THE ROLE OF GEOLOGY IN THE BEHAVIOR AND CHOICE OF PERMEABILITY PREDICTORS

For effective flow-simulation models, it may be important to estimate permeability accurately over several scales of geological heterogeneit y. Critical to the data analysis and permeability prediction are the v olume of investigation and sampling interval of each petrophysical too l and how each relates to these geological scales. We examine these is sues in the context of the As Sarah Field, Sirte Basin, Libya. A geolo gical study of this braided fluvial reservoir has revealed heterogenei ty at a series of scales. This geological hierarchy in turn possessed a corresponding hierarchy of permeability variation. The link between the geology and permeability was found to be very important in underst anding well logs and core data and subsequent permeability upscaling. We found that the small scale (cm) permeability variability was better predicted using a flushed-zone resistivity, R(xo), tool, rather than a wireline porosity measurement. The perm-resistivity correlation was strongest when the probe permeabilities were averaged to best match th e ''window size'' of the wireline R(xo). This behavior was explained b y the geological variation present at this scale. For the larger scale geological heterogeneity, the production flowmeter highlighted discre pancies between flow data and averaged permeability. This yielded a la yered sedimentological model interpretation and a change in averaging for permeability prediction at the bedset scale (ms-10 x ms).