Variations in Formation Water Compositions in a North Sea Oil Field: Trace Waters in Oil

Abstract

Studies of trace waters in oil have direct bearing on our understanding of the origin and evolution of the brines in sedimentary basins. As part of a project to predict carbonate scale formation in oil fields, over the last year we have monitored the changes in the composition of trace water in oil from a producing well located in the North Sea. Using a technique developed at Reading, we are able to analyse trace waters present in the oil even when a separate aqueous phase does not occur. We have been able to identify two distinct mixing trends in the waters implicating the existence of at least three different components. History of water production The well has shown a systematic trend of increasing proportion of water in the total produced fluids (watercut). During the first four months of production, the oil contained less than 0.5 wt.% water whose chloride content increased from 22,000 to 95,000 ppm. Two weeks later, the water-cut jumped to over 5 wt.% and has slowly increased ever since.