Wu, Tao¹, Robert R. Berg¹ 
(1) Texas A&M University, College Station, TX

ABSTRACT: Capillary-Pressure Based, Water Saturation Models for Heterogeneous Sandstone Reservoirs

Knowing the distribution of reservoir-fluid saturations is critical to formation evaluation. Capillary pressure data have been widely used to calculate fieldwide water saturation and hydrocarbon height relationships from core and log information. Most of the commonly used methods are based on empirical correlations among the following parameters: capillary pressure (or converted to height above free water level), water/hydrocarbon saturation (or bulk volume of water/hydrocarbon), porosity, and permeability. One problem is that empirical coefficients have to be adjusted with laboratory measurements from field to field. Furthermore, to be statistically meaningful, a large number of measurements are required. However, laboratory capillary-pressure measurements are usually sparse. In addition, most present techniques work well for homogeneous reservoirs, but are not satisfactory for heterogeneous reservoirs. In this study, several popular saturation-height methods have been reviewed. A new capillary-pressure model is proposed. All the parameters of the model can be related to petrophysical properties, such as porosity, permeability, displacement pressure, and irreducible water saturation. By dividing a heterogeneous reservoir into several flow units according to lithology and physical properties, synthetic capillary pressure curves can be generated for each flow unit. A continuous height-saturation for the whole reservoir can then be established, which makes it possible to estimate free water level (FWL), or predict saturation in the reservoir if the FWL is known. Other applications of the model include estimating permeability when water saturation is known, and calculating hydrocarbon-in-place.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.