Analysis of the Relationship Between Oil Production and CO2 Storage during Low-Salinity Carbonated Waterflooding in Acidic Carbonate Reservoir

Abstract

Carbonated waterflooding (CWF) is an attractive process in the view of EOR (enhanced oil recovery) and CO2 storage. This process can have more advantages in combined with low-salinity waterflooding (LSWF). Low-salinity carbonated water (LSCWF) is a favorable environment to transport of CO2 from brine to oil. However, there is no previous study on the application of LSCWF into acidic carbonate reservoir. When carbonated water is injected into an acidic carbonate reservoir, unlike ordinary reservoir containing neutral formation water, dissolved CO2 in injection water can affect the chemical reaction (mineral precipitation and dissolution). In this aspect, it is necessary to investigate the effect of injection water salinity on mineral reaction in acid condition. In order to analyze the oil production and CO2 storage, LSCWF simulation model was constructed through the history matching process. Then, pH change in acidic and neutral carbonate reservoirs was examined during LSCWF. In acidic condition, mineral precipitation dominantly occurred, reducing the effective permeability, while mineral dissolution was dominant in neutral condition. Meanwhile, CO2 solubility in brine and CO2 mass transfer from carbonated water (CW) into oil was more active in the case of low salinity water due to salting-out phenomena. This transported CO2 contributed to oil swelling and viscosity reduction enhancing oil mobility. In addition, ion exchange results in wettability alteration more in low salinity water. As a result, more additional oil recovery by 3.5% was observed in the neutral reservoir at the injection of lowest salinity of brine compared to acidic reservoir. LSCWF also has significant advantage on CO2 storage. Storage effect was slightly higher in neutral reservoir and highest salinity of injection water due to larger CO2 mass transfer. Finally, we proposed the relationship between oil recovery and CO2 storage according to the salinity of injection water.; 저염탄산수 주입공법은 기존의 탄산수 주입공법과 저염수 주입공법의 시너지 효과를 통해 오일회수율을 더욱 증진시킬 수 있는 하이브리드 오일회수 증진공법이다. 저염탄산수 주입공법을 산성 탄산염암 저류층에 적용할 경우 침전 반응에 의해 투과도가 감소할 수 있으나 이에 대한 연구가 이루어지지 않았다. 탄산수를 산성의 탄산염암 저류층에 주입하게 되면, 주입수 내 용해되어 있는 이산화탄소가 일반적인 중성의 지층수를 함유한 저류층과 다른 화학반응(암석의 용해 및 침전)을 일으키게 된다. 이러한 측면에서 염도가 다른 탄산수가 산성조건에 주입되는 경우 암석에 일으키는 화학반응에 대해 분석을 할 필요가 있다. 이에 본 연구는 저염탄산수공법이 오일생산과 이산화탄소 저장에 미치는 영향을 규명하기 위해 히스토리 매칭과정을 통하여 저염탄산수공법 시뮬레이션 모델을 구축하였다. 이후 구축된 모델을 기반으로 하여 산성저류층과 중성저류층에 염도가 다른 탄산수를 주입하는 시뮬레이션을 수행하였다. 우선, 저염탄산수를 주입 하는 동안의 산성저류층과 중성저류층에서의 pH 변화를 관찰하였다. 산성저류층 내에서는 암석의 침전이 우세하여 유효투과도가 줄어들었으나, 중성저류층에서는 암석의 용해가 우세하였고 두 경우 모두 주입수의 염도가 높을수록 암석의 화학반응이 많이 일어났다. 한편, 염수내의 이산화탄소 용해도와 이산화탄소가 탄산수에서 오일로 이동하는 현상은 salting-out 효과에 의해 가장 염도가 낮은 경우에 활발하였다. 이렇게 이동된 이산화탄소는 오일 스웰링과 점도의 감소에 영향을 끼쳐 오일의 유동도와 접촉효율을 더 좋게 하였다. 또한, 주입수에 의한 이온교환반응은 저염주입수일 경우, 중성 저류층일 경우 더 활발하였다. 이에 따라 잔류오일 포화율의 변화가 크게 나타났으며 이는 상대투과도 곡선이 오른쪽으로 이동함을 의미하므로 암석의 습윤도가 친수성으로 변환된 것을 알 수 있다. 결과적으로, 산성저류층에 비해 중성 저류층에서 3.5% 가량의 더 높은 오일 회수율이 나타나 저염탄산수 주입 시 중성 저류층에서 더 양호한 오일생산 증진 효과를 확인하였다. 이산화탄소 저장 측면에 있어서 저염탄산수의 염도가 높을수록 탄산수에서 오일로의 이산화탄소 이동량이 감소하여 저류층내 이산화탄소 저장량이 증가하였다. 결과적으로 오일회수증진과 이산화탄소저장 효과의 관계를 나타내는 COR 값의 계산을 통해 저염의 탄산수를 주입할수록 이산화탄소 저장 대비 효과적인 오일생산이 가능함을 제시하였다.|Carbonated waterflooding (CWF) is an attractive process in the view of EOR (enhanced oil recovery) and CO2 storage. This process can have more advantages in combined with low-salinity waterflooding (LSWF). Low-salinity carbonated water (LSCWF) is a favorable environment to transport of CO2 from brine to oil. However, there is no previous study on the application of LSCWF into acidic carbonate reservoir. When carbonated water is injected into an acidic carbonate reservoir, unlike ordinary reservoir containing neutral formation water, dissolved CO2 in injection water can affect the chemical reaction (mineral precipitation and dissolution). In this aspect, it is necessary to investigate the effect of injection water salinity on mineral reaction in acid condition. In order to analyze the oil production and CO2 storage, LSCWF simulation model was constructed through the history matching process. Then, pH change in acidic and neutral carbonate reservoirs was examined during LSCWF. In acidic condition, mineral precipitation dominantly occurred, reducing the effective permeability, while mineral dissolution was dominant in neutral condition. Meanwhile, CO2 solubility in brine and CO2 mass transfer from carbonated water (CW) into oil was more active in the case of low salinity water due to salting-out phenomena. This transported CO2 contributed to oil swelling and viscosity reduction enhancing oil mobility. In addition, ion exchange results in wettability alteration more in low salinity water. As a result, more additional oil recovery by 3.5% was observed in the neutral reservoir at the injection of lowest salinity of brine compared to acidic reservoir. LSCWF also has significant advantage on CO2 storage. Storage effect was slightly higher in neutral reservoir and highest salinity of injection water due to larger CO2 mass transfer. Finally, we proposed the relationship between oil recovery and CO2 storage according to the salinity of injection water.