Gravity Drainage in Oil Fields

Abstract

AbstractGravity drainage is the self-propulsion of oil downward in the reservoir rock.Under favorable natural and operational conditions, it has been found to effectrecoveries comparable to water displacement. With modern technical knowledge,the operator can often make a choice between dissolved gas drive, water driveand gravity drainage as the principal recovery agent in a reservoir. It isdesirable that the operator be able to determine which of the three agents willbe most effective under each set of conditions. So far, gravity drainage hasreceived less consideration than the other two. In this paper an endeavor ismade to set forth some of the principles of gravity drainage, to point out thetypes of reservoirs favorable for it, and to show how it may be applied inthem. Field examples are described and discussed.IntroductionSince early in the history of the oil industry, it has been recognized thatgravity is one of the three important natural forces for expelling oil from thereservoir rock. As knowledge of the technology of oil recovery progressed,ideas changed with respect to the relative importance of the three forces andwith respect to the manners in which they functioned. For a time,disproportionate emphasis was placed on the importance and function of gas andthe need for conserving gas pressures. More recently, disproportionate emphasishas been placed on encroaching edge water. Only lately has there been evidencethat the function and importance of gravity was to receive dueconsideration.Study of this 'neglected phase of oil recovery is needed to round out ourknowledge of reservoir behaviors and to gain a balanced viewpoint of theinterplay of forces that may operate in varying importance in reservoirs. Thepurpose of this paper is to discuss mainly in a qualitative way, the functionsof gravity, to present some concrete instances where it has been important andto raise some questions that should be investigated further.Oil-Recovery AgentsThe important natural agents for effecting oil recovery that may be originallyavailable in a reservoir are gas, both dissolved in the oil and free in gascaps, encroaching edge water and gravity. Each of these agents can act in adual capacity-physically to overcome the surface energies that hold the oilwithin the pores and mechanically to overcome viscous resistance and to propelthe oil through the porous reservoir 'rock to the wells.Advancing edge water displaces the oil from the pores and lifts it up dip tostructurally lower wells. Gravity drains the oil from the pores and flows itdown dip to the wells. Gas displaces oil from the pores, entrains it in the gasstream and flows it to the wells, but gas in solution also affects the physicalproperties of the oil and thus can influence oil recovery by the other agents,whether or not the mechanical energy in the gas is utilized.T.P. 1664