Log data from a North Slope field indicated particularly high initial water saturations in the major reservoir zone, often in excess of 50%. Despite this, well tests yielded dry oil from the producing intervals, implying that the mobility of the connate water was negligible. A pore fluid microvisualisation technique was used to examine the initial distributions of fluids within core samples, in order to reconcile the apparent inconsistencies in the field observations.

The amount and distribution of initial water was controlled by the proportion of micro-porosity. Connate water resided primarily within chert, although some was associated with pore-filling kaolinite. This clay mineral was partly water filled and partly oil filled, depending upon the capillary pressure applied to the core sample to acquire Swi. Very little water was held in a conventional manner as thin films on quartz surfaces. Core flow tests confirmed that mobility of water was essentially zero, even at the highest water saturations.

The technique was also used to examine the distributions of fluids at waterflood residual oil saturation. The rock was predominantly water wet in character and classical 'snap-off of residual oil was observed, with the oil being trapped as isolated 'blobs' or as multi-pore 'ganglia'. Saturation and distribution of oil after waterflooding was found to depend upon the flow rate and flow regime used.

Microvisualisations provide a unique insight into fluid distributions at Swj and Sorw. For this reservoir renewed confidence in saturations derived from well logs was realised.

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