Water imbibition, electrical surface forces, and wettability of low permeability fractured porous media

Enhancing oil production from fractured reservoirs is challenging. The flow characteristics observed in naturally fractured reservoirs are complicated due to interaction between the two different flow domains: fracture and matrix. Spontaneous countercurrent imbibition is one of the most important recovery processes in water-wet fractured reservoirs. Oil cannot be spontaneously produced from oil-wet rocks, however, unless the capillary pressure barrier between fracture conduits and matrix is overcome. The flow characteristics in fractured systems thus relate to the wettability of rocks. This dissertation presents experimental oil recovery characteristics of spontaneous imbibition and forced displacement, analytical/numerical approaches to provide an insight into the oil recovery of spontaneous countercurrent imbibition, and evaluation of wettability and fines migration as a function of pH to explain wettability alteration of siliceous shale.;Spontaneous imbibition and forced displacement tests were conducted to investigate and understand oil recovery characteristics of siliceous shale rocks. These reservoir rocks are low permeability and medium porosity. For comparison purposes, Berea sandstone cores were examined. Four brine formulations, brine, carbonated water, acidic and alkaline aqueous solutions, were used to investigate the effects of brine formulation on enhanced oil recovery. The siliceous shale rock was a mixed-wet to oil-wet rock because spontaneous countercurrent imbibition did not occur. On the other hand, oil was produced by low and high pH solutions, especially high pH. This indicated that wettability was altered by changing the pH of solutions. The rate and ultimate oil recovery of spontaneous imbibition were influenced by the rock and fluid properties. In particular, the capillary pressure curves played an important role. This was confirmed by the history-match analysis.;The analytical models assumed to be a piston-like flow were valid for very early times in agreement with the boundary conditions of the core. An analytical model based on the diffusion equation was developed in cylindrical coordinates, and presented reasonable oil recovery over the production time. Comparison of capillary and gravitational forces in intermediate water-wet rocks indicated that there were two flow regimes. Capillary pressure gradients act at short time and gravitational forces act at longer time, and not only capillarity but also gravity determined the ultimate oil recovery. Nonequilibrium effects were investigated to understand the water propagation in spontaneous countercurrent flow. They were important to reproduce the water saturation profiles at early times.;Wettability of siliceous shale rocks was interpreted using the DLVO theory. It describes the surface forces between the rock surface and fluids. It provides us with information on the stability of the thin wetting film. Zeta potential measurements of quartz, silica, and siliceous shale were made using the developed experimental apparatus. The experimental apparatus was verified with quartz and silica materials. The zeta potential of siliceous shale as a function of pH and temperature was in agreement with the literature data; however, it had a different trend in salinity from the literature. The predicted trend in wettability changes, i.e. contact angle, versus pH was in agreement with the oil recovery of spontaneous countercurrent imbibition. DLVO theory reasonably explained the mechanism of wettability change in siliceous shale rock.;The other scenario of wettability alteration is fines migration. The siliceous shale rock contains a small amount of clay minerals (about 2%). The total energy between the clay minerals such as chlorite, illite, and montmorillonite and the siliceous shale rock was calculated. The resulting total energy gave negative values except at very short distances from the surface in any clay mineral and pH solution. These results predict fines to be stable under the conditions examined. The wettability changes in the siliceous shale rock system at the experimental conditions were not induced by the fines migration mechanism.;Results showed that oil was not produced spontaneously from siliceous shale rocks with a neutral pH solution. Additionally, injecting fluids is relatively difficult in order to improve oil recovery, because siliceous shale rocks are low permeability fractured systems. Wettability alteration is effective in improving oil recovery for such reservoirs. Low and high pH solutions influenced wettability of siliceous shale rocks, especially high pH solutions. The surface forces affected by pH explained wettability alteration.