Abstract

Produced water analyses from the Birch Field, UK North Sea have been interpreted and combined with simulation results to explain the causes of changes in produced water compositions over time.Preliminary conclusions are that two formation waters are present in the oil leg, both trapped at the time of oil emplacement.Lower salinity formation water has been expelled from the Kimmeridge Clay Formation (KCF) and dominates shallower sections of the reservoir.Higher salinity formation water is thought to be ancient Brae aquifer water and dominates the deeper sections.Some lateral variability in formation water compositions is evident.Produced water from individual wells is a mixture of the lower salinity formation water, higher salinity formation water and injected water.Trends in produced water compositions over time reflect a relative decrease in formation water production and increase in injection water production.Depending on the constituent, reactions occurring as a result of injection of water into the reservoir also affect the composition of produced water.The results have challenged previous concepts relating to water production at Birch and will be considered in scale management plans in future.They can also be used to constrain reservoir simulations, to aid enhanced oil recovery decisions and to provide more reliable tracking of injection water and formation waters entering the production wells at Birch.

More generally, this study has demonstrated the importance of evaluating produced water analyses as early as possible after water breakthrough.Integration of reservoir simulation studies with the interpretation of produced water analyses can provide information that benefits scale management, STOIIP calculations, reservoir models, and tracking of injection water as well as providing analogue information that can help reduce uncertainties associated with the development of deep water and marginal subsea fields.

Introduction

The composition of produced water is affected by many factors.The more important of which include:

  • The different types of water present in the reservoir (eg formation water from the oil leg or aquifer, injection water, residual drilling mud filtrate fluid, etc)

  • The compositions of these waters, and their proportions in the produced flow.

  • Reactions occurring between injection water and formation water or reservoir minerals.

  • Fluid flow conditions in the reservoir.

Produced water analyses are often obtained regularly for monitoring purposes and their interpretation can provide information on each of these factors.This information has a variety of uses including the identification of scaling in wells[1], constraint ofreservoir simulations[2], understanding the source of reservoir souring[3], identification of injection water breakthrough[4], locating the point of water production in wells[5], and identification of ‘water swept zones’[6] to name a few.

This study has involved the interpretation of produced water analyses from the Birch Field, UK North Sea.The stimulus for the work occurred when unusual (for the field) low salinity water was produced from a well that had just been brought on line after being shut-in for several years.Having established the low salinity was not an artefact, we wanted to understand its source to assess implications for scale mitigation during future production from the well.Produced water analyses for the field were interpreted to determine whether there was evidence for production of this unusual water as a mixture with other waters in the past.Ultimately, this activity helped us determine the source as being a perforated section that had not previously been thought to be contributing significant flow to the well.However, our interpretation also provided a wealth of additional information relevant to other activities on this field.This paper summarises the study, the additional information it revealed, and the benefits to other field activities including future scale management.We also describe a promising method used in this study to estimate the composition of formation waters and to determine the distribution of types of formation water in the reservoir.This involves integration of reservoir simulation results and produced water analyses.

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