Abstract
An analytical model has been developed for quantitative evaluation of vertical sweep efficiency based on heterogeneous multilayer reservoirs. By applying the Buckley-Leverett displacement mechanism, a theoretical relationship is deduced to describe dynamic changes of the front of water injection, water saturation of producing well, and swept volume during waterflooding under the condition of constant pressure, which substitutes for the condition of constant rate in the traditional way. Then, this method of calculating sweep efficiency is applied from single layer to multilayers, which can be used to accurately calculate the sweep efficiency of heterogeneous reservoirs and evaluate the degree of waterflooding in multilayer reservoirs. In the case study, the water frontal position, water cut, volumetric sweep efficiency, and oil recovery are compared between commingled injection and zonal injection by applying the derived equations. The results are verified by numerical simulators, respectively. It is shown that zonal injection works better than commingled injection in respect of sweep efficiency and oil recovery and has a longer period of water free production.
Highlights
Heterogeneity is a common problem encountered in oilbearing formations
It is considerably important to establish a quantitative characterization method for calculating sweep efficiency; by this method, we can determine the criteria of dividing and reorganizing layer series and relieve interlayer interference in stratified waterflooding reservoir development
Vertical sweep efficiency is the fraction of vertical crosssectional area of the reservoir between injection and production wells that is swept by water at a given time
Summary
The most significant property that affects waterflooding performance is the matrix permeability, and its variation in the vertical direction causes displacing fluid to advance faster in zones of higher permeability and results in earlier breakthrough in such layers. This phenomenon negatively affects volumetric sweep efficiency and leads to low ultimate oil recovery [1,2,3,4,5]. For multilayered reservoir development, the production pressure between injection and production well is usually stable, while the rate of liquid produced in each layer is changeable In this model, the reservoir is divided into a number of layers. A mathematical model is developed for calculating waterflood sweep efficiency by extending the Buckley-Leverett displacement mechanism
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