Spontaneous Countercurrent Imbibition and Forced Displacement Characteristics of Low-Permeability, Siliceous Shale Rocks

Abstract

AbstractEnhanced oil production from naturally-fractured low-permeability reservoirs is challenging. A large amount of oil is generally left behind because of heterogeneity and unfavorable rock wettability. Spontaneous countercurrent imbibition in naturally-fractured reservoirs is an important oil recovery mechanism and has been widely studied in the oil and gas industry. Nevertheless, complicated rock properties such as low permeability and other factors controlling flow mechanisms make it difficult to understand the flow characteristics and to generalize expected recovery.Spontaneous countercurrent imbibition and forced displacement tests at reservoir temperature were completed using siliceous shale core plugs that have low permeability, relatively high porosity, and intermediate to oil-wet surfaces. Four brine formulations were examined: carbonated synthetic brine (pH of 3), acidic (pH of 3), neutral, and alkaline (pH of 12).During spontaneous countercurrent imbibition, the oil recovery was about 10% of the oil in place (OIP) or less when the low to the neutral pH synthetic brine was used. On the other hand, with the high pH synthetic brine, the oil recovery increased to 30% of the OIP. In forced displacement after spontaneous imbibition, the final oil recovery was 65%, 70-80%, and 95% of the OIP in the neutral pH, low pH and carbonated water, and high pH brine, respectively.The siliceous shale core used in this study is an intermediate to oil-wet rock as gauged from the oil recovery by spontaneous imbibition in the neutral pH brine. With pH-controlled synthetic brine, the final oil recovery increases; especially using high pH brine. This suggests that the wettability shifts to greater water-wet conditions and the residual oil saturation decreased by exposing high pH brine. In the simulation study, it is inferred that the oil recovery characteristics change by changing the capillary pressure characteristics and the interfacial tension.