Broadly speaking, geothermal drilling is divided into two areas: near-surface geothermal (up to a depth of 400m) and deep geothermal (greater than 400m), the latter of which can produce the high temperatures necessary for electricity production and district heating. Hence, with the growing need for clean, sustainable energy, for the last decade, the global industry has been increasingly exploring and expanding the possible use of deep geothermal on a large commercial scale.

Deep geothermal wells have a lot in common with their oil and gas cousins and, with the downturn in oil prices, there has been a boost in technology transfer between industries and companies diversifying to cover both sectors. However, as companies drill into deeper and hotter zones, new challenges arise. "These deep hot supercritical wells are in a sense uncharted territory," explains Roar Nybø senior business developer at SINTEF Industry, an international research institute delivering contract research within a broad range of areas including sustainable energy. "You get a lot more heat out of supercritical wells, but the well is an extreme environment, challenging us in both material science, downhole electronics and the fundamental knowledge needed to control the drilling and production operations."

Evaluating prospects

When prospecting for new geothermal sites, for example in high-enthalpy exploration, the first wells to be drilled will be purely to explore the subsurface. However, in deep hydrothermal settings, an exploration well normally will also be used as a production or injection well later on. Dr Klaus Dorsch, senior geologist at ERDWERK, the independent planning consultant behind the two record-breaking hydro-geothermal wells in Holzkirchen, Germany, explains: "Ultimately, the techniques and tools are dependent on the geological conditions encountered. For each geological formation or structure, there are often a number of techniques and tools which can be implemented to efficiently and safely drill a well. This means that the first step is to understand the geological conditions along the planned well path as well as possible. Often, however, it is not as easy as it appears at first, especially when lithological and pressure conditions are changing frequently."

 

Holzkirchen

From 2016 to 2017 ERDWERK supervised the successfully drilling of two hydro-geothermal wells in Holzkirchen, south of Munich, Germany. The project was significant for being the deepest hydro-geothermal wells in a sedimentary basin reservoir drilled in Europe to date.

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Drilling 7km into the bedrock of Espoo - deeper than ever before in Finland - the goal of St1 Deep Heat is to build Finland's first industrial-scale heat plant running on an enhanced geothermal system

 

The first well was spudded in January 2016 and the drilling for the first two sections, of five in total, went according to plan. However, following an intense gas kick, the third section had to be abandoned and a sidetrack was drilled following a new well path to avoid the potential gas-bearing high-pressure zone. Despite the geological challenges and resulting delays, the final depth of 5,600m MD (5,079 m TVD) was reached only four months after the start. After successfully testing the first well, the second well commenced in June. Again, in the third section part of a liner as well as a drilling BHA were lost in two separate incidences, due to differential sticking. Two sidetracks were drilled and after a total drilling period of about eight months, the final depth of 6,084m MD was reached, followed by a well test which verified the required productivity and temperature.

Both deviated wells and the drilled sidetracks demonstrated that thanks to RSS-technology the wells could be drilled with very high rates of penetration. The greatest challenges at this location with a deep hydrothermal reservoir in the immediate vicinity to the alpine nappes was the extremely high variance in pore- and formation- pressure in neighbouring sediment layers as well as the difficulty in foreseeing these pressure differences despite data from near hydrocarbon offset wells being taken into consideration. The greatest potential for improvement and the focus for future realisation of deep geothermal wells in this setting has to focus on lowering the drilling flat times caused by the described incidences and to furthermore adapt the drilling design to new information of the local pressure regimes of the different strata.

The exploration phase can help establish what tools should be implemented, if water is circulating and if so, how warm it is. Thus, deciding the viability of the project, keeping exploration costs at a minimum can be vital for more projects to succeed, says Nybø. "These wells can be very narrow, and variants of core drilling have been put forward to keep the cost and complexity to a minimum, even if it's not the fastest option. It sounds like a niche product, but costs accumulate in the exploration phase, and low-cost drilling may determine whether a geothermal project gets to the finishing line."

Deeper faster

Breaking even can also be achieved by drilling wells that will produce more heat. That means drilling deeper and into hotter zones. However, for many companies, the big challenge is not to drill deep, but to drill cheap, and to that extent, hammer drilling holds a lot of promise, suggests Nybø. "When it comes to drilling faster in the hardest rock types, there is a lot of promise in ‘hammer drilling' where the traditional drill bit, which may wear out quickly, is replaced with a ‘hammer' that crushes the rock in front of it."

Dr Jörg Baumgärtner managing director at BESTEC, a renewable energy project company with a focus on geothermal power and heat production, sees great potential in faster drilling methods. "We see serious developments for faster hard rock drilling techniques such as the air hammer used in the Finish ST-1 project or the Thermodrill [http://thermodrill-h2020.org] project which uses an innovative combination of conventional rotary drilling and water jetting," he says.

Set to reach its completion in 2019, the Finish ST-1 project is one of the pilots exploring the possibilities of an enhanced geothermal system (EGS) where high-pressure cold water is pumped down an injection well into the rock to increase permeability. Unlike regular geothermal systems, which require high permeability, this makes it possible to extract heat energy from rock formations with a naturally low permeability.

Oil and gas lessons

According to Nybø, the possibilities of EGS have been expanded even further through fracking as used in the oil and gas industry. "The shale gas boom in the US was made possible by creating fractures in the otherwise impermeable rock, allowing the gas to flow to the well. Geothermal faces a similar challenge," he says. "Around the world, there are rock formations that are warm, but where water can't circulate. It's dry and it's impermeable. Fracking can create passages between wells, allowing water to circulate and pick up heat and so it has become part of the toolbox of enhanced geothermal systems."

Fracking is just one example of a technology previously used exclusively by the gas and oil industry becoming viable in the geothermal sector as larger and larger projects see the light of day. "The most significant development seen in the geothermal market in Bavaria (over the last 10 years) is the implementation of rotary steerable systems over conventional steerable motor systems," says Dorsch "The performance increase with such systems means that wells are able to be more accurately and more efficiently drilled, and the additional cost compared to the conventional systems is balanced out by savings due to reduced drilling time. The technology is nothing new, however, due to the high cost it was previously not viewed as an option and therefore not implemented in geothermal projects until a few years ago."

Adapting to conditions

Despite the crossover between deep geothermal and the oil and gas sector, there are unique challenges to geothermal. One is that most deep geothermal drilling occurs in hard rock at elevated temperatures. Special bits, such as roller cone bits with reliable bearings for high temperatures, are essential, explains Dr Baumgärtner of BESTEC. "On top, these need to be supported by high-temperature resistant mud systems, which are at the same time environmentally friendly and easy to dispose. Good lubricants are another important product. In crystalline rocks, the mud system will not develop a mud cake, thus the friction in the well needs to be reduced by lubricants which are temperature stable and environmentally friendly."

Furthermore, the geothermal well completion requires especially lightweight cements and a casing design which can cope with the high production temperatures. Adding to this challenge is the bigger drilling and casing diameter in the reservoir (due to higher production rates) of geothermal wells compared to the majority of oil or gas wells. "The casing in a geothermal well has to serve for the production of the thermal water; there's no separate production tubing installed down to the reservoir," explains Dorsch. "The selection of the casing material is undertaken in advance and has to be in line with the mechanical and thermal stress as well as the fluid chemistry which is expected at this location. "Consequently, the quality of the casing design strongly depends on the availability, quality and transferability of geological, thermal, hydrochemical and corrosion data from nearby offset wells."

Supercritical wells

Though drilling deeper in general means greater heat energy production, drilling into hotter zones also presents unique technical challenges. Recent years have seen several projects drilling into the "supercritical zone", where the high pressure and temperature forces water into a new phase, which is neither fluid nor gas. One of these projects is the Icelandic Deep Drilling Project (IDDP).

HotCase/IDDP

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The high temperatures of supercritical wells such as the two wells of the Iceland Deep Drilling Project present unique technical challenges

Carried out by an industry consortium led by Equinor with SINTEF and ISOR as research partners (and supported by the research council of Norway and industry partners), the HotCase research project looks at the steel casings which are cemented in place in supercritical geothermal wells and how they fail in the unforgiving environment. The project gathers together suppliers, operators and researchers to explore combinations of cements and steels, herein coating and liner technologies that modify the functional and mechanical performance of the casing system. The solutions have been tested as part of the Icelandic Deep Drilling project. The IDDP-2 well (image) was completed on the 25th of January 2017, at a depth of 4,659m reaching all the project's initial targets. These were to drill deep, extract drill cores, measure the temperature and search for permeability. The temperature at the bottom of the well has already been measured at 427°C, with fluid pressure of 340bar, drill cores were retrieved, and the rocks appear to be permeable at depth. "If your solution does not work, you find out fast," says Roar Nybø of SINTEF.

 

Kristinn Ingason, drilling expert and head of the geothermal department at Mannvit, one of the major consultants for the IDDP consortium, says. "The high temperature (<400°C) which may be associated with some deep drilling is challenging with respect to casing and wellhead design. Cementing of long casings has proven to be challenging bearing in mind that casings in geothermal wells need to be fully cemented."

In a research project for Equinor, SINTEF has looked at the steel casings which are cemented in place in supercritical geothermal wells. "Whether you worry about corrosion on the steel casing, degradation of the cement or minerals that precipitate out and block your well, the common denominator is that we don't know how fluids and materials behave under such extreme temperatures and pressures. It's easy enough to heat a piece of metal in the laboratory, but if we want to expose it to the conditions of a deep geothermal well, with a corrosive mix of gases, minerals and supercritical water, we need to design new laboratory equipment," says Nybø before adding, "as a researcher, I can say we want to build the necessary equipment and get hold of those secrets. But the first supercritical wells are themselves experiments and both researchers and industry are learning a lot from these pilots." 

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Geothermal at a glance 

Geothermal energy is heat energy brought up from the accessible parts of the earth's crust and utilised for a range of purposes most notably to provide sustainable heating and for renewable energy production.

Depending on the depth and the applied technology geothermal can be divided into two areas, near-surface geothermal and deep geothermal:

• Near surface geothermal describes the use of the geothermal heat up to approximately 400m depth through probes, collectors or groundwater wells
• Deep geothermal refers to the thermal use of the underground from 400m and deeper through deep well-drilling technology

The high-temperature geothermal production (>150°C) achievable through deep geothermal is most important for electricity generation while medium-to-low temperature resources (< 150°C) are suited for heating applications.

Enhanced geothermal systems can be used to generate geothermal energy in rock formations lacking the natural permeability required for regular geothermal systems.