ABSTRACT: The Role of Capillary Forces in Diatomite Reservoirs

SALOTTI, CHARLES A., and JAMES R. WOOD, Michigan Technological University, Houghton, MI

Oil exploration and production in the conterminous United States is a mature industry and most domestic production involves secondary and tertiary recovery practices to increase reservoir sweep efficiencies.

Fluid movement in a reservoir can result from: (1) gravity, (2) imposed external pressure, and, (3) capillarity. The importance of capillarity has been largely ignored in reservoirs in which surface-active forces, rather than buoyancy, control the fluid-rock interactions. In reservoirs dominated by surface tension, the size, composition, and geometry of the solid porous media control the fluid micropressures and determine the distribution and variation in fluid saturations.

The potential economic importance of large surface area dominated reservoirs requires that increasing emphasis be directed toward a quantitative understanding of such systems. The Monterey Formation, a California upper Miocene diatomite, contains over one billion bbl of oil. Porosity within parts of the Monterey Formation reach 50% with 50% oil saturations, and matrix permeabilities commonly between 1 to 5 md. It provides an unusual opportunity to study capillary forces, fluid saturations, rock mechanical properties, and especially the changes in surface area and fluid-fluid interfaces as a function of temperature, reservoir, and fluid compositions in a system dominated by surface tension.

During burial, the diatomite undergoes a low-temperature transformation from opal to opal CT to a-quartz. These changes induce fundamental modifications in the solid-fluid relations, particularly those involving capillary forces. These changes and their interrelations will be described. They afford an initial understanding of surface-tension dominated reservoirs and relate to exploitation strategies.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)