Mon, 20 May, 2019
Geothermal Energy, truly is at the heart of our low-carbon energy future. We are sitting on a sustainable energy source that, with careful management, will provide heat and electrical energy for millennia, with almost ZERO carbon emissions.
Unlike wind and solar, there are minimal fluctuations to the energy flowing from geothermal sources, as the earth’s core is basically a large “thermal” power station. The downside, the heat (temperatures) we require is often at considerable depth (>5,000m). The deeper we go the higher the temperature. Some regions have higher temperature gradients than others and some are blessed with very high temperatures at reasonably shallow depths, although these sources are generally volcanic (Iceland, New Zealand, Pacific Rim etc.) so have certain drawbacks.
Geothermal energy also has substantially lower environmental impacts than almost all other low-carbon, sustainable sources.
The common factor for all deep geothermal energy sources is the requirement to drill wells in order to harness the energy stored at depth.
Trying to predict what will be encountered at 5,000m depth, is similar to driving 50Km with only scant information as to where you are going and no clue as to traffic conditions en-route. Nowadays, most people use navigation systems to guide them to their destinations, avoiding hold-ups and wrong turns, minimising the time taken for the journey and avoiding unnecessary costs.
Increasingly reliable geophysical technology, interpretation and modelling is resulting in better prediction for the siting and depth of wells, yet there is still the risk of slow drilling that increases the cost of the project substantially.
Geothermal drilling falls into three main categories:
- High Enthalpy Reserves – Predominantly volcanic or pyro-clastic formations, with temperatures >2500 Mostly drilled with rotary drilling techniques and mud flush. Formations are often very sensitive to mud weights, as they have low fracture gradients, resulting in formation breakdown and mud losses.
- Medium Enthalpy Reserves – Predominantly igneous and metamorphic formations, with temperatures <2500 Formations are often high strength >180MPa and difficult to drill with conventional mud rotary techniques. This sector holds the most promise for sustainable development in areas already highly populated.
- Low Enthalpy Reserves – Predominantly, sedimentary formations with hot brines. Temperatures up to 1000C, with high permeability. Can be drilled with conventional mud rotary, but often the reservoir is overlain by higher strength rocks, which can cause drilling delays, due to slow penetration rates.
The Geo-Drill project is focusing on the Medium and Low Enthalpy sectors, which can be exploited close to large urban populations and in a variety of formation types.
At present, drilling accounts for around 50% of the cost of developing a geothermal resource, with cost over-runs as high as 100% of the original estimates. This uncertainty is holding back the development and utilisation of geothermal energy as a base-load provider, which is why it is so important to bring down the cost of drilling the deeper wells.
The key element of the cost is TIME, as rig operations can be as high as $ 75,000/day, so the less days taken to drill the wells results in substantial savings.
How do we achieve this?
- Drill faster – Seems very obvious, but so many factors can affect how quickly we can drill, such as rock strength, wellbore stability, rock abrasiveness, loss of circulation (which means drill cuttings do not come back to the surface) and so on. So, in order to drill faster, we need to be in control of all the variables.
- Keep the bit on the bottom – Again, seems very obvious, as when the bit is on the bottom, it is making hole which is the main aim. So, we need to make bits and drill assemblies, more robust, leading to reductions in Non-Productive Time (NPT), such as tripping drill pipe in/out of the well, to change them over.
- Keep it Simple – The technology needs to break rock quickly and then remove the resultant cuttings out of the well.
- Cost effective – The technology needs to reduce overall project costs.
In medium to high strength rocks, percussion drilling is far more effective in terms of penetration rates, with up to 10 times the speed of mud rotary. Percussion drill bits also have longer life cycles than rotary bits, resulting in lower NPT.
Percussion hammers are primarily powered by compressed air, a great method of drilling in harder rocks, but they can be limited in depth capability and compressing air to high pressures is expensive and energy intensive.
The alternative to air powered hammers are water powered versions. With operating pressures as high as 100 Bar, high rates of penetration can be achieved, but the water that passes through the hammer has to be of “potable” quality, because the presence of colloids larger than 7 microns, results in excessive wear on internal parts and premature failure.
Recognising the strengths and weaknesses of current hammer drilling technology, Geo-Drill is concentrating on improving the overall system effectiveness and efficiency, extending component life and reducing NPT, leading to faster drilling operations and lower costs.
Using the SatNav analogy, the project team also understands that getting good information quickly, will further enhance the efficacy of the system and the successful drilling of wells. Through the use of sensors and printed circuits, capable of withstanding the harsh environments that drilling operations encounter, the team aims to revolutionise Bit-to-Surface data transfer, as well as other indication systems such as wellbore stability, Lost Circulation Zones, wash-outs and constrictions due to sloughing or hydratable clays.
The Geo-Drill team consists of experts from many fields working together to solve the many problems that deep geothermal drilling presents, leading to a brighter, low-carbon future.
(This content comes courtesy of the Geo-Drill consortium)