Fracture Detection Using the Conductivity Triaxiality Index

Abstract

Over the years, triaxial induction tools have found applications in not only determining formation resistivity, anisotropy, and dip, but also in detecting fractures. Fracture detection techniques provide useful information in identifying fractured reservoirs because fractures provide both space for hydrocarbon storage and also the channel for fluid flow. When fractures are developed in a formation, the formation can exhibit triaxially anisotropic conductivity if the fractures are filled with a fluid with a significantly different conductivity than that of the formation. This is particularly true for transversely isotropic formations. In an isotropic sedimentary formation, fractures can also cause the formation to be triaxially anisotropic if the geometric properties, orientation and porosity of fractures are varied from place to place in the fractured zone. In this paper, we present a new method that uses the information of the conductivity tensor to detect fractures. The new method indicates the presence of contrasting fluid-filled fractures by the difference in three conductivity components of the tensor. This difference is called triaxiality in the paper. The conductivity tensor is found using an inversion approach. In addition to three conductivity components σx, σy, and σz, the inversion also provides three Euler angles γ, α, and β. Multiple initials for Euler angles are used to ensure that a global minimum is obtained. The inversion is followed by a rotation operation on the inverted conductivity tensor to eliminate the ambiguity effect of the principal coordinate system. The new Euler angles are found by solving a minimization problem. We have applied the new method to both synthetic and field examples. Synthetic examples show that the rotation operation is indispensable to remove the ambiguity effect of the principal coordinate system. Furthermore, it is shown that the triaxiality is indeed responsive to the presence of fracture. Previous processing of the field example has showed that fractures exist in many zones. The triaxiality index derived from the new method is found in a fairly good agreement with the fracture index from a previous method.