Petroleum Conference on Production and Reservoir Engineering, 20–21 March, Tulsa, Oklahoma


The only additional recovery resulting from gravity drainage will be that oil which is displaced by gas from within an expanding gas cap volume. Much of the confusion concerning gravity drainage results from not understanding the difference between drainage from within the gas cap and total drainage. Drainage from within the gas cap is directly related to time, while total drainage, which includes oil displaced at the gas-oil contact, is independent of time. Dissimilar movement of the oil and gas phases causes certain difficulties when attempting performance calculations during pressure decline. To date these problems have not been completely solved. A simplified method is presented for calculating future performance when the reservoir pressure is maintained. Results from calculations indicate that the ultimate recovery, to any assumed economic limit, is essentially independent of variations in producing rate. The only factors affecting recovery, which can be controlled, are the reservoir fluid properties.


Gravity drainage is the least understood of all the basic producing mechanisms. Much of the presently published literature pertaining to gravity drainage appears contradictory and a review of the literature often leaves a reader confused. This paper presents certain basic concepts which the authors found extremely helpful in understanding the phenomenon of gravity drainage and its effect on ultimate recovery from a reservoir. It is hoped, that by presenting these concepts, and by pointing out some of the unsolved problems encountered in performance calculations, that an interest in gravity drainage will be stimulated that will lead to the solution of many of these problems.

When the reservoir pressure is maintained by gas injection in a steeply dipping reservoir, the effect of gravity drainage can be readily calculated, under certain conditions, from existing theory. A simplified method is presented which can be used to predict future reservoir performance in a steeply dipping reservoir during pressure maintenance. The calculations can be readily performed using data normally required in the analysis of most reservoir engineering problems.

In a "solution gas drive" reservoir the energy necessary for producing the oil is derived from expansion of the reservoir fluids, which occurs with reduction in the reservoir pressure. Since liquids are only slightly compressible, most of the reservoir energy is obtained from expansion of the gas released from solution in the oil. However, as the reservoir pressure declines and the reservoir gas saturation increases, gas production increases rapidly due to the increasing permeability to gas. This results in the removal of much of the gas from the reservoir, so that most of the potential energy of the reservoir fluids is dissipated.

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