ABSTRACT

The scaling potential of Miller wells is widely accepted as being perhaps the severest in the North Sea, if not the world. This creates a unique chemical challenge, the aim of which has always been to extend the lifetimes of scale squeezes across all the wells. A new scale inhibitor has been deployed which has achieved this goal. The chemistry in question is a novel polymeric chemistry which contains multiple functional groups, including phosphorus tagging.

Two years were spent developing and extensively testing the chemical in the laboratory, and the resulting product was assessed against those submitted as part of an industry wide search. The new chemical is now deployed on all Miller wells and performance has been beyond all expectations. Two wells have already seen a doubling in the treatment lifetimes resulting in thousands of barrels of incremental oil production. This is particularly important for Miller, where total operational efficiency is paramount as the cessation of production date approaches.

This paper documents some of the initial laboratory work involved with the development of the new chemical but mainly dwells upon the field treatments on Miller wells A14(19), A17(04), A18(32), A21(02), A25(29) and A26(08), covering almost 50 squeeze treatments. The paper goes on to describe the management strategy as well as the approach adopted to determine the limit of the squeeze life. In every case the chemical has outperformed the incumbent in terms of barrels of water protected and total scale inhibitor efficiency. This outcome is unprecedented on Miller: No other new chemical has delivered such a dramatic and significant improvement in scale control economics.

MILLER FIELD BACKGROUND

The BP operated Miller field straddles Blocks 16/7b and 16/8b in the UK Sector of the North Sea, approximately 145 miles NNE of Aberdeen. The field produces gas (exported down the Miller Gas Pipeline to Peterhead) and oil (exported via the Forties Pipeline System to Grangemouth). Ten producer wells were drilled with first oil in 1992. Production plateau'd in 1993 at 140 MBOD and remained for approximately three years. Water injection was initially through six injector wells and began one year after first oil, peaking at 300 MBWD.

Steady decline in oil production has been apparent since 1997 being paralleled with high and sudden formation and seawater breakthrough. The Miller field has achieved beyond the expected recoverable reserves having produced a little over 340 million barrels of oil. Currently the field oil potential is 16 MBD and water production a potential 80 MBD. Cessation of production (COP) has been proposed as December 2006 and is based upon low flow trials.

The most noticeable aspect of Miller formation water is the barium concentration. At 650 ppm this presents a very harsh scaling regime in the presence of even minor seawater breakthrough. A complete water chemistry analysis has been summarised in Table 1 for the wells currently squeezed with the new polymer. During the production history however a wide range of barium concentrations have been detected at surface, a maximum being a little over 3,500 ppm. The water is otherwise of moderate salinity with moderate strontium, magnesium, and calcium ions. Seawater breakthrough has been observed on all of the producer wells resulting in barite (BaSO[4]) scale dominating over all others. This should not however detract from the potential for both celestite (SrSO[4]) and calcite (CaCO[3]) to form. It is clear that the control of water production and scaling potential is absolutely crucial to the field producing to the planned COP: even losing one well to scale will have a dramatic impact on the decline trends and late field life.

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