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Geo-Drill : Cost-Effective Approach To Geothermal Drilling

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Wed, 15 April, 2020

“What is interesting about Geo-Drill is that it has brought together experts that are not looking solely at the drilling aspects, but at how their expertise can add value to the drilling process, be this material scientists, advanced coating technologists, data sensor specialists, data transfer expertise and drilling specialists” - Kevin Mallin

As the project completes its successful 1st year, Kevin Mallin, discusses Geo-Drill from a partner and an end users’ perspective. Kevin is a drilling professional with over 40 years of experience across resource drilling and exploration, including geothermal, mineral, oil, and gas. Talking about geothermal drilling in general, Kevin explains the differences between EGS and drilling in geothermal systems such as those found in Iceland, the challenges associated with drilling making it capital intensive, and how Geo-Drill “will bring major changes, but minimal differences to drilling technology, increasing efficiencies and reducing costs.”

What is geothermal drilling?

Geothermal drilling covers a wide range of activity, from shallow (up to 200m deep), through to very deep (>6,000m currently).

The shallow wells use radiated heat from the sun, held in the formation, commonly referred to as ground source heat (GSH), whereas the deep wells use elevated temperatures that are the result of various geological activities that include “volcanic/magmatic” systems, heating of igneous rocks from the Earth’s core and hydrothermal systems, where fluids are heated at depth. Depending on the resource temperature, the heat is used to produce electricity (steam turbines), heat (this is basically the same as heating your house, but spread across many properties) and where possible a combination of the two (CHP – combined heat and power).

Deeper geothermal wells adopt drilling techniques, used in oil and gas (O&G), although they are often not best suited to the types of formations encountered in geothermal fields and can lead to high well costs, relative to the price that geothermal energy can be sold for.

Doubtless, as geothermal energy usage increases, there will be an exponential increase in industry specific equipment. Already we are seeing oil and gas drilling equipment manufacturers moving into the sector, although equipment is often just tweaked and costs remain high.

Whilst many geothermal areas are similar in nature to oil and gas areas, the subtle differences required for a successful well can all too easily be missed. The main areas for consideration/concern are where the geology is wholly different to those normally encountered (avoided) in O&G wells, namely igneous and metamorphic (although shale is now being heavily exploited for gas) where formations have high strengths, highly abrasion, heavily faulted/folded and often zones where fluids can disappear, never to be seen again.

Whilst O&G wells present risks from highly combustible gases being present in the formations (the whole reason for drilling the wells in the first place), geothermal wells also have high risk potential from very hot fluids at very high pressures, that when drilled into, turn instantly to steam, which are difficult to contain/control. Just imagine heating a tin of fruit in sugary water in your oven at 2500C for a couple of hours, taking it out and piercing a hole in the top!

So, in answer to your question – geothermal drilling is the construction of conduits to access heated formations, in order to exploit the resource for “energy” usage.

How deep are these geothermal wells normally and how much does it cost to drill a well of this depth?

As outlined above, geothermal wells vary in depth, but for larger scale energy usage we are usually drilling to depths of between 3km and 5km, although wells are becoming deeper.

Well drilling costs can also vary greatly, depending on formation type (high strength rock, rocks with poor integrity, etc.), the type of well drilled (vertical or radiused, like a letter J), depth drilled and the nature of the resource encountered (high temperature brines causing corrosion, weak rocks that can easily collapse, high strength rocks that can squeeze casing, etc.).

It is difficult to accurately predict the cost of a geothermal well, but currently a figure of between €2,200 and €2,500 per metre drilled and completed (i.e. ready for the exploitation of geothermal energy) is a reasonable estimate. Compare this to a shale gas well with a complex drilling geometry at around €900/metre drilled and completed, again this can vary.

What is EGS drilling? Is it more expensive than drilling in a conventional geothermal resource, e.g. Iceland ?

EGS is the acronym for engineered geothermal systems and often encompass HDR (hot dry rock systems).

Whereas hydrothermal systems have higher temperature fluids present, which will flow from point to point, when there is a pressure change (e.g. drilling a well into the zone), EGS systems have to create flow paths. Two holes are drilled; one for abstracting high temperature fluids and another to reinject the fluid, so that it can be reheated and re-abstracted in a long life cycle. Commonly, EGS systems are used in igneous formations, although not exclusively.

Geothermal systems such as those found in Iceland, are often shallower and have much higher temperatures, due to volcanic/magmatic heating, Because of their proximity to the ocean and often large fluid pathways, extracting high enthalpy resource is relatively easy. However, volcanic formations can range from loose ash to hard basalts and glass like obsidian, so drilling costs can vary wildly.

Because EGS systems are drilled in higher strength rocks, the drilling is slower, so the costs rise. 

What is the reason behind these high drilling costs?

As previously mentioned, the rate of progress is slow and as 'time is money,' the costs of wells increase.

Another factor is that wells need to be drilled close to areas that will utilise the heat and/or energy, which means there are additional health and safety factors to consider – noise, engine emissions, truck movements, well control risks, size of site, etc. All these have a knock-on effect in regard to costs.

Often, drilling equipment that is designed for drilling in O&G basins is used, especially drill bits, that are far from ideal for the higher strength and abrasive rocks that geothermal wells often encounter. This leads to reduced bit life (increased number of bits required) and more time spent pulling drill pipe from the well and re-running, this is known as non productive time (NPT) - where no hole is drilled. One other key factor is that O&G well formations are, generally, at a much lower temperature than geothermal wells, leading to equipment failures, as seals and instrumentation installed near to the bit are very sensitive to heat.

How can the drilling costs be minimised? Is there a threat to health and safety of drillers or environment in efforts to minimise costs?

'Time is money;' hence we need to reduce the time taken to drill wells, particularly wells drilled in the higher strength formations, and the best way of doing this is to speed up the rate of drilling into the rocks (rate of penetration - ROP) and to extend the operating life of all the components that go into the hole, which means less NPT.

Higher strength rocks are conventionally drilled by imposing high bit loadings and “crushing” the formation, this is usually a slow process and bits used are expensive and have relatively short life spans, often lower than 200m and ROPs as low as 3m/hour.

By using DTH hammer systems, bit loadings can be substantially reduced as the hammer induces high shock loadings through the bits and their inserts. This breaks the rock more efficiently and rapidly, so ROPs increase, 10m/hour is not uncommon. The bits also last substantially longer (typical life spans are 500m), so less NPT and substantially lower drilling costs.

Deep drilling is comparatively safe, as there are a large number of risk mitigation strategies in place. Granted, when things do go wrong (Gulf of Mexico, for example) the human and environmental costs are very high and focused, but when you compare the number of metres drilled to the number of serious incidents it is quite low. However, we constantly strive to reach zero incidents.

Cutting corners to reduce costs, inevitably increases risk, but we are not looking to cut corners, we are looking to increase drilling efficiencies, which will in turn lead to increased safety of drill crews and environments, because operating time will be shorter and the wells will be in a safe condition more quickly.

So, to close this point, we are not looking to minimise cost, we are looking to reduce them through increased efficiencies.

How do you think Geo-Drill addresses the issues of these high drilling costs?

What is interesting about Geo-Drill is that it has bought together experts that are not looking solely at the drilling aspects, but at how their expertise can add value to the drilling process, be this material scientists, advanced coating technologists, data sensor specialists, data transfer expertise and drilling specialists.

This is a unique team, which will bring major changes, but minimal differences to drilling technology, increasing efficiencies and reducing costs.

Whilst things may well look the same on a drill site, they will perform on a different level, making geothermal wells a much more attractive proposition for low carbon energy sources and, unlike most other renewables, there will be minimal visual impacts and no environmental impacts associated with the end-of-life disposal issues that a number of RE sources face.

Where do you see geothermal drilling in future?

I see geothermal drilling embracing new technologies, more quickly that O&G, partly because it is still relatively immature, as an industry.

Down hole data acquisition and speed of transfer will lead to more informed decisions, resulting in reduced NPT and possibly full automation, although this may still be some way off, as drilling still has a slight artistic bent, it is not just a matter of science.

Hole geometries will become increasingly complex, so that a resource can be better accessed without the need to continuously move the rig, which will also help to better manage the resource and extend the life of each well drilled; and unlike some other RE sources, climatic changes do not affect energy production.

I also see new materials being used within the well drilling process, leading to smaller rigs and yet deeper wells.

Drilling within urban environments will also become an accepted norm, meaning that the industry will benefit from demand, which will insist on better efficiencies.

Kevin J Mallin

Managing Director, Geolorn Ltd (a project management and Drilling Engineering consultancy).