REVOLUTIONISING SHALLOW GEOTHERMAL BOREHOLE DRILLING
“We have been drilling in London’s complex geology for years now. The progression from
completing only one borehole every two days, to now having the capability of delivering two in a single day, is remarkable really, but it certainly has not happened overnight”, says Alex Bosch, founding partner of GeoTech Developments (GTD).
When GTD was established in the UK 20 years ago, specialised drill rigs for geothermal drilling were limited. The family business struggled to find a rig capable of delivering the level of productivity they knew should be feasible.
Alex explains: “Our early years of drilling exposed so many shortcomings in the available
equipment. We just could not find a rig that would give us the flexibility to adapt our drilling
techniques in response to complex geological conditions in the UK. Most rigs were typically
optimised for specific subsurface settings or restricted drilling methodologies.”
Recognising and responding to the shortcomings, GTD took matters into its own hands and
partnered with a manufacturer to develop its own specialist geothermal rig, to its own specs
and design requirements. This rig, created out of necessity, provided a significant competitive advantage as GTD expanded its drilling operations over the decades.
" Every element of the operation was evaluated and developed with equal importance. Some of our most successful solutions were to simplify and resist the temptation to introduce something new, but instead integrate learnings from the gas and oil sector.
Innovation a necessity
“We field tested, refined, and upgraded our rig consistently, with the sole purpose of increasing productivity and profitability”, Alex continues. “In the fledgling years, our industry was commercially brutal, with many contractors touting that the cost per foot was just too
expensive and consequently not viable. We knew we had to innovate and develop more efficient equipment and processes to prosper.”
“Every element of the operation was evaluated and developed with equal importance. Some of our most successful solutions were to simplify and resist the temptation to introduce
something new, but instead integrate learnings from the gas and oil sector. We realised that
simplifying systems and processes ultimately led to easier on-the-job repairs. Let’s face it, sh*t happens when drilling, the easier it is diagnosed, the earlier parts can be ordered and the quicker a rig turns a profit.”
“This approach led us on a path to design, engineer and develop our own equipment for every stage of the installation process, including solids control, PDC drill bits, installation weights, loop reelers, and high-conductivity grout that is easy to pump. Every product, part or individual component has contributed to both small and giant leaps in productivity, which when combined enabled us to deliver projects with consistency, on budget and on time -crucial in delivering value to both our business and our customers”, Alex emphasises.
Simplified column showing the geology of London up to the Chalk.
Drilling improvement milestones
Let us take the geology of London as an example to illustrate the advances made in GTD’s
drilling techniques. London geology, in simple terms, constitutes a heavy dense clay – the
London Clay – deposited on top of a layer of sand, which in turn is underlain by Chalk that
contains flint fragments of variable size.
The more the clay is re-circulated when drilling, the more it breaks down and the more it
increases the viscosity of the drilling fluids. In addition, the sands at the base of the clay have a tendency to collapse. Finally, the Chalk hardens when drilling deeper into it, with varying
amounts of flint further slowing down progress.
“Initially, we used to be casing off all the way down to the Chalk. The first milestone was drilling down through to the Chalk, running casing to depth, and then drilling the Chalk out, all in one day. This initially doubled production and reduced our risk of lost, stuck-in-hole equipment from leaving casing in the ground overnight”, Alex explains."
“Our continued development of drilling equipment now allows us to drill faster than ever before and has enabled us to dramatically reduce our drilling times and therefore the disturbance of the boreholes. This allows us to minimise the use of casing required on so many of our projects. So much so that often no casing is required.”
“Therefore, we can now drill much smaller holes (5″), which is better for the ground and for the preservation of conductivity. From a drilling contractor’s perspective, it increases our ROP and overall productivity. The smaller hole diameter also means reduced grouting costs and a higher up-flow velocity from our mud pumps, which in turn clears the hole faster of the cuttings without needing to use piston pumps. The speed with which we drill has all but negated the need for a centrifuge to remove the fines, as we don’t allow enough time for the London Clay to break down further. We also never use additives in our drilling fluid”, Alex says.
A London project
A recent project requiring 20 x 181 m (594 ft) geothermal boreholes in Greater London, UK, exemplifies this progress. The site had a challenging geology, requiring drilling 100 m (328 ft) of clay, 4 m (13 ft) of sand, pulling back out, changing drill bit and then continuing back down through the Chalk and flint down to the final depth. However, it didn’t stop Alex’s team completing not one, but two 181 m (594 ft) boreholes in a single day, where previously this would have taken four days. The rig used was the GTD GT35 – 300 hp, single head, 16 ton (35,000 lbs) pull back and 6 m (20 ft) x 88.9 mm (3 ½”) rods with an internal flush diameter of 50 mm(2”). Using only 3 m (10 ft) of surface casing, the rest of the borehole was drilled open hole. The initial clay and sand geology was drilled using a Pink Penetrator® 3-wing PDC bit, down to the Chalk. Then, the string was pulled out to swap to another inhouse developed drill bit – a 537 grade rock roller. The drilling fluids were recirculated in a closed-loop system, which contained no additives. The drill cuttings are processed through the company’s ‘Mud Guzzler’ solids control equipment, featuring a shaker, desilter hydrocyclones and large holding tank.
Accelerating the energy transition
The achievements in productivity described above are a testament to the importance of
innovation, experience, and dedication in the geothermal drilling industry. “I think it is
important to share this with the community in order for other parties to benefit from this too”, Alex concludes. “We are all working on one thing and that is accelerating the energy transition.”
This article could not have been written without significant input from Simon Sergides,
Nicholas Bosch and Alex Bosch from GTD.