Technical- and Comparative Analysis of Water- and Air Hammer Drilling in a Geothermal Environment

Abstract

Various methods of percussion drilling are investigated with an overall goal of delivering cheaper and better wells in the geothermal region Sebechleby. This thesis will focus primarily on the potential and limitations of water- and air driven hammers, but will also evaluate alternative drilling technologies. An electric hammer from Resonator and a mud hammer developed by LKAB Wassara has been included to study how new emerging technologies can expand the operating envelope of percussion drilling.Field experience from Sebechleby showed that the incorporation of an air hammer can increase the penetration rates by more than eight times compared to rotary drilling. Additionally the reduced hydrostatic pressure will mitigate, or at best eliminate the high expected fluid losses in the region. The presence of water in the formation will complicate the hole cleaning process and demands for an effective cuttings removal medium, a polymer-enhanced foam was chosen for this purpose. Analysing the technical performance showed that air hammer drilling would become un-economical at larger depths because of high pressure losses in the system.LKAB Wassaras water hammer is presented as a favourable solution for the bottom hole section. Field experiences showed that penetration rates twice that of Atlas Copcos air hammer and five times that of rotary drilling can be expected. Furthermore the incompressibility of water will enable the hammer to operate with a high efficiency independent of depth. A clean water system will also have positive effects on borehole stability and reduce the environmental impact substantially as the fuel consumption is less than 18% of both air hammer- and rotary drilling.To maximize the production tubing size it will be necessary to backream and expand the down hole section by 2 . This means that the improved performance will be diminished by the need to expand the well. This is however the recommended practice since the value of increased flow rate surpass the costs of expanding the well. Furthermore large vibrational forces and lost circulation problems are expected, these are solved with a thruster system and by drilling without returns. Pumping water at high rates down both the drillstring and annulus enables this approach to be taken.From a technical- and comparative analysis it was concluded that the top hole section should be drilled by means of air hammer drilling as long as the associated risks are kept at acceptable levels. As for the down hole section it is believed that a water driven hammer will provide the best balance between performance, hole stability and safety.