Reports: DNI951561-DNI9: Studies of Streaming Potentials Generated by a Two-Phase Flow with Applications to Detecting Water Encroachment towards Oil Wells

Hui Zhao, PhD, University of Nevada (Las Vegas)

Oil reservoirs with bottom water layers exhibit high oil recovery due to supplemental energy imparted by the aquifer. A large oil production rate may simultaneously drive oil from the reservoir and water from the underlying aquifer towards the lower pressure sink created by the oil well. Typically water moves in a cone shape and such phenomenon is termed water coning referring to the deformation of a water-oil interface which is initially horizontal. Water coning is a serious problem in managing reservoir recoveries since water mobility is much larger than oil mobility in porous rock and water phase may become so dominant in total production to the extent that further operation of the well becomes not economically valuable and the well has to be abandoned. For economic and environmental benefits, solutions have been proposed to prevent or minimize water coning during oil production including artificial barriers, downhole water sink, and injection of polymers or gels. However, in order to control water coning or minimize the impact of unwanted water in oil wells, an effective technique monitoring water flow into oil wells is the first and key step.

The objective of this project is to investigate the feasibility of measurements of streaming potentials to detect water encroachment since when water approaches an initially oil-filled reservoir, it induces changes in the streaming potential.  

During this reported period, we made progress in both experimental and numerical aspects of the project. Experimentally, we constructed a measurement platform that can real-time monitor the streaming potential generated by a two-phase flow. The experimental set-up consists of the electrometer, the digital control pressure, and a channel as a model problem. To prove the concept that the electrokinetic method is able to detect the encroachment of one-phase flow in another phase flow happening in water coning, we measured the streaming potential change induced by the motion of a silicone oil droplet suspended in water. We randomly injected oil droplets into a water stream which is driven by a constant pressure and monitored the streaming potential. Our experiments showed that the streaming potential is significantly increased by the presence of the oil droplet. Meanwhile, once the oil droplet left the channel, the streaming potential quickly returned to its base level. The response time to oil encroachment is within seconds. The large change of the streaming potential due to the oil droplet and the quick response time of the streaming potential to the oil droplet support the possibility of using the electrokinetic potential to detect the two-phase flow.

Numerically, we have implemented the phase-field method to track the motion of a droplet suspended in another liquid in the presence of the electric field. The Laplace equation accounting for the electric field has been incorporated into the model. The model is capable of studying electrokinetic phenomena under the limit of thin double layers. The modeled droplet shape shows good agreement with experimental data.

For the next report period, besides quantitatively comparing experimental results to numerical predictions, experimentally, we will implement our technique into a serpentine channel geometry similar to the porous structure and quantify the streaming potential change in terms of the pressure as a function of the droplet size and channel size. Numerically, we will include ion concentration equations into our model to make it suitable study electrokinetic phenomena in the presence of an arbitrary double layer.

The ACS PRF Doctoral Investigator Grant has a significant impact on PI's career. The grant helped the PI to obtain many important results to study electrokinetic phenomena and gather preliminary results for his NSF proposal on relevant topics. The project is currently supporting a graduate student.