--> ABSTRACT: Consistent Small-Scale Porosity Contrasts in Reservoir Sandstones and Their Relevance to Reservoir Characterization, by E. L. Etris and R. Ehrlich; #91030 (2010)

Datapages, Inc.Print this page

Consistent Small-Scale Porosity Contrasts in Reservoir Sandstones and Their Relevance to Reservoir Characterization

E. L. Etris, R. Ehrlich

Although grain size may gradually increase or decrease vertically through a sandstone reservoir, porosity need not change appreciably. Yet porosity does vary greatly at all scales. Large-scale changes, commonly measured by wireline logs, are used in assessing not only reservoir capacity, but also (not always correctly) reservoir efficiency. Systematic porosity variation at smaller scales, however, is usually not considered in reservoir modeling or assessment because it is thought to be difficult to measure, insignificant compared to variation at larger scales, and difficult to simulate in numerical models. Our data indicate that there is a common mode of small-scale porosity contrast in reservoir sandstones, and that this contrast is important in understanding the behavio of multiphase flow in such rocks.

Using image analysis procedures, we have measured porosity between adjacent laminae in 50 thin sections taken from four contrasting reservoir sandstones (Kekiktuk, Rotliegendes, Wilcox, and Satun), and have found consistent small-scale contrasts in porosity between laminae. In all but a few samples, porosity alternates, with contrasts of a factor of three or more.

The bimodal distribution of porosity in laminated sandstones means that few if any laminae contain the porosity measured from wireline logs. Our data indicate that permeability, pore sizes, and throat sizes also alternate--a fact that can affect the localization of residual oil. Although this phenomenon increases the complexity of the sandstone system, the simple alternation of porosity allows ease in modeling.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.