Technical and economic assessment of adopting under-balanced drilling at the Olkaria geothermal field

Abstract

The current drilling practice at the Olkaria geothermal field is a mixture of overbalance, near balance and balanced drilling. The planned time to drill a well at Olkaria is sometimes exceeded by a big margin when down hole challenges are encountered while drilling. The common down hole problems encountered in the field are loss of circulation, minimal Rate Of Penetration (ROP) and possible formation damage which consequently leads to reduced productivity of a well. It is evident that, the longer the drilling time, the higher the cost of the well. Under-Balanced drilling is the application of drilling fluids to the bore at a pressure slightly less than the formation fluids pressure. If the process is well executed, it may lead to the reduction or even elimination of most down hole challenges. This research was aimed at establishing the effectiveness of Under-Balanced drilling (UBD) in solving the down hole problems experienced at the Olkaria geothermal field. In order to carry out this exercise, simulation of Under- Balanced drilling conditions using the Hydraulic Underbalanced Simulator (HUBS) was done utilizing data from the field. Secondly, the economics of adopting Under-Balanced drilling technology at the field was also done and finally the potential hazards associated with Under-Balanced drilling were assessed. Data from four geothermal wells namely OW 731A, OW 915B, OW 731 and OW 804 was used in the simulation exercise. Simulated annulus volume fraction profiles showed cuttings concentration of less than 5% and an optimum circulation flow rate of 0.03 m3/min. The costs associated with loss of circulation and increased hole cleaning on average represented 3.7% of the total average well cost. The projected benefit cost ratio obtained for the UBD case was 1.40 while the one for the conventional case was 1.25. The Net Present Value for the Under- Balanced drilling case was higher than that of the conventional case. In the initial HAZOP analysis for the UBD procedure, the installation of the rotating control device scored high in operability aspect while choke valve regulation scored high in safety aspect.