Written by S. Q. Sun, and R. Sloan
Published by the Society of Petroleum Engineers
Sourced from the SPE Annual Technical Conference and Exhibition, 5-8 October, Denver, Colorado, 2003
Copyright: 2003. Society of Petroleum Engineers


The most prominent aspects of carbonate reservoirs are their heterogeneity, variable wettability and dual pore network, which collectively contribute to complex fluid flow and uncertainty in reservoir performance and recovery efficiency predictions. A review of ultimate recovery efficiency in 250 mature carbonate fields from around the world provides constraints for quantifying uncertainty in recovery efficiency predictions. Key determinants of ultimate recovery are fluid type, pore network, reservoir heterogeneity, drive mechanism and wettability. Development strategies and reservoir management techniques play crucial roles in maximising expected ultimate recoveries for given reservoir/fluid parameters. Five main fluid type/carbonate reservoir classes, with characteristic ultimate recovery distributions and controls are: (1) heavy and viscous oil reservoirs, in which recovery factor (RF) is controlled by fluid viscosity variations, field size and application of horizontal drilling, (2) karstic/fractured carbonate oil reservoirs, in which RF is controlled by fracture intensity, matrix permeability, wettability and drive mechanism, (3) conventional carbonate oil reservoirs, in which RF is controlled by reservoir connectivity, permeability and mobility ratio, (4) organic buildup oil reservoirs, in which RF is controlled by nature and size of organic buildups and diagenetic modifications, and (5) gas/condensate reservoirs, in which RF is controlled by aquifer encroachment and condensate drop-out. Examination of actual case histories reduces uncertainty in predevelopment recovery efficiency predictions and shows what is possible in new or old fields.

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