This work presents experimental results that demonstrate significantly enhanced imbibition rates and oil recoveries by altering the wettability of the Spraberry rock from weakly water-wet to more strongly water-wet conditions. The Spraberry reservoir of the Permian Basin in west Texas is characterized as an areally extensive, low net-to-gross oil reservoir with low matrix permeability (< 0.1 md) that produces owing to a well-developed natural fracture system. The natural fractures coupled with a weakly water-wet rock-fluid system in a high salinity environment, and the low matrix permeability make it challenging to efficiently waterflood the Spraberry. Such conditions lead to poor sweep efficiency, but low waterflood recovery can also be attributed to limitations of the spontaneous imbibition process under the mixed-wettability, low matrix permeability conditions existing naturally in Spraberry. Spontaneous imbibition experiments were performed on Spraberry plugs for a range of injectant brine compositions by varying brine salinities or by adding surfactants. Recovery improvements were observed. Surfactants and lower-salinity brines both alter the wetting properties of the Spraberry to more water-wet conditions, and these laboratory experiments show that stronger water-wet conditions result in significant increases in oil recovery by the spontaneous imbibition process. The results may assist the interpretation of potential field tests in future.
The Spraberry field is believed to originally contain about 40 billion barrels of oil in the Spraberry and Dean formations. To increase field recovery, large areas of the field were unitized in the early 1960's, and secondary recovery projects involving pattern waterfloods were implemented. Current projections estimate that 10 to 20% of the oil originally in place (OOIP), will be recovered with waterflood developments using predominantly 40 acre well spacing and some 20 acre infill drilling. This is largely due to the presence of a well-developed natural fracture network which leads to poor sweep efficiency of the rock matrix by the injected water. The problem is further compounded by the fact that the Spraberry formation has low matrix permeabilities (typically < 1 md) and tends to be weakly water wet, so that the imbibition that does occur between the fractures and matrix is slow, and is inhibited relative to the imbibition that could occur in a more strongly water-wet rock-fluid system.
The objective of this study is to investigate ways in which the performance of the Spraberry waterfloods can be improved. Laboratory experiments are conducted to study ways of enhancing oil recovery via secondary imbibition either using low salinity brine or by using dilute surfactant systems. The results from the lab study will facilitate design a field pilot flood.