Charles O. Grigsby

Valles Caldera geothermal systems, New Mexico, U.S.A.

Abstract Valles Caldera is part of a Quaternary silicic volcano in northern New Mexico that possesses enormous geothermal potential. The caldera has formed at the intersection of the volcanically active Jemez lineament and the tectonically active Rio Grande rift. Volcanic rocks of the Jemez Mountains overlie Paleozoic—Mesozoic sediments, and Precambrian granitic basement. Although the regional heat flow along the Rio Grande rift is ~2.7 HFU ∗ , convective heat flow within the caldera exceeds 10 HFU. A moderately saline hotwater geothermal system ( T > 260° C , Cl ⋍ 3000 mg / l ) has been tapped in fractured caldera-fill ignimbrites at depths of 1800 m. Surface geothermal phenomena include central fumaroles and acid-sulfate springs surrounded by dilute thermal meteoric hot springs. Derivative hot springs from the deep geothermal reservoir issue along the Jemez fault zone, 10 km southwest of the caldera. Present geothermal projects are: (1) proposed construction of an initial 50-MWel power plant utilizing the known geothermal reservoir; (2) research and development of the prototype hot dry rock (HDR) geothermal system that circulates surface water through deep Precambrian basement (∼5MWth); (3) exploration for deep hot fluids in adjacent basin-fill sediments of the Rio Grande rift; and (4) shallow exploration drilling for hot fluids along the Jemez fault zone.

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.

Geology, water geochemistry and geothermal potential of the jemez springs area, Canon de San Diego, new Mexico

Abstract Studies of the geology, geochemistry of thermal waters, and of one exploratory geothermal well show that two related hot spring systems discharge in Canon de San Diego at Soda Dam (48°C) and Jemez Springs (72°C). The hot springs discharge from separate strands of the Jemez fault zone which trends northeastward towards the center of Valles Caldera. Exploration drilling to Precambrian basement beneath Jemez Springs encountered a hot aquifer (68°C) at the top of Paleozoic limestone of appropriate temperature and composition to be the local source of the fluids in the surface hot springs at Jemez Springs. Comparisons of the soluble elements Na, Li, Cl, and B, arguments based on isotopic evidence, and chemical geothermometry indicate that the hot spring fluids are derivatives of the deep geothermal fluid within Valles Caldera. No hot aquifer was discovered in or on top of Precambrian basement. It appears that low- to moderate-temperature geothermal reservoirs (

Rock-water interactions in hot dry rock geothermal systems: field investigations of in situ geochemical behavior

Abstract Two hot dry rock (HDR) geothermal energy reservoirs have been created by hydraulic fracturing of Precambrian granitic rock between two wells on the west flank of the Valles Caldera in the Jemez Mountains of northern New Mexico. Heat is extracted by injecting water into one well, flowing it through the fractured region and recovering the heated water from the second well. The produced fluid is cooled on the surface and reinjected into the system. The first reservoir was formed by fracturing the injection well at a depth of 2.75 km where the initial rock temperature was 185° C. A heat-extraction experiment conducted in this reservoir was run from January 27 to April 13, 1978. A second, larger reservoir was created after cementing the fracture-to-wellbore connections at 2.75 km in the injection well and refracturing 180 m deeper. This second reservoir was tested from October 23 to November 16, 1979. During each of these experiments, samples of the geothermal fluids and gases were collected at regular intervals from the injection wellhead, the production wellhead, and at the make-up pump which provided the water from storage ponds to replace the water lost downhole by permeation into the reservoir walls. Changes in the composition of the produced fluid provide a means for studying the reservoir behaviour under normal (recirculating) operating conditions. Certain of the dissolved species appear to be derived by displacement of an indigenous pore-fluid, while others appear to be derived by dissolving minerals known to be present in the reservoir rock. In this paper we describe the results of the chemical analysis of the geothermal fluids and relate the fluid and gas chemistry to geochemical processes that result from the heat- extraction experiments. In particular, the implications of the silica and NaKCa geothermometers and the pore-fluid displacement theory are examined for insight about the long-term effects of fluid geochemistry on heat extraction from HDR reservoirs.

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

Experiment 2033. Proposed Injection Test into the Upper Fracture Zone in EE-3

This is a detailed examination of microseismic event locations obtained from experiments 2018, 2020, 2023 and 2025. It suggests that the major difference in pressure behavior between systems that open at lower pressure and those that open at higher pressure can be ascribed to a lithologic boundary. The boundary appears to be defined by a discontinuity in joint orientation rather than an abrupt change in stress field. Examination of rock types determined from cuttings in EE-2 and EE-3 have defined two major intrusive events that contact roughly coincides with the inferred fracture orientation boundary.

Fracture Volume Growth and Fluid Mixing in the Phase I System

A complete review of the tracer test data from Segments 2 through 5 has revealed pertinent information regarding the growth of the reservoir. Several parameters have been correlated with observed changes in the flow through reservoir volume as measured by both sodium fluorescein and bromine (Br82) tracers. These include the effects of thermal energy removal, pressurization including massive hydraulic fracturing, and wellbore separation distance on the reservoir volume. Ultimately, we would like to correlate measured tracer volumes with effective heat transfer surface. In addition, the interpretation of tracer volume changes could be used to develop improved methods of reservoir operation for example, remedial pressurization for stress relief (like SUE) or a huff-puff operation mode in contrast to our normal (stress-constrained) continuous mode of extracting heat.