Reservoir Performance of a High Relief Pool

Abstract

Abstract A method is presented for evaluating the effect of gravity drive upon thereservoir performance of a high relief pool. Conventional forms of reservoiranalysis do not consider the alterations in the basic material balance datacaused by gravity segregation of reservoir fluids. A procedure is outlined forstructurally weighting physical and chemical data for use in the materialbalance equation. It is demonstrated how actual pool performance data can beutilized to evaluate the future reservoir performance of a gravity drivepool. Introduction Conventional reservoir engineering procedure is inadequate for the analysisof an oil pool which has considerable structural relief, steep dips, and goodpermeability development. In pools of this type, gravity drainage has animportant part in the movement of oil to the wells and the effects of gravityon the overall pool performance should be included in any analysis of reservoirbehavior. Many engineers have the opinion that the force of gravity in themovement of oil is not important until the later life of a pool. Probably thebasis for this belief is that gravitational effects may not be readilydiscernible until a pool is nearing depletion. This would be especially truefor pools not having a high degree of structural relief and permeabilitydevelopment. Actually the effects of gravitational forces are at a maximum whenthe pool pressure is high, for during this period the hydrostatic head of theoil column is at a maximum and the viscosity of the oil is at a minimum. Oil recoveries from pools having favorable gravity drive characteristics mayequal or even exceed recoveries which might be expected from waterdisplacement. Field evidence indicates that in some reservoirs gravity drivehas resulted in recoveries greater than that which could have been expectedfrom gas expansion or water drive. Unfortunately, the possible effects ofgravity drive on pool performance have been underestimated and other reasonshave been sought to explain the high recoveries obtained. There areunquestionably many reservoirs to which the principles of gravity drainage canbe effectively applied. It is the purpose of this paper to illustrate one method whereby gravitydrive is included in the reservoir analysis of an oil pool. A hypotheticalpool, typical of many California reservoirs, is used as an example. As used inthis paper, "gravity drive" is defined as the overall effect ofgravitational influences on the recovery of petroleum from the reservoir;"gravitational segregation" as the gravity separation of oil and gaswithin the reservoir; and "gravity drainage" as the downward movementof oil as caused by the force of gravity. T.P. 2645