Scale control is a key production engineering challenge for Shell operated oil fields of the ETAP project (i.e. the Heron cluster). The formation waters are found to be high in sodium, calcium, barium and strontium contents. Computer modelling indicates that this chemistry, combined with the large pressure and temperature gradients anticipated over life of field, leads to a unique scaling environment throughout the production system which warrants unprecedented scale control measures in the North Sea.
Halite salt deposition is perceived as the main threat to production. Carbonate scales are expected to occur due to depressurisation of the produced fluids. The scaling potential is expected to increase significantly as gas breaks out from solution as CO2 can vent into a gas space. The scale control strategy adopted for this Skagerrak reservoir relies primarily on minimising water production, prevention of halite scaling by downhole dilution of the formation water with low salinity water; inhibition of carbonate and sulphate scales with chemicals. Complementary remedial removal treatments will also be carried out as and when required. The temperatures anticipated within the reservoir are at the stability limit for present scale inhibitors. Sulphate scaling will be obviated by removing sulphates from the sea water used as the diluent. Organic scales i.e. wax and asphaltenes are also anticipated - a subject not discussed in this paper.
The 1.6 billion Eastern Trough Area Project (ETAP) is located 150 miles east of Aberdeen and involves up to six oil fields, two condensate fields and one gas field. It is intended to develop those fields over a number of years, phasing them to make optimum use of the Central Processing Facility (CPF) which is situated over the Marnock field. Export oil will be piped to Cruden Bay through the Forties pipeline system and gas will go through the Amoco Central Area Transmission System (CATS) to Teesside.
Shell and Esso through their Shell Expro venture have equity in the Marnock Field and a share in the Central Processing Facility which will handle all the ETAP fluids and which will be operated by BP on behalf of the equity holders: BP, Shell, Esso Agip, Murphy, BHP and Mitsubishi. Shell Expro have 100% equity of three of the satellite fields, Heron, Egret and Skua, known collectively as the Heron Cluster. Shell Expro also has equity in the Scoter and Mirren fields which are nominated as future ullage fillers to the project.
The combined ultimate recovery for the seven ETAP fields is projected at some 400 million barrels of oil and 1.1 trillion cubic feet of sales gas. Peak production will be 210,000 bid with an average gas export volume of 360 million scf/d. The Heron Cluster will produce almost 30% of the oil and 23% of the gas. First production from Heron and Egret is scheduled for 4Q 1998 with Skua expected to come on stream four years later.
The Heron field was discovered in 1988 by well 22/30a-2 and is located some 17 km south-east of Marnock. The discovery well found both the Pentland and the Skagerrak formations to be oil bearing. The Skagerrak tested at a maximum oil rate of 14,000 bid. Appraisal well 22/30a-6 tested the Pentland at a stable rate of 580 b/d. The Skagerrak was water bearing and was production tested to acquire formation water samples. In 1994 the field was further appraised by the well 22/29–5 and showed better than expected reservoir quality, producing at rates in excess of 10,000 bid from a limited reservoir interval.