Development of the Drilling Support System as a Part of the MWD System for Geothermal Wells

Abstract

Many geothermal wells are drilled in formations of hard volcanic rock, with open fractures, at high temperatures and pressures. Operating companies and government agencies spend a lot of money dealing with frequent lost circulation and other problems. Since many geothermal targets are located in high angle fractures, and most wells are drilled on sites of limited size in mountainous areas in national parks, directional drilling technology is very important in order to hit the target from a restricted location. ìMeasurement While Drilling (MWD)î technology, where the drilling direction and the tool face direction are known ìin real timeî during drilling, is the most effective method for serious directional, controlled drilling. However, MWD technology is based on oil and gas technology and the newest MWD systems on the market have maximum operating temperatures of almost 180 Deg.C. Therefore, we can not use these MWD systems to control the drilling direction in the higher temperature zones of the target. GERD and Mitsui Zosen Group (Akishima Laboratory Co. and Mitsui Zosen Co.) are developing a high temperature MWD system that can be operated under a condition of 200 Deg.C in the ìDevelopment of High Temperature Measurement While Drilling System for Geothermal Wellsî Project (a part of ìNew Sunshine Projectî of MITI). This project has been conducted by NEDO since 1991.We are developing a ìDrilling Support Systemî that consists of the ìDirectional Control Support Systemì and the ìFormation Evaluation Support System ì through this project. The Directional Control Support System consists of a ìWell Trajectory Planning Systemî, a ìWell Trajectory Visualization Systemî, and a ìWell Trajectory Prediction Systemî. The system can present the measured data and the well trajectory with a 2-dimensional plane view/Cylinder view/3-dimensional birdís-eye ìin real timeî. In addition, the system can be used to plan and predict well trajectory and use a new BHA that the driller can select on site. The Formation Evaluation Support System consists of a ìFormation Temperature Analysis Systemî and a ìFormation Pressure Analysis Systemî. The ìFormation Pressure Analysis Systemî is still in its basic design, but the ìFormation Temperature Analysis Systemî is almost completed. The ìFormation Temperature Analysis Systemî is the prediction system for equilibrium formation temperature while drilling, based on the borehole temperature simulation code ìMWDTEMP2î that is modified from ìGEOTEMP2î (Mondy & Duda, 1984); using a non-linear least square inversion technique. This paper shows the function comparison of the ìDrilling Support Systemî, analysis of results using actual data, and consideration for the effect of drilling efficiency by applying the system to directional drilling. Figure 1 shows concept of MWD system, and figure 2 shows concept of the ìDrilling Support Systemî.