Improved Oil Recovery in Chalk: Wettability Alteration or Something Else?

Abstract

Abstract Improved oil recovery from low permeable chalk reservoirs is regarded as a great challenge because of their complexity and heterogeneity. Historically seawater salinity has not been considered as an important factor in determining the amount of oil recovered. Over the last decade, a number of studies have shown that waterflooding performance is dependent on the composition of injecting brine solution. Extensive laboratory research has been carried out in order to understand improved oil recovery from chalk using surfactant solutions and later on using the modified sea water. The researchers suggested wettability alteration towards more water wetting conditions to be the reason for improvement in oil recovery. In this paper, we are further investigating the reasons of observed improvement in oil recovery with sulphate ions. Is it really wettability alteration or something else also? Most of the previous work has been made on the basis of spontaneous imbibition using core plugs aged in crude oil. Our study is based on flooding and utilizes completely water wet cores without being aged in crude oil, just saturated with crude oil under vacuum. This would exclude alteration of wettability as a positive factor. Brine without sulphate is considered as the base injected fluid. Waterflooding experiments were carried out with brines without sulphate, as well as brines having different concentrations of sulphate. The effect of temperature, injection rate, crude oils and different sulphate concentration on the total oil recovery and the recovery rate was investigated. In Stevens Klint outcrop chalk samples, 10% increase in oil recovery was observed with sulphate enriched brine flooded core as compared to core flooded with brine without sulphate ions. We observed also 4-6 % increment of OOIP using brine with sulphate for water wet chalk at high temperatures. This clearly indicates improvement in oil recovery without wettability alteration, because chalk is already water wet. No increment in oil recovery was observed at 40 ºC and, neither, in experiments with North Sea crude oil. This clearly explained that crude oil type also plays a significant role in the effect that brine composition has on waterflood oil recovery. This study will help in getting more in depth understanding of seawater improved oil recovery process for chalk reservoirs.