Feasibility Study of Single-Well Dual-Cable DAS for Micro-seismic Monitoring of Geothermal Operations

Abstract

Distributed acoustic sensing (DAS) that uses optical fibres as sensing units is attracting increasing interest for micro-seismic monitoring in geothermal projects. Standard optical fibres provide one-component measurements along the fibre and this pose challenges in determining certain characteristics of the source, such as its azimuth and its full moment tensor. Full source characteristics can be obtained via offset downhole measurements and/or measurements from horizontal well sections but these come with substantial extra costs. This paper proposes a single-well dual-cable DAS configuration to reduce the need for drilling additional wells or sections, where two DAS cables are assumed to be positioned within a single vertical well at opposite sides of the well. Synthetic DAS signals are generated by an open-source code that assumes plane-layered media and are used to study the feasibility of the dual-cable DAS for localising a seismic source and resolving its moment tensor. A localisation procedure is presented, and a sensitivity analysis of localisation accuracy is conducted with respect to source parameters and noise levels. In addition, an analysis is performed to assess the resolvability of the moment tensor components from the dual-cable DAS configuration. Results suggest the source location can be fully determined, yet low signal-to-noise ratio and azimuth close to 0∘ (North, aligned with the two cables) lead to a decrease in accuracy. The full moment tensor can be resolved only if the epicentral distance is 5 m or less, while non-double-couple components can be reliably resolved with an epicentral distance up to 20 m, showing improvement compared to installations with a single cable. Consequently, near-borehole failures, regardless of the source mechanisms, can be characterised within an epicentral distance of 5 m. With epicentral distance increasing, resolvability of the mix-mode failures is reduced first, followed by the resolvability of the pure shear or tensile failures, which depends on the azimuth. Overall, the results demonstrate that a single-well dual-cable configuration has the potential for monitoring and understanding near-borehole micro-seismic events induced during geothermal reinjection and stimulation operations.