유가스정 주변의 코닝 현상과 컨덴세이트 뱅크 현상에 의한 생산성 분석

Abstract

저류층의 유가스를 지상으로 생산하는 동안, 생산정의 압력강하는 피할 수 없지만, 과도한 압력강하가 발생되면 유전에서는 대수층의 물이나 가스층의 자유가스가 오일과 함께 생산되는 코닝 현상이 나타나며, 가스전에서는 PVT 상변화로 인하여 가스로부터 응축된 컨덴세이트가 가스와 함께 유동되어 저류층의 생산성이 크게 저하되기 때문에 오래전부터 연구의 대상이 되었다. 본 연구에서는 유전에서 발생하는 가스·물코닝의 동시 거동과 가스전에서 형성되는 컨덴세이트 뱅크 현상을 시뮬레이션을 통해 확인하고, 유가스의 생산성에 미치는 영향과 이를 제어하기 위한 방안에 대한 분석을 수행하였다. 유정 주변에서의 코닝 현상을 분석한 결과, 임계생산량 이론해에 근거한 오일량에서도 코닝이 발생하였으며, 이는 이론해가 저류층의 비정상상태, 모세관압, 젖음성 등을 반영하지 못하기 때문이다. 오일층 두께에 따른 분석결과, 얇은 오일층에서는 생산 초기부터 물 및 가스의 BT가 발생되고, WOR가 증가하여, 오일의 생산성이 저하되었다. 반면, 200 ft로 두꺼운 오일층은 물코닝이 생산정에 유입되지 않은 채, 안정된 상태로 존재하였다. 코닝은 수직 방향으로 거동하므로 수직투과도에 큰 영향을 받았으며, 낮은 수직투과도에서 BT가 지연되었고, 높은 수직투과도에서는 생산 초기에 Concave 형태로 발생되었던 물코닝이, 2년 시점에서는 오일-대수층 경계면이 전체적으로 상승하면서 WOR이 급증하고 생산성이 저하되는 것을 확인하였다. 대수층의 크기에 따른 WOR이나 물 생산량의 변화는 미미하여, 오일의 생산성에 미치는 영향이 작았지만, 대수층 크기가 큰 경우는 대수층에 의한 저류층 압력 유지 효과로 가스 생산량이 절반 가까이 감소되는 것을 알 수 있었다. 유정의 천공 길이가 짧을수록, 코닝 발생 시점은 지연되었지만, 너무 짧으면 오일 생산구간의 감소로 압력강하가 크게 발생하여 오히려 생산성이 악화되었다. 유정에서 오일 생산량을 감소시킴으로써, 코닝을 효과적으로 방지하고 생산성을 증진시킬 수 있었지만, 오일 생산량은 유전의 경제성과 직결되는 문제이므로, 중·장기적인 생산 계획을 고려하여 적정유량을 선정하는 작업이 필요하다. 가스전의 컨덴세이트 뱅크 현상을 분석한 결과, 공저압이 이슬점 압력보다 낮기 때문에 생산과 동시에 가스정 주변으로 컨덴세이트가 급격히 형성되었고, 가스의 유동을 방해하여 생산성이 크게 저하되었으며, 생산 초기에 가스-오일비의 변동이 크게 일어나는 컨덴세이트 가스전의 복잡한 거동을 관찰할 수 있었다. 저류층의 불균질성이 높은 경우, 컨덴세이트 뱅크 반경이 생산 1일 만에 100 ft까지 증가되었고, 생성된 컨덴세이트의 포화도는 최대 0.61까지 높아져서 가스의 유동성이 저하되고, 가스의 회수율이 감소되는 것을 확인할 수 있었다. 일반적으로 가스정에 가까운 지역에서 컨덴세이트 포화도가 높게 나타나지만, 저류층의 불균질성이 심하면 가스정에서 먼 지역에서도 지역적인 투과도 분포에 따라 포화도가 높게 나타났다. 컨덴세이트로 인한 생산성 저하를 줄이기 위해 가스정의 공저압을 단계적으로 감압시켜 운영한 결과, 생산 초기에 큰 압력강하를 유지한 경우에 유가스 생산량이 최대가 되어, 생산성이 높은 것을 알 수 있었다. 그러나 컨덴세이트 형성 시점이 빨라지고, 잔류 컨덴세이트의 증가로 인해 생산 가스-오일비가 높아지므로, 해당 가스전의 중·장기적인 생산 계획과 경제성을 종합적으로 고려하여 가스정의 운영 압력과 유량을 최적화하는 생산 전략의 수립이 필요하다.| While producing oil and gas from the reservoir to the surface, there exists a unavoidable pressure drop near a wellbore. Due to an excessive pressure drop, the water and gas coning phenomena occur in oil reservoirs with producing water from an aquifer and gas from the gas zone. In condensate gas reservoirs, a liquid condensate drops out from a gas phase and flows with gas towards a well because of compositional changes in PVT phase behavior. Since these all make a productivity of reservoir worse and reduce an overall recovery, many studies of coning phenomena and condensate bank phenomena have been performed for a long time. In this study, a reservoir simulation has been conducted to investigate the effect of well's productivity due to the behavior of coning and condensate bank near well. Also this study presents conformance control methods to minimize coning problems and condensate bank problems effectively. From simulational results of coning behavior near well, gas and water coning were observed even with the critical oil rate obtained analytically. It is because the critical rate is calculated using a steady state expression. In order to examine the coning behavior, firstly, we have run for various oil layer thicknesses. The result in case of thin layer shows early breakthrough of gas and water cones and the increase in water-oil ratio which worsens the well's productivity. Meanwhile, for the thick case of 200 ft, there is no water breakthrough observed even though water cone has been already formed because it is stable. Since cones move mainly in a vertical direction, the development of cone is greatly affected by a vertical permeability. The breakthrough time is getting delayed as the vertical permeability is smaller. In case of a high vertical permeability, the shape of water cone is developed in a form of concave at the beginning, however, after 2 years of production, this cone shape becomes almost flat since the oil-water contact is elevated uniformly throughout the whole reservoir while reducing the productivity. In the analysis of coning behavior for different aquifer sizes, it is found that aquifer size does not affect both water-oil ration and water production rates. But, with a strong bottom aquifer, gas production rates are decreased by half since the pressure is maintained by the active aquifer. As the completion interval is decreased, the breakthrough time is delayed. However, a large pressure drop is occurred in the shortest interval so that it worsens the well productivity. The most practical method to control coning is the oil production rate. By keeping oil rates as low as possible, production of gas and water can be minimized and the productivity of reservoir is improved. But, the low rate is directly linked to well's economics, and therefore, the optimizing process for production rate is essential with the consideration of mid and long-term production plan. From the analysis of condensate bank phenomena in a gas-condensate reservoir, the condensate forms rapidly in the vicinity of the well as soon as the gas production since the bottom-hole pressure is much lower than the dew-point pressure. The accumulated condensate causes a blockage effect and reduce the effective permeability appreciably, and also causes the decline of well's productivity. The trend of producing GOR shows the abrupt changes which represents typical complex behaviors of gas-condensate reservoirs. In a very heterogenous reservoir, the bank radius expands up to 100 ft in a day and the maximum saturation of condensate reaches 0.61. Therefore, the mobility of gas is dropped and the overall recovery of oil and gas is decreased. While oil saturation has a highest value near the well in general, in case of reservoir with a strong heterogeneity, some regions away from the wellbore show high level of oil saturation according to local distributions of permeability. To increase the productivity, a different producing strategy was conducted by changing the manner in which the well is brought into flowing condition. The total gas and oil production can be achieved by lowering the BHP quickly in the first half of production period instead of dropping the BHP quickly in the last half of production time. However, it makes the formation of condensate bank earlier and increases the producing GOR due to the growth of condensate bank. Accordingly, gas productivity can be maximized by optimizing the producing sequence, such as operating pressures and flow rates by considering mid and long-term production plan and well's economics.; While producing oil and gas from the reservoir to the surface, there exists a unavoidable pressure drop near a wellbore. Due to an excessive pressure drop, the water and gas coning phenomena occur in oil reservoirs with producing water from an aquifer and gas from the gas zone. In condensate gas reservoirs, a liquid condensate drops out from a gas phase and flows with gas towards a well because of compositional changes in PVT phase behavior. Since these all make a productivity of reservoir worse and reduce an overall recovery, many studies of coning phenomena and condensate bank phenomena have been performed for a long time. In this study, a reservoir simulation has been conducted to investigate the effect of well's productivity due to the behavior of coning and condensate bank near well. Also this study presents conformance control methods to minimize coning problems and condensate bank problems effectively. From simulational results of coning behavior near well, gas and water coning were observed even with the critical oil rate obtained analytically. It is because the critical rate is calculated using a steady state expression. In order to examine the coning behavior, firstly, we have run for various oil layer thicknesses. The result in case of thin layer shows early breakthrough of gas and water cones and the increase in water-oil ratio which worsens the well's productivity. Meanwhile, for the thick case of 200 ft, there is no water breakthrough observed even though water cone has been already formed because it is stable. Since cones move mainly in a vertical direction, the development of cone is greatly affected by a vertical permeability. The breakthrough time is getting delayed as the vertical permeability is smaller. In case of a high vertical permeability, the shape of water cone is developed in a form of concave at the beginning, however, after 2 years of production, this cone shape becomes almost flat since the oil-water contact is elevated uniformly throughout the whole reservoir while reducing the productivity. In the analysis of coning behavior for different aquifer sizes, it is found that aquifer size does not affect both water-oil ration and water production rates. But, with a strong bottom aquifer, gas production rates are decreased by half since the pressure is maintained by the active aquifer. As the completion interval is decreased, the breakthrough time is delayed. However, a large pressure drop is occurred in the shortest interval so that it worsens the well productivity. The most practical method to control coning is the oil production rate. By keeping oil rates as low as possible, production of gas and water can be minimized and the productivity of reservoir is improved. But, the low rate is directly linked to well's economics, and therefore, the optimizing process for production rate is essential with the consideration of mid and long-term production plan. From the analysis of condensate bank phenomena in a gas-condensate reservoir, the condensate forms rapidly in the vicinity of the well as soon as the gas production since the bottom-hole pressure is much lower than the dew-point pressure. The accumulated condensate causes a blockage effect and reduce the effective permeability appreciably, and also causes the decline of well's productivity. The trend of producing GOR shows the abrupt changes which represents typical complex behaviors of gas-condensate reservoirs. In a very heterogenous reservoir, the bank radius expands up to 100 ft in a day and the maximum saturation of condensate reaches 0.61. Therefore, the mobility of gas is dropped and the overall recovery of oil and gas is decreased. While oil saturation has a highest value near the well in general, in case of reservoir with a strong heterogeneity, some regions away from the wellbore show high level of oil saturation according to local distributions of permeability. To increase the productivity, a different producing strategy was conducted by changing the manner in which the well is brought into flowing condition. The total gas and oil production can be achieved by lowering the BHP quickly in the first half of production period instead of dropping the BHP quickly in the last half of production time. However, it makes the formation of condensate bank earlier and increases the producing GOR due to the growth of condensate bank. Accordingly, gas productivity can be maximized by optimizing the producing sequence, such as operating pressures and flow rates by considering mid and long-term production plan and well's economics.