Microhole arrays for improved heat mining from enhanced geothermal systems

Abstract

a b s t r a c t Keywords: Geothermal energy Heat extraction Microholes Numerical modeling EGS 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.

A hybrid optimisation approach to improve long-term performance of enhanced geothermal system (EGS) reservoirs

“…In general, simulation of heat extraction in an EGS reservoir involves thermal-hydraulic-mechanical and chemical (THMC) coupled processes [39,40]. However, the coupled hydraulic and thermal processes, which represent the fluid flow and heat transfer, play the most significant role in the heat extraction stage for the long-term performance compared to the other processes [41][42][43], see further…”

GeoVision Analysis: Exploration Task Force Report

“… Using microholes (boreholes with diameter less than 10 cm) to enhance fluid flow and heat exchange within an EGS: Multiple microholes are drilled as sidetracks off of injection or production wells (Bracke and Wittig, 2011). Because microholes can be spread widely, thus intersecting a larger portion of the fracture network, the use of microholes increases the rock volume that is accessed by the circulating working fluid, therefore extracting more heat from the geothermal reservoir (Finsterle et al, 2013). Additionally, if some of the microholes miss the stimulated fracture zone, the circulating fluid will selfregulate and flow through the remaining microholes that intersect the fracture zone.…”

A review on heat transfer and energy conversion in the enhanced geothermal systems with water/CO2as working fluid

“…or 10.2-cm diameter) rather than a few conventionally drilled wells [33]. or 10.2-cm diameter) rather than a few conventionally drilled wells [33].…”

“…(bores less than 4 in. or 10.2-cm diameter) rather than a few conventionally drilled wells [33]. Their numerical simulation results showed that the microholes were 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.…”