Thief Zones and Effectiveness of Water-Shut-Off Treatments under Variable Levels of Gravity and Reservoir Heterogeneity in Carbonate Reservoirs

Abstract

Abstract Progressive percentage of total fluids produced in the oil industry is formation brine. Ever, increasing water cut will reduce oil recovery, diminish wells' productivity and increase the cost of eventual artificial lift and produced water handling. In addition to mobility ratio, the predominant factors that cause and aggravate irregular movement of injected water resulting in early water breakthrough and high water cut wells is gravity and reservoir heterogeneity. Through very comprehensive simulation study, performed on a sector model extracted from a layered and heterogeneous carbonate oil reservoir, this paper investigated the evolution of injected water front and the level at which high permeability layers will act as a thief zone causing early water breakthrough. The paper determined the effect of different reservoir heterogeneity parameters on the behavior thief zones, including thief zone's horizontal permeability, kv/kh ratio, and thickness. The threshold of these parameters and their various combinations that cause a higher permeability layers to act as thief zones were established and for different levels of reservoir permeability (very low, low, medium, high, and very high). Moreover, the paper investigated the effect of gravity on the process by comparing the behavior of top versus bottom thief zones. The difference in the combination of reservoir parameters that cause their different behavior is reported. In order to generalize the outcomes of this paper, all results are presented in terms of dimensionless variables. Water-shut-off, WSO, treatments were then simulated to determine their effectiveness in delaying the water breakthrough time, and reducing high water cuts, under different thief zones' conditions. Extreme thief zone cases were selected from the first part of the study for this purpose. The paper finally summarizes the applicability of these WSO treatment methods and at which conditions of reservoir heterogeneity and gravity, they are or are not effective.