Numerical Modelling Analysis of Borehole Electromagnetic Methods for Imaging Serpentinized Zones During Stimulated Geological Hydrogen Production

Abstract

The exploration and stimulation of geological hydrogen within ultramafic rocks represent an emerging frontier in energy resources. Stimulating hydrogen production involves injecting water and catalysts to accelerate serpentinization, a mineral alteration process that significantly reduces the electrical resistivity of the host rock often by several orders of magnitudes while altering other geophysical properties. Monitoring the spatial and temporal evolution of these serpentinized zones is essential for optimizing fluid flow, tracking fracture propagation and maximizing hydrogen yields. In this study, we numerically evaluate the feasibility of two inductive borehole electromagnetic (EM) techniques, specifically cross-well and borehole-to-surface EM configurations utilizing magnetic sources and receivers, for imaging the electrical resistivity changes associated with induced serpentinization. We construct time-lapse subsurface electrical resistivity models representing varying degrees of serpentinization over time within a crystalline basement overlain by a conductive sedimentary cover. We simulate forward responses and perform inversions to assess the resolving power of each EM method. Our numerical inversion results demonstrate that cross-well EM methods, operating at a high source frequency of 10,000 Hz, provide comprehensive tomographic coverage and accurately image the geometry and resistivity of the 20–30 m thick serpentinized zones even during the early stages of stimulation. In contrast, the borehole-to-surface EM approach requires lower source frequencies of 1000 Hz to ensure the EM energy penetrates the conductive overburden, which makes it less sensitive to thin, early-stage serpentinized zones. However, as the stimulated zone expands to 60–80 m in thickness over time, the borehole-to-surface EM method successfully delineates the broader serpentinized volume. We conclude that cross-well EM monitoring is the preferred high-resolution tool when multiple wells are available, whereas borehole-to-surface EM monitoring remains a practical alternative in single-well scenarios during advanced stages of stimulation.