Complex conductivity tomography using low-frequency crosswell electromagnetic data

Abstract

The complex conductivity of partially saturated siliciclastic sediments can now be reasonably predicted through recently developed petrophysical models like the POLARIS model. However, classical crosswell-induced polarization tomography (using a galvanometric approach) is characterized by a poor sensitivity map far from the wells, and thus other methods should be analyzed for potential improvements. The presence of low-frequency ([Formula: see text]) polarization effects in earth porous materials noticeably increases the amplitude and decreases the phase of measured electromagnetic (EM) fields. As such, the quadrature conductivity (directly associated with the low-frequency polarization effect) yields a significant contribution to the EM fields. We demonstrate that these contributions can be observed in crosswell EM data in terms of signal-to-noise ratio. With a realistic amount of noise, we can recover the distribution of the in-phase and quadrature conductivities for crosswell EM tomography. We use an integral equation approach for the forward modeling and a gradient-based approach with Tikhonov regularization for the inverse problem. We also develop a new inversion algorithm to invert time-lapse frequency domain EM data using an active-time-constrain approach. This information may be used in turn to improve our ability to monitor saturation changes in enhanced oil reservoir production, the remediation of oil spills, and the exploration and production of geothermal fields.