MULTIPLE-LENGTH-SCALE STUDY OF THE ORIGIN OF CAPILLARY HETEROGENEITIES IN SANDSTONE

Understanding multi-phase flow dynamics in rocks and quantification of the underlying correlated capillary heterogeneities plays a crucial role in various research areas, ranging from Carbon Capture and Storage, and Enhanced Oil Recovery to vadose zone environmental remediation. Recent developments in micron-scale X-ray CT give insight into the morphological composition of rocks and, combined with results from medical CT imaging, provide a window into micro-morphological features controlling mm- to cm-scale flow properties. Furthermore, used as input for different modeling approaches, they allow simulation of various pore space properties, such as porosity, permeability, capillary pressure and relative permeability.

The object of investigation in this study was a heterogeneous sandstone from the Heletz site in Israel (provided by courtesy of the MUSTANG project). Fluid flow and rock properties of the sandstone were evaluated at several length scales, ranging from permeability measurements on the core scale, capillary pressure measurements on the cm-scale, porosity and capillary pressure measurements on the mm-scale, down to micro CT imaging of morphological features on the micron-scale. The results of this study are twofold:

1) Statistical evaluation of the results at different scales allowed us to find correlations between mm-and cm-scale multi-phase flow properties and micron-sized morphological pore space features. In particular, we were able to identify capillarity-controlling rock and pore morphologies and quantify their impact on local CO₂ saturations measured on the mm-scale, which confirms the dominantly structural origin of capillary heterogeneity. We will present correlations and discuss implications on multi-phase fluid flow in sandstone.

2) In addition, we present a new numerical approach that can provide capillary pressures on the cm-scale without complex simulation, solely using the rock morphology provided by micro CT. Our approach requires neither solving conservation equations, nor depends on the extraction of pore-networks or simplification of process-controlling physics. We present the methodology underlying our new approach and demonstrate its potential at the example of a capillary pressure curve of the Heletz sandstone using respective 3D micro CT images.