An Experimental and Theoretical Investigation of Gravity Drainage Performance

Abstract

Theoretical and experimental investigations of a constant pressure gravitydrainage system are reported. Experimental data are presented to show thatrecovery to gas breakthrough by gravity drainage is inversely proportional torate. The gravity drainage reference rate, which is numerically equal to theso-called "maximum theoretical rate of gravity drainage" is shown tohave no particular significance from a recovery standpoint. Before this ratecan be used as a basis of comparison for recoveries, it is necessary that therelative permeability and capillary pressure characteristics and displacingfluid viscosities be identical for the systems compared. A method is presented by which accurate prediction of the performance of agravity drainage system can be made. Close agreement between experimental andcalculated drainage performance shows that steady state relative permeabilityand static capillary pressure data can be used to describe fluid displacementbehavior. The very wide range of liquid recoveries before gas breakthroughwhich result from production rate variation alone demonstrates the importanceof this factor in planning depletion of a gravity drainage reservoir.Calculated results are presented which show that little additional recovery canbe expected from a high pressure gravity drainage system between the times ofgas breakthrough and attainment of such high gas/liquid ratios as to makefurther pressure maintenance impractical. Introduction It has been recognized for some time that gravity forces play an importantpart in the recovery of oil from some types of reservoirs. Field experience hasshown that under certain conditions, gravity drainage can result in very highoil recoveries. Qualitative reasoning has led most engineers to the generalconclusions that:Where gravity drainage is important, the reservoir pressure should bemaintained by gas injection at the crest of the structure to prevent shrinkageof the oil in place and to keep a low viscosity so the oil can drain at thefastest possible rate.Recovery by gravity drainage is rate sensitive. A survey of the literature indicates that while considerable work has beendone on the effects of gravity in oil production problems, no satisfactorymethod of calculating the performance of gravity drainage reservoirs has beenreported. In the absence of any proven method of calculating reservoirperformance, the level at which pressure should be maintained and the rate ofproduction for most efficient operation has been open to debate. T.P. 3199

Development and Optimization of Gas-Assisted Gravity Drainage (GAGD) Process for Improved Light Oil Recovery

“…Many authors suggest the drainage process to be a type of displacement mechanism with the classical theories of Buckley-Leverett (1942), Darcy's law, relative permeability, continuity equation, and decline curve analysis (material balance equation) to be applicable (Terwilliger et al, 1951;Hagoort, 1980;.…”

“…A gravity drainage model similar to that of Cardwell and Parsons (1948) was proposed by Terwilliger et al (1951). Terwilliger et al (1951) applied the B-L immiscible displacement theory and the 'shock-front' technique (using fractional gas flow equations (Welge, 1952)) to match the steady state gravity drainage laboratory experiments (assuming steady-state relative permeability and static capillary pressure distribution).…”

“…Terwilliger et al (1951) applied the B-L immiscible displacement theory and the 'shock-front' technique (using fractional gas flow equations (Welge, 1952)) to match the steady state gravity drainage laboratory experiments (assuming steady-state relative permeability and static capillary pressure distribution). Terwilliger et al (1951) also showed that recovery by gravity drainage is inversely proportional to production (conversely, injection) rates and recommended a "maximum rate of gravity drainage" or "gravity drainage reference rate" (Equation 3.7).…”

“…Terwilliger et al (1951) applied the B-L immiscible displacement theory and the 'shock-front' technique (using fractional gas flow equations (Welge, 1952)) to match the steady state gravity drainage laboratory experiments (assuming steady-state relative permeability and static capillary pressure distribution). Terwilliger et al (1951) also showed that recovery by gravity drainage is inversely proportional to production (conversely, injection) rates and recommended a "maximum rate of gravity drainage" or "gravity drainage reference rate" (Equation 3.7). Equation 6 appears to be the theoretical basis for the "critical injection rate" and "frontal stability" equations developed by various researchers (Hill, 1952;Dietz, 1953;Perkins and Johnston, 1963;Dumore, 1964;Brigham, 1974;Moissis et al, 1987;Ekrann, 1992;Virnovsky et al, 1996) for commercial gravity drainage applications.…”

“…Although Cardwell and Parsons (1948) and Terwilliger et al (1951) models first presented the governing equations for the gravity drainage process, the non-linearity of the equations forced them to ignore two important parameters: (i) the capillary pressure variation with saturation and (ii) capillary pressure dependence on permeability. Although, Nenniger and Storrow (1958) provided an approximate series solution (obtained from film flow theory) to predict the gravity drainage rates on a glass bead pack, the next important development in gravity drainage modeling was the generalization of the Cardwell and Parsons (1948) theory (Dykstra, 1978) by improving the capillary pressure representation in the governing equations.…”