01 July 2020

Strada: How Our Geothermal Drilling Technology Works

By Taras Olijnyk, CTO Strada Global

The energy potential that lies beneath our feet has historically been measured by what can be achieved – and what can’t be. Strada exists because of that challenge. We’ve identified the issues raised by ultra deep geothermal drilling, and set out to solve them through game changing technology. That’s why we’re focused on hard rock drilling methods capable of drilling deeper, faster and more efficiently than ever before.


Our technology animation (watch above) is designed to show the benefits of Strada’s tested and patented Fluid Hammer Operating System, hardware and methodology in a format that both industry and individuals can understand. Read on for a deeper dive into our patented technology in action, and how this new style of drilling can be implemented within our industry, for geothermal applications but also within critical infrastructure projects that can make a difference.


Although still in its infancy, the results of our deployment of our Fluid Hammer Operating System (FHOS) are proving to have superior penetration rates, particularly in hard formations. Efficient borehole cleaning is another indicator that ‘deep hole percussion drilling’ is now an achievable reality. We keep reminding ourselves that we could be the first people to drill with two fluids and a down the hole (DTH) percussion hammer.


Over the past few months, we have put the theory into practice and then optimised penetration rates with hole cleaning to prove the technology. Through determination and dedication, we have fast-tracked our system into something that is game-changing for the drilling industry. To bring our audiences up to speed with how Strada’s FHOS system works, I’ll endeavour to explain from a basic level.


How our Geothermal Drilling Technology Works

At the heart of the FHOS is the DTH percussion hammer. As referenced
here, the DTH hammer drilling principle has been around since 1850 and uses a reciprocating mass and anvil principle to effectively spud the bit buttons by adding a vertical impact force to fracture the rock and increase the penetration rate.


Generated by the fluid flow and pressure, the reciprocating mass moves in the range of 15 to 30 beats per second. The anvil – or bit as it is known – is attached to the hammer body via a spline and retainer rings. There are no moving parts in the bit and its face is populated with a spread of fixed ultra-hard tungsten buttons. The hammer and bit won’t operate until fluid is flowing to it and it is pressed in contact with the rock to be drilled.


Upon every blow of the reciprocating mass, the buttons of the bit penetrate and fracture the rock breaking it into chips the size of your fingernail. With the rotation of the drill string from the surface and flushing fluid to clear the rock chips the buttons are exposed again to a clean rock surface. Weight on the bit (WOB) of up to five tons and a rotational speed of two revolutions for every inch of penetration is required to follow the advancement of the hole. The cuttings are pushed to the outside of the drill string with the flush fluid where they combine with the exhausted power fluid of the hammer to be flushed to the surface up the annulus of the borehole.


Strada’s Fluid Hammer Operating System


Let me explain the features of Strada’s FHOS technology and why it matters – from the bottom up. Firstly, maintaining hole diameter is of the utmost importance when drilling deep holes. That’s why we have incorporated gauge wear limit buttons into our drill bit design that create an increase in rotational torque that indicates to the driller the bit needs to be changed.


Like all drill strings, Strada’s is made up of manageable pieces that transmit rotation, weight and a fluid path to the hammer. The difference here is that our drill string creates two isolated fluid paths, one round and the other annular in section. Being more restrictive, the annular path was chosen to deliver clean working/power fluid to drive the reciprocating hammer mass to avoid wear of its internals. The inner pipe is chosen to deliver the appropriate amount of flushing fluids, whether salt or freshwater, drilling muds – or even air should it be desired.


As with any drilling method, the drill bit will want to follow the path of least resistance and deviate should it encounter fractures or softer formations. To maintain the hole’s directional stability, the FHOS is fitted with a near bit stabiliser and higher up in the bottom hole assembly (BHA) is a close-fitting spiral stabiliser. The near bit stabiliser also incorporates bit-head retention. Should the bit shank fail, the head of the bit is retained from being left at the bottom of the hole. The combination of stabilisers, spread, near fit and relatively low weight on bit (WOB) results in an incredibly accurate hole which will be important when drill holes are only separated by a few metres.


Installed on all drill rigs is a rotational drive system and these days the majority of them are top drive. Rotational top drives are located in the derrick of the rig and travel its length to advance the drilling process or retrieving the drilling tools. Top drives are fitted with a fluid supply rotary swivel. The FHOS requires an additional rotary swivel and piping to supply the second fluid. This can be either located on top or below the top drive depending on the configuration of the rig. On our test rig, we have utilised a swivel that sits below the top drive due to the confines of the top drive design.


Besides a much lower WOB, the drilling methodology essentially remains the same as per mud rotary drilling. The drill rig provides rotation and tension in the drilling string to prevent it from buckling and only allows enough WOB to keep up with the rate of penetration.


Something interesting that we learned during FHOS trials, is that having the ability to optimise the speed of the fluid flushing from the borehole eliminates the re-crushing of the cuttings and a good size cutting is found to be exiting the hole. The larger particle size tends to settle out completely in our dams and tanks before the fluid is reused again in the system.

Typically, mud rotary drilling rigs will be equipped with existing solids control systems which we believe will be adequate to use with the FHOS so that the fluid can be recycled. An additional centrifuge and basket filters are needed to ensure the power fluid is cleaned correctly, these are commonly used in the international drilling industry.


When drilling these vastly deep wells a three safety barrier philosophy needs to be implemented since higher pressures in the formation may force fluids into the hole and end up on the surface. This is the basic concept of a blowout. The primary barrier to the wellbore is the ability to displace its contents with mud that is matched to the formation pressure, thus keeping the well balanced. Unlike other percussion drilling systems, the FHOS it has a direct, unobstructed path connecting the surface to the bottom of the well so this crucial function can still be performed. The second defender to your wellbore is the steel casing cemented to a predetermined depth, and thirdly, a blowout preventer is connected to the wellhead to close off the well in case of an emergency. 


The FHOS has an annular check valve located within the hammer along with check valve subs that can be placed in the drill string as desired. Every FHOS drill string connection has three metal to metal sealing barriers to the outside. Strada is developing a dual circulation surface drill stem valve (Kelly-valve) used to prevent flow out of the drill pipe should a blowout occur during a tripping operation.


Deep hole percussion drilling is now possible after the testing of Strada’s FHOS technology. Combining the proven safety aspects of the mud rotary system with the penetration rates of percussion drilling, it certainly has heat explorers, subsea engineers and research scientists taking notice.


Our mission is certainly to see our technology deployed effectively and at scale to create maximum benefits for our planet. But along with that, we are focused on developing awareness that fully baseload ‘always on’ energy can now be accessed quickly and cost-effectively, everywhere. Unlocking geothermal energy through groundbreaking technology innovation can be the solution to the world’s clean energy needs. Together with progressive drilling partners and governments, we can accelerate our decarbonised future.