BREAKTHROUGH ON A ROTARY STEERABLE SYSTEM FOR GEOTHERMAL DRILLING IN A WEST JAVA FIELD

Abstract

Geothermal wells are often deviated and most of the directional work is generally performed in the intermediate hole section (17 ½”), conventionally using a mud motor. However this has some limitations, such as a small RPM range available for cleaning the hole while rotating and the risk of getting stuck due to pack-off (especially in the presence of total losses while sliding (particularly for inclinations around 35 degrees and above) in the formations of high heterogeneity encountered in the volcanic environment).
To overcome these drilling challenges, a Rotary Steerable System (RSS) has been utilized, specifically a RSS Xceed900. This is a “point the bit” system which ‘directs’ the bits in the desired direction, either through bending a shaft or having a built in offset, as opposed to a “push the bit” system, which utilizes “pads” to allow steering. This is the first time a “point the bit” RSS has been used in a geothermal well, worldwide.
To allow the successful deployment of this new technology in a geothermal application, a thorough technical/commercial analysis and several simulations and iterations were performed, which included:
• Gyro and Magnetometer drilling modes due to the expected cross-magnetic interference from the formation.
• IDEAS (Integrated Dynamic Engineering Analysis System - Schlumberger proprietary software) simulations to ensure both directional and dog-leg capabilities in the anticipated drilling conditions.
• Well plan optimization targeting a dog-leg severity (DLS) below 3 deg/30m.
• Flow rate optimization during aerated drilling together with survey considerations (downlink).
The actual field results can be summarized as follows:
• Higher on bottom ROP (15%).
• Optimization of parameters for hole cleaning - RPM 100-140, flow rate 900-1100 gpm.
• Minimization of back reaming.
• Optimization of Hi-Vis pills usage.
• Smoother T&D trend (including micro dog-leg avoidance).
• Implemented surveying strategy and QuikSurvey application.
The RSS Xceed900 showed several benefits that offset its higher cost compared to conventional mud motors in directional wells. This technology proved, in meeting specific drilling challenges, to be a viable alternative to mud motors for improving both performance and hole cleaning.