The Gravity Drainage Mechanism

Abstract

Richard H. Smith, general production superintendent for the Signal Oil and Gas Co., presented the eighth talk in the series on "Elements of Petroleum Production" at a recent meeting of the Pacific Technology Forum. Smith discussed the mechanism of gravity drainage as it affects production from petroleum reservoirs. Smith first defined gravity drainage as differential movement of oil and gas within an oil reservoir caused by density differences such that counterflow is achieved. He then discussed the balance between gravity forces and capillary forces that controls the counterflow of oil and gas in a gravity drainage system; and, on the basis of this discussion, presented an analysis of gravity drainage in oil reservoirs. Characteristics of 12 oil pools were presented to illustrate a method for determining whether or not gravity drainage was a major portion of the producing mechanism for each pool. He then presented methods for detecting gravity drainage early in the life of an oil pool and discussed briefly the necessity for detecting gravity drainage or a lack of gravity drainage before a gas injection program is begun: One of the first field studies that indicated the presence of gravity drainage as a production mechanism was made by H. C. Miller, who studied such pools as Yates, Shuler-Jones and Oklahoma City. He noted that early in the life of these pools reservoir pressure declined rapidly, but that later in the life of the pools pressure declined only slowly although substantial quantities of oil continued to be produced. Miller concluded that this later production was the result of gravity drainage of oil into the wells.