Successful Drilling of an Underbalanced Horizontal Well in the Rigel Halfway Pool - Laboratory Screening and Field Results

Abstract

Abstract The Middle Triassic Halfway formation in northeastern British Columbia is a high quality, well sorted quartzose sublitharenite. Past drilling operations using conventional overbalanced technology in the reservoir had indicated significant formation damage occurred due to a combination of mechanical and chemical formation damage effects associated with the loss of drilling fluid to the formation. A series of compatibility and special core analysis tests were conducted to refine an underbalanced drilling procedure with natural gas for a 300 m long horizontal well in the formation using a natural gas-mist hydrocarbon system at a 1400 kPa underbalance pressure level. The well was drilled successfully and flowed at peak rates of up to 1100 m3/day (almost 7000 bbl/day) of oil and 340,000 m3/day (12,000,000 scf/day) of gas during the underbalanced drilling operation. Post test pressure transient and interference tests indicated an in-situ permeability of 150 mD with zero skin factor indicating that the well had been drilled successfully in an undamaged fashion. The well has currently produced close to 13,000 m3 of oil (80,000 bbl) and over 11,000,000 m3 (400 MMscf) of gas with initial AOF potential of approximately 520 m3/day (3300 bbl/day). Introduction The Middle Triassic Halfway formation of the Rigel field located in northeastern British Columbia is part of a regressive sequence, bounded stratigraphically below by silts and shales of the offshore Doig formation and above by continental hypersaline anhydrites, dolomites, silts and shales of the Charlie Lake formation. The formation exhibits a regional southeasterly dip with an average depth of 1266 meters. The pronounced north-south orientation of the pool is thought to be influenced by rejuvenation of deep-seated faults. Although not visible on seismic traces, small scale reactivation may have controlled the axis of deposition and subsequently aided in the preservation of reservoir facies. The Rigel Halfway C pool is fine-grained, well-sorted quartzose sublitharenite with patchy calcite cementation. It is interpreted as a cross-cutting tidal channel fill deposit, juxtaposed against tight sands and shales of the Halfway barrier-bar system. Using a permeability cutoff to oil of 1.0 mD, a porosity cutoff of 8% was determined for two wells including C-20-L (from which experimental sample selections were taken). Based on these cutoffs, net pay of the reservoir ranges from approximately 1.1 meters to 7.8 meters. Rock quality of the formation is characterized by an average permeability to air of approximately 100 mD, with core sample permeabilities increasing from 25 to 85 mD in the upper interval (1263 to 1266 meters) to 100 to 900 mD in the lower interval (1266 to 1270 meters). Porosity over the entire interval increases with depth from 15% to 25%, and original oil in place for the pool is estimated to be 1.6 × 106 m3. Drilling Strategy Utilizing drilling and production experience gained through other wells in the area, Samedan Oil of Canada set out to drill a horizontal well (3–33). The objective of drilling the 3–33 well was to target the recoverable oil by drilling a non-damaged horizontal well with high deliverability for the least drawdown and GOR performance. Experience with the C Pool indicated that formation damage had occurred when drilling previous wells, and that damage was severe enough to prevent water injection for a waterflood even after perforating the injector. P. 245