Novel method for identifying phase state of high-pressure reinjection associated gas

Abstract

A novel high-pressure reinjection associated gas phase state identification method comprises the following steps: describing P, V and T phase state characteristics of a fluid by using a mathematical equation, and judging the phase state of the fluid by using an actual solution of the equation; the mathematical equation is a unary cubic equation, and the phase state characteristics of the well flow can be judged through the solution of the equation; in other words, when two actual solutions are solved, vapor-liquid two-phase coexistence is shown; when a real solution is solved, the single gas phase or liquid phase is represented, when the real solution is the minimum real root, the single liquid phase is represented, and when the real solution is the maximum real root, the single gas phase is represented. In order to verify the accuracy of the method, a formula calculation value is compared with an experimental value, and the precision can meet the engineering requirements.

Description

Technical Field

[0001] The present invention relates to the technical field of oil field production, and in particular to a new method for identifying the phase state of high-pressure reinjection associated gas. Background Art

[0002] Due to the low permeability of shale oil reservoirs, conventional energy replenishment methods cannot be used, resulting in a rapid decline in single-well oil production and generally low recovery rates. To increase oil well production, the conventional method is to reinject associated gas produced by shale oil to replenish formation energy, which has the effect of reducing viscosity and expanding volume, thereby increasing single-well oil production. Field test measures have achieved good results in increasing oil production. Due to the high pressure of shale reservoirs, the wellhead injection pressure is high. In addition, the content of heavy components in the oilfield associated gas is high. During the pressurization process, the heavy gas component is very likely to liquefy, resulting in a gas-liquid two-phase flow state in the compressor cylinder, posing a liquid hammer safety hazard. Therefore, it is necessary to accurately identify the phase changes of shale oil associated gas and promptly adjust the compressor operating parameters to prevent the liquefaction of high-pressure associated gas during compression, thereby maintaining a single gas phase for safe and stable compression.

[0003] Many equations of state are commonly used to describe gases, such as the linear equation: PV = ZRT, and the cubic equations: VDW, RK, and SRK. These equations can meet engineering precision requirements. However, when these equations are used to calculate P, V, and T for liquid, gas-liquid, or two-phase conditions, they can produce significant errors and fail to meet field requirements. Summary of the Invention

[0004] In order to solve the technical problems existing in the background technology, the present invention provides a new method for identifying the phase state of associated gas in high-pressure reinjection. This method identifies the phase state of associated gas by calculation, that is, the above state equation is used to determine the fluid state. In order to verify the accuracy of this method, the calculated value of the formula is compared with the experimental value, and the accuracy can meet the engineering requirements.

[0005] The technical solution provided by the present invention is: a new method for identifying the phase state of associated gas in high-pressure reinjection, comprising the following steps: using mathematical equations to describe the P, V, and T phase characteristics of the fluid, and using the real solutions of the equations to determine the phase state of the fluid;

[0006]

[0007] Where: is the model parameter of a fluid in the mixed fluid, is the model parameter of the mixed fluid, cm 3 / mol; It is the relative pressure of a fluid in the mixed fluid, dimensionless; is the temperature of a fluid in the mixed flow, is the critical temperature of a fluid in the mixed fluid, K; is the critical pressure of a fluid in the mixed fluid, MPa; is the thermal expansion coefficient of a fluid in the mixed fluid, is the eccentricity factor of a fluid in the mixed fluid, dimensionless; is the model parameter of a fluid in the mixed fluid, is the model parameter of the mixed fluid, MPa×cm 6 / mol 2 ; is the content of a certain fluid in the mixed fluid, dimensionless, P is the mixed fluid pressure, MPa; T is the mixed fluid temperature, K; R is a constant 8.314; V is the mixed fluid volume, M 3 ; ci represents the critical parameter of a component.

[0008] When the values ​​of P and T are given, the mathematical equation is a cubic equation of one variable, which is solved by iterative calculation. The engineering accuracy requirement of 5% can be met by controlling the number of iterations. The following is the Newton iterative calculation formula:

[0009]

[0010] In the above formula: is the approximate value of the nth iteration; is the function value at; is the derivative value at; is the approximate value of the n+1th iteration.

[0011] The above mathematical equation is a cubic equation of one variable. The phase characteristics of the well flow can be determined by the number of real solutions of the equation; that is, when it solves two real solutions, it indicates the coexistence of gas and liquid phases; when it solves one real solution, it indicates a single gas phase or liquid phase; when the real solution is the smallest real root, it indicates a single liquid phase; when the real solution is the largest real root, it indicates a single gas phase.

[0012] The beneficial effects of the present invention are: a new method for identifying the phase state of associated gas in high-pressure reinjection, which identifies the phase state of associated gas through a calculation method, that is, using the above state equation to determine the fluid state. In order to verify the accuracy of this method, the calculated value of the formula is compared with the experimental value, and the accuracy can meet engineering requirements. DETAILED DESCRIPTION

[0013] The present invention is further described below in conjunction with examples:

[0014] Example: A new method for identifying the phase state of associated gas during high-pressure reinjection includes the following steps: using mathematical equations to describe the P, V, and T phase state characteristics of the fluid, and using the real solutions of the equations to determine the phase state of the fluid;

[0015] (1)

[0016] (2)

[0017] (3)

[0018] (4)

[0019] (5)

[0020] (6)

[0021] (7)

[0022] Where: is the model parameter of a fluid in the mixed fluid, is the model parameter of the mixed fluid, cm 3 / mol; It is the relative pressure of a fluid in the mixed fluid, dimensionless; is the temperature of a fluid in the mixed flow, is the critical temperature of a fluid in the mixed fluid, K; is the critical pressure of a fluid in the mixed fluid, MPa; is the thermal expansion coefficient of a fluid in the mixed fluid, is the eccentricity factor of a fluid in the mixed fluid, dimensionless; is the model parameter of a fluid in the mixed fluid, is the model parameter of the mixed fluid, MPa×cm 6 / mol 2 ; It is the content of a certain fluid in the mixed fluid and is dimensionless.

[0023] When the values ​​of P and T are given, the mathematical equation is a cubic equation of one variable, which is solved by computer iteration. The engineering accuracy requirement of 5% can be met by controlling the number of iterations. The following is the Newton iteration calculation formula:

[0024]

[0025] Considering that the mathematical equation is a cubic equation of one variable, the phase characteristics of the well flow can be determined by the number of real solutions of the equation; that is, when it solves two real solutions, it indicates the coexistence of gas-liquid phases; when it solves one real solution, it indicates a single gas phase or liquid phase; when the real solution is the smallest real root, it indicates a single liquid phase; when the real solution is the largest real root, it indicates a single gas phase.

[0026] When calculating, the characteristic values ​​of this gas component are all known parameters, and y i Also provided as known data, the following are the characteristic values ​​of each gas, all of which are known constant values.

[0027] Basic data statistics table 1

[0028]

[0029] In this way, the values ​​of a and b in the above formula (1) can be calculated by the given constants and gas component contents, and R is a constant of 8.314.

[0030] When the values ​​of P and T are given, the mathematical equation is a cubic equation of one variable, which can be solved through iterative calculation. The phase characteristics of the well flow can be determined by the number of real solutions of the equation. Moreover, by controlling the number of iterations, the engineering accuracy requirement of 5% can be met. The following is the Newton iterative calculation formula:

[0031]

[0032] When the associated gas composition is 65%C1+15%C2+9%C3+5%C4+1%C5+5%CO2: b=31.323 cm 3 / mol, a=389249.154 MPa×cm 6 / mol 2 , V=771.221 cm 3 / mol, T=288.15K.

[0033] When the associated gas composition is 80% C1+10% C2+5% C3+1% C4+1% C5+3% CO2: b=35.127 cm 3 / mol, a=419259.732 MPa×cm 6 / mol 2 , V=730.437 cm 3 / mol, T=287.95K.

[0034] In order to verify the accuracy of the mathematical equation in predicting the associated gas phase state, we conducted a PVT indoor experiment. The results showed that the calculation accuracy can meet the actual engineering requirements.

[0035] Comparison of simulated and measured values ​​of associated gas phase state Table 2

[0036]

Claims

1. A new method for identifying the phase state of associated gas during high-pressure reinjection, comprising the following steps: using mathematical equations to describe the P, V, and T phase characteristics of the fluid, and using the real solutions of the equations to determine the fluid phase state; （1） （2） （3） （4） （5） （6） （7） Where: is the model parameter of a fluid in the mixed fluid, is the model parameter of the mixed fluid, cm 3 / mol; It is the relative pressure of a fluid in the mixed fluid, dimensionless; is the temperature of a fluid in the mixed flow, is the critical temperature of a fluid in the mixed fluid, K; is the critical pressure of a fluid in the mixed fluid, MPa; is the thermal expansion coefficient of a fluid in the mixed fluid, is the eccentricity factor of a fluid in the mixed fluid, dimensionless; is the model parameter of a fluid in the mixed fluid, is the model parameter of the mixed fluid, MPa×cm 6 / mol 2 ; is the content of a certain fluid in the mixed fluid, dimensionless, P is the mixed fluid pressure, MPa; T is the mixed fluid temperature, K; R is a constant 8.314; V is the mixed fluid volume, M 3 ; ci represents the critical parameter of a component; When the values ​​of P and T are given, the mathematical equation is a cubic equation of one variable, which is solved by iterative calculation. The engineering accuracy requirement of 5% can be met by controlling the number of iterations. The following is the Newton iterative calculation formula: （8）； In the above formula: is the approximate value of the nth iteration; is the function value at; is the derivative value at; is the approximate value of the n+1th iteration.

2. A new method for identifying the phase state of high-pressure reinjection associated gas according to claim 1, characterized in that: The mathematical equation is a cubic equation of one variable. The phase characteristics of the well flow can be determined by the number of real solutions of the equation; that is, when it solves two real solutions, it indicates the coexistence of gas and liquid phases; when it solves one real solution, it indicates a single gas phase or liquid phase; when the real solution is the smallest real root, it indicates a single liquid phase; when the real solution is the largest real root, it indicates a single gas phase.

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