Petrophysical study of the Glorieta-Clearfork dolomite in the Monahans Field, Ward County, Texas
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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 (core)) and water saturation calculated by the Archie equation (Sw(Archie)). Because core data are often unavailable, the author substituted dielectric water saturation (Sxo(dielectric)) for core-derived water saturation to determine m and n, and applied the technique to the Glorieta-Clearfork dolomites in the Monahans field, Ward County, Texas. 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 (errorfunction) between Sxo (dielectric) and Sxo (Archie). Two mporosity 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 mporosity 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 errorreduction 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).