RECENT ADVANCES IN THERMO-HYDRO-MECHANICAL MODELLING OF COLD WATER STIMULATION AND TRIGGERED EARTHQUAKES

Abstract

Perturbation to ambient stress conditions within a geothermal reservoir is of increasing interest in the context of cold water stimulation of reinjection wells and associated microseismicity. Cold water injected at elevated pressure reduces the effective normal stress across in situ fractures, which promotes shear failure. When fractures fail in shear, there is the possibility for self-propping and permeability enhancement and, if the event is large enough, a detectable microearthquake. Representing these processes in a numerical model allows us to probe the dynamics of coupled fracture-flow systems and improves our forecasting abilities. Existing thermo-hydro-mechanical (THM) reservoir simulators typically account for poroelastic and thermal stress effects in coupling the flow to the mechanical system. Constitutive models for permeability and porosity that employ either failure criteria or damage models are used to implement the reverse coupling. However, an explicit description of the earthquake process, including slip and stress drop, is much less common and an area of considerable interest. This paper reviews recent advances in modelling cold water stimulation of geothermal reinjection wells using a constitutive model for permeability enhancement that honours the concept of self-propping shear failure of in situ fractures. The contribution of thermal stresses is highlighted and we find a close correlation between the volume of cold injectate and the volume of shear-enhanced permeability. Similarly, in several geothermal systems, correlations are observed between the volume of reinjection and cumulative measures of microseismicity. Extension of our existing reservoir simulation capability to capture these effects requires a simplified description of the earthquake process. The Hilbert transform method coupled to a rate-and-state friction model is one possible avenue for achieving this goal.