RECONSTRUCTION OF THE S-WAVE VELOCITY LOG IN CARBONATE FORMATIONS

摘要

Seismic prospecting and evaluation of the rock mechanical properties require knowledge of S-wave velocity. However, frequently the S-wave logs are not available and their reconstruction is a topical problem especially in carbonate formations characterized by a complex double-porosity microstructure. In this paper we propose a technique for determining the S-wave velocity using the joint inversion of conventional logs (P-wave slowness, micro-resistivity log, total porosity, density and gamma ray logs) for carbonate formations with different types of secondary porosity. The calculation of the theoretical logs is based on the model of a double-porosity medium that consists of a homogeneous isotropic matrix with a primary-pore system and secondary pores approximated by spheroidal inclusions. Selecting the aspect ratio of inclusions we can simulate different types of the secondary porosity such as vugs (close to sphere inclusions), vugs connected by channels (prolate spheroids), and cracks (flattened spheroids). To calculate different physical properties from this model, the self-consistent method of the symmetrical effective medium approximation is applied. The joint and simultaneous inversion procedure consists in the minimization of differences between the measured and calculated logs. As an inversion result we obtain the following parameters of pore microstructure: the matrix and secondary porosity values, and secondary-pore shapes. Then, the S-wave logs are simulated using the model parameters determined by the inversion. The technique was verified in the boreholes from the South Zone of Mexico where both P- and S-wave logs were measured. The comparison of the predicted and experimental S-wave logs has shown good agreement and the errors of reconstruction are less than 8%.