Summary

In this paper, we study the performance of time-lapse inversion of borehole magnetometric resistivity data in the context of quantitative monitoring of CO2S storage. We have considered the injection of CO2S in a thick sandstone aquifer at a depth of 300 m, as is planned for a controlled release experiment at the Carbon Management Canada Field Research Station (FRS) in Alberta, Canada. The CO2S plume extension is modeled as a resistive zone with increased lateral extensions. Inversion results show that MMR can be a robust tool to follow the evolution of CO2S plume propagation. Structure orientation bias that affects the MMR method can be reduced using a distance weighting function. Also, the resistivity of the plume is generally under-estimated.

Introduction

The geological sequestration of Carbone dioxide (CO2S) in deep saline aquifer requires the development of geophysical monitoring techniques that allow detecting and imaging the spatial extent of the CO2S plume to ensure longterm integrity of the storage. Electrical and electromagnetic methods appear appropriate for such problem because a high resistivity contrast exists between brine and the injected supercritical CO2S. For that reason, time-lapse electrical resistivity tomography (ERT) is being used in a few brine pilot tests (e.g. Schmidt-Hattenberger et al., 2012).

In this paper, we present the results of a numerical feasibility study for borehole magnetometric resistivity (MMR) monitoring of CO2S sequestration at the Field Research Station (FRS), Alberta, Canada. Instead of measuring the electrical potential field caused by current injection, MMR method measures the associated magnetic field. The latter is representative of current channelled through the ground and is a function of acquisition geometry and resistivity contrast. Besides, the MMR method is sensitive to current density variations and not to the absolute resistivity values. Consequently, the MMR response is not affected by the problem of noise in conductive media, as is the case of ERT survey (very weak electrical potential in conductive environments). MMR measurements are carried out using three-component fluxgate magnetic probe, which reduces the need for borehole electrode installation and suppresses corrosionrelated problems that can bias time-lapse measurements. Another important MMR feature for time-lapse CO2S monitoring is that the vertical magnetic field resulting from the current distribution in the earth is only dependent on zones where lateral resistivity contrasts exist (Acosta and Worthington, 1983). The vertical magnetic component for a homogeneous or a layered earth is zero.

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