Relative Permeability Anisotropy Measurements in Tensleep Sandstones
Authors: Iverson W.P. , Dunn T.L. , Ajdari I. 
Published: December 2007

Relative permeability and anisotropy of absolute permeability are commonly measured quantities in the oil industry. The anisotropy of relative permeability, however, has not been accurately documented previously. Recent measurements of relative permeability on Tensleep sandstone samples from Wyoming demonstrate the magnitude and character of anisotropy that can be expected. Core plugs were cut both parallel and perpendicular to the eolian sandstone bedding planes, then prepared for standard absolute gas permeability measurements, and run through an unsteady state relative permeability apparatus. When flooding perpendicular to bedding. the fractional flow curve is steeper than the curve obtained when flooding parallel to bedding. In other words, more piston like behavior is observed perpendicular to bedding. Such anisotropy characteristics can be used in a simple reservoir simulation to demonstrate the expected changes for a producing reservoir. A significant change is predicted when considering the recovery of oil from horizontal wells in a waterflood as opposed to vertical wells. Properly placed horizontal wells can enhance oil recovery in even the most mature waterflood projects.

Introduction

Relative permeability studies have been conducted through countless variations since the pioneering work of Buckley and Leverett. Their work, however, and most subsequent studies treat relative permeability as a scalar quantity that simply adjusts permeability k to account for a partial fluid saturation. Absolute permeability, on the other hand, is readily recognized as an anisotropic quantity being slightly different depending on the direction of flow parallel or across sedimentary bedding surfaces. Normally, when bedding surfaces are well defined, the permeability parallel to bedding is consistently higher than that measured perpendicular (flowing across) layers. This type of intrinsic permeability anisotropy exists regardless of fluid saturations. and is often measured in dried cores at 100% gas saturation. Relative permeability is then a function of water (or gas) saturation and can possibly exhibit a new dimension in permeability anisotropy.

Previous work on relative permeability anisotropy is scarce. Data is abundant on relative permeability and measurements of absolute permeability anisotropy But the combination of relative permeability anisotropy is difficult to find. Corey and Rathjens show one core measurement where relative permeability measured in the vertical direction is slightly different than that measured parallel to bedding. That study. however, presented an insufficient number of measurements to conclusively determine the characteristics of such relative permeability anisotropy Kortekaas used a reservoir simulator to show the effects of relative permeability anisotropy through cross-bedding in sandstones. Orientation of bedding was shown to be important, but there were no actual laboratory measurements of relative permeability anisotropy.

This current work provides fundamental laboratory data on relative permeability anisotropy, the interpreted characteristics of such anisotropy, and some suggestions as to how it affects actual oil recovery in waterflood projects.

Sample Collection

Dunn et al present most of the details regarding the geologic considerations of sample collection. The samples used for this study are Tensleep sandstones of Permian age from the Big Horn Basin in northwest Wyoming (Fig. 1). Tensleep sandstones and equivalents represent the largest producing formation in Wyoming. with over a billion barrels cumulative oil production.

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