Investigations on spontaneous imbibition and the influencing factors in tight oil reservoirs

Spontaneous imbibition has become a role on the development of tight oil reservoirs. Many oilfield cases have confirmed the validity of soaking to produce oil depending on imbibition. Considerable yield has been recovered from soaking for a period of time after hydraulic fracturing by fracturing fluid. In order to study the spontaneous imbibition and its influencing factors in tight oil reservoir with stimulated reservoir volume (SRV), we first conducted wettability and imbibition experiments with/without surfactant treatment using different tight cores. Then, we established a mechanism model of imbibition including the influencing factors in tight oil reservoirs based on the experiments. The model was also validated using the published results. After that, numerical simulation was employed to study the impacts of some essential factors on the effects of spontaneous imbibition. The results indicate that no matter mixed-wet or water-wet tight cores, water can be spontaneously imbibed into the tight cores, and consequently expel the oil. The wettability alteration agent has slight influence on contact angle and imbibition recovery for the water-wet core, but it has a distinct influence on those of the mixed-wet or oil-wet core. Natural fractures in tight rock can promote the imbibition. The size of the tight rock after hydraulic fracturing is a significant factor of imbibition recovery. Interfacial tension is crucial to imbibition in tight reservoir because capillary force becomes more significant in tight rocks than that in conventional reservoirs. Wettability intensely decides the imbibition recovery, and it is indispensable to add modifying agents to enhance imbibition for mix-wet or oil-wet rocks. The reduction of temperature in tight oil reservoir due to fracturing and huff-n-puff may decrease the recovery by more than 1% after 5-6 cycles.