Stratified reservoirs may have different types of heterogeneity in terms of grain size distribution in vertical direction. Geological surveys (i.e. using well logs) have long recognized the existence of fining upward and coarsening upward formations. In this study, such formations refer to as systems with decreasing upward and increasing upward permeability trends, respectively. Many waterflood candidate reservoirs have been found to follow either classification. However, the awareness of including this distribution classification as one of the screening criteria prior to waterflooding has not been established in the oil industry.
A simulation study using a number of conceptual stratified reservoir models has been conducted. The results show that grain size distribution classification should have significant impact on waterflood performance. Each of the two classifications yields different effects on vertical sweep efficiency resulting from different crossflow mechanisms, which consequently gives different waterflood performance. It has been found that the oil recovery from waterflooding a reservoir with coarsening upward formation will always be higher than that from waterflooding exactly the same reservoir but with the opposite classification (i.e. fining upward formation) even though the two reservoirs are of the same level of heterogeneity (i.e. similar values of coefficient of permeability variation). In this study, the degree of heterogeneity effects on recovery were investigated as well as the magnitude of vertical-to-horizontal permeability ratio effects. Also, permeability noise was addressed as the reservoir may contain contrast permeability streaks in between the adjacent layers.
A correlation has been derived based on the simulation results and has been proven to be able to predict simulation results with relatively good accuracy. Validations performed by comparison to actual production from a simple injection scheme (one injector and one producer) in Bangko Field, Indonesia, indicate that oil production rates obtained from the correlation show good agreement with those of production history.