Fractured reservoirs have always been considered poor candidates for enhanced oil recovery. This is mainly due to the complexities involved in predicting performance in such reservoirs. A good understanding of multiphase flow in fractures is important to reduce oil bypass and increase recovery in these reservoirs. This paper presents CO2 flooding experiments in homogeneous and fractured rocks with in-situ saturation and porosity measurements using an X-Ray CT scanner. We found that injection rates played an important role in the recovery process, more so in the presence of fractures. At high injection rates we observed faster CO2 breakthrough and higher oil bypass than at low injection rates. But very low injection rates are not attractive from an economic point of view. Hence we injected viscosified water to reduce the mobility of CO2, similar to the WAG process. Breakthrough time reduced significantly and a much higher recovery was obtained. Saturation measurements were made from the CT scans and were found to be in good agreement with those obtained from effluent data.
Fractured reservoirs form a large percentage of the world's hydrocarbon reserves. However, in spite of their wide occurrence and huge reserves, the oil recovery from most of these reservoirs is extremely low. This can be attributed to their poor response to both secondary and tertiary recovery operations. In a fractured system, the displacement process is dependent on the fracture-matrix geometry, size and interaction apart from other physical phenomena (1). Uleberg and Hoier (2) suggest that the injection fluid tends to flow through the highly permeable fractures, often resulting in early breakthrough and poor sweep efficiency. This is especially true when the displacing phase is a highly mobile fluid like CO2. In order to improve the sweep efficiency and delay breakthrough, the mobility of the displacing fluid in fractures, has to be controlled.
Several field cases (3)(4)(5)(6) and laboratory experiments (7)(8)(9) indicate that the water-alternating-gas (WAG) process has been an effective mobility control method in most cases. In a fairly homogeneous system, water invades the zones bpreviously invaded by the gas, subsequently diverting the gas into other zones (10). But a completely different situation prevails in the presence of extreme heterogeneities like fractures. In such a case, the conformance control agent must be able to effectively divert the fluid into the matrix, thereby delaying breakthrough and reducing oil bypass. But the performance of WAG in terms of mobility control in fractures has not been adequately studied.
The goal of this work is to investigate CO2 flow in fractures, in the presence of water as a mobility control agent. Immiscible CO2 flooding experiments were first conducted using homogeneous cores (in the absence of fracture) at different injection rates to serve as a comparison for thefractured core experiments. The cores were then fractured using a core splitter. Experiments were then conducted in fractured cores with continuous CO2 injection and injection of specific pore volumes of water and CO2. The cores were then fractured using a core splitter.
