Capillary imbibition is one of the major recovery mechanisms in naturally fractured reservoirs (NFR) where most of the oil is stored in tight matrices. Most of the imbibition studies of NFR assume uniform distribution of fractures. However, in reality most of NFR are partially fractured with various degrees of fracture intensities. Studies on imbibition phenomena in partially naturally fractured reservoirs (PNFR) are yet to be fully investigated. Thus, this paper investigates the effectiveness of capillary imbibition phenomena for PNFR with various fracture intensities (FI). We define FI as the ratio of fractured portion of the reservoir to total reservoir volume. Moreover, the study shed lights on the effect of water injection rate on the performance of PNFR.
In this paper, a random distribution of fractures is assumed to simulate irregularity of fracture network. A dualporosity/ dual-permeability model is used to simulate the wateroil displacement phenomenon. Results show that the FI significantly affects the reservoir performance. Reservoirs with high FI are dominated by counter-current capillary imbibition phenomenon. Conversely, reservoirs with low FI are dominated by co-current capillary imbibition phenomenon. For reservoirs with intermediate FI, both phenomena have a critical role and the recovery is adversely affected.
Performance prediction of NFR depends upon the ability to simulate various inherent driving mechanisms of the reservoir such as water imbibition, gas-oil and water-oil gravity drainage, molecular diffusion and convection. The contrast in capillary pressure of the matrix and fractures constitutes the basic difference between recovery performance of NFR and non- NFR1–4. Oil displacement by water occurs either in the presence of an active water aquifer or during water flooding process. If the rock is water-wet, the displacement of oil by water is known as imbibition. Imbibition due to capillary forces is known as capillary imbibition. Oil displacement from porous medium by external force establishing a pressure gradient is called forced imbibition. Imbibition can be co-current and counter-current modes. Brownscombe and Dyes5 conducted core flooding experiments on cores from Spraberry reservoir of West Texas. They concluded that if most of the oil is stored in the matrix and enough volume of water is supplied to the fractures, capillary imbibition dominates the recovery mechanism. Since 1952, various experimental, analytical and numerical studies regarding imbibition phenomena in fractured porous media have been published. Experimental studies have been concerned primarily with the imbibition aspect of the flow mechanism in the matrix compared to total flow problem in the fracture - matrix system6–10 while the analytical studies have been mainly concentrated on developing matrix/fracture transfer functions, scaling formulations and extensions of Buckley-Leverett11 approach to imbibition in NFR6,12–18.
Numerical studies have been helpful in understanding the imbibition mechanism in NFR both at single block scale and field scale. Three major approaches have been used to model the fracture/matrix system. The first approach, known as "single porosity modeling" provides good accuracy in simulating complex fracture network but requires very large number of grids19–20. The second approach, called "dual porosity modeling" has been widely used in simulating flow mechanics in NFR13,21–22.