Abstract:

The Raft River geothermal field is the site of an innovative Department of Energy Enhanced Geothermal System (EGS) project to determine the viability of using combined thermal and hydraulic stimulation techniques to improve energy production. Well RRG-9 is currently undergoing a stimulation program using injectate from the US Geothermal Raft River Power Plant and cold water from a cooling tower make-up water well. The stimulation began on 13 June 2013 with injection from the power plant at a temperature of about 39 °C and a pressure of 275 psig. Next, two positive displacement plunger type pumps were used to increase the injection pressure and flow rate for about one month. The highest rate achieved was 258 gpm at a pressure of 741 psig. During this time, fluid from the cooler water well was injected for about 2 weeks at various pressures. Then, the pumps were removed and plant injection resumed on 25 September. Plant injection will continue until the spring of 2014, when a high pressure hydraulic stimulation will be conducted. A series of seismic monitoring stations deployed around the well are providing data on seismic events occurring at the site. Over the past year, 51 microseismic events have been recorded, all less than Magnitude 1.
During injection, several diagnostic tests were conducted to gain a better understanding of the well and reservoir. A step-rate test was performed on 22 August to measure the in-situ stress and aid in modeling in-situ fractures. A tracer was injected into the well on 9 September. No tracer was detected in adjacent production wells after several months. A second borehole televiewer survey was conducted for comparison to pre-stimulation images. A third borehole televiewer survey is planned after the high pressure stimulation.
Injection test data is evaluated in real time. A modified Hall plot analysis indicates the effective permeability is increasing. The injectivity index supports the results of the modified Hall plot analysis. As the thermal stimulation has continued, the injectivity index has consistently followed an upward trend from 0.1 gpm/psi to 0.53 gpm/psi.

Abstract:

In August 2012, the City of Akutan completed an exploration program to further characterize the geothermal resource and to select drilling targets in the geothermal resource area on Akutan Island near Hot Springs Bay Valley. The exploration program included geologic mapping, a magnetotelluric (MT) survey, and a gravity survey. The program built on previous exploration, which included an MT survey, a geologic reconnaissance field study, soil and soil gas geochemical surveys, a satellite remote sensing study, a review of existing hot springs geochemistry data, drilling of two temperature gradient wells, and development of a conceptual model. The culmination of the 2012 work was to use 3D visualization of the data to advance the conceptual model and select deep drilling targets. Access requirements were taken into account in selection of the surface locations; underground targets will be reached using directional drilling.

Abstract:

The Raft River geothermal field, located in Cassia County in southwestern Idaho, is the site of a Department of Energy Enhanced Geothermal System project. U.S. Geothermal, Inc. currently produces about 11 MWe from Precambrian metamorphic rocks. These lie beneath ~5,000 ft of Quaternary and Tertiary volcaniclastic and volcanic deposits. Maximum temperatures range from 271o F to 300o F.

Well RRG-9 ST1, the well targeted for stimulation is located approximately 1 mile south of the main bore field. The open hole section of the well, from 5,551 to 5,900 ft MD, consists of Precambrian Elba Quartzite, the stimulation target, granite and minor diabase. Prior to setting the casing acoustic, gamma ray, and density logs were run. After completing the well, a step rate/step down test was conducted. The maximum injection rate achieved was 18 bpm at a wellhead pressure of 1,150 psig.

A borehole televiewer run in the open hole section showed evidence of more than eighty fractures. The majority of these fractures trend from N20⁰W to N20oE and dip from 40o to 60oW. Permeable fractures were encountered in the Elba Quartzite at 5,640-5,660 ft MD. Analysis of the injection test indicates that the minimum in-situ principal stress in this zone is 3,050-3,200 psi, corresponding to a fracture gradient of 0.59-0.62 psi/ft. A discrete fracture network model was developed using

measured and inferred fracture orientations, distributions and dimensions.

A three-phase stimulation program is proposed for RRG-9 ST-1. During the first two stages, water at 140oF, and later 40oF, will be injected to pre-condition and thermally fracture the reservoir. The third stage will consist of a high rate, large volume conventional hydraulic stimulation.

Abstract:

The Salton Sea Geothermal Field is one of the largest geothermal resources in the world. Recent changes in leasehold positions, changes in lake management due to Colorado River water transfers, a transition to renewable energy resources and the clean energy initiatives of California, have prompted renewed interest in development of the field for baseload power generation. The receding shoreline of the Salton Sea is now exposing areas previously inaccessible, and exposing large tracts of land for development. Since the last conceptual model and resource estimate for the Salton Sea Geothermal Field was published in 2002, significant additional data has become available, including publicly available seismic surveys over the resource area, experiences of developers and operators at the field, and recent research related to seismicity and tectonics of Southern California. In this study, we integrate these data sets in an updated conceptual model and a revised estimate of the power generation potential of the field. The result is a model that can serve as the basis for further exploration and development in the field. Our study increases the power generation potential of the field to 2950 MWe.

Abstract:

The Salton Sea geothermal field is one of the largest geothermal
resources in the world. It was discovered in the 1950s
but has only been developed on the southeastern lake shore and
the actual field boundaries are not well known. In this paper we
describe a combined offshore/onshore Magnetotelluric (MT)
survey made over the known geothermal field and surrounding
region to determine the formation resistivity signature of the
geothermal field and to use this signature to map the external
field boundaries and internal structure.
The survey was made with land, marine and hybrid MT
field systems. These instruments use a portable, low-power
digital data acquisition system with sensors deployed on land,
and on the shallow sea bed. The survey consisted of 70 sites
arranged in 4 profile lines; 3 of these profiles cross the northeast
trending geothermal field in a NW-SE direction and the
4th profile crosses in a NE-SW direction. The data from the
sites were processed to provide apparent resistivity and phase
as functions of frequency for each site. The sites were then
grouped into profiles, and a 2D inversion code was applied to
provide a resistivity versus depth section along each profile.
The MT profiles show that the geothermal reservoir has a
lower resistivity than the background. This difference is largely
due to the higher temperatures and higher formation water
salinity. Based on the low resistivity signature, we estimate
that the field encompasses more than 200 km2, over half of
which lies offshore. Within the field, the MT profiles match the
known geology and borehole induction resistivity logs well.
The general stratigraphic section can be divided into three
vertical horizons: a shallow mud and silt cap rock, an upper
reservoir zone consisting of high temperature sand and clays
and a deeper, more continuous reservoir zone of consolidated
sand and silt.

Abstract:

Five geothermal wells in the southern Salton Sea geothermal
field (SSGF) penetrate at least 30 m of vein-controlled
and stratabound, epithermal lead-zinc mineralization with
at least 2.5 wt.% sphalerite plus galena. The richest of these
intervals is hosted by a steeply-dipping fracture zone, and assays
an average 3.5% Zn, 1.4% Pb, and 0.8 oz/T Ag over the
depth range 305-369 m. The 17.4-m true thickness of this
shallow mineral deposit is impressive at these grades even by
mining-industry standards.
The base-metal sulfides of this deposit formed during
a complex paragenesis marked by alternating precipitation
and dissolution of ore and gangue minerals, and by mildly
fluctuating temperatures and salinities of the mineralizing
hydrothermal fluids. Sphalerite and galena were preceded by
anhydrite, then barite, and were followed in sequence by quartz,
fluorite, and mixed-layer chlorite/smectite. Fluid-inclusion
data demonstrate: (1) that sphalerite crystallized primarily
from 192-218oC brines having apparent salinities in the range
13-19 equivalent wt.% NaCl; (2) that slightly hotter (up to
223oC) and much cooler (173-178oC) fluids intermittently
accessed the sphalerite as it was crystallizing; and (3) that following
sphalerite precipitation, hydrothermal brines in the ore
zone maintained similar solute concentrations but experienced
a gradual warming trend culminating in the modern reservoir
temperature range (228-237oC) and salinity (14.4 wt.%).
Ore-zone mineralogy, paragenesis, and fluid-inclusion
systematics for this Pb-Zn deposit point to sulfide precipitation
through fluid mixing and fluid-rock chemical interaction
leading to cooling, pH change, and H2S enrichment in the
upper levels of (for this geothermal field) an unusually shallow thermal-fluid upflow zone beneath an impermeable mudstone
caprock. Based on the calculated maximum age of its host
siliciclastic strata, the mineralization is no older than 68-96 ka.
The deposit shares a compelling number of physical-chemical
attributes with the fossil hydrothermal systems that formed
“higher-salinity” epithermal silver-base metal orebodies like
those at Creede, Colorado, and Fresnillo, Mexico.