Selection of proper enhanced oil recovery fluid for efficient matrix recovery in fractured oil reservoirs

As most of the oil is stored in matrix due to its higher storage capacity than fracture network of naturally fractured reservoirs (NFR), reservoir development plans will aim at maximizing the matrix oil recovery. An enhanced oil recovery (EOR) application principally targets (a) to minimize the residual oil in matrix depleting the matrix as effective as possible and/or (b) to accelerate the recovery rate for rapid production of oil cost efficiently. For reservoirs with high recovery factor, minimizing matrix residual oil saturation is a critical issue to extend the life of the reservoir. For reservoirs with low recovery factor, accelerating the production rate is more vital. For each of these reservoir types, different FOR methods should be considered and implemented accordingly. This paper addresses and discusses these two issues and identifies selection criteria for different FOR methods in NFR, namely chemical (surfactant and polymer) and hot water injection. The focus is specifically on matrix type (permeability and wettability), oil and water viscosities, matrix boundary conditions, transfer type (co- or counter-current imbibition), and interfacial tension (IFT). For different values of these properties, the most proper injection fluid type to be used as an FOR fluid is identified to obtain an effective matrix recovery. Co-current and counter-current capillary imbibition experiments at static conditions are conducted to show how effective and how useful these applications are for different rock and fluid types and matrix boundary conditions. For experimentation, strongly water wet Berea Sandstones and oil-wet carbonates (cores from an oil formation) are used. Light crude oil, kerosene and engine oil are selected as the oleic phase. Proper project implementation (adjustment of injection rate and/or concentrations) and selection of the injection fluid for a cost efficient management are also discussed for different conditions outlined above. Finally, the selection criteria of FOR methods based on the rock and fluid properties are defined. This will provide an insight into an effective management of NFRs and reservoir depletion strategies, if the matrix oil recovery is the main target. (C) 2003 Elsevier B.V. All rights reserved.