UK SOUTHERN GAS BASIN Gas price crash forces rethink of long-term field development

Low gas prices are nothing new in the UK. They held back development of Ranger's Anglia Field north of Norfolk, which was discovered originally in 1972 but was not brought onstream until 1991.

Difficulty of access to the reservoir's various gas pockets was another factor in the delay, but thanks to innovative horizontal drilling techniques, the project has since progressed as planned. The six-slot tripod platform in the more easily producible eastern sector (in block 48/19b) has since been joined by two subsea tie-ins, with a third subsea well possible next year to tap the field's northwest extension.

Further development is possible through neighboring block 48/22 to the southwest, where Ranger recently bought a 75% working interest off Enterprise. A known structure has been drilled here already, possibly containing up to 60 bcf, which could be developed eventually via a 20 km tieback to the Anglia platform.

The company plans a further 3D shoot next year around Anglia, and is keen to gain access to more acreage in adjoining blocks through farm-ins. At this stage any extra gas discovered would probably be used to extend plateau production from Anglia: Ranger's share of the field's known reserves of 235 bcf are committed to National Power for the next 20 years.

Reservoir model

Current output is around 58 mcf/d, exported via Conoco's LOGGS system. In the early days of Anglia's fast-track development - 13 months up to 1991 - Ranger was setting new records in the UK for long-reach horizontal wells. The first one in 1990 was "physically a lot easier than anyone thought possible," according to the company's UK managing director Robert McCrackin. "We have built up a sophisticated model of layers in the Anglia Field to get into the sweeter spots and duck in and out of the tighter spots."

This model was used to bring onstream the western part of the reservoir in 1993, through a subsea well, B1. Another subsea well in the same region, B2, was successfully tied back a year ago using pre-packed screens to cater for sand production.

This year de-watering and de-sanding equipment have also been retrofitted to the tripod platform, managed by Amec under an EPIC contract. To cut down production downtime, hot work offshore was kept to a minimum. Amec also designed the packages so that they could be lifted from the barge by the platform's 8-ton crane, avoiding the expense of a heavy lift crane vessel.

The wellhead protection structure installed on B2 last year also houses spare slots for two further remote wells. This compensates for the limited number available on the platform, should a plethora of new discoveries emerge around Anglia, but as McCrackin points out, "what is a well these days? How many slots do you need to develop a sizable gas pool with muiltilateral techniques?"