Fracture Characterization Using Single-Hole EM Data

Abstract

A simple method of mapping fractures in terms of a few fracture parameters is presented in this paper. Frequencydomain 3-component electromagnetic (EM) fields measured in a single-hole configuration are used for the analysis. In actual producing fields the fracture geometry and surrounding medium can be very complicated. For precise imaging of such a fractured medium, it will take a full-fledged 3-D EM inversion that will involve a great deal of computing resources. Success of such an effort may not be guaranteed because of the lack of data inherent to the single-hole survey method. For simplicity, it is assumed that the medium is a uniform whole space, and the parameters to be inverted are the background conductivity, the distance, conductance and dip angle of a single fracture of large extent. In obtaining these parameters, a combination of simulated annealing (SA) and nonlinear least-squares (NLSQ) methods have been used. The SA is an excellent searching algorithm leading to the general vicinity of the global minimum, thereby providing reasonably good initial guess for the NLSQ. Detailed fine-tuning of inversion parameters can then be accomplished through NLSQ with an accelerated convergence. The approach has been very successful in determining fracture parameters. Data used for the tests are synthetic, generated by a 1-D medium with a thin layer and by a thin sheet in a whole space. The proposed method is useful for analyzing models of simple geometry. But since the approach is simple and computationally efficient, it can be used real-time in producing fields for fast evaluation of the reservoir in which fluid flow is controlled by fractures. Additionally, results obtained in this manner may be useful in preparing an initial model for a more rigorous 3-D EM inversion at a later time.