The first well utilizing a novel drilling fluid based on polyalphaolefin (PAO) chemistry has been drilled in the North Sea. In addition to excellent drilling performance, PAO drilling fluids exhibit low toxicity, low bioaccumulation, and are biodegradable. Success of the PAO mud has established the system as a viable synthetic-based alternative to oil-based muds.

The non-aqueous PAO system was used to drill the 12.25-in. section of a well in the Central Basin of the North Sea. The interval from 6,300 to 15,067 ft (1,920 m to 4,592 m) was drilled at an angle of 55 deg. The formation consisted primarily of tertiary shales mixed with limestone stringers. Mud weight range was 15.0-15.5 ppg (1.8-1.86 s.g.).

Comparisons with oil-based muds used in this area of the North Sea indicated the PAO fluid helped increase penetration rates and reduce torque and drag. Other performance benefits included superior hole cleaning and no evidence of weight material sag. Additionally, cuttings discharged into the sea retained an average of less than 10% PAO (<100 g/kg dry retort solids).

Comparisons with other non-aqueous alternatives such as ester and ether-based fluids show PAO systems provide better elastomer compatibility, lime and cement tolerance, surface handling, synthetic/water ratio flexibility, and temperature stability.

This paper covers drilling and fluid performance results of the first field trial of a PAO-based mud. Environmental data including toxicity, bioaccumulation, and cuttings beds studies also are discussed. Chemical and practical differences between PAO and other synthetic-based muds are presented.

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