Petrophysical study of the Glorieta-Clearfork dolomite in the Monahans field, Ward County, Texas

Calculation of a reservoir's water saturation using the Archie equation requires the values for cementation exponent (m) and saturation exponent (n). Determination of these two parameters, particularly in carbonate reservoirs, is often difficult. Recently a new method (CAPE) for estimating m and n has been proposed by Maute et al. (1992). In the CAPE (Core Archie Parameter Estimation) method, m and n are determined by minimizing the error between laboratory derived water saturation (Sw;The Permian (Leonardian) Glorieta-Clearfork dolomites in the Monahans field represents an upward-shoaling carbonate platform sequence. The predominant rock type is dolostone and the major mineral constituents are dolomite and anhydrite. Petrographic analysis reveals mainly intercrystalline/intergranular pore geometry with minor vuggy/moldic porosity.;In this study the author applied three techniques: (1) non-linear, (2) linear, and (3) m-porosity transform to determine m and n values that minimize the error (error-function) between Sxo (dielectric) and Sxo (Archie). Two m-porosity transforms were established, but the transform that represented the majority of the data (85%) was used to derive m values. Using data from the Glorieta-Clearfork dolomite in the Monahans field, the non-linear method resulted in the minimum error between Sxo (dielectric) and Sxo (Archie).;The m and n values determined by the non-linear and linear methods probably do not represent physical rock characteristics but are only values that minimize the error functions. In contrast, m and n values determined by the m-porosity transform method should represent physical attributes of reservoirs such as pore geometry or wettability.;In order to further reduce the error between Sxo (dielectric) and Sxo (Archie), m and n were approximated by mathematical functions (polynomial and fourier series) to model the vertical variation of m and n in the reservoir (variable m and n method). This variable m and n method based on a fourier series resulted in the greatest error-reduction when compared to the non-linear method.;After determining m and n values that result in the minimum error between Sxo (dielectric) and Sxo (Archie), these values can then be used to calculate the water saturation in the uninvaded zone (Sw).