Capillary Heterogeneity Trapping within the Captain Sandstone - a Core to Field Scale Study

Abstract:

The aim of this paper is to understand the effect of natural rock heterogeneities on capillary trapping over different length scales, during CO2 sequestration. A multiscale approach is applied, combining petrophysical characterisation and numerical modelling, to create a physics-based representation of capillary heterogeneity trapping at the field scale. Previous research focusing on the impact of capillary heterogeneity during drainage is extended to imbibition processes, accounting for relative permeability and capillary pressure hysteresis. Two-region models have been created using a commercial reservoir simulator to study the physical basis of capillary heterogeneity trapping. The parameters influencing the length scale of capillary heterogeneity trapping are explored to define an improved dimensionless capillary number, to develop a better understanding of the controls on capillary heterogeneity trapping. Additionally, the parameter space in which capillary heterogeneity trapping dominates pore scale trapping is studied.

The insights from the two-region models are applied to the Captain Sandstone, to study the effect of natural rock heterogeneities on capillary trapping within a target storage site. A comprehensive database has been produced through characterisation of 48 core plugs over a 65m interval of the Captain D Sandstone, Goldeneye formation, UK North Sea. Steady-state core flooding experiments using medical x-ray CT provide a detailed characterisation of continuum multiphase flow properties, including residual trapping characteristics, over cm scales. The data are used to create 1D fine models of the Captain sandstone to evaluate capillary heterogeneity trapping during imbibition. Our results demonstrate that heterogeneity results in a higher initial saturation from which imbibition occurs hence the residual saturation is naturally higher relative to the homogenous case. In addition, the residual saturation is higher than predicted by a typical residual trapping model because capillary pressure barriers trap CO2 at saturations greater than expected from residual trapping processes alone.