Heat Production On From A Geothermal Reservoir Formed By Hydraulic Fracturing - Comparison Of Field And Theoretical Results
Authors: Murphy Hugh D. 
Published: December 2007

Murphy, Hugh D., and Tester, Jefferson W., Members SPE-AIME Univ. of California/Los Alamos Scientific Laboratory

Abstract
Results of preliminary heat extraction testing of a prototype hot dry rock geothermal energy reservoir are presented. This reservoir was produced by hydraulic fracturing of hot, otherwise non-porous, granite at a depth of 3 km in northern New Mexico. Heat was extracted from the reservoir during closed loop operation using two wells, one for injection and one for production. During 75 days of operation the steady flow impedence decreased by a factor of five as thermal contraction resulted in the opening of natural joints that provided additional communication with the producing well. Water losses to the rock surrounding the fracture steadily diminished and eventually this loss rate was less than 1% of the injected rate. The geochemistry of the produced fluid was benign and the seismic effects associated with heat extraction were immeasurable small. The relatively rapid thermal drawdown of the produced water, from 175 to 85 degrees C (345 to 185 degrees F), indicated that the effective hydraulic fracture radius was approximately 60 m (200 ft). The average thermal power extracted was 4 MW. Reservoir modeling was performed with a numerical simulator which handles two-dimensional fluid transport and heat conduction equation for the rock surrounding the fracture. The match obtained with field data indicates that the model is based upon a fundamental correct balance of the heat transport processes, convection and conduction, and provides bounds for the hydraulic fracture geometry.

Introduction
A program designed to demonstrates the feasibility of extracting energy from hot, dry rock has been initiated at the Los Alamos Scientific Laboratory (LASL). Basically, it is proposed that man-made geothermal energy reservoirs can be created by drilling into relatively impermeable rock to a depth where the temperature is high enough to be useful, creating a reservoir by hydraulic fracturing, and then completing the circulation loop by drilling a second hole to intersect the hydraulically fractured region. Thermal power would be extracted from this system by injecting cold water down the first hole, forcing the water to sweep by the freshly exposed hot rock surface in the reservoir-fracture system, and then returning the hot water to the surface where the thermal energy would be converted to electrical energy or used for other purposes. System pressures would be maintained such that only one phase, liquid water, would be present in the reservoir and the drilled holes. The concept is described in more detail by Smith et al.

DRILLING AND COMPLETIONS
The first reservoir is being investigated at Fenton Hill, located on the west flank of a dormant volcano, the Valles Caldera, in the Jemez mountains of northern New Mexico. The first deep borehole GT-2 (Geothermal Test-2) was drilled to a depth of 2.929 km (9609 ft) in granite, where the temperature was 197 degrees (386 degrees F), cased to 2.917 km (9571 ft), and then fractured in the open hole below the casing. The variation of equilibrium temperature and geology with depth is shown in Fig. 1. The Precambrian crystalline rocks were encountered at 730 m Precambrian crystalline rocks were encountered at 730 m (2400 ft). From this depth to the bottom the geothermal gradient is reasonably constant. The steeper gradient in the sediments above 730 m is caused by their lower thermal conductivity at 2.77 km (9100 ft), the geothermal heat flow at this site is 0.16 W/m2, 2-1/2 times the worldwide average.
Following a series of hydraulic fracturing experiments at deeper depths, a new, larger fracture was created at 2.81 km (9220 ft), where a milling operation had accidentally broached the casing. This fracture became the main exit point when water was injected in GT-2. The granitic rock in point when water was injected in GT-2. The granitic rock in which these fractures were created is relatively homogeneous and unstratified, so it is assumed that all the fractures discussed here are approximately circular in shape, rather than rectangular as is usually assumed in oil and gas reservoirs.