Matrix-fracture interaction due to capillary forces (spontaneous imbibition) is a commonly encountered process in fractured subsurface reservoirs during oil, gas, and geothermal fluid recoveries and contaminant transport. It occurs in counter-current manner if the interaction between matrix and fracture takes place from the same side. In this study, the effects of the matrix shape factor, wettability, and interfacial tension on the rate of imbibition and development of residual non-wetting phase saturation were studied experimentally.
To achieve this, experiments were conducted on Berea sandstone samples. Cylindrical samples with different shape factors were obtained by cutting the plugs _, 1 and 2 inch in diameter and 2, 4, and 6 inches in length. All sides were coated with epoxy except one end. Static imbibition experiments were conducted on vertically and laterally situated samples where the imbibition took place upward and lateral directions, respectively. Water-air, brine-kerosene, brine-mineral oil and surfactant solution-mineral oil pairs were used as fluids. The rate of imbibition was evaluated for different matrix size, shapefactor, wetting/non-wetting phase types and IFT. Conditions at which the shape factor controls the residual oil development were identified.
Capillary imbibition can be an effective recovery mechanism if matrix is water wet and enough amount of water exists in fractures of naturally fractured reservoirs. Mechanism is counter-current, i.e., displacement of non-wetting phase by wetting phase takes place from the same side of the matrix, if the boundary conditions physically restricting the contact of wetting phase with matrix from the other sides exist. Despite the recognition of the process for quite a long time [1–5], yet many issues are unanswered in regards to the dynamics of the spontaneous counter-current interaction between matrix and fracture. Some of these issues include the effect of the matrix shape factor, wettability, and IFT (when surfactants are used as the wetting phase) on the rate of imbibition (progress of the front) and the development of residual oil saturation in both swept and unswept zones. Previous studies mostly dealt with counter-current imbibition in uncoated matrices (all surfaces open to flow). Ma et al.  conducted counter-current imbibition experiments both with all surfaces open and partially coated cores. They used coated cores ranging between 4 and 6 cm in length and a fixed 11/2 inch in diameter and were able to apply scaling equations first proposed by Mattax and Kyte . Babadagli  used various configurations of matrix shape. He investigated co- and counter-current flow for surfactant and polymer solutions, and concluded that the modified form of Mattax and Kyte's scaling group is applicable for high-IFT cases. Bourbiaux and Kalaydjian  conducted counter-current imbibition experiments and estimated the relative permeabilities using numerical simulations. They observed that the relative permeabilities to the phases are smaller when compared to the ones for co-current flow. They stated that, although experimental and numerical results are consistent with the proposed theory, further investigation on counter-current interaction is needed. Schembre et al.  performed CT scanning experiments using Berea sandstone and diatomite.