The concept of triaxial induction measurements can betraced back to 1960s. The first commercial triaxialinduction tool has been in service for more than adecade. The tensor type of measurements acquired bythe triaxial induction tool enables extracting not onlythe resistivity anisotropy but also the dip of asubsurface formation. The first anisotropic resistivitymodel that was developed is the transverse isotropic(TI) model, in which the principal axis associated withthe vertical resistivity (R_v) is perpendicular to thebedding plane. This model is in widespread use incommercial products by oilfield service companies toprovide resistivity anisotropy and the dip of aformation.Image data and outcrops show that crossbedding occursin multiple depositional environments. The applicationof an existing fast forward computer model forcrossbedded formations has been limited to verticalwells. We have developed a new fast computer modelfor the same problem that works not only in verticalwells but also in deviated wells. This new forwardmodel enables the user to study the triaxial toolresponse more efficiently in multilayer formations. Inparticular, it supports detailed investigation of the toolresponse where both dipping beds and crossbeddingexist in the model.We have used this model to simulate the triaxial toolresponse extensively in some typical formations. Thesimulation results demonstrate that the dipping bedeffect is tightly coupled to the effect of anisotropy dueto crossbedding in thin-layer formations. Thecomparison of the new crossbedding model with theprevious TI model suggests that the inversion based onthe TI model inversion is not adequate in the presenceof strong crossbedding, and both resistivity and dipresults are adversely affected.The new crossbedding forward model makes it possibleto have an accurate interpretation in dippingcrossbedded formations. A new inversion-basedinterpretation workflow is developed using the newcrossbedding model. The new method delivers onebedding dip and multiple bed-to-bed anisotropy dipsover the zone of interest. Results show that in thepresence of crossbedding, the method is able todistinguish between bedding dip and anisotropy dip,and provides better resistivity interpretation.
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