Is Theis Flow Modelling Sufficient for EGS/HSA Geothermal Energy Production?

Abstract

The energy density of hot water is ~1% of the energy density of hydrocarbons. To produce comparable wellbore energy output, geothermal subsurface-to-surface fluid flow rates must greatly exceed those of hydrocarbon fluids. The high-flow constraint on geothermal energy production places a considerable burden on EGS/HSA projects. We propose that the burden arises in part because the complex, spatially unpredictable nature of in situ fluid flow is not adequately understood. We illustrate our proposition by considering the commonly used Theis (1935) expression purporting to relate in situ fluid flow parameters to wellbore pressure histories. The relation is valid if permeability heterogeneity can be ignored via spatial averaging. The relation can, however, badly mislead in the presence of in situ fracture-induced permeability heterogeneity likely to be encountered in realising EGS/HSA flow systems. We assess the Theis relation using an empirically-based physio-numerical computational framework capable of generating high degrees of in situ flow heterogeneity. In this framework, numerical flow simulations connect pressure to flow in the presence of spatial fluctuations of porosity ? and permeability ? obeying two ‘rules’ widely attested by well-log and well-core data: • Fracture-density/porosity spatial fluctuation power S(k) scales inversely with spatial frequency k, S(k) ~ 1/k, over 5 decades, ~1/km < k < 1/cm. • Fracture-density/porosity spatial fluctuations ?? control permeability spatial fluctuations ?? via ?? ~ ?log(?). Our pressure/flow simulations demonstrate that, in Theis terms, in situ wellbore pressure data can imply a quasi-uniform formation flow capacity ?h when in fact the in situ fluid flow is highly non-uniform. Wellbore pressure histories thus are suspect in yielding useful information about in situ heat transport between wellbores in an EGS/HSA heat exchange flow system. Failure to recognise the likely decoupling of in situ pressure fields from in situ flow/heat-transport fields can, we suggest, lead to unnecessary complications in realising EGS/HSA heat energy extraction.