Development of the Pekisko Dolomite in the Bigoray Field. Issues in Formation Damage Mechanisms, Drilling Fluid Selection and Well Stimulation

Abstract The Pekisko formation in the Bigoray field in central Alberta is an oil-bearing dual permeability/porosity dolomite. A portion of the reservoir has permeabilities in the range of 1-2 Darcy and porosities approaching 25-30% while the rest of the reservoir features 10-30 millidarcy average permeabilities, 8-12% porosities and low initial water saturation. It is suspected that both rock types contribute to the flow of oil. Drilling of horizontal wells in the Pekisko formation offers technological challenges in terms of field development and formation impairment. Damage mechanisms for the two rock types are expected to be different yet need to be addressed in one drilling fluid system. Open hole completion/stimulation to repair any formation damage and the avoidance of underlying water adds to the drilling challenges. This paper describes the results of laboratory formation damage studies to both identify damage mechanisms and evaluate which drilling fluid system to be used in the horizontal well section. The higher permeability/porosity dolomite core plugs were damaged by solids plugging while aqueous phase trapping and solids plugging were responsible for damage in the lower quality rock. A total of eight horizontal wells have been drilled in the field as of December 1997. The horizontal wells were drilled with three types of drilling fluids: oil, oil-soluble resin and a carbonate bridging system containing an aqueous phase trap reduction agent. The horizontal wells drilled with the carbonate/aqueous phase trap drilling fluid system had better oil production and less water than the wells drilled with the other drilling fluids. Discussion will also focus on stimulation of those horizontal wells that were damaged in the drilling process. P. 161