ＪＵＤＧＥ井（１０ｋｍ，４００℃）掘削の困難性と実現に向けての基本戦略の提案

Abstract

Japanese Ultra-deep Drilling and Geo-scientific Experiments (JUDGE), which aim to drill into the Philippine-Sea-Plate subduction zone from the Japanese Island, had been proposed by scientists more than ten years ago. Scientists estimate the plate can be reached with a 10-km deep borehole, and they predict that the formation temperature will increase from 200 to 400°C at that depth. Some studies were made about this project several years ago, however, no drilling proposal has been submitted to conduct this project.This paper discusses: What are the major technical issues to carry out this project? How deep can we drill into the formation with estimated formation temperature gradients?, and How should we plan this project? Numerical simulations were conducted to find out the temperature changes in the borehole during drilling operations. The results are: A 7-km hole can be drilled, but the 10km hole will be very difficult to drill with conventional drilling methods and existing muds and tools.Based on simulation results a 12-year project plan, consisting of 3-phases, was proposed. Phase-1 is 5 years and assigned to be a planning and engineering stage. A 6-km to 7-km hole was planned to determine actual formation temperature and bore hole stabilities. Based on the obtained information, necessary research will be conducted in phase-2. To test the temperature capabilities of drilling and logging tools, a 4km-deep high-temperature well will be drilled in a geothermal area. Phase-3 is the execution stage to drill the 10-km well in four years.Temperature simulation results showed that it would be very difficult to drill 10km-JUDGE well with conventional method, however new drilling methods such as casing-drilling, expandable-tube, TDS-cooling-method etc., have been developed recently. So, it seems that the JUDGE project is no longer just a scientists' dream, but could be feasible in the near future.