Title:

The Radiator-EGS System: A Fresh Solution to Geothermal Heat Extraction

Authors:

Peter GEISER, Bruce MARSH, Markus HILPERT

Key Words:

EGS, thermal diffusivity, hydro-thermal

Conference:

Stanford Geothermal Workshop

Year:

2015

Session:

Enhanced Geothermal Systems

Language:

English

Paper Number:

Geiser

File Size:

1032 KB

View File:

Abstract:

The standard Enhanced Geothermal System (EGS) has repeatedly been hobbled by the inability of rock to conductively transfer heat at rates sufficient to re-supply heat extracted convectively via artificially made fracture systems. At the root of this imbalance is the basic magnitude of thermal diffusivity for most rocks, namely ~10-6 m2/s, which severely hampers heat flow once the cooled halos about fractures reach ~0.1 m or greater. This inefficiency is exacerbated by the standard EGS design of mainly horizontally constructed fracture systems with inflow and outflow access at the margins of the fracture network. We introduce an alternative system whereby the heat exchanger consists of; 1] The style of a conventional radiator in an internal combustion engine, which we call a Radiator-EGS (i.e., RAD-EGS). That is, the heat exchanger is built vertically with cool water entering the base and hot water extracted at the top. The intervening distance is designed for the local prevailing crustal thermal regime. The RAD-EGS itself consists of a family of vertically interconnected vanes produced through sequential horizontal drilling and frac-induced rubblization. 2 ] The observations that natural hydrothermal systems ultimately depend on heat conduction for their source of thermal energy and successfully transfer the heat necessary for generating commercial quantities of electrical energy. This suggests that an EGS emulating a hydro-thermal system may offer a solution to the low thermal conductivity of rock sufficient for commercial electrical generation. The key elements of the RAD-EGS are; 1] a set of “manufactured” vertical permeable fracture zones (the radiator “vanes”) that circulate fluids through a rock volume such that its final temperature is sufficient for commercial energy production. 2] The location of the producing well at the Tmin (150o C) isotherm and the injector at a sufficient depth below the Tmin isotherm such that the temperature of the convecting fluid is more than Tmin when it arrives at the depth of the Tmin isotherm. We refer to the injector depth as the Tmax isotherm. 3] The orientation of the manufactured permeable fracture zones requires they have the same orientation as that of natural transmissive fracture systems with respect to the ambient stress field. The strike orientation of transmissive fractures is controlled by SHmax e.g. Barton et al, 1995, dip is controlled by the orientation of S1. Below about 700 m, S1 is vertical (Fisher and Warpinski, 2012) and the average strike of transmissive fractures parallels SHmax. Creating vertical fractures that include S1 and SHmax requires drilling stacked laterals parallel to SHmax at depths more than 700m. This design has several fundamental advantages over conventional EGS: 1] the system uses the natural vertical crustal geothermal gradient for re-supply of heat from below and laterally, which also naturally induces a continual vertical flow and delivery, through buoyancy, of all heated fluids to the capping extraction zone; 2] it incorporates the flow associated with preexisting transmissive fracture/fault zones into the heat exchanger, thus using them to advantage rather than short-circuiting the desired flow pattern; 3] it can be deployed in basins with HSA where the overall modest background or regional fluid flow (~10-7 m/s) will help to overcome the sluggish rates of thermal diffusion and greatly sustain the longevity of heat extraction. Key to creating a RAD-EGS is a fracing system with a somewhat surgical precision in stepping upwards in the system from one lateral to the next. We suggest the use of propellant-fracing to achieve the desired level of control.


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