Engineered Geothermal Systems

In EGS systems, Heat-In-Place is essentially the heat stored in the reservoir rock because EGS rocks are typically tight. Pathways for fluid flow are made by hydraulic fracturing (either creating new fractures or enlarging existing ones) but whilst key to permeability, their volumetric contribution is minimal.

Heat recovery mechanism is a combination of conduction and convection where conductive heat-flux from the matrix warms up the fracture fluid-fill that flows (convection) from injector to producer. In theory and given enough wells drilled and time available, conductive/convective flux could draw down all reservoir heat from initial to abandonment temperature. In practice, how much heat can be recovered depends on the fracture network (length, height, spacing) and on and how direct or diffuse is the fracture-connection pathway from injector to producer.

One of the most challenging aspects of the resource assessment of EGS systems is to assess the likely range in effectiveness of thermal-energy yield (i.e., the “thermal recovery factor”). Because heat recovery is through a combination of convection and conduction processes, numerical simulation of recovery efficiency requires coupled thermal-hydraulic modelling. However, given the large uncertainties regarding fracture network geometry and properties that apply to most settings especially early-stage, an elegant alternative to complex numerical modelling is to deploy analytical approximations. Solutions like the Gringarten et al (1975) mathematical model (graph below) are relatively quick and straightforward to deploy, and they provide good first-pass insights into the anticipated decline in produced fluid temperature over time and the impact of key uncertainties (see sample graphs further below). Analytical assessments can capture the uncertainties well and as such, underpin the initial assessment of thermal recovery and resources of an EGS play. Firming up by numerical simulation can be done at later stage when more conclusive data (and/or more time) is available.

Please click this link to assess a copy of the EGS case-study AEGeo presented at the EAGE 8th annual meeting in Vienna, June 2021.

The abstract link is here: https://doi.org/10.3997/2214-4609.2023101061