Integrating New Technology - The Development of the Barque Extension Structure
Authors: Neate C.J. 
Published: December 2007

Integrating New Technology - The Development of the Barque Extension Structure.

Abstract
The Barque Extension PL well head jacket recently came on stream with the opening up of well PL01 on 31/10/95. This was a successful culmination of the efforts of the Southern Field Unit teams that designed and executed this project on time and under budget. The reservoir will supply gas to the Bacton plant over the next 25+ years forming an important part of Expro's long term gas supply from the Southern North Sea basin. Review of the evolution of the subsurface development plan highlights the effective integration of innovative technology to improve Project economics.

Geological Setting
Barque Extension is situated in the Greater Sole Pit area of the Southern North Sea some 50 miles NNE of the Bacton gas plant. (Figure 1.) The reservoir is a fault bounded southward dipping structure in the Lower Permian Rotliegend sands. These aeolian sandstones form the main reservoir body, consisting of dune, interdune and sabkha deposits. The reservoir is subdivided based into the upper A-sand unit (mediocre reservoir properties, but carrying a large part of the reserves), the underlying B-sand (dune slipface sands with better reservoir properties allowing much higher production rates) and the basal C-sand unit (the poorest properties but only occasionally above the GWC). Palaeo-burial at double the present depth of + 8000 ft tvss has reduced the porosity and permeability, resulting in average in-situ effective matrix permeabilities of only 50 D. Faulting and folding during the burial and subsequent inversion of the reservoir resulted in a complex natural fracture system of NW-SE striking faults. Although the fracture swarms are far apart, they provide a good connection between dune bodies, thus improving transmissibility in both vertical and horizontal directions.
Field Infrastructure and Operating Philosophy There are existing fields in the area, which supply gas to the Clipper complex, which is the offshore end of the Sole Pit Offshore Transportation System (SPOTS), that transports the (wet) gas back to the Bacton shore terminal. See Figure 2. The Clipper complex is one of Shell/Esso's 3 nodal platforms in the Southern North Sea that are permanently manned, with the other satellite platforms operated as Normally Unmanned, with only emergency overnight accommodation. Visits to the Normally Unmanned Installations (NUI) are performed by intervention teams as required. Against the background of this operating philosophy the platform for Barque Extension structure was designed to be a NUI tied back to the SPOTS export system.

History of Well Planning
The original development concept (1988) for the Greater Sole Pit area assumed that the majority of reservoir potential would result from direct intersection of deviated wells with natural fractures and indirectly through massive hydraulic fracture treatments. It was estimated that up to 40% of the wells would fail to intersect fractures even after stimulation and only produce at low rates from the matrix. Early deviated and fracture stimulated wells displayed poor inflow performance and the approach was therefore changed to horizontal wells that could increase the probability of intersecting the slipface sands and open fracture networks.
The Development Plan for the Barque Extension structure (Oct. '93) therefore called for 8 single horizontal wells, with a projected drilling cost of some 60 million. The plan has matured further, being reduced to three single horizontal and three dual lateral horizontal wells. The projected drilling cost is now of the order of 37 million, still accessing the same reserves. See Table A.
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