Three-dimensional anisotropy modelling and simulation of gas hydrate borehole-to-surface responses

Abstract

Electromagnetic (EM) methods have provided an alternative means of detecting and estimating gas hydrate, especially in places where hydrates have been established to exist but the gas hydrate shows no bottom-simulating reflection. However, most EM numerical studies being done focused on surface/seafloor Controlled Source Electromagnetic (CSEM) studies with little consideration for borehole EM studies. The assessment and interpretation methods of data from EM surveys require fast and accurate forward modelling algorithms integrated with the complexity of the earth's geological formation. This study develops 2D and 3D anisotropy forward modelling codes using a high-order finite difference frequency domain (FDFD) method for borehole surface EM studies. The newly developed numerical codes can model the effects of anisotropy on borehole-surface earth responses using gas hydrate models. The simulation results revealed that the newly developed numerical codes are highly efficient and suitable for simulating borehole surface CSEM data, especially for marine gas hydrate exploration and other complex earth models. An increase in the coefficient of anisotropy resulted in an increase in the magnitude of the electric field at the surface for the x and z components. Changes in layer resistivity affect the EM field diffusion, recorded Borehole Surface Controlled Source Electromagnetic (BSCSEM) responses, and the overall data interpretation. Also, amplitude of the 3D simulation response increases in the anisotropic faulted earth systems. Lastly, the results imply that Borehole Surface Electromagnetic (BSEM) simulation results can assist with the location and extent of gas hydrates in subsurface.