Abnormal Permeability Behavior of a North Sea Sandstone Reservoir

Abstract

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.

Two-Phase Flow in Porous Media with Slip Boundary Condition

“…It is usually assumed that the relative permeabilities 0 ≤ k r ≤ 1. However, there are several reports in the literature that explicitly show endpoint relative permeabilities k r > 1 (Odeh 1959;Dullien 1992;Anderson 1987;McPhee 1994;McPhee and Arthur 1994;Karabakal and Bagci 2004;Honarpour et al 2000;Arps and Roberts 1995;Ehrlich 1993;Mikkelsen and Scheie 1991). The term "endpoint" refers to the relative permeability at the saturation endpoints, S w,c and S o,r .…”

Relative Permeability Measurements: An Inter-Laboratory Comparison

“….Wiiku non-wetting phase penneabilitim greatir ihan 1 have been noted by McPhee (12) for strongly oil-wet carbonates but at matched viscosi~ratios, "iiidby Mikkekon et al (13)for North Sea sandstones, who ascribed this to reduced "lnterfacialtension at @h temperatures or the effects of I@-ram oilflood~g during"dyn.aniicdisplacement. l+oi grf=ter than 1, and they omsidired that this maybe due m the strong waterwetting characteristics of Ihe test cores.…”