The completion of High Temperature High Pressure (HTHP) gas condensate wells has historically been limited to techniques that are considered sub-optimal when compared to techniques used on lower pressure and temperature wells. These limits have been driven by the extreme nature of the wellbore conditions and the resultant high stresses and difficult operating conditions placed on well bore equipment.
In developing the Elgin and Franklin fields, Elf Exploration UK plc opted to use a completion design that provided a more optimised approach utilising larger bore tubing and associated equipment. A method of perforation that maximised reservoir productivity and completion efficiency whilst providing the necessary operational safety was developed.
In order to utilise this approach, much of the completion equipment and techniques had to be specifically developed and rigorously qualified for this harsh environment. The resulting design and techniques provide a simplified and optimised completion methodology for HPHT wells.
The Elgin and Franklin fields are the largest and highest pressure HPHT gas field to be developed in the British Sector of the North Sea.
The fields lie approximately 100 miles North East of Aberdeen in the Central Graben region of the North Sea (Fig 1).
The fields will be produced via two unmanned wellhead platforms tied back to a central processing facility. Drilling and completion of the wells has been carried out by two heavy duty jack-ups in cantilever mode over the wellhead jackets.
The primary reservoir is the Fulmar Sands located at an approximate depth of 5300m (17400ft) TVDSS. Reservoir fluids within the Fulmar are gas condensate with a bottom hole pressure of 1100 bar (16000psi) and temperature of 190°C (375F). Production will give surface conditions of 860 bar (12500psi) wellhead pressure with an associated surface temperature of 170°C (340F).
The Fulmar reservoir is underlain by the Pentland reservoir with bottom hole conditions of 1140bar (16500psi) and temperatures of 200°C (392F). The Pentland reservoir will be produced on the Franklin field.
The individual wells on the field will produce up to a maximum gas rate of 3 × 106 m3/day (100mmscf/d) gas with associated condensate.
The completion philosophy for the Elgin and Franklin fields was determined during the conceptual phase of the development and encompassed the desire to develop a design that optimised both the operational and functional aspects of the completion whilst giving due consideration the significant safety implications of working in the difficult HPHT environment.
The basic premises on which the design was to be determined were:
Maximum bore consistent with the constraints of the casing architecture.
To be as simple as possible.
To avoid if possible the use of elastomeric and/or dynamic seals
To allow reliable and safe displacement to an underbalanced packer fluid.
To optimise the quality of the reservoir to wellbore interface by allowing optimal perforation of a zone up to 200m in length.
To minimise the requirement for well intervention.