It has long been known that spontaneous imbibition is the major oil recovery mechanism from Naturally Fractured Reservoirs (NFRs), if the matrix is water-wet. The rate of oil recovery by imbibition mechanism depends on several parameters including oil viscosity. If a NFR contains heavy oil, the rate of oil recovery by imbibition mechanism decreases significantly. To alleviate this problem, the matrix is heated by hot waterflooding or steam injection in which oil viscosity is reduced.
In this work, both static and dynamic experiments were conducted to investigate the effects of various parameters on imbibition mechanism. The effect of temperature on imbibition rate was investigated by conducting static experiments at different temperature levels under the boiling temperature of water. In these experiments, only imbibition and thermal expansion of oil are effective mechanisms in oil recovery. Results from these experiments indicated that the imbibition rate and ultimate recovery increase as the temperature increases. Numerical simulations were also conducted to explain effects of some parameters (such as interfacial tension and oil viscosity) on imbibition rate and recovery when interpreting the experimental results obtained with temperature effects. Further, another set of static experiments (at ambient temperature) was conducted to study the effect of interfacial tension (IFT) on imbibition by using surfactant solutions. Results indicated that recovery increases as IFT decreases. Dynamic experiments were conducted on fractured two dimensional glass bead models to observe the effects of fracture and flow properties on the matrix-fracture transfer function. In these experiments, the effects of the injection rate and fracture configuration on the capillary imbibition behavior and saturation distribution in the matrix were visually studied.
The imbibition process has been studied in the past both theoretically and experimentally for various purposes. However, the physics of capillary imbibition and its influence on oil recovery still need additional investigation because of the complexity of the mechanism. The rate of imbibition depends on several parameters including matrix and fluid properties in static conditions. Under dynamic conditions, fracture properties and injection rate also affect the mechanism.
Most of the experimental studies of spontaneous imbibition were conducted at room temperature. As shown by Briggs et. al., the rate of imbibition substantially decreases as oil viscosity increases. When thermal oil recovery methods such as hot waterflooding and steam injection processes are used, imbibition mechanism may become important for oil recovery in NFRs. When these methods applied to NFRs, imbibition mechanism takes place in both hot water and cold water zones.
The main objective of this study is to investigate the effect of temperature on the imbibition transfer efficiency. For this purpose, static experiments were carried out using Berea sandstone samples. Experimental runs were performed at different temperatures under the boiling temperature of water. Therefore, only imbibition and thermal expansion of oil become effective mechanisms. Other mechanisms caused by temperature increase may become important at much higher temperatures than the temperature range applied during the experimental runs in this study. The amount of the changes of IFT and oil viscosity with temperature affects the imbibition rate. Since the rate of changes of IFT and oil viscosity with respect to temperature was expected to affect the imbibition rate in a different manner, variations of these properties with temperature were also measured in this study.
The ratio of gravity to capillarity directly influences imbibition recovery. In the second group of static experiments, surfactant solution was used as the water phase.