Wettability alteration is one of the most promising techniques to enhance oil recovery of oil-wet reservoirs. As contact angle decreases within the matrix, the flux of fracture-matrix counter-current imbibition increases significantly (Sedaghat et al., ECMOR 2016). However, wettability altering agent only influences the wettability of a portion of the matrix region, and not necessarily uniformly. The agent (water phase) makes the fractures and surrounding matrix water-wet, but this alteration is gradually reduced with the distance from the fracturematrix interface. Beyond the fracture-matrix imbibition halo, wettability (contact angle) does not change anymore. So, this wettability transition zone needs to be considered when simulation of multiphase flow in naturally fractured rocks is performed.
Utilizing a Finite-Element-Centered-Finite-Volume (FECFV) numerical approach, we simulated a waterflooding scenario on a discrete fracture and matrix (DFM) model built based on an outcrop analogue. First, a wettability altering agent is flooded into the system until the change in the saturation gets negligible. The system is then equilibrated in a given time period. By weighting contact angle with wetting fluid saturation, wettability is updated for each element at the equilibrium state. Finally, waterflooding is performed with the updated wettability. Assuming that the wettability altering agent has 100% performance, it does not influence water properties and requires a considerable time to change the wettability. Therefore, reactive transport is left out of the computations.
Compare to the results of base case model in which wettability transition zone was neglected (Sedaghat et al., ECMOR 2016), partial wettability alteration associated with a non-homogenous distribution of contact angle significantly influences the oil recovery, fracture-matrix counter-current imbibition, and ensemble relative permeability. Moreover, velocity fluxes and flow behaviour is influenced dramatically as it leads to a heterogeneous distribution of capillary pressure over the matrix within the fracture-matrix counter-current imbibition halos.