| Abstract |
Geothermal development has been restricted in Japan for many years because approximately 80% of the vast amounts of geothermal resources are located in natural parks. Under these circumstances, an R&D project on geothermal well drilling was started and is under way in Japan which aims to develop an environment-friendly low-cost, extended-reach drilling technology that enables access to geothermal resources from outside the natural parks. One of the key issues in extended-reach drilling is hydraulics design. Particularly in geothermal wells with subnormal pressure and lost circulation zones, low-density, low-viscosity drilling fluid, or water in some cases is usually used which is disadvantageous for obtaining good hole cleaning. Although the largest possible flow rate is needed for sufficient hole cleaning with low-density, low-viscosity drilling fluid and for avoiding an increase of equivalent circulating density (ECD) due to cuttings deposition on the low side of borehole in inclined sections, excess flow rate unexpectedly increases ECD which may cause lost circulation and borehole instability problems. Implementation of both effective hole cleaning and maintenance of an appropriate ECD in geothermal wells is contradictory and much more difficult than in oil and gas wells. In this study, we conducted a number of cuttings transport experiments using a large-scale flow-loop apparatus, numerical simulations using a transient two-layer model hydraulics simulator we have developed, and field measurements of annular pressure using APWD (annular pressure while drilling) and cuttings flow at the outlet of the shale shaker in a geothermal directional well recently drilled in Japan. Based on these measurements and studies, optimum hydraulics conditions for good hole cleaning and appropriate ECD management for the targeted long extended-reach geothermal well with total depth 3,000 m, horizontal departure 2,500 m, and maximum hole inclination angle 70 degrees are comprehensively discussed. And recommendations for preventing drilling problems such as poor hole cleaning, high torque and drag, stuck pipes, borehole instability and lost circulation are presented. |