Hugh D. Murphy

Hot dry rock geothermal reservoir testing: 1978 to 1980

Experimental results and re-evaluation of the Phase I Hot Dry Rock Geothermal Energy reservoirs at the Fenton Hill field site are summarized. Reservoir growth is traced. Reservoir growth was caused not only by pressurization and hydraulic fracturing, but also by heat extraction and thermal contraction effects. Reservoir heat-transfer area grew from 8000 to 50,000 m/sup 2/ and reservoir fracture volume grew from 11 to 266/sup 3/m. Despite this reservoir growth, the water loss rate increased only 30%, under similar pressure environments. For comparable temperature and pressure conditions, the flow impedance (a measure of the resistance to circulation of water through the reservoir) remained essentially unchanged, and if reproduced in the Phase II reservoir under development, could result in self pumping. Geochemical and seismic hazards have been nonexistent in the Phase I reservoirs. The produced water is relatively low in total dissolved solids and shows little tendency for corrosion or scaling. The largest microearthquake associated with heat extraction measures less than -1 on the extrapolated Richter scale.

Preliminary Assessment of a Geothermal Energy Reservoir Formed by Hydraulic Fracturing

Two, 3-km-deep boreholes have been drilled into hot (approximately 200/sup 0/C) graphite in northern New Mexico in order to extract geothermal energy from hot dry rock. Both boreholes were hydraulically fractured to establish a flow connection. Presently this connection has a large flow impedance which may be improved with further stimulation. Fracture-to-borehole intersection locations and in situ thermal conductivity were determined from flowing temperature logs. In situ measurements of permeability show an extremely strong dependence upon pore pressure--the permeability increased by a factor of 80 as the pressure was increased 83 bars (1200 psi). An estimate of the minimum horizontal earth stress was derived from fracture extension pressures and found to be one-half the overburden stress.

Hot Dry Rock Reservoir Development and Testing in the USA

Two hot dry rock geothermal reservoirs have been created, or are being created, in hard impermeable rock at a site in the Jemez Mountains of New Mexico. In the first reservoir, created for research purposes, the technical feasibility of this new energy source was established during three years of intensive testing. Now the Phase II reservoir, intended as the first step in demonstrating commercial viability, is being created.

Hot dry rock reservoir engineering

The development of geothermal energy has been limited historically to the use of natural hot water and steam contained in what are referred to as hydrothermal geothermal reservoirs. However, as indicated by Roland Home elsewhere in these proceedings, probably less than 5% of the energy in a hydrothermal reservoir is contained in the extractable fluid. What about the 95% remaining?

Experiment 2039 – Diagnostic Logging in EE-3

Experiment 2039, diagnostic logging in EE-3, was conducted in two parts. Part A, temperature and collar locator surveys while injecting water was run from ~16:00, 4-Apr-84 through ~11:00, 5-Apr-84. Part B, tracer surveys, run from ~8:15 - 21:00, 6-Apr-84. The purpose of the surveys was to determine fluid entry locations in the open hole sections of EE-3

Current Summary of Hydraulic Fracturing Experiments in Phase II Reservoir

To aid those interested in wading through the old experimental data regarding Phase II fracturing, we have compiled the following summary of the experiments. A similar summary, but one slanted from a surface operations viewpoint, was reported by A. Richard Sinclair in reference 1. Our summary is abstracted from the individual experiment reports, when available, the daily status reports, and other available information. Whenever possible we have tried to acknowledge individual authors, but occasionally we may have been incomplete in our attribution, and in the cases we offer our apologies. In the near future we intend to compile temperature and friction-corrected downhole pressures so that a complete categorization of fracture pressures will be available.

Results of Experiment 2035, Preliminary Analysis

The purpose of this experiment was to run temperature, spinner and tracer surveys in EE-2, while injecting water, so as to determine the extent, if any, of damage to the casing cement, and also to determine locations and flow fractions of the fracture between the bottom of the casing and top of the sand.

Experiment 2035: Investigation of the EE-2 Casing Shoe and Openhole Interval

The primary objectives for Expt. 2035 are to determine the amount and distribution of the postulated flow behind the EE-2 casing and determine the flow split between the several openhole fracture entrances.

Experiment 2003 – First Pressurization of EE-2

Water was pumped into EE-2 at a nominal rate of 9gpm, to a final pressure of 2070 psi. The wellbore was exceptionally tight we might just have well pumped into a steel pressure vessel not only was there no evidence of breakdown, but only a total of about 30 gallons of water permeated the rock during the 2-1/2 hour-long pressurization.

Fracture Evaluation Procedures

Following fracturing it will be necessary to evaluate the success of each treatment. The primary method will be to measure impedance.