Extended reach wells drilled from the Goodwyn Alpha platform are highly aggressive and have been drilled with a variety of invert-emulsion mud systems. Historically, mud system selection for these wells was based on compliance to environmental regulations and satisfaction of technical criteria. In order to take a more proactive approach to mud system selection and to raise environmental standards to new levels, an initiative was undertaken to replace the environmentally acceptable, and technically competent mud system being used on Goodwyn with a new ester-based system. Ester-based mud systems are considered the "system-of-choice" in terms of environmental acceptability and possess inherent lubricating qualities for torque and drag reduction.
This paper describes the development process and 4 successful field applications of the new ester-based mud system. A new ester, which exhibits the lowest kinematic viscosity of commercially available esters, was identified and selected for use. New and innovative technologies were introduced in the laboratory development and field applications to address barite sag, hole cleaning efficiency and hydrolytic stability.
The system development and field testing processes were enhanced by daily communication with the development teams in Aberdeen, Houston and Perth. Down-hole friction factors were significantly lower than modeled predictions. Barite sag problems encountered on previous wells were eliminated, and hole cleaning efficiency was optimized.
The Goodwyn field is located off the Northwest Shelf of Western Australia. The field was discovered in 1971 by the Goodwyn-1 well, and is one of several large fault blocks located on the Rankin Trend, which form the northern margin of the Dampier sub-basin. The Goodwyn Field Development Plan was written in 1989 to encompass drilling 47 wells: 26 wells from Goodwyn A, 19 wells from a 2nd platform and 2 subsea wells. The plan was to drill the wells at maximum hole angles and horizontal departure of 55° and 4,000 meters, respectively. With the introduction of extended reach drilling, to 7,372 m horizontal step-out, the requirement for the second platform has possibly been eliminated.
The complexity of the Goodwyn extended reach drilling program increased as the wells progressed. The evolution of invert-emulsion fluids used on Goodwyn, from low toxicity oil-based muds (LTOBM) to ester-based muds (EBM), had a significant impact on the ability to expand the horizontal reach from the platform and the length of reservoir penetrated. At the conclusion of GWA Phases I & II, a total of 17 wells had been drilled to a maximum step-out of 7,372 m and horizontal section lengths of 2,300 m.
Extended-reach drilling is a form of deviated drilling characterized by lengthy horizontal departures and aggressive hole angles. With consideration to eliminating drilling problems such as excessive torque and drag, poor hole cleaning, stuck pipe, low rates-of-penetration and well-bore stability, increasingly these wells are being drilled with invert-emulsion muds.1,2,3
Invert-emulsion muds have evolved over the past several decades, primarily in response to government-imposed restrictions on the discharge of contaminated cuttings. LTOBM were introduced into the UK sector of the North Sea in the early 1980's and the number of wells drilled with LTOBM steadily increased in the late 1980's, with growing concern about the impact of discharges on aquatic life.