| Abstract |
As a part of the "Development of high temperature MWD system for geothermal wells", well trajectory prediction system, one of the drilling support system, has been developed by NEDO for well trajectory prediction while drilling and for designing BHA before drilling. This system was designed to be applied for DHM (down hole motor) drilling and for rotary drilling in geothermal areas. Computer programs of the system have been developed based on equations of equilibrium for BHA (bottom hole assembly) (Jogi et al., 1988) to calculate the force acting on a bit, definition to determine the direction of drilling (Ho, 1987) and equation to find resonant RPM (Yew and Zhang, 1994). We verified the program using actual directional drilling data in a geothermal area. Regarding DHM drilling, as well trajectories calculated by the program were mostly consistent with actual field data, we considered that this program could support efficient DHM drilling. However, in some cases for rotary drilling the results from the program were not consistent with actual data. We considered the actual well trajectory in rotary drilling had the tendency to be affected not only by force acting on the bit, but also geological effects. For estimating and rejecting geological effects, we developed a new program to calculate bit and rock anisotropy index by an inversion method and tested it using actual rotary drilling data. We found it possible to improve the well trajectory prediction by applying bit and rock anisotropy index. We have examined such geological effects with this program using drilling and geological data of deep geothermal wells (WD-1a and WD-1b) drilled by NEDO. In this study, anisotropy indexes of these adjacent two wells (WD-1a and WD-1b) were similar. And, strong correlation was observed between bit anisotropy index calculated by the inversion program with actual directional drilling data and geological boundary as confirmed by drilling. Well trajectory was seriously affected by geological effect at the geological boundary. If we can estimate the geology of a new well from other nearby well data, it will be possible to improve drilling efficiency by this system. Next our task is to study other geological and downhole factors affecting well trajectory and reflect such study results to improvement of geothermal drilling. |