Contribution of Underbalanced Drilling To Production Capacity at Olkaria Geothermal Steamfield in Kenya

Abstract

The planned time to drill a geothermal well at Olkaria is sometimes exceeded by a big margin when down hole challenges are encountered whilst drilling. The geographical coordinates of Olkaria are 0 o 53'09S and 36 o 16'12E. The current drilling practice at the field is balanced pressure drilling. The common down hole problems encountered in the field are loss of circulation, well bore collapse, 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. Underbalanced drilling is the application of drilling fluids to the bore at a pressure slightly lesser than the formation fluids pressure. If the process is well executed, it may lead to the reduction/elimination of most down hole challenges. This research was geared to 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. Data from four geothermal wells namely OW 731A, OW915B, OW731 and OW804 was used in the simulation exercise. Analysis of loss of circulation to well productivity for a sample of 19 wells was also undertaken. Simulated annulus volume fraction profiles showed cuttings concentration of less than 5% and an optimum circulation flow rate of 0.03 m 3 /min. The costs associated with loss of circulation of drilling fluids on average amounts to USD 127,731.00. Most of the wells sampled had over 70% of the drilled depth experienced no loss of circulation during drilling. Wells which had a circulation regime of full returns, partial returns and no returns have shown that their productivity is higher compared to the other categories and had an average production capacity of 12.03 Mw Keywords:Underbalanced drilling, Formation damage, Rate of penetration, Loss of circulation and down hole challenges.