Geothermal power plants date back to a 1904 italian city, Larderello. Based at that dry stream field, the first geothermal generator was able to produce 10 kW of energy that was used to power five light bulbs. The plant gradually grew throughout the years and Larderello now includes 34 plants operated by Enel Green Power (EGP). The site has an 800MW capacity and has helped Italy become the sixth-largest producer of geothermal energy in the world, as it constitutes nearly 2% of Italy’s energy mix.

Due to high costs and limited locational applicability, the industry has traditionally held back in the development of Geothermal energy, but now new technologies are shaking this paradigm and everyone's watching to see what will happen next.

A couple of weeks ago we discussed Enhanced Geothermal Systems. Today, we would like to shine a light on all unconventional geothermal systems and the new possibilities they bring. Initial complications with unconventional systems were related to the lack of subsurface knowledge and the costs to study it. However, this is where possibilities for other industries arise.

1. The O&G industry has the know-how of drilling, completion and imaging technologies as well as seismic studies.
2. The Power sector has gained better experience planning facilities, and the need for geothermal attributes within the power system is greater than ever.

Types of Geothermal Energy

There are five principal types of geothermal systems based on geologic, geophysical, hydrologic, and engineering criteria that can be divided into 2 main groups: conventional and unconventional:

Conventional systems commonly named “hydrothermal systems” require a large amount of fluid immersed in a porous and permeable rock. In these cases, groundwater is heated by underline magma or by circulation through deep fractures or fissures. Production wells drilled into the earth´s surface tap into the hot fluid of the naturally occurring hydrothermal reservoirs. Under pressure, heated fluid flows up these production wells to the surfaces and into a power plant. They can be classified into the following categories:

• Young igneous systems: these are the hottest ones (more than 370°C & 689°F), located in volcanic areas with relatively shallow depths (less than 1,5 km / 0,9 miles deep)
• Tectonic systems: these have elevated heat flow but are essentially devoid of igneous activity. They usually have reservoir temperatures ≤250°C / 482 °F and occur at depths ≥1.5 km / 0,9 miles
• Geopressured systems: these are found in sedimentary basins where subsidence and deep burial of fluid-bearing strata have formed hot, “over-pressured” reservoirs. Typical depths and temperatures are 1.5 to 3 km (0,9 to 1,9 miles) and 50°C to 190°C (122 F to 374 F)

Unconventional systems lack either an underground fluid and/or rock permeability / porosity. Thus, in these systems, the underground must be stimulated somehow to obtain energy. Though technically feasible under these conditions, these systems used to be commercially impractical under previous economic conditions. However, new innovators suggest that this is no longer impossible. These new innovators can be classified into the following technical categories:

• Hot Dry Rock (HDR): As the name references, hot dry rock with no/low porosity. In these cases, the energy can be produced in closed or open systems. Closed systems can use any working fluid and can be applied to a wider range of depths according to the fluid and the amount of energy desired. They are usually called Closed Loop systems. Open systems need to create small fractures in the rock so that water can be injected, then heated via convection and finally extracted from a production well. The drilling depth depends on the type of rock, the temperature in the underground and the amount of energy desired. The open systems are called Enhanced Geothermal Systems (EGS)
• Magma Tap: the newest technology still, it implies drilling deep enough to reach a depth in which magmatic activity rises temperatures of 400 to 100°C (752 F to 1832 F). Usually more than 5 km / 3,1 miles deep. Given the underground temperatures in the USA, it is here, in the unconventional systems, that we see a cross-over possibility for both the energy and O&G sector. Commercial electricity generation is generally economic from geothermal resources at temperatures above 150°C/302 F. Geothermal resource temperatures at a depth of 7 km (about 4 miles) are accessible with existing drilling technology. For comparison, the average depth of onshore oil and gas wells drilled in the United States in 2017 was about 3 km (just under 2 miles):

The following image shows the different applications of geothermal systems, both conventional and unconventional:

As it can be seen, according to the depth and temperatures available, different applications can be found. The main categories fall under:

• Geothermal Heat Pump: for home consumption. Its opportunity lies in the cooling and heating systems for each home in geothermal heat exchangers.
• Hydrothermal Systems
• Enhanced Geothermal Systems (EGS)

In our forum next week (February 25th) we will be introducing Jim Hollis from Geothermal Technologies and he will be giving further information on their technology, applications, opportunity for the Oil&Gas sector and future expectations.