The concept of shale oil EOR by adding surfactant into fracturing fluid became quite popular in the recent decade. Mechanisms of the improved productions are either Interfacial Tension (IFT) reduction or Wettability (WTB) Alteration. While surfactant based shale oil EOR has been proven to be an effective approach by several scholars, the dominating mechanism is still vague and needing a better understanding for project optimization. In addition, as most current studies focused on the spontaneous imbibition under atmospheric pressure, it is important to investigate the process with external pressures because hydraulic fracturing is a high-pressure driven operation. By performing both experimental and numerical simulation studies, this work is to explore the feasibility and potential of surfactant, as a liquid phase substance, to improve the recovery from shale matrix with the assistance of cyclically injected external forces. Further, the relative contributions of IFT reduction and WTB alterations are separately investigated.
The experimental work was performed with a self-assembled set-up allowing high external pressures to be applied. Cyclic injections with up to 3000 psi soaking pressure were conducted with a series of surfactant candidates vary in IFTs. The experiments followed a 12-hour-soaking / 12-hour-depletion schedule with 8 cycles in total for each case. The oil recovery after each cycle was recorded visually to provide more reliabilities. A lab-scaled simulation model that is able to separately reflect the effects of IFT reduction and WTB alteration was built and tuned to match our experimental results. The individual effects through soaking and depletion stages were further analyzed.
The experimental results showed that it is possible to improve oil recovery from shales by cyclically injecting a liquid phase with the assistance of surfactant, and WTB alteration is still the dominating effect in this process. The recovery factor (RF) was as high as 38% after 8 cycles by injecting the surfactant candidate that altered the wettability of shale from oil-wet to water-wet without prominently reducing the IFT. Through the simulation study, the soaking time showed major effects on the recovery in a short period, while the depletion time didn't affect the results too much. However, the times of soaking-depletion cycle will not change the final recovery in the long run. Cyclic Surfactant Injection (CSI) technique will expedite the production, but the potentially enhanced recovery depends on the properties of selected surfactants. Our results and observations provided suggestions to successfully perform a surfactant based EOR project in shale oil reservoirs.