| Abstract |
Directional drilling technology is now popular in oil and gas industry because extended-reach and horizontal wells improve the productivities of oil and gas reservoirs. Also in geothermal development, directional wells are expected to enable environmental-friendly cost-saving developments by means of aggregation of well site drilling and production facilities outside the nature conservation geothermal areas, and enhancement of steam productivity through the greater penetration of high-temperature hydrothermal reservoirs. Particularly in enhanced geothermal systems (EGS), to economically extract heat from the reservoirs, directional drilling technology is indispensable for high-quality fracture creations and effective fracture penetrations of production wells. However, there are not much records of extended-reach and horizontal drilling applications in geothermal development so far. One of the key issues in extended-reach and horizontal drilling is hydraulics. In geothermal drilling with subnormal pressure formations and many lost circulation zones, low-density and low-viscosity drilling fluids are usually used which is, however, disadvantageous for obtaining good hole cleaning. On the other hand, excessive flow rate to compensate the poor hole cleaning may increase equivalent circulating density (ECD) more rapidly in extended-reach and horizontal wells than in vertical wells. In this presentation, informations from experimental measurements, numerical simulation studies and analysis of field pressure while drilling (PWD) data conducted by the authors research group for over 10 years are summerised, and the optimum hydraulics design and operation for cost-saving, safe extended-reach and horizontal geothermal drilling are discussed. What the differences of cuttings transport and ECD behaviors between oilwell and geothermal drillings are, and how we can avoid hole cleaning, lost circulation and borehole instability problems are focused in the discussion. |