EFFECTS OF THE RATE OF NATURAL GAS PRODUCTION ON THE RECOVERY FACTOR DURING CARBON DIOXIDE INJECTION AT THE INITIAL GAS- WATER CONTACT

The object of research is gas condensate reservoirs, which is being developed under the conditions of the manifestation of the water drive of development and the negative effect of formation water on the process of natural gas production. The results of the performed theoretical and experimental studies show that a promising direction for increasing hydrocarbon recovery from fields at the final stage of development is the displacement of natural gas to producing wells by injection non-hydrocarbon gases into productive reservoirs. The final gas recovery factor according to the results of laboratory studies in the case of injection of non-hydrocarbon gases into productive reservoirs depends on the type of displacing agent and the level heterogeneity of reservoir. With the purpose update the existing technologies for the development of fields in conditions of the showing of water drive, the technology of injection carbon dioxide into productive reservoirs at the boundary of the gas-water contact was studied using a digital three-dimensional model of a gas condensate deposit. The study was carried out for various values of the rate of natural gas production. The production well rate for calculations is taken at the level of 30, 40, 50, 60, 70, 80 thousand m3/day. Based on the data obtained, it has been established that an increase in the rate of natural gas production has a positive effect on the development of a productive reservoir and leads to an increase in the gas recovery factor. Based on the results of statistical processing of the calculated data, the optimal value of the rate of natural gas production was determined when carbon dioxide is injected into the productive reservoir at the boundary of the gas-water contact is 55.93 thousand m3/day. The final gas recovery factor for the optimal natural gas production rate is 64.99 %. The results of the studies carried out indicate the technological efficiency of injecting carbon dioxide into productive reservoirs at the boundary of the gas-water contact in order to slow down the movement of formation water into productive reservoirs and increase the final gas recovery factor.


Introduction
The oil and gas industry is the main branch of the national economy of many countries of the world. The main attention in every society is paid to improving pro duction efficiency as a source of welfare growth.
The development of hydrocarbon deposits is compli cated by many geological and technological factors. How ever, the problem of flooding of productive deposits and producing wells is extremely urgent and important at present for the world practice of natural hydrocarbon pro duction [1].
The complexity of natural gas production is due to uneven drainage of productive deposits, which leads to uncontrolled movement of the gaswater contact along the productive section and the area of gas content in the wa ter pressure regime [2]. The heterogeneity of productive deposits both in thickness and in area leads to the cutting off by the formation water front of the productive reservoir areas with high residual gas saturation [3].
Under the conditions of the manifestation of the wa terdriven regime of watering of production wells, it is a natural and inevitable process [4]. Usually this can be associated with both an «emergency» breakthrough of water from aquifers due to poorquality casing of the wells, and breakthrough of formation water through highly perme able interlayers. Breakthrough of formation water leads to a decrease in the current production of hydrocarbons and the production capacity of the field [1].
The natural gas recovery factors from deposits, which are characterized by a water pressure regime, are 70-85 %. The more active the water pressure system, the greater the saturation with residual gas and the lower the hy drocarbon recovery factor [5,6].
Hydrocarbon fields discovered in recent years are cha racterized by a complex structure, large depths of occur rence of productive deposits, insignificant reserves and can't significantly affect the maintenance of hydrocarbon produc tion [7]. That is why there is a need to develop optimal ways to maintain hydrocarbon production from existing, ISSN 2664-9969 already to a certain extent depleted hydrocarbon fields. Increasing the efficiency of the development of hydrocarbon deposits for the water pressure regime at different stages of their development is possible subject to the introduc tion of scientific achievements and the latest technologies.
A promising direction for increasing the hydrocarbon yield of gas condensate deposits is the introduction of secondary development technologies using nonhydrocarbon gases [8,9].
An important task in the design of the secondary hy drocarbon production technology is the choice of the dura tion of the injection periods of nonhydrocarbon gases, the number and system of location of production and injection wells in the area of the reservoir, as well as the techno logical parameters of their operation [10].
In this work, the influence of the rate of natural gas production during the injection of carbon dioxide into pro ductive deposits on the verge of the initial gaswater contact on the gas recovery coefficient is investigated.

The object of research and its technological audit
The object of research is gas condensate fields develo ped under the conditions of the manifestation of a water pressure regime and the negative effect of formation water on the process of natural gas production.
To improve the existing and develop new technologies for intensifying hydrocarbon production under conditions of a water pressure regime by injecting nonhydrocarbon gases into productive deposits, additional research is required. On the basis of the studies carried out, it is necessary to substantiate the optimal methods and technologies for which it is possible to ensure maximum hydrocarbon recovery factors. Solving the problem of regulating the process of flooding of productive deposits and production wells is one of the directions of energysaving development of the economy of any state that is engaged in the production of natural hydrocarbons.

The aim and objectives of research
The aim of research is to study the efficiency of the technology of injecting carbon dioxide into the reservoir at the initial gaswater contact to regulate the process of formation water inflow into gassaturated horizons using numerical modeling.
To achieve this aim, it is necessary to complete the following objectives: 1. Investigate the effect of the rate of natural gas pro duction during the injection of carbon dioxide into the reservoir on the activity of the water pumping system.
2. Establish the optimal rate of natural gas produc tion when injecting carbon dioxide into the productive reservoir on the verge of the initial gaswater contact.

Research of existing solutions to the problem
The overwhelming majority of oil and gas condensate fields is confined to reservoir waterpressure systems and is developed according to the waterpressure regime, the essence of which is the movement of formation water into gassaturated horizons and the restriction of residual hy drocarbon reserves in a porous medium. A significant number of studies have been carried out to develop optimal ways to extract micro and macroentrained gas from productive deposits. On the basis of the results of the studies carried out, the mechanism of the behavior of restrained gas by formation water in a porous medium has been revealed [11,12]. However, until now, the problem of increas ing the hydrocarbon production from hydrocarbon deposits developed under the water pressure regime remains important.
According to the analysis of numerous studies, a pro mising direction for increasing the hydrocarbon production from depleted, watered productive deposits is the displace ment of residual gas by injecting nonhydrocarbon gases. Nitrogen, carbon dioxide, flue gases, mixtures of various gases, etc. are used as injection agents into productive deposits in order to increase the hydrocarbon yield [13,14].
In 1941, it was first proposed to inject carbon diox ide under high pressure into depleted deposits [15]. For the first time, work on injecting carbon dioxide into the reservoir was carried out in the United States in 1949.
The studies were carried out on oil reservoirs that were previously flooded and reached the limit of profitable hy drocarbon production under such conditions. An industrial experiment increased the final oil recovery factor by 10 %.
The positive results of the work carried out led to the introduction of the technology under study in ten more different fields. Due to the injection of carbon dioxide, additional production of hydrocarbons was provided, which determined significantly higher production capabilities of the fields. The results achieved made it possible to use carbon dioxide in the fields of the USA and Canada [16,17]. Also, carbon dioxide is successfully used in Norway to increase the production of carbohydrates from productive deposits under the North Sea [18].
Thus, the technology of injection of carbon dioxide is currently one of the most successful technologies in the field of secondary hydrocarbon production. Nume rous studies of the process of injection of carbon dioxide into productive deposits with the aim of increasing their hydrocarbon production confirm its effectiveness [19,20].
The carbon dioxide injection technique is to create an artificial barrier between water and natural gas, due to which it is ensured that formation water is blocked from entering gassaturated horizons.
In the case of the introduction of the technology of carbon dioxide injection in the flooding of a part of the deposit, part of the restrained gas is displaced into the production wells. In the zone of injection of carbon dioxide, reservoir pressure increases, causing the creation of an additional hydrodynamic barrier, due to which the process of inflow of reservoir water into gassaturated horizons is partially complicated. The faster the technology of inject ing carbon dioxide in the field is implemented, the higher the efficiency of this technology and the much higher the final coefficients of hydrocarbon recovery [7].
According to the results of modeling the process of de veloping productive deposits, it was found that the highest coefficient of hydrocarbon production is provided in the case of developing a deposit for depletion to an economi cally viable boundary with subsequent injection of carbon dioxide into the formation [21,22].
The results of laboratory studies [23] indicate that the final gas recovery factor in the case of its displace ment using nonhydrocarbon gases depends on the type of displacing agent and the degree of reservoir heterogeneity. TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 1/3(57), 2021

ISSN 2664-9969
The study of the process of developing hydrocarbon fields indicates that in order to ensure a more complete coverage of productive deposits by development, it would be desirable to completely prevent the movement of produced water. To date, no practical solution to this problem has been found [10,24]. Taking into account the above, in order to improve the existing technologies for the development of natural gas fields, it is advisable to carry out additional research using the tools of hydrodynamic modeling.

Methods of research
To assess the effect of the rate of natural gas pro duction on the gas recovery rate when injecting carbon dioxide into the reservoir, the main Eclipse and Petrel hydrodynamic modeling tools from Schlumberger (USA) were used. The study was carried out on the basis of a digital threedimensional model of a gas condensate field. To reproduce the phase transformations that take place in the pore space with a decrease in reservoir pressure, a composite PVT model was used [25,26].
The development of the gas condensate bed is carried out using 5 production wells, and the injection of carbon dioxide is carried out using 6 injection wells located at the initial gaswater contact. The acceptance rate of injection wells is taken at the level of 50 thousand m 3 /day. Calcu lations were carried out for production wells flow rates at the level of 30, 40, 50, 60, 70, 80 thousand m 3 /day.
The research methodology and processing of the hy drodynamic modeling results are given in [10].

Research results
Based on the results of calculations of technological indicators of reservoir development, it was found that due to the injection of carbon dioxide at the boundary of the initial gaswater contact, additional gas production is provided and the volume of formation water produc tion is reduced.
Comparison of the dynamics of accumulated hydrocarbon production during the injection of carbon dioxide and dur ing the development of a reservoir for depletion for a gas production rate of 40 thousand m 3 /day is shown in Fig. 1. Using the results of modeling, the analysis of the de pendence of the breakthrough time of carbon dioxide in production wells on the rate of natural gas production was carried out. According to the results of the calcula tions, it can be concluded that due to the increase in the rate of natural gas production, the period of development of the productive position decreases by the time of the breakthrough of carbon dioxide into the producing wells. The higher the rate of natural gas production, the faster the injection agent breaks into the production wells and the faster it leads to their decommissioning.
During the operation of production wells with a gas flow rate of 30 thousand m 3 /day, carbon dioxide reaches the production wells in 70 months. An increase in the production rate of production wells to 80 thousand m 3 /day leads to a decrease in the duration of their operation period to 31 months.
Analyzing the dependence of reservoir pressure on the rate of depletion of the productive reservoir, it should be noted that the higher the rate of production, the more intensively the reservoir pressure in the reservoir decreases. It is also necessary to pay attention to the nature of the dependencies obtained for different rates of natural gas production. This nature of the change in reservoir pressure over time is explained by the shutdown of production wells due to the breakthrough of carbon dioxide, or watering. If the well is shut down for one of the above reasons, the production of natural gas from the reservoir decreases, which leads to a decrease in the rate of reservoir pressure drop. The inflow of formation water into the productive horizons after stopping production wells continues, which leads to an intensive increase in reservoir pressure at the final stages of development.
The dynamics of reservoir pressure depending on the rate of natural gas production are shown in Fig. 2.  Based on the results of the studies carried out, the calculation of the accumulated production of formation water at the time of the breakthrough of carbon dioxide in the production wells was carried out. Based on the analysis of calculations, it was found that an increase in the rate of natural gas production leads to an increase in the production of formation water at the time of break through of carbon dioxide into production wells.
The dependence of the cumulative water production on the rate of natural gas production at the time of the TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 1/3(57), 2021 ISSN 2664-9969 breakthrough of carbon dioxide into production wells and during the development of the reservoir for depletion is shown in Fig. 3. Analyzing the results of modeling the development of deposits for depletion and with injection of carbon dioxide, it should be noted that an increase in the rate of gas production during injection of carbon dioxide leads to a decrease in the production of produced water. Consider ing the difference in the densities of carbon dioxide and natural gas, as well as its solubility in formation water, it can be argued that the injection of nonhydrocarbon gas provides blocking of selective watering of the reservoirs. If the technology under study is introduced, an artificial barrier is created at the interface between the two phases, which minimizes the negative impact of formation water on natural gas production. This ensures stable and water less production of production wells for a long period of development of gas condensate deposits. The calculated results indicate the technological efficiency of injection of carbon dioxide into the reservoir in order to regulate the flow of aquifer waters into the gassaturated horizons.
The accumulated production of water during injection of carbon dioxide, depending on the rate of natural gas production at the end of the development of the produc tive position, varies within wide limits and is 30 thou Using the results of the studies carried out, the cal culation of the gas recovery coefficients was carried out during development for depletion and at the moment of breakthrough of carbon dioxide into a number of production wells. The results of the calculations are given in Table 1.
According to the simulation results, it was found that an increase in the rate of natural gas production leads to an increase in the gas production coefficient. Also, it should be noted that if the technology of injecting nonhydrocarbon gas into the reservoir is introduced, significantly higher gas recovery factors are provided at the time of carbon dioxide breakthrough compared to depletion development. The dependences of the gas recovery factor on the rate of gas production at the time of the breakthrough of dioxide and during the development of the reservoir for depletion are shown in Fig. 4. Based on the results of the calculated data, the optimal value of the rate of natural gas production was determined when carbon dioxide is injected into a productive reservoir at the initial gaswater contact, outside of which the gas recovery coefficient changes insignificantly. The optimal value of the production well flow rate at the moment of carbon dioxide breakthrough into a number of production wells is 55.93 thousand m 3 /day. The predicted coefficient of gas recovery for the given optimal value of the rate of natural gas production when injecting carbon dioxide into the reservoir is 64.99 %, and when developing for depletion -58.34 %.
The modeling results indicate a high technological ef ficiency of injection of carbon dioxide into the reservoir at the initial gaswater contact in order to regulate the process of watering gassaturated horizons and increase the final hydrocarbon recovery factors for the conditions of a particular field.

SWOT analysis of research results
Strengths. Based on the results of the conducted stu dies, a high technological efficiency of the introduction of ISSN 2664-9969 secondary technologies for the development of hydrocarbon deposits using nonhydrocarbon gases was established. The simulation results show that the injection of carbon dioxide into the productive reservoir, which is being developed ac cording to the water pressure regime, allows to significantly intensify the process of hydrocarbon production. Thanks to the introduction of carbon dioxide injection technolo gies, it is possible to regulate the process of watering of gassaturated horizons and production wells. Based on the results of the studies carried out, it was found that in the case of injection of carbon dioxide, an increase in the natural gas recovery factor by 6.65 % is achieved in comparison with the development for depletion.
Weaknesses. The study of the effect of the rate of natural gas production during injection of carbon dioxide on the efficiency of regulating the process of watering the productive position was carried out on the basis of a hypothetical homogeneous threedimensional model. The heterogeneity of productive deposits introduces significant uncertainty in the process of justifying optimal technolo gies. That is why, in order to develop optimal ways to increase the hydrocarbon production from the fields, it is necessary to conduct additional research using permanent geological and technological models of real fields.
Opportunities. The results of the studies carried out make it possible to improve the existing technologies for the development of gas and gas condensate fields accord ing to the water pressure regime. Low coefficients of hy drocarbon recovery under conditions of active inflow of formation water into productive deposits make this kind of research promising. The main task of future research is to establish optimal operating modes for injection wells to ensure reliable hydrodynamic and filtration barriers in waterhazardous areas.
Threats. Carbon dioxide injection technologies are well known and widely used all over the world to increase the coefficients in the hydrocarbon production of depleted deposits. However, the effectiveness of these technologies depends solely on the availability of a reliable source of supply of carbon dioxide. Failure to provide the necessary volumes of nonhydrocarbon gas for injection into productive reservoirs may lead to a decrease in the predicted effect.

Conclusions
1. Based on the results of the studies carried out, it was found that an increase in the rate of natural gas production with the injection of carbon dioxide leads to a decrease in the accumulated production of formation water. If the technology of injecting carbon dioxide into a productive reservoir is introduced, on the verge of the initial gaswater contact, the creation of hydrodynamic and filtration barriers is ensured, due to which the flow of aquifer waters into gassaturated horizons is partially blocked. Based on the simulation results, it was also es tablished that with an increase in the rate of gas pro duction, the accumulated gas production increases, and, consequently, the final gas production coefficient. The results of the studies carried out indicate the high technological efficiency of the investigated method for increasing the coefficient of gas recovery in the water pressure regime.
2. Using the results of hydrodynamic modeling, the optimal value of the rate of natural gas production when injecting carbon dioxide into the productive reservoir at the initial gaswater contact was determined. At the time of the breakthrough of carbon dioxide into a number of production wells, the optimal production rate of a production well is 55.93 thousand m 3 /day. The gas recovery factor for the given optimal value of the gas production rate when injecting carbon dioxide is 64.99 %, and when developing for depletion -58.34 %. The above research results indi cate a high technological efficiency of the technology for injecting carbon dioxide into a reservoir at the initial gas water contact in order to control and regulate the process of formation water inflow into gassaturated horizons.