Modelling and optimization of geothermal drilling parameters : a case study of well MW-17 in Menengai Kenya

Abstract

Several factors come into play when a drill bit is crushing the rock at the bottom of the hole. To effectively drill geothermal wells, these factors must be carefully considered and combined in an optimum manner. The characteristic of geothermal formations is such that it is composed of different layers of rocks alternating from the surface to the final depth. Some rocks are highly temperature altered while others are highly fractured and unconsolidated. A careful approach has to be devised while drilling through the different sections to avoid problems which lead to delays in drilling. At the same time drilling parameters have to be applied according to the rock types in such a way that the well is drilled in the shortest time possible and in the most cost effective manner. The following factors have been mathematically modelled by Multiple Linear Regression and shown how they affect the overall drilling rate: Formation strength, Depth, Formation compaction, Pressure differential, Bit diameter and Weight on bit (WOB), Bit rotation (RPM), and Bit hydraulics. This modelling approach has been adapted for geothermal drilling from the Oil and Gas drilling as first applied by Bourgoyne and Young. Optimization of WOB and RPM showed most of these parameters are in some cases applied too low and in others too high. Data captured while drilling of well MW-17 in Menengai geothermal field was used in making the drilling model. A combination of Excel and Matlab was used in the data analysis.