A generic term referring to hard, coarsely crystalline plutonic rock that is produced by intrusion of magma followed by a long period of slow cooling and solidification below the surface (>10 km depth). The mineralogy and composition are variable, ranging from felsic (e.g., granite) to mafic (e.g., diorite).
Did you know... geothermal wells can be highly deviated too?
Just as in the oil industry, the first geothermal wells were all vertical, which remains common practice mainly because it is cost-effective. The maximum depth is typically about 10,000 feet (3 km). Deviated geothermal wells have been drilled too, extending laterally over horizontal distances up to about 5,000 feet (1.5 km) and dipping at angles of less than 45° as measured from the vertical. In most geothermal fields, the rock formations are made up of volcanic and sedimentary rocks.
New groundbreaking developments are now happening in the geothermal industry borrowing methods used for unconventional hydrocarbon development. Recently, a well having a long horizontal leg was drilled for the DEEP geothermal project in the Williston Basin, Saskatchewan, Canada.
The type of rocks being drilled into for geothermal development are relevant because up until now very few have reservoirs hosted by granitic rock, which is abrasive and hard on wear and tear of downhole equipment. Examples of such wells include 14-2 at Roosevelt Hot Springs, WD-1A at Kakkonda, Habanero 1 in the Cooper Basin, 33A-7 at Coso and OTN-3 in Finland. Of these, WD-1A has the hottest bottom hole temperature (~500°C), and OTN-3 is the deepest, but for this rock type, highly deviated wells are absent.
The new deep well at Utah FORGE, 16A(78)-32 is thus notable. It shows that the drilling of sub-horizontal well trajectories in granitoid are achievable. Such highly deviated wells are required for EGS wells in order to intersect a large number of sub-vertical fractures and to maximize energy production.
Figure showing the geothermal well profiles, host rocks and deviation angles: conventional wells in red; sedimentary basin wells in green (Saskatchewan, Canada; Groß Schönebeck, Germany); metamorphic-plutonic well in blue (Helsinki, Finland); granitoid wells in black (Roosevelt Hot Springs, Utah; Kakkonda, Japan; Cooper Basin, Australia; Soultz-sous-Forêts, France; Coso, California); granitoid well in pink (Utah FORGE).
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Sabin et al 2016, Proceedings Stanford Geothermal Workshop (https://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2016/Sabin.pdf)
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The process of operationally isolating specific intervals or zones along a wellbore to perform well intervention activities, such as stimulation.
Binary Cycle Power Plant
A power plant in which the produced geothermal fluid passes through a heat exchanger and heats a low boiling point liquid, commonly an organic compound such as pentane, to vaporize and condense as it passes through a turbine and generate electricity. This is a closed loop power cycle, in which organic working fluid is confined and reused through the heating and cooling process.
an operation carried out on a well that increases production or injection by improving the flow characteristics of the reservoir and enhancing the flow between the reservoir and the wellbore.
Enhanced Geothermal System
unconventional geothermal resources that contain heat similar to conventional hydrothermal resources but lack the necessary groundwater and/or rock characteristics (e.g., permeability) to enable energy extraction without stimulation.
Did you know that the first wells were drilled over 2000 years ago?
Drilling is an ancient technology and it has long been used to explore for natural resources and to produce fluids such as water, brine, oil and gas that occur underground. The Chinese drilled shallow wells over 2000 years ago to produce brine. The first oil wells were drilled in the 1800s and up through the early 1900s, wells were vertical and limited to depths of a few hundred to a couple of thousand feet. By the 1970s, depth records were being broken starting with Bertha Rogers No. 1 which was drilled to over 31,000 feet (9.5 km) to explore for gas in the Anadarko basin, Oklahoma, USA. In 1979, the Kola Superdeep scientific well in Russia was drilled to over 40,000 feet (12.2 km), making it the deepest well in the world. In 2009, the deepest oil well was completed to 35,000 feet (10.6 km) from the Deepwater Horizon platform in the Gulf of Mexico.
The idea of drilling a slanted deviated well by directional drilling was realized in the 1930s. Today, the drilling of highly deviated wells is commonplace in the exploration and production of oil and gas reservoirs. The Chayvo oil field in Russia is the site of several record-breaking deviated wells that are drilled to depths of about 3,000 feet (0.9 km) with a long horizontal reach exceeding 40,000 feet, the longest of which is O-14. For comparison, geothermal wells are generally drilled to no more than 10,000 feet (3 km), and if deviated, they are done so at modest angles of less than 45°.
There are several reasons for drilling deviated wells such as increasing the section or length of well interval through rocks that are rich in oil (or gas). In some cases, there are obstacles (e.g., town or lake), which means the resource has to be accessed from the side rather than vertically from the surface. In other cases, there are logistical advantages to clustering a number of deviated wells on a single pad as is common in offshore oil platforms.
Figure shows the depth ranges of the deepest and longest wells in comparison to wells that are commonly drilled in geothermal production fields.