ABSTRACT

Formation bulk density over a radius of at least 50 feet from the wellbore is routinely obtained using the BHGM. The bulk density equation for this volume of rock contains terms for porosity, fluid densities, matrix density, and fluid saturations. Assuming the component densities are known, the bulk density equation can be reduced to one containing only two unknown parameters - porosity and water saturation. Archie's equation relates water saturation, formation resistivity, water resistivity, and porosity. By properly weighting the formation resistivities measured by a deep investigating resistivity log, one can obtain an equivalent resistivity for a bulk Archie's equation that applies to the same volume that is sampled by the BHGM. If the cementation factor, saturation exponent, and water resistivity are independently known, Archie's equation reduces to one containing two unknowns-porosityand water saturation. A simultaneous solution of the bulk density and Archie equations is possible, and this solution provides values of porosity and water saturation within the volume sampled by the BHGM and resistivity log. Hydrocarbon saturation is then easily obtained, since it plus water saturation must equal one. It is important to note that porosity values are obtained from this approach alone and are not required from any other external source such as logs or cores. One advantage of this technique is that the computed porosities and saturations are very insensitive to wellbore conditions, filtrate invasion, and other drilling disturbances. Another advantageous result is the investigation of a large, representative volume of reservoir rock that is not normally accessible by conventional logging methods. Actual field results for several wells are presented. Applications include the detection of bypassed hydrocarbons and the determination of residual oil saturation.

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