The Influence Of Reservoir Wettability On Waterflood And Miscible Flood Performance

Laboratory waterflood and miscible flood data of three Alberta reservoir systems are compared with their respective field projections. Trends in laboratory waterflood and miscible flood recoveries were found to correspond closely with the expected ultimate recoveries in the respective fields and to correlate well with reservoir wettability. The wettability, as characterized by measuring water-oil relatie permeabilities, ranged from strongly water-wet to strongly oil-wet with one system of intermediate wet/ability and another of probable mixed wettability. Miscible flood oil recoveries were high in systems possessing other than strongly water-wet nature. In addition to the water-shielding phenomenon, interfacial adhesion is also considered to gain a better understanding of the wettability effects in miscible gas processes. The results of this study could be used to identify candidate reservoirs for carrying out miscible floods and to re-evaluate the need for secondary waterfloods in certain reservoirs. In some reservoirs where mixed wettability conditions exist extended waterfloods, within economic limits, may not adversely affect the potential for future miscible floods. This paper brings forth the significant role played by wettability in miscible processes and also offers insight into strategic alternatives for field applications. Introduction Reservoir wettability is an important factor among the many that affect the design of a miscible enhanced oil recovery process. Wettability influences the extent of access of oil by the injected miscible solvent through its effect on the relative location of oil and water. This, in turn, will influence miscible flood performance. The influence of wettability on waterflood performance has been studied for several decades(l) and the published literature supports widely differing conclusions leading to some uncertainty. These uncertainties did not seem important as long as there existed a belief that all petroleum reservoirs were strongly water-wet. Many researchers have contributed significant findings over the last few decades to dispel this belief. It is now generally accepted that reservoir wettabilities vary widely between the extremes of strongly water-wet and strongly oil-wet states, with many falling in the mid-band of neutral or intermediate wettability where the rock shows equal preference to both oil and water- None of these homogeneous wettability states could explain the high waterflood recoveries in the East Texas Woodbine core displacement tests. Efforts to explain these results culminated in the concept of "mixed wettability" which was introduced by Salathiel in 1973(2). This mixed wettability state has also been recently assigned to the Pembina Cardium field in Alberta(3). What is the influence of these widely differing wettability states of petroleum reservoirs on their waterflood and miscible flood behaviour? Despite such a wide variation in reservoir wettability, should we continue to adopt the same sequence of production strategies namely secondary waterfloods and tertiary miscible floods? Or could we optimize our production strategies for maximum benefit by a better understanding of the influence of wettability on secondary and tertiary oil recoveries? This study attempts to address these concerns. This paper begins with the role of wettability in miscible enhanced oil recovery (EOR) operations wherein the literature dealing with this subject is reviewed.