Microhole arrays for improved heat mining from enhanced geothermal systems

Microhole arrays for improved heat mining from enhanced geothermal systems Stefan Finsterle a , Yingqi Zhang a,∗ , Lehua Pan a , Patrick Dobson a , Ken Oglesby b a b Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 74-120, Berkeley, CA 94720, USA Impact Technologies LLC, Tulsa, OK 74153, USA 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-Forets. 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. 1. Introduction The fact that an EGS reservoir needs to be engineered by drilling wells for stimulation, injection, and production leads to consid- erable technical challenges, but at the same time provides unique opportunities to design and optimize the system for maximum heat extraction and sustainability at minimum risk and cost. Advances in drilling technology are essential for the technical success and economic viability of EGS. Specifically, the development of drilling technology for slimholes (bores less than 6½ inches or 16.5 cm diameter), microholes (bores less than 4 inches or 10.2 cm diame- ter), or ultra-slim diameter wells (bores between 1 and 3 inches (2.5 and 7.4 cm) diameter) (Pritchett, 1998; Finger et al., 1999; Moe and Rabben, 2001; Garg and Combs, 2002; Sanyal et al., 2005) may offer the flexibility needed to design well configura- tions that enable optimization of geothermal heat mining using EGS. Pritchett (1998) showed considerable potential of slimhole drilling for smaller geothermal projects (100–1000 kWe) in off- grid remote areas. Slimhole drilling technology for the operation of small geothermal power plants is summarized in the Slimhole Handbook (Finger et al., 1999), which contains case studies and recommendations for slimhole drilling practice. Moreover, the U.S. Department of Energy sponsored a Microhole Initiative to promote the development of technology and tools for microhole drilling where the advanced FLASH ASJ TM drilling technology for microhole drilling was developed (Oglesby, 2009; Summers et al., 2007). Geothermal energy is considered to be a clean, carbon-neutral, renewable and stable form of baseload energy. High-enthalpy heat mining for electrical energy production requires systems consisting of (1) a heat source with sufficiently high temperature at accessible depths, (2) fluids to absorb the heat from the hot rock matrix and to transport it to the surface, and (3) interconnected pore space that allows fluid flow at acceptably high rates between injection and production wells. While heat is present essentially every- where in the subsurface (MIT, 2006), the requirements for sufficient water and permeability are only met at locations where the tec- tonic, structural, and geologic conditions favor the development of a hydrothermal system. In the absence of such favorable condi- tions, the concept of enhanced geothermal systems (EGS) provides a means to create a reservoir by hydraulically or thermally frac- turing the rock or shearing existing fractures, thus generating or increasing permeability and inter-well connectivity. The injected working fluids then pick up reservoir heat as they flow along the created permeable pathways toward production wells. ∗ Corresponding author. Tel.: +1 510 495 2983; fax: +1 510 486 5686. E-mail address: YQZhang@lbl.gov (Y. Zhang).