Geothermal Drilling: No theoretical depth limit exists

The efficiencies and economical gain in the use of Down the Hole (DTH) percussion hammer drilling in hard rock formations are well known to us at Strada Global. Conventional DTH percussion hammers utilise compressed air as the means to drive the percussion action of the hammer at the borehole face and then utilise the expanding exhausting air to clear the cuttings out at high speed. Although they have high penetration rates in hard rock their means of drilling to great depths is very inefficient and in a lot of cases impossible to access geothermal resources where they are needed.

Conventional DTH air hammers are limited by groundwater entering the hole and power required to charge up the supply air to working pressure as the hole becomes deep. Although they have a good uphole velocity for clearing cuttings, as the hole gets deeper this efficiency drops, and foam additives have to be introduced to clear suspended cutting blockages. There can also be concerns regarding personnel safety and the dust oil mist pollution that is created when working alongside compressed air equipment.

From first-hand experience, we recognise the limitations of DTH percussion air hammer drilling and are adopting water-powered DTH percussion hammer drilling technology and methodology to unlock geothermal potential. Percussion Water Hammer Drilling (WHD) adopts clean water to drive the percussion action of the hammer drill. It is proven to be very effective in hard rock conditions, able to work underwater and potentially there is no theoretical depth limit exists. They have similar penetration rates as the air hammers, but they are significantly cheaper to run. Because of this, it means they are better for the environment and additionally, there is no dust produced nor lubricating oil released and much safer for the operators. The lower uphole velocity and density of the water improve cutting transport, which also has benefits such as increased hole stability and significantly less erosion on the borehole and hammer body, BHA and drill pipe. When water is used as the drilling fluid the use of close-fitting stabilisers on the hammer body, and in the BHA, become more practical. This enhanced hole deviation, when compared to air hammer drilling, is another positive reason to adopt the technology.

There are also considerable cost savings in the BHA requirements, as the DTH percussion drilling requires a much lower weight on bit. In addition, the WHD method can be utilised with a smaller truck-based drill rig and corresponding smaller site requirements, which again results in overall savings. However there are some operational restrictions, which include;

• The requirement for large volumes of high-quality clean water to drive the hammer tool which sometimes creates disposal challenges — tit means the use of cleaning system in locations where supply is limited and in order to prevent rapid wear of the hammer.
• Clean water lifting the cuttings has limited ability to effectively deal with lost circulation in the borehole.
• They have limited ability to introduce additives that can lessen the risk of stuck drill pipe by controlling sloughing and swelling formations.
• Water hammers have limited well control ability — when using the mud rotary method, denser mud can quickly be introduced to the well to equal or overcome the pressure of the formation, thus controlling the well. This option is not available when using the water hammer method because the tool constitutes a form of orifice at the bottom of the well, thus limiting fluid flow through the system. Hence limiting the ability of the driller to kill the well.
• Water hammers have multiple moving parts, internal sealing surfaces and since they operate at significantly higher pressures the tolerances between the machine parts are much higher.

At Strada Global we believe that something needs to be done in making geothermal a more attractive investment. A water-powered DTH percussion hammer has the potential to make drilling for geothermal more economically viable, and environmentally friendly. By decreasing the drilling time and fuel expenditure, and overall project, this approach could reduce payback periods.

The downsides of the water-powered percussion method, namely well integrity and ability to kill a well, are still present. However, we are currently working on a drilling methodology that can potentially overcome these pitfalls. The system is currently in a proof of concept stage of development, but we are looking forward to publishing the results of our testing soon.

Taras Olijnyk, Project Engineer, Strada Global