Interview in GeoDrilling International
The December version of GeoDrilling International magazine includes an interview on "Deep geothermal drilling" with two of Mannvit's geothermal drilling experts Mr. Kristinn Ingason and Mr. Thoroddur Sigurdsson. The article talks about the main requirements of, and the latest trends in, deep geothermal drilling equipment and technology. The experts from Mannvit also answer questions on the comparison of geothermal drilling versus oil & gas drilling.
Excerpt from the GDI article: The Drilling techniques in the geothermal field include over-balanced, hammer and underbalanced drilling, among others.“The most common method is rotary drilling by direct circulation of water-based drilling mud or water in the production part. Other frequently used methods are the aerated drilling ones. Reasons for introducing air into the drilling fluid might be highly permeable formations causing loss of circulation or to minimise formation damage to enhance possible productivity of the wells,” explain Kristinn Ingason and Thoroddur Sigurdsson from the geothermal drilling department of Mannvit, an Iceland-based engineering consulting firm.
The top sections (for surface casings) of geothermal wells may also be drilled by percussion methods, such as the air-hammer method with air and foam, especially in hard-rock formations.”The requirements of the geothermal and O&G industries are practically the same. Both industries face the same challenges and have the same end goals: drilling a well into rock to a specific depth in order to reach a reservoir."
The interview continues, explaining the differences between O&G and high-enthalpy geothermal drilling related to the following topics:
"Explosive atmosphere: Because of the possibility of flammable gases, the O&G industry must have its drilling equipment ATEX-certified, and employs degassers and flare pits to handle gas that reaches the surface while drilling. Geothermal drilling generally does not have to account for flammable gases unless the wells are being drilled in O&G areas.
High temperature: Mud chillers (usually air-cooled) are generally not required in O&G drilling but are necessary in geothermal drilling. Temperature limitations of downhole equipment can be a limiting factor in high-enthalpy geothermal wells, especially for directional tools (MWD) and wireline-logging equipment. High pressures: The downhole pressures encountered in O&G drilling are generally much higher than in geothermal wells. Therefore the blow-out preventers (BOPs) are usually rated higher (5,000-10,000psi/345-690 bar) in O&G compared to geothermal (2,000-3,000psi/138-207 bar). Formation pressures in geothermal drilling are usually less than hydrostatic.
Directional accuracy: The drilling targets in geothermal wells are generally much bigger than in O&G drilling. Therefore, the level of directional accuracy is much more important in the O&G industry, which uses state-of-the-art RSS, while the geothermal industry uses the cheaper alternative of positive displacement motors (PDMs).
Well completions: The well completions are quite different in the geothermal field compared to O&G. There is no production tubing in geothermal, therefore the produced fluid comes in direct contact with the production casing. Larger production casings and production hole sections are required to accommodate the high flowrates required to produce geothermal wells economically. The geothermal production sections have uncemented perforated liners or just an open-hole completion (‘barefoot’). Multilateral and/or horizontal wells are rare in the geothermal industry.
Drill bits: High-enthalpy geothermal wells are generally drilled in igneous rock formations usually with tungsten-carbide roller-cone bits. PDC bits, frequently used in O&G, have rarely been applied to high-enthalpy geothermal drilling but are more common in low-enthalpy geothermal wells (especially those drilled in sedimentary formations)."
The entire 5 page article from GDI can be found here as pdf file. The whole issue is available at www.geodrillinginternational.com