Abstract
Recent observations have indicated surprisingly high oil recoveries for gravity drainage. This has prompted us to reexamine gravity drainage as an prompted us to reexamine gravity drainage as an oil-recovery process. It is shown that oil relative pumeability is a key factor. For accurate measurement pumeability is a key factor. For accurate measurement of this oil relative permeability, we have developed a new method based on centrifugal gas/oil displacement in small cores. Several oil relative permeabilities for various rock types are presented. permeabilities for various rock types are presented. These relative permeabilities confirm that gravity drainage in water-wet, connate-water-bearing reservoirs can be very effective for oil-recovery.
Introduction
Gravity drainage is a recovery process in which gravity acts as the main driving force and where gas replaces the voidage volume. In other words, it is a gas/oil displacement in which gravity forces are dominating. It may occur in primary stages of oil production (gas-cap expansion drive or segregation production (gas-cap expansion drive or segregation drive), as well as in supplemental stages where gas is supplied from an external source.Two recent studies have indicated surprisingly high oil recoveries for gravity drainage. During their capillary-pressure studies, Dumore and Schols discovered that residual oil saturation after gas invasion in highly permeable sandstone cores containing connate water can be extremely low (5%). They also found low residual oil saturations in sandpack gravity-drainage experiments.A field study by King and Stiles concerns the East Texas Hawkins reservoir, in which a very high displacement efficiency for gravity drainage was reported (87%).These findings prompted us to reexamine the gravity-drainage process. Using the classical description of immiscible two-phase flow applied to a one-dimensional vertical gravity-drainage system, we easily can pinpoint the key factors of the process. One of the most important quantities appears to be the oil relative permeability. For quick and accurate measurement of the oil relative permeability, we present a new technique based on centrifugal gas/oil present a new technique based on centrifugal gas/oil displacement.The results of this study enable us to evaluate gravity drainage better. Using the results of the simple one-dimensional description, we can make quick first judgements of the process. Further, the new measuring technique provides basic input data for more comprehensive mathematical simulation studies.
Vertical Displacement Efficiency of Gravity Drainage
Classical Description
We can think of gravity drainage as a displacement process in which gas displaces oil. Then, by using the process in which gas displaces oil. Then, by using the concepts of relative permeability and capillary pressure, together with the continuity equation and pressure, together with the continuity equation and Darcy's law, the displacement can be described mathematically. For the simple case of a stable, vertical, downward displacement of oil by gas (Fig. 1), we end up with a single equation:(1)
where fo is the oil fractional flow function as defined in Appendix A.To gain more insight into the effect of the various parameters, we put Eq. 1 in dimensionless form. parameters, we put Eq. 1 in dimensionless form. Therefore, we introduce the reduced oil saturation and reduced porosity:
SPEJ
P. 139