of formation water. Background Art

[0002] In recent years, in order to cope with the rapid growth of natural gas consumption demand, solve the contradiction between regional seasonal supply and demand, and ensure energy security, my country has accelerated the construction of natural gas storage and supply facilities such as underground gas storage, and dozens of gas storages have been newly built or expanded in various regions. The vast majority of gas storages under construction or in operation are converted from depleted gas reservoirs. There is usually a certain amount of formation water remaining in the storage space of such gas storages. Affected by the hydrocarbon-water phase equilibrium, during the injection and production cycle of the gas storage, the formation water remaining in the storage space around the well will gradually evaporate and be produced with the natural gas to the ground, causing the formation water saturation in the storage space to gradually decrease, expanding the storage space and seepage channels of natural gas. In addition, since a large amount of inorganic salts are usually dissolved in the residual formation water, with the continuous evaporation of formation water, the inorganic salts dissolved in the formation water are precipitated due to supersaturation, and gradually occupy the storage space, causing inorganic salt blockage in the storage space around the well, thereby affecting the flow and storage of natural gas.

[0003] However, there are few studies on the water saturation prediction methods for gas storage reservoirs considering the evaporation and salting out of formation water. Usually, a rough estimate of the drying range of the gas storage reservoir is made, which makes the research results limited in guiding the operation of the gas storage reservoir. Summary of the invention

[0004] The purpose of the present invention is to provide a method for predicting the water saturation of a gas storage reservoir taking into account the evaporation and salting out of formation water, so as to solve the problem that there are few studies on the method for predicting the water saturation of a gas storage reservoir taking into account the evaporation and salting out of formation water, and usually a rough estimate is made of the drying range of the gas storage reservoir, resulting in limited role of the research results in guiding the operation of the gas storage reservoir.

[0005] To achieve the above object, the present invention provides a method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water. The method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water comprises the following steps:

[0006] S1: Calculate and establish a chart of saturated water vapor content in natural gas based on the formation temperature and formation water salinity of the gas storage reservoir;

[0007] S2: Calculate the drying and salting range of the reservoir space around the well during the gas storage stage according to the gas injection volume, operating pressure range and geological parameters of the gas storage reservoir;

[0008] S3: According to the gas peak-shaving plan of the gas storage, taking into account the effects of salt precipitation and evaporation, calculate the average formation pressure at different stages of the gas storage;

[0009] S4: Calculate the formation pressure at different radii of the gas storage based on the daily production of the gas wells in the gas storage and the average formation pressure of the gas storage;

[0010] S5: Calculate the water saturation at different radii of the gas storage reservoir according to the formation pressure at different radii of the gas storage reservoir;

[0011] S6: Repeat steps S2 to S5 to calculate the water saturation of the gas storage reservoir in different operation cycles.

[0012] Wherein, in step S1: the calculation formula for establishing the saturated water vapor content map in natural gas is:

[0013]

[0014] Where: M mw is the molar mass of pure water, p is the formation pressure, Z is the deviation factor, T is the formation temperature, p fws is the saturated vapor pressure of formation water, R is the gas constant, H(X s ,T) is the correction term for the salt concentration and temperature to the saturated water vapor content, and the subscript sc is the standard condition.

[0015] Wherein, in step S2: the calculation formula for calculating the drying and salting range of the storage space around the well during the gas storage stage is:

[0016]

[0017] Where: r ds is the radius of the drying and salting-out range in the gas storage stage, g inj is the gas injection volume in the gas storage stage, w tp is the saturated water vapor content in natural gas at the upper limit of gas storage pressure, w inj is the water vapor content of the natural gas injected during the gas storage stage, π is the circumference, h is the effective thickness of the gas storage reservoir, φ is the effective porosity of the gas storage reservoir, S fw is the formation water saturation, ρ fw is the formation water density, X s is the salt concentration of formation water.

[0018] Wherein, in step S3: the calculation formula for calculating the average formation pressure of the gas storage reservoir at different stages is:

[0019]

[0020] Where: S s is the volume ratio of salt in the pore space, y w is the mole fraction of water vapor in natural gas, g pro is the gas production in the peak-shaving stage, g sis the gas storage capacity of the gas storage, the subscript i is the initial state of the gas storage during the peak-shaving stage, and the symbol “—” represents the average.

[0021] Wherein, in step S4: the calculation formula for calculating the formation pressure at different radii of the gas storage reservoir is:

[0022]

[0023] Where: m(p) is the pseudo-pressure function, A and B are coefficients of radius r, q g is the daily production of the gas well, subscript r is the radius r, and μ is the gas viscosity.

[0024] Wherein, in step S5: the calculation formula for calculating the water saturation at different radii of the gas storage reservoir is:

[0025]

[0026] In the formula, Δr is the change of r, t is the gas storage in q g continuous production time.

[0027] The present invention provides a method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water. The method establishes a saturated water vapor content map in natural gas and calculates the drying and salting out range of the storage space around the well in the gas storage stage. Then, the gas production and peak regulation scheme of the gas storage reservoir is used to consider the effects of salting out and evaporation, and the average formation pressure of the gas storage reservoir at different stages is calculated. The formation pressure at different radii of the gas storage reservoir is calculated by combining the daily production of the gas wells of the gas storage reservoir with the average formation pressure of the gas storage reservoir. Then, the water saturation at different radii of the gas storage reservoir is calculated by the formation pressure at different radii of the gas storage reservoir. The above steps are repeated to calculate the water saturation of the gas storage reservoir in different operation cycles. The above method is used to quantitatively study the water saturation at different reservoir positions after evaporation and salting out of formation water in the gas storage reservoir, so as to correctly understand the distribution characteristics and change laws of water saturation of the gas storage reservoir at different stages, and provide a basis for carrying out gas storage capacity accounting and gas well injection and production operation parameter evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 It is a flow chart of the steps of the method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water provided by the present invention.

[0030] Figure 2It is a schematic diagram of the saturated water vapor content in the natural gas of gas storage reservoir A in a specific embodiment of the present invention.

[0031] Figure 3 It is a schematic diagram of the formation pressure distribution at the end of the gas production period after 5 operation cycles of the A gas storage in a specific embodiment of the present invention.

[0032] Figure 4 It is a schematic diagram of the water saturation distribution of the gas storage reservoir A after 5 operation cycles in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0033] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0034] See also Figure 1 The present invention provides a method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water. The method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water comprises the following steps:

[0035] S1: Calculate and establish a chart of saturated water vapor content in natural gas based on the formation temperature and formation water salinity of the gas storage reservoir;

[0036] S2: Calculate the drying and salting range of the reservoir space around the well during the gas storage stage according to the gas injection volume, operating pressure range and geological parameters of the gas storage reservoir;

[0037] S3: According to the gas peak-shaving plan of the gas storage, taking into account the effects of salt precipitation and evaporation, calculate the average formation pressure at different stages of the gas storage;

[0038] S4: Calculate the formation pressure at different radii of the gas storage based on the daily production of the gas wells in the gas storage and the average formation pressure of the gas storage;

[0039] S5: Calculate the water saturation at different radii of the gas storage reservoir according to the formation pressure at different radii of the gas storage reservoir;

[0040] S6: Repeat steps S2 to S5 to calculate the water saturation of the gas storage reservoir in different operation cycles.

[0041] In this embodiment, by establishing a saturated water vapor content map in natural gas and calculating the drying and salting-out range of the storage space around the well during the gas storage stage, the gas production peak-shaving plan of the gas storage reservoir is used to consider the effects of salting-out and evaporation, and the average formation pressure of the gas storage reservoir at different stages is calculated. The formation pressure at different radii of the gas storage reservoir is calculated by the daily production of the gas wells of the gas storage reservoir and the average formation pressure of the gas storage reservoir. Then, the water saturation at different radii of the gas storage reservoir is calculated by the formation pressure at different radii of the gas storage reservoir. The above steps are repeated to calculate the water saturation of the gas storage reservoir at different operation cycles. The above method is used to quantitatively study the water saturation at different reservoir positions after evaporation and salting-out of formation water in the gas storage reservoir, so as to correctly understand the distribution characteristics and change laws of water saturation of the gas storage reservoir at different stages, and provide a basis for carrying out gas storage capacity accounting and gas well injection and production operation parameter evaluation.

[0042] Wherein, in step S1: the calculation formula for establishing the saturated water vapor content map in natural gas is:

[0043]

[0044] Where: M mw is the molar mass of pure water, p is the formation pressure, Z is the deviation factor, T is the formation temperature, p fws is the saturated vapor pressure of formation water, R is the gas constant, H(X s ,T) is the correction term for the salt concentration and temperature to the saturated water vapor content, and the subscript sc is the standard condition.

[0045] Wherein, in step S2: the calculation formula for calculating the drying and salting range of the storage space around the well during the gas storage stage is:

[0046]

[0047] Where: r ds is the radius of the drying and salting-out range in the gas storage stage, g inj is the gas injection volume in the gas storage stage, w tp is the saturated water vapor content in natural gas at the upper limit of gas storage pressure, w inj is the water vapor content of the natural gas injected during the gas storage stage, π is the circumference, h is the effective thickness of the gas storage reservoir, φ is the effective porosity of the gas storage reservoir, S fw is the formation water saturation, ρ fw is the formation water density, X s is the salt concentration of formation water.

[0048] Wherein, in step S3: the calculation formula for calculating the average formation pressure of the gas storage reservoir at different stages is:

[0049]

[0050] Where: Ss is the volume ratio of salt in the pore space, y w is the mole fraction of water vapor in natural gas, g pro is the gas production in the peak load phase, g s is the gas storage capacity of the gas storage, the subscript i is the initial state of the gas storage during the peak-shaving stage, and the symbol “—” represents the average.

[0051] Wherein, in step S4: the calculation formula for calculating the formation pressure at different radii of the gas storage reservoir is:

[0052]

[0053] Where: m(p) is the pseudo-pressure function, A and B are coefficients of radius r, q g is the daily production of the gas well, subscript r is the radius r, and μ is the gas viscosity.

[0054] Wherein, in step S5: the calculation formula for calculating the water saturation at different radii of the gas storage reservoir is:

[0055]

[0056] In the formula, Δr is the change of r, t is the gas storage in q g continuous production time.

[0057] See also Figures 2 to 4 , the specific embodiment is:

[0058] Take gas storage A as the gas storage to be predicted. Gas storage A is rebuilt from an abandoned gas reservoir. The basic parameters of gas storage A are: the average formation pressure when the gas reservoir is abandoned is about 4MPa, the operating pressure range of the gas storage is 40MPa~15MPa, the gas production of the gas well is 15×104m3 / d, the average porosity of the gas storage is 12.5%, the average effective permeability is 1.72mD, the average effective thickness is 69m, the average temperature is 120℃, the gas well spacing is 400m, the formation water saturation is 35%, and the formation water salt concentration is about 26%.

[0059] According to the basic situation of gas storage A, the water saturation prediction method of gas storage considering formation water evaporation and salting-out is used to predict the distribution of water saturation of the gas storage after 5 operation cycles. The results show that after the gas injection of the 5th operation cycle, the drying and salting-out range of the reservoir space around the well extends to 18.77m, while after the production of the 5th operation cycle, the drying and salting-out range of the reservoir space around the well is about 20m. The "pressure drop funnel" caused by radial seepage is the key to determining the gradual distribution of water saturation of the gas storage.

[0060] What is disclosed above is only a preferred embodiment of the present invention, and it certainly cannot be used to limit the scope of rights of the present invention. Ordinary technicians in this field can understand that all or part of the processes of the above embodiment and equivalent changes made according to the claims of the present invention still fall within the scope of the invention.

Claims

1. A method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water, characterized in that: The steps include: S1: Calculate and establish a chart of saturated water vapor content in natural gas based on the formation temperature and formation water salinity of the gas storage reservoir; S2: Calculate the drying and salting range of the reservoir space around the well during the gas storage stage according to the gas injection volume, operating pressure range and geological parameters of the gas storage reservoir; S3: According to the gas peak-shaving plan of the gas storage, taking into account the effects of salt precipitation and evaporation, calculate the average formation pressure at different stages of the gas storage; S4: Calculate the formation pressure at different radii of the gas storage based on the daily production of the gas wells in the gas storage and the average formation pressure of the gas storage; S5: Calculate the water saturation at different radii of the gas storage reservoir according to the formation pressure at different radii of the gas storage reservoir; S6: repeating steps S2 to S5 to calculate the water saturation of the gas storage reservoir in different operation cycles; In step S1: the calculation formula for establishing the saturated water vapor content map in natural gas is: Where: M mw is the molar mass of pure water, p is the formation pressure, Z is the deviation factor, T is the formation temperature, p fws is the saturated vapor pressure of formation water, R is the gas constant, H(X s ,T) is the correction term for the saturated water vapor content due to salt concentration and temperature, and the subscript sc refers to standard conditions.

2. The method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water according to claim 1, characterized in that: In step S2: the calculation formula for calculating the drying and salting range of the storage space around the well during the gas storage stage is: Where: r ds is the radius of the drying and salting-out range in the gas storage stage, g inj is the gas injection volume in the gas storage stage, w tp is the saturated water vapor content in natural gas at the upper limit of gas storage pressure, w inj is the water vapor content of the natural gas injected during the gas storage stage, π is the circumference, h is the effective thickness of the gas storage reservoir, φ is the effective porosity of the gas storage reservoir, S fw is the formation water saturation, ρ fw is the formation water density, X s is the salt concentration of formation water.

3. The method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water as claimed in claim 2, characterized in that: In step S3: the calculation formula for calculating the average formation pressure of the gas storage reservoir at different stages is: Where: S s is the volume ratio of salt in the pore space, y w is the mole fraction of water vapor in natural gas, g pro is the gas production in the peak load phase, g s is the gas storage capacity of the gas storage, the subscript i is the initial state of the gas storage during the peak-shaving stage, and the symbol "—" represents the average.

4. The method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water as claimed in claim 3, characterized in that: In step S4: the calculation formula for calculating the formation pressure at different radii of the gas storage reservoir is: Where: m(p) is the pseudo-pressure function, A and B are coefficients of radius r, q g is the daily production of the gas well, subscript r is the radius r, and μ is the gas viscosity.

5. The method for predicting water saturation of a gas storage reservoir taking into account evaporation and salting out of formation water according to claim 4, characterized in that: In step S5: the calculation formula for calculating the water saturation at different radii of the gas storage reservoir is: In the formula, Δr is the change of r, t is the gas storage in q g continuous production time.

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