Essay Sample about the Geothermal Activities of Imperial Valley

Location of the region

This report will predominantly focus on the geothermal activities of Imperial Valley in the California region. The Imperial Valley geothermal field is situated in the southern part of California and consists of 10 generating plants based in the Salton Sea, feeding the consumption grid with approximate 327 net megawatts ("Imperial Valley Geothermal Area | Department of Energy", 2018). California is a Northern American State situated at Longitude and Latitude of 120.0 W, 37.0 N, covering a boundary length of 3299 kilometers. On the East and Southeast, the State is bordered by Arizona State while on the North it is bordered by Oregon State. On the western border, however, California lies along the Pacific Ocean with an 840 miles coastline, which is the third longest coastline in the entire United States ("Location of California", 2018).

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Importance of The Region

Imperial Valley region is of particular interest because it lies right on the Pacific's "ring of fire", shown in Figure 1, with its constantly shifting and colliding tectonic plates consequently producing earthquakes and volcanic eruptions. The location of the valley on the sprawling ring that extends 25,000 miles from New Zealand through Indonesia, Japan and Philippines to Alaska, West Coast and Canada down to the South American continent, makes California the largest generator of geothermal electricity in the country due to the volcanic action beneath its surface.

Figure 1: Map showing the "ring of fire" source: (Location of California, 2018)

According to the California Geothermal Energy Statistics & Data (2018), the 25 Known Geothermal Resource Areas (KGRAs) shown in figure 2 in 2016 alone produced 11582 gigawatt-hours (GWh) electricity from the current 43 operational geothermal plants. But of even more importance out of the 25 KGRAs, six of them are in Imperial Valley with four of these, the Salton Sea, East Mesa, the Heber and Brawley KGRA, showing great amounts of exploitable energy (Layton,1978).

Figure 2: Showing the 25 Known Geothermal Resource Areas Source: ("Energy Maps of California - California Energy Commission", n.d.)

Information Covered by This Report

This case study report will explore the various aspects of the Imperial Valley geothermal field including the Hudson Ranch Geothermal plant situated in the Salton Sea. In details, this report covers a brief history of the field which will include the exploration information and the dates of discovery. It will also investigate and report on the geologic setting of the region which will include the plate tectonic setting, the type of rocks as well as the relationship between the rock structures, the fluid flow and geothermal activities of the field. Additionally, it will examine and report on the hydrothermal fluid characteristics of the fluids in the filled and give a detailed report on the power plant designs of the stations in the field. Finally, the report will explore the environmental impact of geothermal production activities in this field to the surrounding environment while incorporating seismic information due to the development of these plants.

Exploration History

Discovery History

According to Kaspereit et al. (2016), geothermal exploration in Imperial Valley started in 1927 but commercial production began in 1982.First to be developed in the Imperial Valley region was the North Brawley Geothermal Power Plant as an experimental power plant in 1980.The power plant, which at the time produced 10MW, however, was operational for only five years after which it was decommissioned for further expansion development. The new North Brawley generating 50MW was completed in 2008 under operation and ownership of Ormat Technologies as stipulated in the 20-year agreement with Southern California Edison (SCE) in 2010 ("North Brawley Geothermal Area | Open Energy Information", 2018).

Discovery Technique

Geothermal production capability in the Imperial Valley was discovered in 1957 when Sinclair No.1 was drilled while exploring the area for oil-gas. Coincidentally, instead of getting oil, a natural underground heated water reservoir was discovered. The same phenomena occurred when the subsequent wells were drilled near the Salton Sea and the corridor extending from Heber south of the valley and East Mesa up to the north of the Salton Sea (Quintero et al.,2015).

Exploration Techniques

The main exploration technique used in Imperial Valley for detecting hidden geothermal systems was gravity surveys. Biehler (1971) carried out the gravity survey to identify the seven highs at North Brawley, East Brawley, the Buttes, Border, Mesa, Dunes and Heber the techniques were discovered efficient for geothermal reservoirs overlaid by a sediment layer of thousands of feet. Another crucial method used to identify the hidden geothermal systems was through temperature gradient well where heat anomalies could easily be identified and temperatures measured. This method was used to explore Brawley, East Mesa and Heber (Dobson,n.d).

Exploration Status

As of 2016, Imperial Valley region had 11 plants with a combined capacity of 403.4 MW, all installed in or near the Salton Sea Known Geothermal Resource Area (SSKGRA). However, more plants are scheduled to be installed as the sea further shrinks with already three given to the Controlled Thermal Resource (CTR) company to implement the Hell's Kitchen Project with a projected capacity of 250 MW ("Hell's Kitchen Project", 2018).

Geological Setting

Plate Tectonic Setting

Imperial Valley region comprises of an extended fault zone bound by two plates, the Pacific Plate and the Northern American Plate. The Salton Sea Geothermal Field (SSGF), where most of the plants in the Valley reside, is located in the Salton Trough in between the two active plates constantly moving apart. At this transitional area, the southern tip of San Andreas fault system meets with the Northern Imperial Fault as it steps over the continental drift. The faulting plates, however, result in secondary faults that do not manifest in surface expressions but rather cause numerous and intense earthquakes ("North Brawley Geothermal Area | Open Energy Information", 2018). Figure 3 shows the Tectonic map of Salton Trough in Imperial Valley, California.

Figure 3: Showing the Tectonic map of Salton Trough in Imperial Valley, California. Source : (Sass et al.,1988)

Major Rock Types

Imperial Valley bed mainly comprises of a trough filled with aged, lacustrine, deltaic sands and shales while its surface is layer consists of volcanic and alluvial lacustrine rocks. These rocks form clay-dominated thick layers comprising of more than 75% of Clay, which is a clear indicator that the major rock type of the area is sedimentary rocks (Jennings & Thompson 1986).

Source of Geothermal Heat in Imperial Valley

According to Edmunds (1977), the main heat source that heats up the groundwater for geothermal fluid application originates from the plates movement. When the plates move apart, the valley spreads, which causes the basement rocks located at five to six kilometres to fracture and allow the hot basaltic magma at the core of the earth to seep up to the deep groundwater reservoirs.

Types of Faults Present

The main fault is known as the Imperial Valley, which is a strike-slip fault resulting from the Pacific and North American Plates shifting away from each other, with a length of 69km running from Mexicali Valley to El Centro. The average slip rate measure by shoreline deposit of this fault is estimated at 15-20 mm per year (Lyons et al.,2002).

The Relationship Between the Fluid Flow and Rock Structures

The flow of groundwater in the imperial valley is towards the central point of the valley which then flows to the northwest directed toward the Salton Sea. The sedimentary-rock nature of the region allows water from Colorado River, canal seepage and irrigation water from nearby farms into groundwater reservoirs which reside within quartz and arkosic-dominated sandstone laying on an impermeable clay rock layer situated 300-600m deep ("North Brawley Geothermal Area | Open Energy Information", 2018).

Geology of the Main Geothermal Reservoir

According to Lippmann (1999), Salton Trough is the most important geothermal reservoir in Imperial Valley and it comprises of sedimentary rocks with strata of shale and siltstones and interbedding lacustrine muds. The trough which covers more than 300km is characterized by rapid crust thinning that allows core magma upwelling, consequently resulting in a new oceanic crust. The North Brawley is the largest plant in this trough comprising of 5 wells have a depth range of 967 to 1372 m ("North Brawley Geothermal Area | Open Energy Information", 2018).

Hydrothermal Fluid Characteristics

Temperature of the fluids

The Salton Sea at the Imperial Valley geothermal fields provides the best conditions for the generation of geothermal energy. The temperature of the fluids in the region range from 600 degrees Fahrenheit and above, which is an indication of the large energy potential in the region and economical in terms of production of energy (Trabish, 2012). This makes it easy for the system used in the geothermal field to easily pump the fluids for generation of energy. The fluid used to generate water in these fields is mostly liquid-dominated since the fluids have a lower boiling point. The fluids in the field have to be liquid for them to generate the vapour required to rotate the turbines to generate the required energy. This vapour is obtained when the working liquid is vaporized by the geothermal liquid but is later condensed for reuse.

Chemistry of fluids

The fluid obtained is highly saline due to the high concentration of brine, with a range of 20 to 30 percent alkalinity in the fluid and low concentration of carbon dioxide. The fluid also contains a high concentration of total dissolved salts (TDS), a factor that has resulted in the constant accumulation of brine in the pipes of the production wells during energy production. The stable isotope signatures of the fluids are not known

Power Plant Designs

The Salton Sea KGRA has 11 power plants, with the most recent addition being Hudson Ranch I. The designs of these plants vary, with the existence of double and triple flash systems among them.

Functioning of the Plant

The Imperial Valley geothermal power field operates using the binary cycle system to generate energy. This system uses water that exists at lower temperatures with the fluid containing an organic compound (Renewable Energy World, N.d.). It requires two fluids, the geothermal fluid and the binary fluid, which is also referred to as the working fluid. The geothermal fluid is usually moderately heated and its temperature is usually below 400 degrees Fahrenheit, while the binary fluid has a lower boiling point that allows it to vaporize easily. As the fluids flow from the production well, the high temperature of the geothermal fluid causes the binary fluid to evaporate and this vapour is used to drive the turbines that are used to generate electricity which is supplied to the generators and used to supply electricity from the plant (Electricity Generation, N.d.). However, unlike other system used to generate energy, the binary system has a closed loop that ensures that the fluids are re-injected into the ground to prevent pollution of the environment with the by-products contained in the saline fluids. The plant thus uses water from the Salton Sea to condense the fluids after they pass the heat exchanger and as they flow back into the ground to be reheated and reused to generate more energy (Renewable Energy World, N.d.)

Figure 3: Geothermal power plant at Imperial Valley, California (Source: Renewable Energy World,...