Tight Reservoir Stimulation for Improved Water Injection - A Novel Technique

Abstract

Abstract Among the emerging technologies in the petroleum industry is the application of electro-kinetic phenomena to enhance oil recovery from tight heavy sandstone reservoir, which has been reported to yield technical and commercial success in some of the North American oil fields. The basic theory behind the stimulation effect is predicted to be the colloidal movement of pore lining clays that results in widening of pore throats and/or opening new flow tunnels. Nevertheless, few works have been performed on its applicability to water injection wells. This paper investigates the effect of electrokinetics on improving water injectivity in tight sandstone reservoirs. Two sets of experiments were conducted. In the first set, the DC potential is varied and optimized during the water injection. In the second set, the DC potential is kept constant and the injection rate is varied to determine the hydrodynamic effect on clay movement. The core plugs and liberated clays were characterized through size exclusion micro-filtration and ICP-MS analysis. The Joule heating phenomena associated with electrokinetics is also studied during the entire injection period. Results showed that several folds (up to 152%) apparent increase of core permeability could be achieved. Some of the experiments were more efficient in terms of dislodgement of clays and enhanced stimulation which is supported by produced brines analysis with higher concentration of clay elements. The results also showed larger quantity of clay elements in the produced brines in the initial periods of water injection, prior to the stabilization of differential pressure and electrical current, implying that the stimulation effect stops when the voltage gradient and flow rate values are no more able to remove additional clays. Additionally, fluid flow temperature measurements showed an increasing trend with the injection time and direct proportionality with applied voltages. Introduction and Background In general, the three phases of petroleum recovery processes are primary, secondary and tertiary oil recovery. The primary recovery phase is mainly driven by the natural energy present in the reservoir due to dissolved solution gas pressure, pressure from the overlain gas cap or due to the pressure from an active aquifer below the oil zone. In most cases, the natural driving mechanism is a relatively inefficient process and results in a low overall oil recovery (Ahmed, 2001). The lack of sufficient and consistent natural driving energy is compensated by supplementing with injection of water or gas (or a combination of both) which is the initiation of the secondary recovery phase. Due to several techno-economic factors, the most widespread secondary oil recovery process and responsible for most of world's oil production is the waterflood recovery (Willhite, 1986). Nevertheless, water injection may be associated with numerous hurdles related to fluid incompatibility, reservoir heterogeneity, early breakthrough through thief zones, permeability damage due to suspended particles and clay swelling. Poor water injectivity is one of the problems that is often encountered by the operators especially in tight permeability formations (Pang and Sharma, 1997). This problem may further aggravate, when swelling type clays are present upon which pore throat blockage occur (Tchistiakov, 2000). This situation is often faced in clastic shaly sandstones. The swelling and release of clay particles from pore walls and their subsequent redeposition downstream in smaller pore throats would induce unexpected injectivity damage (Miniawi et al., 2007; Yi, 2001).