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Geological Society, London, Special Publications; 2004; v. 237; p. 99-113;
DOI: 10.1144/GSL.SP.2004.237.01.07
© 2004 Geological Society of London

Physical, Chemical and Numerical Models

Rates of reservoir fluid mixing: implications for interpretation of fluid data

Craig Smalley1, William A. England1, Ann Muggeridge2, Yafes Abacioglu3 & Stephen Cawley4

1 , BP, Chertsey Road, Sunbury, Middlesex TW16 7LN, UK smalleypc{at}bp.com
2 Department of Earth Science & Engineering, Imperial College, London SW7 2AZ, UK
3 , BP 200 Westlake Park Bvd, Houston, Texas 77079, USA
4 , BP, Dyce, Aberdeen AB21 7PB, UK

This paper highlights the benefits of using knowledge of the rates of fluid mixing in the interpretation of reservoir fluid data. Comparison of the time it would take for a fluid difference to mix with the actual time available for mixing to occur allows two significant advances over a purely statistical analysis of reservoir fluid data: (1) differentiation of a step in fluid properties, indicative of a barrier to fluid communication, from a gradient indicative of incomplete mixing; and (2) quantitative estimation of the degree of compartmentalization that can readily be adapted into models for prediction of reservoir production performance.

We review the existing equations that estimate the mixing times for three main types of variation in fluid properties (fluid contacts, fluid density and fluid chemistry). In addition, a new relationship for fluid pressure mixing is presented. In each case the relationships were validated by comparison with numerical simulation. The different fluid mixing processes were compared by applying the equations to a range of simple fluid scenarios in one simple reservoir description. This shows that mixing times for fluid mixing processes are diffusion > fluid density > fluid contacts > fluid pressure. For each scenario, the processes were analysed in terms of the volume of fluid that must move in order to bring the system to equilibrium and the drive for fluid mixing (pressure difference x permeability/viscosity). Perhaps surprisingly, there is an excellent linear relation between fluid mixing times (a) calculated from the mixing equations and (b) estimated from volume/drive. This indicates that fluid volumes and mixing drive are the main controls on fluid mixing times. This can be used to derive simple interpretation guidelines to estimate mixing rates even in the absence of quantitative modelling. A simple field case study demonstrates how this understanding of fluid mixing times can add value to the interpretation of reservoir fluid data.





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