Microhole arrays for improved heat mining from enhanced geothermal systems

Numerical simulations are used to examine whether microhole arrays have the potential to increase the heat mining efficiency and sustainability of enhanced geothermal systems (EGS). Injecting the working fluid from a large number of spatially distributed microholes rather than a few conventionally drilled wells is likely to provide access to a larger reservoir volume with enhanced overall flow distances between the injection and production wells and increased contact area between permeable fractures and the hot rock matrix. More importantly, it reduces the risk of preferential flow and early thermal breakthrough, making microhole array-based EGS a more robust design. Heat recovery factors are calculated for EGS reservoirs with a conventional well configuration and with microhole arrays. The synthetic reservoir has properties similar to those of the EGS test site at Soultz-sous-Forêts. The wells and microholes are explicitly included in the numerical model. They intersect a stimulated reservoir region, which is modeled using a dual-permeability approach, as well as a wide-aperture zone, which is incorporated as a discrete feature. Local and global sensitivity analyses are used to examine the robustness of the design for a variety of reservoir and operating conditions. The simulations indicate that the flexibility offered by microhole drilling technology could provide an alternative EGS exploitation option with improved performance.