Drainage Performance and Capillary- Pressure Curves With a New Centrifuge
Authors: Firoozabadi Abbas , Soroosh Hossein , Hasanpour Gholambhossein 
Published: December 2007

Summary
A new centrifuge has been used to measure the drainage performance and the drainage capillary-pressure function, P, of various rock samples. The permeabilities of the rocks used for this study range from 0.1 to 700 md. A unique feature of the apparatus is that the rock sample is always in contact with the wetting phase at the outlet face so that the saturation boundary condition is known at this end. The drainage performance and P data for a variety of reservoir rock samples reported in this paper are expected to provide the basis for added insight into the drainage of oil from porous media.

Introduction
Gravity drainage is believed to be one of the most effective processes of producing oil reservoirs. In some of Iran's giant fractured processes of producing oil reservoirs. In some of Iran's giant fractured reservoirs where oil has been produced for several decades and the reservoir has been subject to both waterdrive from the bottom region and gravity-drainage mechanism from the top region, material-balance calculations indicate higher oil recoveries in the gravity- drainage areas than the water-displacement regions. As an example, one particular reservoir has a bottomwater-drive oil recovery efficiency of about 30%, whereas the top gravity-drainage-zone efficiency is some 45 % (these reservoirs have low matrix permeabilities on the order of 1 md and oil columns ranging from 1,000 to several thousand feet thick).
Many papers have been published that describe high recovery efficiencies where gravity effects influence the production mechanism. In an early paper, Katz reported recoveries of more than 90% from a completely drained portion of the Oklahoma City Wilcox sand. Haldorsen et al. recently revealed recoveries on the order of 60% for the gravity-drainage areas of the Prudhoe Bay field.
The understanding of the gravity-drainage mechanism is not as complete as that of other production processes. Relative permeability and capillary pressure are believed to be the two major permeability and capillary pressure are believed to be the two major parameters that determine the recovery performance of a porous parameters that determine the recovery performance of a porous medium for this process. Data in the form of drainage capillary pressure and drainage performance of various rock samples will pressure and drainage performance of various rock samples will provide the basis for added insight into the oil drainage from porous provide the basis for added insight into the oil drainage from porous media.
Dykstra reviewed the limited experimental data on drainage performance of unconsolidated sand columns in his examination of the theory of gravity drainage. He commented that the data lacked some required information (i.e., permeability, fluid properties). Dykstra further suggested a more thorough research effort to obtain all required information. For consolidated porous media and for permeabilities less than 100 to 500 md, the gravity-drainage data are basically measured from centrifuge runs. Marx used a centrifuge to establish the gravity-drainage performance of a Torpedo sandstone of 1.15-in. [2.92-cm] length. He also measured the drainage performance of the same rock with a length of 60.63 in. [154 cm] in the normal gravity field. Marx's experiments showed that recovery data from the centrifuge and normal gravity environment compare well when the time is scaled. He did not report pertinent fluid and rock data. Hagoort reported the pertinent fluid and rock data. Hagoort reported the gravity-drainage performance of nine core samples with permeabilities ranging from 40 to 2,400 md when a centrifuge was used. Capillary-pressure curves were not measured for these core samples.
The purpose of this research project is to provide experimental data on the gravity-drainage performance and drainage capillary pressure of a variety of sandstone, limestone, and dolomite reservoir pressure of a variety of sandstone, limestone, and dolomite reservoir rock samples by use of a new centrifuge. Two of the core samples have been selected to examine the effect of capillary pressure on wetting- and nonwetting-phase relative permeabilities. The result of this study are presented elsewhere.