Cyclic Water Injection Improves Oil Production in Carbonate Reservoir

Abstract

Abstract The cyclic injection process to improve water flooding in carbonate reservoir was evaluated in laboratory experiments, as well as analytical and numerical simulations. Laboratory cyclic injection experiments were performed on a carbonate rock sample with an artificial horizontal fissure. Live oil was established by in-situ recombination of dead oil with hydrocarbon gas at reservoir temperature and bubble point pressure. The experiments were designed by numerical simulation of the cyclic process. Analytical modelling was done to evaluate cyclic injection sensitivity to critical reservoir and process parameters. The cyclic water injection experiments above oil bubble point pressure increased oil recovery by additional 2.9% of Oil Originally In Place (OOIP) above ordinary water flood. This effect can be attributed to the hasten imbibition of water into the matrix during pressurisation half-cycles and capillary retained water in the fine pores in the matrix resulting in oil counter current flow from matrix to the fracture during de-pressurisation half-cycles. Cyclic injection below bubble point pressure, designed to ensure gas saturation not exceeding the critical level, yielded additional recovery of 5.9% of OOIP. This effect may be credited to the energy of released gas, expelling the matrix oil into the fracture. The experimental results indicate significant potential of cyclic injection to improve micro level displacement efficiency of waterflood in carbonate reservoirs. If cyclic water injection is applied at field scale, sweep efficiency improvement from flow pattern redistribution could make an additional contribution. This has been reported by several field cases in the US, China and Former Soviet Union (FSU). Introduction The concept of cyclic injection is based on (1) pulsed injection and (2) alternating waterflood patterns [1–4]. Cyclic injection appears to have the greatest potential in heterogeneous, permeability-contrast reservoirs, with light, high compressibility fluids. Cyclic or pressure pulsing injection has been successfully applied in a number of sandstone and fractured carbonate oil fields in Russia, USA and China [1, 5–8]. Reduction of water production and acceleration of oil extraction rates were observed at several field applications. In the cyclic process, water injection rates are changed between high and low values, in a periodic fashion. The cycle periods at field scale are typically in the range of days to months, much different from the known pulsed pressure technique, where several pressure pulses are applied in time intervals of minutes. Laboratory and field applications of cyclic water injection indicate that additional oil recoveries in the range of 2–15% can be achieved with significant reduction in water cut levels, making the process very attractive and profitable [13,14,15].