Interpretation of produced water analyses is a relatively inexpensive method of obtaining a wide range of information that can be used to aid scale management, constrain the reservoir model, provide information on barriers to flow and zones of connectivity in the reservoir, and identify zones of water production in wells. In this paper we report on the interpretation of produced water analyses to help understand why unusual low salinity water was produced from well Z3, a sub-sea well in the Birch Field with two production zones (Brae Conglomerate and Ryazanian Sandstone), when production was re-started in March 2004 after a three-year shut-in period. In this case, of particular interest were the implications for future scale management on the well.
We have shown that the low salinity water is likely to be formation water from the Ryazanian Sandstone. Production of this water from well Z3 is declining over time, which may reflect the limited extent of the Ryazanian Sandstone, a nearby barrier to flow or calcium carbonate deposition adjacent to the well during pressure drawdown. Future water production from well Z3 is expected to be dominated by production from the Brae Conglomerate with production from the Ryazanian Sandstone only being important after lengthy shut-in periods. Current squeeze treatments are based on pre-development scaling predictions and have been designed to prevent moderate barite deposition. These treatments are likely to mitigate the mild barite scale deposition predicted to be associated with future production of water from the Brae and Ryazanian intervals. As the future scaling potential is predicted to be much less than originally planned for, the possibility of reducing the current MIC or discontinuing future squeeze treatments is being considered. Further work is required to fully assess the carbonate scaling risk associated with mixing of these waters but there are currently no indications that carbonate scale is forming in the well.
This study has demonstrated the value of interpretation of routinely collected produced water analyses in that it helped us understand and act on significant changes in produced water compositions from a sub-sea well without resorting to an expensive well intervention.
Interpretation of produced water analyses is a relatively inexpensive method of obtaining a wide range of information including the time of injection water breakthrough, the proportion of injection water in produced flow, evidence for the presence of barriers to flow and zones of connectivity in the reservoir, the source of produced water, estimated compositions of formation water, evidence for reactions occurring in the reservoir during waterflood, fractions of produced water derived from different production intervals, and evidence for scale deposition in the well[1–6]. Generally, interpretation of produced water analyses has been confined to fields with dry wellheads where samples have been obtained from individual wells, occasionally completed with two or more production zones. Interpretation of produced water analyses from sub-sea fields is more complicated in that many of the samples obtained are usually mixtures of water produced from several different zones in each well, several different wells and sometimes more than one field.