Abstract:

Prolific geothermal well Magmamax-6B (M-6B), in the southwestern Salton Sea geothermal field, yields initially 290-298oC hypersaline brine from an interval of hydrothermally altered mudstone and sandstone disrupted by veinlets and breccias mineralized with specular hematite and anhydrite. About half the well’s total production is from one such breccia zone that coincides with an abrupt, 8°C reversal in the static-temperature profile. Converging textural, petrologic, chemical, and thermodynamic evidence suggests that: (1) the M-6B breccias were formed in a strike-slip fault zone as a result of hydraulic implosion abetted by explosion of overpressured, high-temperature pore fluids; (2) the hematite and anhydrite cementing these breccias were precipitated from a downflowing brine originally cooler than the 300°C reservoir rock which the fluid eventually invaded; (3) the downflowing brine may have slightly cooled and mixed with a counterflow of hot brine already ascending the fault zone; (4)
the M-6B temperature reversal may be a further consequence of the above process; (5) the reversal nonetheless is only a minor local perturbation in a system that is still thermally prograde; (6) the observed brecciation and mineralization as well as the inferred brine downflow are geologically recent phenomena; (7) sandstones in the reservoir interval are productive only where comparatively shallow and just incipiently altered; and (8) a late-stage assemblage of anhydrite and hematite in this geothermal system may signal favorably rejuvenated porosity and permeability.

Abstract:

A working conceptul model has been developed for the southwestern portion of the Salton Sea geothermal system, the region encompsing CalEnergy Operating Company’s imminent “Unit 6” field expansion (185 megawatts). The model is based on examination and analysis of several thousand borehole rock samples combined with a wealth of subsurface information made available for the first time from the databases of present and prior field operators.

The Unit 6 sector of the system is hosted by fluvial and lacustrine, silciclastisc sediments and sedimentary rocks of Quaternary age. These strata are gently folded and cut by high-angle fault zones with a component of strike-slip displacement. The thicker of these zones (1) are mineralized and enriched in gouge and crush breccia (both commonly slickensided) as well as dilational microbreccia with a “jigsaw-puzzle” texture; and (2) are hosts for the most productive thermal-fluid conduits yet encountered in this part of the field. Much of this production is derived from major faults apparently forming the upper portion of a “negative flower structure”, a common feature of transtensional wrench-fault regimes like the one in which the field is situated.

A unique, ~100-200 m-thick, evaporitic anhydrite-rich layer in the mudstone capping the sedimentary sequence is continuous except above the faults most productive at depth. We believe that only these faults penetrate significantly upward into the cap, providing ingress for cooler, sulfate-dissolving waters from above.

Unit 6 as drilled to date shares numerous attributes with the broader Salton Sea geothermal resource. The production fluids are hypersaline brines (total dissolved solids content 20-25%) circulating at temperatures generally in excess of 290°C. Porosity and Permeability for fluid flow and storage are provided primarily by fractures, breccias, and veinlets, but also, in the upper part of the reservoir (and in a supra-reservoir outflow plume), by porous sandstones in which calcite has been hydrothermally dissolved. Overlying strata have not only retained their calcite, but have been mineralized locally with intergranular anhydrite, therefore providing an effective reservoir cap.

In addition to a paucity or absence of calcite, the following hydrothermal features are closely correlated with the Unit 6 geothermal reservoir: (1) pervasive veinlets of various compositions; and (2) widespread and commonly abundant epidote, accomponied locally at deeper levels by actinolite and clinopyroxene. The most prolific thermal-fluid channels coincide with fault-controlled concentrations of veinlets and dilational breccias mineralized with post-calc-silicate specular hematite +/- anhydrite.

The foregoing observations and deductions are consistent with a conceptual geothermal reservoir centered above a still-cooling granitic pluton at least 2 km in diameter and -3.5 km below the modern ground level. Major zones of buoyantly upwelling hot brine above the intrusion are focused along faults. More diffuse upflow occurs in a stockwork of interconnected, mineralized fractures (veinlets). This stockwork probably formed by hydraulic rock rupture induced by explosion of isolated, fluid-filled pores heated and consequently overpressured at an expanding (prograde) thermal front emanating from the magmatic heat source. Subhorizontal stratigraphic permeability in this model is concentrated in the upper portion of the reservoir, where the balance between carbonate dissolution and calc-silicate mineralization has favored formation of sandstone aquifers. Local downflow and warming of initially cooler brine from below the cap along major faults leads to slight (5-10°C) cooling of the upflow concomitant with open-space hematite +/- anhydrite mineralization.

The Unit 6 geothermal reservoir is clearly open at depth and for at least a kilometer to the northwest. It extends to the southwest and (especially) to the northeast for considerably greater distances. The reservoir plunges abruptly to the southeast, but even here, by analogy with the northern part of the geothermal field, there is a high probability for encountering productive reservoir rock at depths below 2 km. Our conceptual model, coupled with documented reservoir behavior here and elsewhere in the field, strongly suggests that the immediate resource is more than sufficient to support the planned expansion.

Abstract:

The Hot Springs Bay Valley (HSBV) geothermal resource area on Akutan Island, Alaska, has been exploredsince 2009. Geological, geochemical, geophysical surveys and the drilling of two thermal gradient wellssuggest a mature neutral-chloride reservoir between 240 and 300◦C, with outflow temperatures ∼180◦C. A network of regional and local structures control near-surface permeability. Alteration, mineralizationand geophysical patterns, including high-temperature hydrothermal minerals at unexpectedly shallowdepths of formation, indicate a poorly to moderately developed clay cap, likely the result of erosion ofthe upper portion of an older, well developed system.

Abstract:

During the drilling of injection well KS-13 in 2005 at the Puna Geothermal Venture (PGV) wellfield, on the island of Hawaii, a 75-meter interval of diorite containing brown glass inclusions was penetrated at a depth of 2415 m. At a depth of 2488 m a melt of dacitic composition was encountered. The melt flowed up the wellbore and was repeatedly redrilled over a depth interval of ~8 m, producing several kilograms of clear, colorless vitric cuttings at the surface. The dacitic glass cuttings have a perlitic texture, a silica content of 67 wgt.%, are enriched in alkalis and nearly devoid of mafic minerals with the exception of rare pyroxene phenocrysts and minor euhedral to
amorphous magnetite. The melt zone is overlain by an interval of strong greenschist facies metamorphism in basaltic and dioritic dike rock. The occurrence of an anhydrous dacite melt indicates a rock temperature of approximately 1050oC (1922oF) and sufficient residence time of underlying basaltic magma to generate a significant volume of highly differentiated material. Heat flux from the magma into the overlying geothermal reservoir is ~3830 mW/m2, an order of magnitude greater than for mid-ocean ridges. The geologic conditions at PGV combine tensional tectonics with magmatic temperatures at readily drillable depths (<2500 m).

Abstract:

Increasing power generation at the Soda Lake geothermal power plant requires thorough understanding of the well field and reservoir structure. A three dimensional drill-hole and geologic model was created, incorporating data from temperature surveys, wireline geophysical logs, mudlogs, narratives indicating suspected fault and fracture zones, and numerous technical papers. Once all information was compiled, it was used with other geospatial data to create a drill-hole project in Geosoft’s Target software, with wells oriented in 3D space, and a variety of down-hole data displayed in both 2D and 3D. CSAMT and seismic profiles were dropped into the 3D model, where they could be viewed in conjunction with other data. For the first time, geoscientists could visualize the subsurface locations of all wells with respect to each
other, geology, geophysics, and the temperature anomaly. The result was dozens of data layers that could be toggled on and off and rotated in space. This model was then imported into the Oasis Montaj project, where it was combined with geophysical data to offer a more comprehensive view of the Soda Lake geothermal field. Cross-sections, fence diagrams, and strip logs were easily produced and updated in Target via user-defined templates. Slices of 3D objects such as temperature voxels and geologic surfaces were also displayed in 2D cross sections.
This model has made a valuable contribution to the overall understanding of the Soda Lake geothermal system. As a result, the team has implemented changes leading to increased production at Soda Lake. Two wells that had initially failed as producers were revisited after using the 3D model to study them in the context of their surroundings. One is now hooked into the plant as a producer, while the other is currently undergoing injection testing, with initial tests indicating improvement. An older well was also revisited, and flow tests dramatically demonstrated that a steam cap had developed beneath the well. The 3D model was used in conjunction with geophysical data to study the steam cap, and Magma now has plans for direct use of the steam. In addition, the first of several potential new wells has been targeted and is due to spud in June, 2010.

Abstract:

During the 1980s the apparently low permeability 77-29 and 84-33wells at the Soda Lake field were improved into useful long-term production or injection wells with standard perforation recompletions of cemented casings. In the early 1990s long term low pressure injection into well 81-33 either created or restored permeability that had not been recognized or had been damaged during drilling operations. Because of this historic response to stimulation, a technique that employs deflagration
(the targeted ignition of a propellant to force a pressure wave into the surrounding formation) has been performed on 3 Soda Lake wells. In 2009 and 2010 relatively small charges had uncertain success in wells 45A-33 and 41B-33 due to the fact that other well modifications occurred at the same time. Charges 2.3 times larger became available in 2011 and three of these were used in the 25A-33 well. Immediately following the deflagration of the propellant charges, 5 days of low pressure (<150 psi) injection improved the injectivity by 20 fold from 30 gpm to over 600 gpm. Magma ignited the larger charges in three perforated sections of the well at 4153', 4575' and 4911'. Within two weeks of the stimulations using deflagrating materials the injection capacity of the well steadily increased from an initial 30 gpm to a steady flow over 600 gpm, with a maximum of about 750 gpm, all of this occurring at normal injection system pressures less than 150 psig.