Reducing geothermal drilling problems to improve performance in Menengai

Abstract

Drilling geothermal wells is one of the main construction activities of a geothermal project. The aim is to access the available steam in the production zone of a geothermal reservoir. The discharged wet steam is separated into steam that is channelled to a power plant, where it drives turbines to produce electricity, and water which is normally injected back into the reservoir. Several challenges are encountered while drilling geothermal wells. These challenges have resulted in drilling timelines not being met as well as increases in drilling costs. This report describes the causes of some of these challenges and how they can be minimized or reduced so as to improve performance. Challenges encountered in Menengai geothermal field in Kenya are the main subject of this study. These challenges are either natural or due to human error. This report found that drill string sticking accounted for 12% of the total drilling time in Menengai. Sticking is caused by poor well bore cleaning, especially when there is lost circulation and long or periodic waiting for water. Lost circulation is caused by intersecting fractures or permeable zones. Measures for mitigating problems of lost circulation so as to avoid sticking have been discussed.

This report found that the most troublesome zone in Menengai is at a depth of 2100 m where the drill string got stuck in Wells MW03, MW04, MW07 and MW12. Wells MW01 and MW06 had drill string sticking at a depth of 2206 and 2202.96 m, respectively. A comparison was made with the 2104 m depth of an Iceland Deep Drilling Project well at Krafla in Iceland. An analysis indicated that the troubles were related to intersecting magma intrusions, shown by fresh glass in the cuttings that were observed at the shale shakers. The magma pushed the drill string up (large drop in hook load) and it got stuck, blocking the circulation at the same time. Temperature surveys later showed the fluid near the bottom was superheated. Metal fatigue was found to be another problem, as shown by shears that were experienced on cross-over box ends. To minimize this, the report suggests the use of drill string risk management for close monitoring of any defects.