Efficient Hierarchical Processing and Interpretation of Triaxial Induction Data in Formations with Changing Dip

Abstract

Abstract Triaxial induction tools have contributed considerably to the understanding of thinly bedded reservoirs, in which the use of both horizontal and vertical resistivity (Rh and Rv, respectively) reduces the uncertainty in laminated sand petrophysical analysis of hydrocarbon in place. At the same time, existing answer products, based on a windowed 1D inversion, return dip and azimuth of the formation as well as Rh and Rv. The dip and azimuth, however, are constrained to be constants over the length of the inversion window, usually at least 10 ft, and often much longer. For many formations in complex environments, the necessary assumptions for using a 1D inversion are at odds with the actuality of the formation, and the results are of questionable quality. An inversion based on a homogeneous anisotropic formation model was introduced as part of the oil-based mud (OBM) borehole correction for the multiarray triaxial induction tool. It has been observed that this simple model can give better results in some 3D formations than those from the 1D inversion. The authors have taken this observation and have developed a new zero-D inversion that is divorced from the borehole correction algorithm so it can work in all environments, both OBM and water-based mud (WBM). This new process has been implemented in a hierarchical manner with a second-generation 1D inversion algorithm to produce a workflow that allows the best estimates for dip, azimuth, Rh, and Rv in complex reservoirs with changing dip. The results from the zero-D inversion are used to identify the complexity of the formations. The 1D inversion is run or displayed only through those zones in which the 1D model is valid, namely beds with relatively constant dip and azimuth. This new workflow combines the results from both the zero-D and 1D algorithms to produce the best results. Tests with model data of fully 3D formations show that this hierarchical processing workflow returns better results than those from the 1D inversion alone without the guidance of the zero-D inversion. The results in several complex reservoirs illustrate the applicability of the new algorithms and workflow. Backed by the new process, we expect the triaxial induction measurements to become routinely used in a wide variety of formations.