This paper highlights the role which laboratory analyses together with in-situ monitoring surveys have helped in the scale management programme for the Dunbar Field, a very water sensitive reservoir situated in the South-Central part of the Shetland Basin, block 3/14. The major formation damage mechanism in Dunbar is that on production, the mixing of separate formation and injection brines drops out a sulphate scale within the near wellbore and wellbore itself. In order to solve this problem, TOTAL in conjunction with COREX (UK) Ltd., Aberdeen designed a laboratory programme to best mimic field scale treatments under reservoir conditions. Two separate water based scale inhibitors were assessed to evaluate their compatibility within the reservoir and identify any damaging mechanisms, which could result from their squeeze application. The interpretation of the permeability data obtained from the coreflood tests was aided by X.R.D. (x-ray diffraction), dry and cryogenic S.E.M. (scanning electron microscopy) and thin section analyses to determine the nature of the formation damage mechanism(s).

The coreflood test results suggest that the major control on permeability impairment was the retention of a compound believed to be derived from the scale inhibitor which occurred as a thin film, hemispherical globules with a moderate wetting preference and/or irregular shaped globules with a high wetting preference. Retention of this substance reduced pore volume and narrowed pore throats, hence reducing permeability.

In conjunction with the laboratory testing wellbore monitoring surveys were undertaken. Production and caliper logging tools were run to gather wellbore data and to give an indication of any decrease in wellbore diameter with time through sulphate scale drop out. Wellhead pressure and topside sampling of the connate/injection water mixture was monitored to determine if any scale precipitation was occurring and of what type.

The evaluation of the laboratory data plus the information obtained from wellbore monitoring surveys were complemented by the more global recommendations issued by a task-force which includes, in particular, TOTAL's partner Elf Exploration UK PLC. It is on this basis that Total based its decision on the most suitable downhole scale management programme for the Dunbar Field.

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