Electrical Property Estimation For Complex Carbonate Reservoirs Using Wireline Logs Available to Purchase

Carbonate rocks are usually characterized by a variety of porosity types resulting from depositional and diagenetic processes. The variation of the pore geometry, connectivity and size controls the electrical properties of carbonate rocks. The cementation exponent (m) is controlled by various properties including pore connectivity and tortuosity whereas the water saturation exponent (n) is more controlled by the pore size and wettability. For exploratory wells in carbonates, it is essential to estimate electrical properties for appropriate formation evaluation. Instead of using only analogues, the models developed in this study are shown to be valid for the evaluation of different rock types in exploration wells. This increases the confidence level in initial estimates of water saturation and hydrocarbons-in-place. A triple porosity model is used to calculate m of complex carbonate reservoirs in the Middle East using conventional well log data. The emphasis is on exploration wells as the uncertainty associated with m generally decreases during the development and production stages of the reservoirs. The carbonate rocks in this study are characterized by a combination of interparticle, fracture and non-connected porosity (e.g., vuggy and fenestral) that changes continuously with depth. This increases uncertainty in the estimation of m because, for this complex composite carbonate system, m can be larger, equal to, or smaller than the cementation exponent of the single porosity matrix blocks (mb). The variation in m depends on the relative contribution of natural fractures, interparticle porosity and non-touching vugs to the total porosity of the triple porosity reservoir. The validity of the model is demonstrated through comparison of porosity types calculated from well logs and from direct sources including core samples and thin sections. A continuous curve of m for the whole carbonate reservoir is obtained using this approach.