Improved Recoveries in the Pickerill Field from Multilateral Drilling into Tight Gas Sands
Authors: Bokhari S.W. , Hatch A.J. , Kyei A. , Werngren O.C. 
Published: December 2007

In summer of 1996 a suspended well in the Pickerill Field in the UKCS Southern North Sea was re-entered and two sidetracks were drilled into underdeveloped tight gas sands. This well, A3ZY, was one of the first multilateral wells to be drilled in the Southern North Sea and several lessons were learned about the multilateral drilling technology and the performance of various zones within the tight Rotliegendes formation at Pickerill. The well showed a way to develop marginal reserves from tight gas sands using multilateral sidetracks and as a result additional opportunities have been identified in the Pickerill Field and surrounding area that will benefit from this approach.. The well is expected to recover an additional 10 BCF in reserves and has added 20 MMCFD to the daily production rate. The work was successfully completed in 61 days resulting in two laterals of around 1800 and 1500 feet.

This paper covers the justification, planning and evaluation of the results from this multilateral well. In addition, it discusses the successful use of the medium radius drilling technique, high angle drilling through potentially unstable Zechstien salt horizons, the use of low-weight/low-invasion drilling fluids in extreme overbalanced conditions, and the successful deployment of the long slotted liner with pre-installed formation saver valve.

Introduction

The Pickerill Field is a dry gas accumulation situated in the Southern North Sea straddling Blocks 48/11a, 48/11b, 48/12c and 48/17b (Figure 1). The field was discovered in 1966 after which 12 further appraisal wells were drilled. A total of 15 producing wells have been drilled from two platforms (Figure 2). ARCO British Ltd. operates the field for a consortium of seven companies. The field has been on production since August 1992 and the maximum contract rates were 210 mmscfd. The daily contract production rate for the current contract year is 149 mmscfd. The gross GIIP is estimated to be 900 BCF and proven reserves are 550 BCF.

The platforms are located some 40 miles offshore in 82 feet of water and are normally unmanned Gas flows onshore to the Theddlethorpe Gas Terminal where it is processed, compressed and shipped primarily to the Killinghome Power Station (Figure 3).

The Pickerill Field produces from the Rotliegendes, Leman Sandstone formation in the western margin of the Southern North Sea. The reservoir has been described by six vertical zones, with most of the GIIP in the top three zones (Figure 4). The reservoir is compartmentalized with the field divided in two main sections, Pickerill East and Pickerill West, separated by a sealing fault (Figure 2). Production is mostly from aeolian dune sandstones. The field has an average porosity of 12% and the core permeability ranges from 0.1 to 10 md. Initial pressure and temperature were around 3995 psia and 204 F respectively at a depth of 8900 feet TVDSS. The gas gravity is around 0.61 and the liquid yield is around 7 bbls/mmscf at separator conditions.

A detailed Pickerill reservoir study 1 known as the "Pickerill Phase 3 study" was completed in late 1994 and showed that the top two Rotliegendes zones were of poor quality in the eastern section. In some areas of the field, a Sabhka unit provided a vertical permeability barrier between zones 2 and 3. The original Pickerill Field reservoir model, which was based on Phase 3 geological model, showed that the area around the suspended well 48/11b-A3 was not being adequately depleted by the existing producers and had near virgin reservoir pressures in the top two zones. In addition, the reservoir simulation model showed that a multilateral well in this area could improve the recoveries significantly and recover an additional 15 BCF of reserves. In the summer of 1996, dual laterals were successfully side tracked from the 48/11b-A3 well and the well has now been producing for eight months at an average rate of 18.5 mmscfd (Figure 5) and is expected to recover an incremental 10 BCF.

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