INTEGRATING MULTI-SENSOR ACOUSTIC AND RESISTIVITY DATA FOR IMPROVED FORMATION EVALUATION IN THE PRESENCE OF DRILLING INDUCED FRACTURES

摘要

Drilling induced fractures are frequently encountered in wells that are drilled overbalanced. These fractures are typically close to vertical and have the same direction as natural or hydraulically induced fractures that extend parallel to the maximum formation stress direction. The appearance of fractures filled with a mixture of oilbased mud and formation fluid changes resistivity distribution around the borehole. This resistivity change affects the multiple components and multiple depths of investigation measurements of the modern induction tools differently depending on fracture orientation, fracture length, and formation resistivities. The fractures also create anisotropy in acoustic properties around the borehole, enabling them to be effectively detected/measured using a cross-dipole acoustic tool. Analysis of the measured induction data using advanced inversion and modeling techniques allows recovery of the fracture orientation and length as well as horizontal and vertical resistivities of the undisturbed formation. Analysis of the measured cross-dipole data allows the determination of the fracture intensity and orientation, which can be jointly used with the multicomponent induction (3DEX) results to reduce interpretation uncertainties and better characterize the fracture parameters. In this case study, we discuss an application from offshore India where a 12.25” vertical well was drilled overbalanced with a 13.2 ppg synthetic oil-base mud. A wide spectrum of logging while drilling (LWD) and wireline logs was acquired, including LWD propagation resistivity, wireline array induction (HDIL), multi-component induction (3DEX), and cross-dipole acoustic measurements (XMAC Elite). The presence of drilling induced fractures in some intervals was immediately apparent based on the large difference between LWD resistivity and array induction resistivity data, separation of the shallow and deep array induction resistivity curves, and different responses of Hxx and Hyy multi-component measurements (3DEX). We devised an enhanced data processing scheme to derive accurate information from the formation over the fractured intervals: - Joint interpretation of orientation measurements and the differences in 3DEX multi-frequency focused Hxx and Hyy components reliably provided fracture azimuth. The successful determination of the fracture orientation based on the minimum, maximum, and zero crossing points of the Hxx and-Hyy data were carried out. - A cross-dipole acoustic azimuthal anisotropy analysis also provided the fracture orientation that agreed well with the orientation determined from 3DEX. - Analyses of the array induction measurements with different depth of investigation yielded an estimate of the radial fracture extent by comparing the measured data with pre-calculated 3-D models with different fracture lengths. - Development and application of the new 3DEX data processing scheme utilizing LWD resistivity data and a special rotation of the 3DEX Hxx, Hyy, and Hxy components for obtaining reliable computation of horizontal and vertical resistivities unaffected by fractures. The computed formation resistivity parameters were then successfully applied in the final petrophysical analysis.