This paper summarises the factors influencing the well design for a high pressure high temperature (HPHT) field development using a Not Normally Manned Installation (NNMI) in the UK sector of the Central North Sea (CNS).
The Erskine gas condensate field is a 50% Texaco / 50% BP venture and will be the first HPHT field developed in the North Sea with first gas scheduled for 1997 (Fig. 1). The field development concept is to install a not normally manned installation (NNMI) with multiphase export of produced fluids to the Lomond platform from six platform wells. Drilling and completion operations will be carried out using a harsh environment jack-up rig in cantilever mode.
Primary functional requirements for the wells include high reliability, high productivity and the ability to perform through tubing plug-backs. Reserves in the core area are found in three separate but generally overlying Jurassic sandstone producing horizons, the Kimeridge, Erskine, and Pentland sands.
A multi-discipline project team consisting of reservoir, production, drilling and facilities engineers was set up to progress the development concept. Specific well design principles were adopted and an iterative approach was used to produce a robust and reliable drilling and completion design that is compatible with the overall development concept and provides reliability on a NNMI in HPHT service conditions.
Jackup drilling will commence over the platform jacket which will be installed over an existing sub-sea appraisal well in the spring of 1996 (Fig. 2). Two wells will be predrilled through the jacket structure and suspended prior to the platform topsides deck being lifted into place in 1997. The appraisal well will be tied back and wells will then be completed ready for commercial gas export in late 1997. Further wells will then be drilled and completed as required.
Platform Concept Outline
By North Sea standards Erskine is a marginal field (335 MMSCF gas, 66.5 MMBBL oil). The resultant development has revolved around this in order to make development economic. The following lists the main features of the development.
1) Simple NNMI 12 slot wellhead platform providing unprocessed multiphase fluids export to host platform with a projected field life of 15-20 years.
2) Platform design and slot layout which enables access to all slots for cantilevered jack-up drilling in the 300 ft water depth.
Well Design Considerations
Primary considerations for the well design are:
1) Reservoir fluids containing H2S and CO2. Reservoir pressure +/-l4,000 psi, temperature 350 F, initial surface shut-in pressure 10,600 psi (Table 1).
2) Design flow rates required from each well will be up to 60 mmscf/day of gas.
3) The wells that are initially completed as Pentland sand producers will water out and require to be plugged back and recompleted as Erskine sand producers. Due to availability and cost of large jack-up rigs, this requires to be a rig-less operation.