Modelling of Drainage Capillary Pressure: Using the Modified Carman-Kozeny Purcell Model to Identify the Type of Pore Throat Distribution Inherent in Laboratory Data

Capillary pressure relationships (drainage) involve key SCA parameters measured in the laboratory. Reservoir engineers use capillary pressure relationships in reservoir engineering calculations and dynamic reservoir simulation and petrophysicists use such relationships in saturation-height modelling. The objective of this paper is to describe a novel technique to identify pore throat or grain size distributions from capillary pressure profiles. Capillary pressure, laboratory data are traditionally obtained from three different types of experiments: centrifuge, porous plate and mercury injection. The modified Carman-Kozeny Purcell (MCKP) model may be used in matching various laboratory results. Initial application of the MCKP model involved a standard matching procedure without differentiating according to the type of pore throat or grain size distribution. In this paper a refined, more powerful analysis technique is described in detail where matching of any laboratory data with the MCKP model is obtained in model space, showing various characteristics. Characteristic profiles observed in model space can be directly attributed to the type of pore throat or grain size distribution: homogeneous (narrow), broad (poorer sorting) and bimodal (two distinct distributions), or a mixture of distribution types. Comparison of model generated profiles with laboratory curves indicate a high degree of congruence, typically R2 > 0.99, consistently better than any other capillary pressure formulation. The technique is also able to pick up artefacts, such as a sample fracture or laboratory procedural issues. It is shown how the MCKP model may be used in predicting capillary pressure relationships from basic parameters by building a framework based on end-point correlations derived from available laboratory data. The paper presents various examples and a detailed case history for the Bayu-Undan gas field, covering various pore structure and diagenetic characteristics.