A method for identifying CO2 gas breakthrough

Abstract

The present invention provides a method for identifying CO2 drive gas channeling, comprising the following steps: step 1, calculating a crude oil dissolved gas-oil ratio curve of a target oil layer; step 2, calculating a formation water dissolved gas-oil ratio curve; step 3, calculating a critical gas-oil ratio curve under different water content conditions; step 4, judging gas channeling based on the relative position of the critical CO2 gas channeling gas-oil ratio on the curve obtained in step 3 under different water content conditions; the present invention effectively solves the problems of the current method for judging gas channeling, such as high testing cost, long time consumption and inability to be directly used for judging gas channeling with gas having good oil-water solubility. The method has both accuracy, timeliness and economy, and has the characteristics of being intuitive, fast, simple and low-cost.

Description

Technical Field

[0001] The present invention relates to the technical field of improving oil recovery rate in oil and gas field development, and in particular to the technical field of improving oil recovery rate in CO2 flooding. Background Art

[0002] As an effective method for the resource utilization of greenhouse gases, CO2 flooding and enhanced oil recovery technology has received increasing attention. This technology has good application prospects in the development of low-permeability and ultra-low-permeability reservoirs. Gas channeling refers to the phenomenon that the injected gas is produced in a continuous phase. It is an inevitable result of gas drive development to a certain stage. Among them, objective factors such as CO2 viscous fingering, gravity overlap and reservoir heterogeneity will aggravate gas channeling. Gas channeling generally occurs in the late stage of gas drive. CO2 gas channeling leads to the ineffective utilization of injected CO2 gas, aggravates the contradiction of CO2 drive development, and there are safety risks in the large-scale production of CO2 after gas channeling. How to quickly and accurately judge CO2 gas channeling is a technical problem that needs to be solved urgently. Analysis shows that: under a certain temperature and pressure, the maximum amount of CO2 dissolved in the formation fluid (including formation water and formation crude oil) (saturated state) is the critical point of CO2 gas channeling, that is, the maximum gas-oil ratio of the formation fluid saturated with CO2 is the critical point of gas channeling.

[0003] At present, the main methods for judging gas channeling are the characteristic component method and the gas-oil ratio method. Among them, the characteristic component method generally uses gas / liquid chromatography to test the characteristic components in the produced gas; the gas-oil ratio method is to judge gas channeling based on the gas-oil ratio in the production well. These two methods are suitable for judging gas channeling of air, N2 and other gases that are insoluble or difficult to dissolve in formation fluids. For CO2 flooding, since CO2 has good solubility in formation fluids (including formation water and formation crude oil), the presence of CO2 in the produced gas is not gas channeling. Only when the CO2 content reaches a certain value or the gas-oil ratio reaches a certain value (the maximum amount of CO2 dissolved in the formation fluid under this pressure) is it gas channeling. This critical value can be tested by an indoor high-pressure PVT instrument, but the cost is high and the workload is large. How to accurately, quickly and cost-effectively judge gas channeling has not yet been reported. Summary of the invention

[0004] The purpose of the present invention is to provide a method for identifying CO2 purge gas crossover, which solves the defects of the existing method for identifying CO2 purge gas crossover, such as high cost and heavy workload.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is:

[0006] The present invention provides a method for identifying CO2 purge gas breakthrough, comprising the following steps:

[0007] Step 1, calculating the crude oil dissolved gas-oil ratio curve of the target oil layer;

[0008] Step 2, calculating the formation water dissolved gas-oil ratio curve;

[0009] Step 3, calculating the critical gas-oil ratio curve under different water content conditions;

[0010] Step 4, judging gas cross-talk based on the relative position of the critical CO2 gas cross-talk gas-to-oil ratio on the curve obtained in step 3 under different water content conditions.

[0011] Preferably, the specific method for calculating the crude oil dissolved gas-oil ratio of the target oil layer in step 1 is:

[0012] S1, obtaining the crude oil total hydrocarbon component data and basic physical property parameters of the target oil layer, wherein the basic physical property parameters include crude oil viscosity, density, volume coefficient and compressibility coefficient;

[0013] S2, splitting the crude oil components according to the total hydrocarbon components of the crude oil of the target oil layer obtained in S1, and obtaining several groups of crude oil models with different splitting;

[0014] S3, fitting physical properties according to the crude oil models with different splits obtained in S2 to obtain characteristic parameters of each pseudo-component;

[0015] S4, using the characteristic parameters of each pseudo-component obtained in S3, calculate the bubble point pressure of CO2 at different concentrations and the crude oil, and obtain the maximum solubility of CO2 in the crude oil at different pressures;

[0016] S5, using the oil-gas separation calculation module to perform oil-gas separation calculations for CO2 saturated crude oils with different concentrations, and obtain the surface production gas-oil ratio of CO2 saturated crude oils at different pressures at the same reservoir temperature.

[0017] Preferably, in S3, the specific method for obtaining the characteristic parameters of each pseudo-component is:

[0018] The basic physical parameters of each pseudo-component are continuously adjusted until the error between key physical parameters such as crude oil saturation pressure, crude oil viscosity curve, volume coefficient and compressibility coefficient and the high temperature and high pressure test results is less than 5%, thereby obtaining the characteristic parameters of each pseudo-component.

[0019] Preferably, in step 2, the specific method for calculating the dissolved gas-oil ratio of formation water is:

[0020] The OGW / EOS multiphase separation calculation module in the Li and Nghiem model is used to calculate the dissolved gas-water ratio of CO2 in formation water at different pressures at the same reservoir temperature.

[0021] Preferably, in step 3, the critical gas-oil ratio R under different water content conditions is calculated by the following formula: g / o , forming the critical gas-oil ratio curve under different water content conditions:

[0022]

[0023] in, f w is the moisture content, %; r g / o is the dissolved gas-oil ratio of CO2 in the formation crude oil m 3 / m 3 ; r g / w is the dissolved gas-water ratio in the formation water and the converted critical gas-oil ratio at different water contents m 3 / m 3 ; V w is the volume of water; V o is the oil volume; V go V is the maximum volume of dissolved CO2 in a certain volume of crude oil under ground conditions; gw It is the maximum volume of dissolved CO2 in a certain volume of formation water under ground conditions.

[0024] Preferably, in step 4, according to the bottom hole pressure of the production well after gas injection, the wellhead gas-oil ratio and the water content value, the relative position of the critical CO2 gas breakthrough gas-oil ratio curve under different water content conditions is used to judge the gas breakthrough. When the point is above the curve, it is judged as gas breakthrough, and when it is below the curve, there is no gas breakthrough.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention provides a method for identifying CO2 flooding gas breakthrough, which calculates the dissolved gas-oil ratio of crude oil and the dissolved gas-oil ratio of formation water in the target oil layer, and converts them into the critical gas-oil ratio R under different water content conditions. g / o , and then identify CO2-driven gas channeling. Supplement: In view of the shortcomings of the current characteristic component method and gas-oil ratio method, such as long testing cycle, high cost and inability to achieve early warning of gas channeling, this method uses numerical simulation to calculate the solubility of CO2 in crude oil and formation water and the maximum dissolved gas-oil ratio of gas channeling under different pressures and water contents. There is no need for on-site sampling and analysis, which basically eliminates testing costs, saves time, achieves early warning of gas channeling, is conducive to the timely adjustment of on-site injection and production policies, and greatly improves the utilization rate of injected CO2. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram for judging gas breakthrough at a certain moisture content. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0029] The present invention provides a method for identifying CO2 drive gas channeling. The solubility of CO2 in formation water and crude oil at different pressures at reservoir temperature is calculated according to numerical simulation; the gas-oil ratio of saturated CO2 formation water and crude oil at ground conditions at different pressures is calculated according to the gas-liquid separation method, and then the gas-oil ratio curve of critical gas channeling under different pressures and water content conditions is calculated. Gas channeling is identified based on the relative positions of the bottom hole flow pressure, wellhead gas-oil ratio and water content of the gas-producing well after gas drive on the critical gas channeling curve. When the actual production data falls above the curve, it is gas channeling, otherwise it is non-gas channeling. The specific calculation process is as follows:

[0030] Step 1: Calculate the dissolved gas-oil ratio of the target oil layer

[0031] S1, obtaining the crude oil total hydrocarbon component data and basic physical property parameters of the target oil layer, wherein the basic physical property parameters include crude oil viscosity, density, volume coefficient and compressibility coefficient;

[0032] S2, according to the total hydrocarbon components of the crude oil in the target oil layer obtained in S1, the crude oil components are split to obtain a seven-pseudo-component split crude oil model, which includes CO2, N2, CH4, C2-C6, C6-C 10 , C 10 -C 20 , C 20+ ;

[0033] S3, fitting the physical properties of crude oil according to the split crude oil model obtained in S2, and continuously adjusting the basic physical property parameters of each characteristic component until the error between the key physical property parameters such as crude oil saturation pressure, crude oil viscosity curve, volume coefficient and compressibility coefficient and the high temperature and high pressure test results is less than 5%, and obtaining the characteristic parameters of each pseudo-component, including molecular weight, molecular weight, critical temperature, critical pressure, deviation factor, etc.;

[0034] S4, using the characteristic parameters of the splitting model obtained in S3, calculate the bubble point pressure of CO2 at different concentrations and the crude oil, and obtain the maximum solubility (mol percentage) of CO2 in the crude oil at different pressures.

[0035] S5, using the oil and gas separation calculation module to perform oil and gas separation calculations for CO2 saturated crude oils with different concentrations, and by setting different pressures and temperatures before oil and gas separation (the reservoir temperature is a constant), the ground production gas-oil ratio of CO2 saturated crude oil under different pressures is obtained.

[0036] Step 2: Calculate the dissolved gas-oil ratio of formation water

[0037] According to the results of formation water salt content analysis, the Li and Nghiem model was selected, and the OGW / EOS multiphase separation calculation module in the Li and Nghiem model was used to calculate the dissolved gas-water ratio (m / s) of CO2 in formation water at different pressures at reservoir temperature. 3 / m 3 ).

[0038] Step 3: Calculate the critical gas-oil ratio under different water content conditions

[0039] Assume that the oil-water system is independent of each other, CO2 dissolves in crude oil first, and then dissolves in water after saturation, and in the entire oil-gas-water system, natural gas is dissolved in crude oil. Among them, the water content, dissolved gas-oil ratio and dissolved gas-water ratio are known, and the CO2 dissolved in the formation water is converted into crude oil to obtain the critical gas-oil ratio R at different water contents g / o curve:

[0040]

[0041] in, f w is the moisture content, %; r g / o is the gas-oil ratio of CO2 dissolved in formation crude oil, m 3 / m 3 ; r g / w is the dissolved gas-water ratio in formation water, m 3 / m 3 ; R g / o is the critical gas-oil ratio at different water cuts, m 3 / m 3 ; V w is the volume of water; V o is the oil volume; V go V is the maximum volume of dissolved CO2 in a certain volume of crude oil under ground conditions; gw It is the maximum volume of dissolved CO2 in a certain volume of formation water under ground conditions.

[0042] Step 4: Quickly determine gas channeling

[0043] like Figure 1 As shown in the figure, according to the bottom hole pressure of the production well after gas injection, the wellhead gas-oil ratio and the water content, the relative position of the critical CO2 gas breakthrough gas-oil ratio curve under different water content conditions is used to judge the gas breakthrough. When the point is above the curve, it is judged as gas breakthrough, and when it is below the curve, there is no gas breakthrough.

[0044] At present, the main methods for judging CO2 gas channeling are the characteristic component method and the gas-oil ratio method. The characteristic component method uses chromatographic analysis to test the content of characteristic components in the produced gas. This method can accurately judge the amount of characteristic components in the produced gas, but the test time is long, the cost is high, and it cannot be predicted in advance. When using this method to detect CO2 gas channeling, the maximum content (dissolved amount) of CO2 in the produced gas components must be determined. Similarly, when using the gas-oil ratio method to identify CO2 gas channeling, the production gas-oil ratio of the crude oil after saturation with CO2 must be determined. It can be tested using a PVT instrument, but the cost is high and the testing workload is large.

[0045] The present invention uses numerical simulation to calculate the gas-oil ratio of dissolved CO2 in the formation crude oil and formation water respectively based on the formation crude oil components, high-pressure physical properties and formation water ion components, and obtains the gas-oil ratio curve of dissolved CO2 under different bottom hole pressures and different water cuts at the reservoir temperature, that is, the critical CO2 gas channeling gas-oil ratio curve. According to the bottom hole pressure of the production well after gas injection, the gas-oil ratio at the wellhead and the water cut value, the relative position on the critical CO2 gas channeling gas-oil ratio curve under different water cuts is used to judge gas channeling. This effectively solves the problem that the current method for judging gas channeling has high testing cost, long time consumption and cannot be directly used for judging gas channeling with gas with good oil-water solubility. This method combines accuracy, timeliness and economy, and has the characteristics of being intuitive, fast, simple and low-cost.

Claims

1. A method for identifying CO2 purge gas breakthrough, characterized in that: The following steps are involved: Step 1, calculating the crude oil dissolved gas-oil ratio curve of the target oil layer; Step 2, calculating the formation water dissolved gas-oil ratio curve; Step 3, calculating the critical gas-oil ratio curve under different water content conditions; Step 4, judging gas cross-talk based on the relative position of the critical CO2 gas cross-talk gas-to-oil ratio on the curve obtained in step 3 under different water content conditions; In step 3, the critical gas-oil ratio R under different water content conditions is calculated by the following formula: g / o , forming the critical gas-oil ratio curve under different water content conditions: in, f w is the moisture content; r g / o is the dissolved gas-oil ratio of CO2 in formation crude oil; r g / w V is the dissolved gas-water ratio in the formation water and the converted critical gas-oil ratio under different water contents; w is the volume of water; V o is the oil volume; V go V is the maximum volume of dissolved CO2 in a certain volume of crude oil under ground conditions; gw It is the maximum volume of dissolved CO2 in a certain volume of formation water under ground conditions.

2. A method for identifying CO2 purge gas breakthrough according to claim 1, characterized in that: The specific method for calculating the crude oil dissolved gas-oil ratio of the target oil layer in step 1 is: S1, obtaining the crude oil total hydrocarbon component data and basic physical property parameters of the target oil layer, wherein the basic physical property parameters include crude oil viscosity, density, volume coefficient and compressibility coefficient; S2, splitting the crude oil components according to the total hydrocarbon components of the crude oil of the target oil layer obtained in S1, and obtaining several groups of crude oil models with different splitting; S3, fitting physical properties according to the crude oil models with different splits obtained in S2 to obtain characteristic parameters of each pseudo-component; S4, using the characteristic parameters of each pseudo-component obtained in S3, calculate the bubble point pressure of CO2 at different concentrations and the crude oil, and obtain the maximum solubility of CO2 in the crude oil at different pressures; S5, using the oil-gas separation calculation module to perform oil-gas separation calculations for CO2 saturated crude oils with different concentrations, and obtain the surface production gas-oil ratio of CO2 saturated crude oils at different pressures at the same reservoir temperature.

3. A method for identifying CO2 purge gas breakthrough according to claim 2, characterized in that: In S3, the specific method for obtaining the characteristic parameters of each pseudo-component is: The basic physical parameters of each pseudo-component are continuously adjusted until the error between key physical parameters such as crude oil saturation pressure, crude oil viscosity curve, volume coefficient and compressibility coefficient and the high temperature and high pressure test results is less than 5%, thereby obtaining the characteristic parameters of each pseudo-component.

4. A method for identifying CO2 purge gas breakthrough according to claim 1, characterized in that: In step 2, the specific method for calculating the dissolved gas-oil ratio of formation water is: The OGW / EOS multiphase separation calculation module in the Li and Nghiem model is used to calculate the dissolved gas-water ratio of CO2 in formation water at different pressures at the same reservoir temperature.

5. A method for identifying CO2 purge gas breakthrough according to claim 1, characterized in that: In step 4, according to the bottom hole pressure of the production well after gas injection, the wellhead gas-oil ratio and the water content value, the relative position of the critical CO2 gas breakthrough gas-oil ratio curve under different water content conditions is used to judge gas breakthrough. When the relative position is above the curve, it is judged as gas breakthrough, and when it is below the curve, there is no gas breakthrough.

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