Down-Dip Waterflooding of an Oil Reservoir Having a Gas Cap
Authors: Dandona A.K. , Morse R.A. 
Published: December 2007

The presence of gas in a reservoir under waterflood can reduce the residual oil saturation significantly. A study, of the feasibility, of trapping gas ahead of a waterflood front in dipping oil reservoirs is accompanied by an investigation using numerical models to study the rate sensitivity of down-dip waterflooding oil reservoirs with gas caps.

Introduction
That the presence of gas in a reservoir during a waterflood can significantly reduce the residual oil saturation is well known in the oil industry. In 1922, Russell first found experimentally that waterflooding of oil reservoirs by a mixture of water and gas yielded higher oil recoveries than water or gas displacement alone. In 1973, Dandona and Morsel studied the effects that water injection rates and gas saturation have on ultimate oil recoveries for horizontal reservoir systems. Meltzer reported on additional oil recoveries obtained by waterflooding in the presence of gas in the Brookhaven oil field in Mississippi. Other investigators have also established that the presence of gas during a waterflood results in more oil recovery than when no gas is present.
The purpose of this work is to study the feasibility of trapping gas ahead of a waterflood front in dipping oil reservoirs by forcing a zone of high gas saturation ahead of the waterflood front. Such a reservoir may have a sufficient quantity of gas available in a gas cap. If it does not, a high gas-saturation zone can be introduced by injecting gas into the crest of the structure. Special problems exist when a waterflood is contemplated in dipping problems exist when a waterflood is contemplated in dipping oil reservoirs with an initial gas cap. Although such a displacement involves the movement of all three fluids (gas, oil, and water), analysis can be made under certain conditions by using available theories for two-phase flow. If a zone of high gas saturation exists between the oil zone and the waterflood front, the gas-oil and watergas displacement processes can be considered separately. At the gas-oil contact, oil is being displaced by gas, while at the water-gas contact gas is displaced by water. Available theories can be used to predict each of these displacements provided the velocities of the gas-oil and the water-gas contacts are such that distinct interfaces are formed.
One of the earliest methods of predicting the displacement of one fluid by another from porous media was provided by Buckley and Leverett. This method is provided by Buckley and Leverett. This method is applicable to one-dimensional flow systems only. Dietz developed a theory that is applicable to two-dimensional flow systems. He indicated that, within a range of fluid flow rates, the interface between two moving fluids in an inclined system is a plane inclined at an angle that can be predicted. Hawthorne applied the Dietz theory to derive equations that were analogous to the Buckley-Leverett equations but, unlike Buckley-Leverett, included gravitational effects perpendicular to the angle of dip. He also developed a theory for calculating the slope of a fluid interface under stable displacement conditions. The results predicted by this theory were in good agreement with the performance of a laboratory model.
Wilson applied the Dietz theory to find the conditions under which stable displacement will exist when all the three phases have mobility in the reservoir system.
JPT
P. 1005