Analysis of Migration Characteristics of Supercritical CO2 and Mechanism of Oil and Gas Production in Condensate Gas Reservoirs

At present, circulating gas injection is the most effective development method for condensate gas reservoirs in China, but the phenomenon of dry gas overlap has seriously affected the development effect of circulating gas injection in condensate gas reservoirs, while CO2 injection is used to drive gas, which can make CO2 widely used in removing retrograde condensation of condensate gas reservoirs, improving the development effect of gas reservoirs. This paper by comparing the migration laws of injected dry gas and injected CO2 obtains that the dry gas with a smaller density flows to the upper space of the reservoir, while the CO2 with a larger density settles to the lower part. With the injection of CO2 into the reservoir, the content of CO2 in the crude oil increases, the volume of condensate oil expands significantly, and the flow capacity of condensate oil in the formation is improved. Injecting CO2 into the formation can reduce the viscosity of condensate. The mixture of CO2 injected into the formation and formation water is slightly acidic, which can dissolve some cement in the rock. With the increase of CO2 injection, the interfacial tension between oil and water will also increase, which is unfavorable for the stability of the oil-water system and is easy to emulsify. After demulsification, the viscosity will decrease and the condensate oil will flow more easily. The dissolved carbon dioxide in the water is in a weak acid state, which is easy to corrode and shed particles, improving the reservoir space and seepage conditions near the well. It will provide theoretical support for the effective development of a condensate gas reservoir.


Introduction
A condensate gas reservoir is a special gas reservoir with extremely complex fluid phase state changes, which is very difficult to develop.During the development process, with the decrease of pressure, condensate will come out and adhere to the rock surface, and bound water will also start to participate in the flow to form an oil-water emulsion, leading to a sharp decline in gas phase permeability.For condensate gas reservoirs with low porosity and low permeability, once they are blocked, it is difficult to leave enough channels for the gas phase, and the productivity will decrease, which may lead to sudden gas well shutdown [1][2] .For condensate gas reservoirs with high condensate oil content, circulating gas injection is usually used to maintain formation pressure and improve the recovery of condensate oil and gas.At present, the better injection medium of the condensate gas reservoir is a dry gas, which belongs to hydrocarbon gas.It can effectively extract the heavy components in the condensate gas of the reservoir to achieve gas-liquid phase equilibrium, making the pressure-maintaining development of dry gas injection an effective development method adopted by most condensate gas reservoirs.
However, in recent years, it has been found that the phenomenon of dry gas overlap has seriously affected the development effect of circulating gas injection in condensate gas reservoirs, reducing the utilization rate of injected dry gas and increasing the production cost.However, CO 2 has a large solubility in crude oil and has a strong ability to extract hydrocarbon substances and reduce the dew point pressure of condensate gas.CO 2 injection is used to drive gas during the exploitation of condensate gas reservoirs, which can make CO 2 widely used in removing retrograde condensation of condensate gas reservoirs, improving the development effect of gas reservoirs, and enhancing recovery [3][4] .This paper compares the migration laws of injected dry gas and injected CO 2 , analyzes and compares the differences between injected dry gas and CO 2 -produced condensate gas, and analyzes the CO 2 storage mechanism, which provides a theoretical basis for establishing a mathematical model of CO 2 overlap.

The migration characteristics of injected dry gas
After the injected gas enters the reservoir to displace the formation fluid, due to the low density of dry gas, it is overlaid on the condensate gas by buoyancy.Due to molecular diffusion, the injected dry gas cannot be rapidly miscible with the condensate gas.The migration in the direction perpendicular to the reservoir is mainly affected by gravity and buoyancy.The migration speed in the vertical direction depends on the size of the density difference.In addition, the physical parameters of the reservoir also have a certain impact on the vertical seepage of the injected gas, which will lead to the phenomenon that the injected dry gas overlaps condensate gas, and affect the development effect of cyclic gas injection.
It is found through experiments that if the condensate gas reservoir is divided into upper, middle, and lower layers, the bottom of the reservoir is a high permeability layer with an average permeability of 831×10 -3 μm 2 , while the production gas oil ratio is only 2,530; The top of the reservoir is a low permeability layer with an average permeability of 40×10 -3 μm 2 , while the production gas oil ratio is up to 12,500, as shown in Table 1.It can be found that after the injection of dry gas, the fluid distribution in the reservoir has changed greatly, and the gas density variation has a great impact on the flow of the injected gas.More injected dry gas will migrate to the upper low permeability reservoir, which does not fully conform to the flow law of the high permeability reservoir.

The migration characteristics of supercritical CO 2 gas
At present, a large number of studies have found that for natural gas reservoirs with formation temperature and pressure higher than the critical point of CO 2 , During CO 2 injection and natural gas recovery, there is no large-scale first miscibility between supercritical CO 2 and natural gas under formation conditions, but they flow in the way of displacement.
Under the reservoir pressure and temperature, CO 2 is a supercritical fluid, its density is close to the density of the liquid.The main component of natural gas is CH 4 , its density and viscosity are small, and the physical properties of the two fluids are quite different.Supercritical CO 2 with high density tends to migrate under the light natural gas components in the displacement process, while the two fluids are mixed transition zones.
Due to the density differentiation, the supercritical CO 2 subsides downward and is located at the lower part of the reservoir, flows forward, and displaces the natural gas to achieve burial, and the natural gas is gathered at the higher part of the gas reservoir to be recovered.The contact part between the mixing zone and the natural gas is called the displacement front, and the contact part between the mixing zone and the supercritical fluid is called the burial front.The whole reservoir can be divided into three different phase zones, namely, the purely natural gas zone above the displacement front, the mixing zone in the middle, and the supercritical CO 2 gas zone below the burial front, as shown in Figure 1 [5][6] .
Figure 1 Schematic Diagram of Supercritical CO 2 Displacement of Natural Gas Therefore, from the migration characteristics of natural gas displaced by supercritical CO 2 gas, it is found that the density difference is the main factor affecting the downward overlap of supercritical CO 2 gas.
Near gas recovery wells, the reservoir pressure drops greatly, and the condensate gas has a large phase change.Considering the influence of gravity, the condensate oil and condensate gas with high density gather at the bottom of the reservoir, while the injected CO 2 tends to flow at the bottom of the reservoir due to its slightly higher density.The phase change of condensate gas increases the effect of density differentiation and affects and changes the flow track of injected CO 2 and CO 2 overlap characteristics.Therefore, the phase change of condensate gas should be emphatically considered in the study of CO 2 injection overlap.

Expansion mechanism of CO 2 injection
After CO 2 is dissolved in the condensate oil, the volume will expand.The greater the expansion, the less condensate oil will remain in the reservoir and the less pore space occupied by the condensate oil.When a certain volume of CO 2 is dissolved in the condensate oil, its volume will increase by 10~100%.At the same time, it can also increase the kinetic energy in the liquid, reduce the blockage caused by condensate oil accumulation, and play a certain role in removing or reducing the damage of condensate oil to the formation.The gas injection expansion experiment was carried out under the condition of formation pressure of 17 MPa and temperature of 61.2 ℃ to study the influence of CO 2 injection on the expansion property of condensate oil and gas system.The experimental results show that with the increase of the molar content of CO 2 in the crude oil, the volume of the crude oil expands obviously.When the molar content of injected CO 2 rises to 15%, the expansion coefficient of crude oil volume is 1.3 (Figure 2).The greater the volume expansion of condensate oil, the less residual condensate oil in the reservoir.In addition, the expansion energy formed by CO 2 dissolved in crude oil can change part of the residual oil in the formation into flowable oil, which improves the flowability of condensate oil in the formation [7][8] .
Figure 2 The Variation Curve of Molar Content of Injected CO 2 and Expansion Coefficient

Plug removal mechanism of CO 2 injection
Injecting CO 2 huff and puff into the gas reservoir can remove the blockage near the wellbore zone.The role of each mechanism in the process depends on the characteristics of oil and gas reservoirs and fluids, together with injection and production conditions.These mechanisms mainly include the following aspects: ① Viscosity reduction: CO 2 injection reduces the viscosity of the condensate oil, which is conducive to the connectivity of the rock pores and the fluidity of the condensate, to restore the effective permeability of the gas phase reduced by the retrograde condensation to a certain extent, and reduce the damage to the formation caused by the condensate oil accumulated at the bottom of the well due to the retrograde condensation.
② Acidizing plugging removal: after CO 2 dissolves in water, the viscosity of water increases, which can improve the oil-water viscosity ratio and reduce the amount of residual condensate oil.The mixture of CO 2 injected into the formation and formation water is slightly acidic, which can dissolve some cement in the rock, remove the formation plugging near the bottom of the well, and improve the formation permeability [9][10] .
③ Interfacial tension: For an oil-water system containing dissolved gas, the content of dissolved gas plays a decisive role in the interfacial tension between oil-water phases.When CO 2 is injected into the formation, with the increase of injection, the greater the solubility of a gas in condensate oil makes the polarity difference between oil and water larger, and the interfacial tension between oil and water also increases, which is unfavorable to the stability of the oil-water system and easy to demulsify.After demulsification, the viscosity of the oil decreases and the oil flows more easily.And the dissolved carbon dioxide in the water is in a weak acid state, which may cause corrosion.In the process of huff and puff flow back, the particles shed due to corrosion is brought out, which will improve the reservoir space and seepage conditions near the good zone.

Conclusion
(1) Through the comparative analysis of the migration characteristics of supercritical CO 2 gas and the migration characteristics of dry gas injected into a condensate gas reservoir, the results show that the dry gas with smaller density flows to the upper space of the reservoir, while the CO 2 with larger NESP-2023 Journal of Physics: Conference Series 2592 (2023) 012019 density settles to the lower part.Under the action of gravity and buoyancy, different fluids exist in a differentiation state, thus revealing that the density difference between different fluids is the main internal cause of the upward and downward overlap of the injected fluid.
(2) Considering that injected CO 2 and condensate gas are not immediately miscible, there is the dynamical behavior of convection-diffusion between the two fluids.Affected by thermodynamic properties, different gas diffusion coefficients affect the vertical concentration distribution of dry gas.In addition, when the pressure is lower than the dew point, the condensate gas will have a phase change behavior, and a large amount of retrograde condensate liquid at the bottom of the reservoir will change the migration trajectory of injected gas and promote the sinking of injected CO 2 .
(3) With the increase of CO 2 content in crude oil, the volume of condensate oil expands significantly.The greater the volume expansion of condensate oil, the less the residual condensate oil in the reservoir.In addition, the expansion energy formed by CO 2 dissolved in crude oil can change part of the residual oil in the formation into flowable oil, which improves the flowability of condensate oil in the formation.
(4) Injecting CO 2 into the formation can reduce the viscosity of condensate.The mixture of CO 2 injected into the formation and formation water is slightly acidic, which can dissolve some cement in the rock, remove the formation plugging near the bottom hole, and improve the formation permeability.

Table 1
Gas oil ratio of reservoir fluid driven by dry gas injection