Accurate Estimation of Water Saturations in Complex Carbonate Reservoirs

Abstract

Abstract Large errors in the computed values of water saturations in the uninvaded zone (Sw) and invaded zone (Sxo) will occur if some of the parameters used in the interpretation are treated as constants. In particular, the application of constant values of the Archie factor (m) in the saturation equations has been found to introduce major errors. A method was developed to calculate a "variable m" using resistivity and electromagnetic propagation logs. This method evaluates in detail the mixing in the invaded zone between the invading filtrate and the formation water. This allows representative values of the fluid properties in the invaded zone, such as resistivity and electromagnetic propagation time, to be obtained and used in the interpretation. The computed results using this technique were verified in water bearing zones, tar zones, zones with production tests and by comparison with core analysis, Examples of well data are included where comparisons were made between the more conventional interpretation method, using constant values form and fluid properties, and this new interpretation method. The validity of the assumptions made, particularly with regards to the potential mismatch particularly with regards to the potential mismatch in the depth of investigations of the various measurements, were analysed in detail. This method has now been tried on a large number of wells, and has been shown to improve the accuracy of the computed values of saturations over the conventional method. 1 - Introduction 1.1 - General Background. Variations in Sw with porosity and the measured resistivity have been the subject of many research and publications since Archie proposed his equation in publications since Archie proposed his equation in 1942. Archie's equation is defined below for the uninvaded and invaded zones: Uninvaded zone- (1) phi m ⋅ Snw a ⋅ RwRt = --------------- Invaded zone - (2) phi m ⋅ Snxo a ⋅ RmfRxo = --------------- (Note: the Archie factor is commonly referred to as the cementation factor - this is following an early publication which concluded that m variations were related only to the cementing of the matrix grains, and ignored more important factors such as texture and formation heterogeneity caused by the presence of vugs and fractures.) The standard practice in log interpretations is to obtain values of porosity (phi) and resistivity in the uninvaded and invaded zones, Rt and Rxo respectively, from log measurements. P. 199