Wellsite Interpretation of Multicomponent Induction Log Data in Anisotropic Media

Electrically anisotropic reservoirs are encountered frequently in hydrocarbon exploration. For accurate saturation estimation and optimum hydrocarbon recovery from these reservoirs, it is essential to detect and properly describe their electrical properties. For example, in laminated sand/shale sequences or sands with different grain size distributions the resistivity parallel to the formation bedding is usually smaller than the resistivity perpendicular to the bedding. Conventional induction logging tools with transmitter and receiver orientation parallel to the borehole axis provide insufficient information to accurately evaluate hydrocarbon saturations in anisotropic reservoirs. These reservoirs usually show low resistivities from conventional induction logs, where the hydrocarbon content can be economically significant and are called low resistivity pay. Under the sponsorship of Shell Technology EP, Baker Atlas developed and field-tested a new induction logging tool that comprises three mutually orthogonal transmitter-receiver configurations yielding all necessary data to derive the horizontal and vertical resistivities of an anisotropic formation. Interpretation of these measurements is complicated because the magnetic field responses of horizontal induction coils are more sensitive to shoulder bed and borehole effects than conventional, vertical induction sensors. To provide a fast, yet reliable, interpretation of the formation parameters, we developed an enhanced apparent parameter estimation procedure that also applies shoulder bed corrections to the measured field data. We match the data acquired at each logging depth with responses of an anisotropic, whole-space model optimizing both horizontal and vertical resistivities in a least-squares sense. This process incorporates information about formation dip and azimuth as well as borehole trajectory and tool rotation. Then, we integrate both horizontal and vertical resistivities over layer intervals derived from the horizontal magnetic field data. After that, we apply shoulder bed corrections to the measured data, based on the synthetic layered host model response and the corresponding whole-space responses. Finally, we match the shoulder bed corrected data at the center of each layer with synthetic responses of an anisotropic, whole-space model. These newly derived resistivities in both horizontal and vertical direction are then used to refine both, the layered earth model and the shoulder bed corrections. The process comprising whole-space response matching and refining shoulder bed corrections is repeated until convergence is achieved. This interpretation scheme developed with synthetic data and tested with field data has proven to be fast and efficient, thus allowing us to provide reliable estimates of both horizontal and vertical resistivities at the wellsite.