Geological Applications of Capillary Pressure: Taking the Mystery out of Basic Rock Properties

John G. Kaldi
Australian School of Petroleum, University of Adelaide, Australia

Capillary pressure is a basic rock property. Capillarity within the pores and pore throats of rocks controls the static distribution of fluids in the reservoir prior to production as well as on the remaining hydrocarbons after primary recovery. While porous-plate and centrifuge (air-brine or oil-brine) techniques are the most common analyses, mercury injection capillary pressure (MICP) is typically favored for geological applications, such as inferring the size and sorting of pore throats, especially in fine-grained rocks.

Capillary pressure concepts can be used to evaluate reservoir rock quality, expected reservoir fluid saturations, depths of reservoir fluid contacts, thickness of the transition zone and to determine seal capacity (height of hydrocarbon column that a caprock can hold before leaking). The relationship between the injection (drainage) and withdrawal (imbibition) curves approximates recovery efficiency and residual saturation during primary production. The height above free water level can be determined by comparing capillary pressure data to hydrocarbon shows and/or log-derived fluid saturations.

Capillary pressure data can also be used to distinguish reservoir rocks from non-reservoir rocks and pay from non-pay, based on wetting phase (water) vs non-wetting phase (hydrocarbon) saturations. Other applications of capillary pressure data include relating capillary pressure to permeability and relative permeability values and to investigate pore-level heterogeneity.

AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010