Special core analyses on sandstone samples from a North Sea oil reservoir showed unusually large variations in absolute and effective permeability. Abnormally high endpoint relative permeability to oil was observed during primary drainage oilfloods. Absolute air permeability was in many instances several times higher than the absolute water permeability. In order to improve the understanding of the causes for this particular behaviour, a comprehensive laboratory core study was carried out.
Comparative studies on several suites of parallel samples showed that air permeability and primary drainage endpoint oil permeability were strongly dependent on experimental procedures. Miscible cleaning and critical point drying are preferred to conventional methods. Establishment of initial water saturation by oil flooding gave less change in permeability than centrifuge and porous plate methods. Water permeability was less sensitive to experimental procedures, but significant changes were still observed. A reversibility in permeability was noted with many samples; decreases were often observed when going from air or oil saturated plugs to water saturated plugs, whilst increases occured when going the opposite way.
The rock material was characterized with respect to mineralogy, morphology and pore size distribution. The study showed that the permeability effects seem to be dependent on the content of filamentous illite. Analyses of fluid effluents showed significant particle migration only in cases of two-phase flow.
The study demonstrated that control over experimental effects is important and demanding, but still achievable, for this type of sensitive rock. It contributed to an improved data basis for further reservoir evaluation and better understanding and procedures for future core analysis on this rock type.