Recovery Sensitivities of High-Relief Reservoirs

Abstract

ABSTRACT Of the mechanisms for oil recovery, the greatest challenge to reservoir engineering is a reliable determination of the expected performance of areservoir subject to gravity control. With the increasing number of high relief reservoirs recently developed in the Rainbow-Zama area, in addition to theolder but important D-3 reef reservoirs, it is increasingly important to the Alberta oil industry to gain a better understanding of the factors affecting the efficiency of gravity drainage. This paper presents the results of asensitivity study to determine the effects of rate, permeability and relative permeability on the recovery performance for such high-relief reservoirs. Theuse of a mathematical reservoir simulator has permitted the quantitative assessment of each of these factors and the study has shown that relative permeability is by far the most important. In fact, under gravity-controlledconditions, conventional relative permeability data obtained by the Welge method do not cover the low oil saturation range needed for accurate recovery predictions. Relative permeability curves may be extrapolated in this range, but a better method would be a steady-state determination of relative permeability at the low oil saturations. INTRODUCTION OIL PRODUCTION FROM HIGH-RELIEF RESERVOIRS forms an appreciable part of thetotal oil production in Alberta. The recovery mechanism in these reservoirs isprimarily one of gravity drainage. Gravity forces in these reservoirs tend tosegregate the fluids according to their densities, causing the oil to move vertically ahead of the displacing gas or water. Although the general influence of rate, permeability and relative permeability on gravity is intuitively obvious, there is little quantitative data available to show the magnitude andrelative effects of these factors. This paper presents a study of the sensitivity of oil recovery to these factors. The study was carried out using aone-dimensional, three-phase reservoir simulation mathematical model of a typical Rainbow reservoir. Although it used only the description of one reef, the conclusions of this study are generally applicable to most reef reservoirs which are being produced by gravity drainage; for example, the Rainbow-Zamareservoirs or D-3 reefs in Alberta. The problem was simplified by assumingone-dimensional (vertical) flow; however, the work done by McCulloch et al.shows that the results from one-dimensional simulation of high-relief reservoirs are in good agreement with three-dimensional simulation results.