Pore Size Characterization of Vuggy Carbonate Material

Vuggy carbonate material pose significant challenges when it comes to characterization at the pore scale level. The material under study is a highly porous and connected vuggy system with vugs in the millimetre range and permeabilities correspondingly in the tens of Darcy range. Depending on vug accessibility and connectivity, pore size distributions from capillary pressure based methods such as mercury capillary injection curves can greatly underestimate the larger elements of the actual pore size distribution, even when viewed as a pore throat distribution. Comparison of results from capillary pressure curves and pore size distributions obtained through a combination of NMR relaxation and diffusion measurements show that the mercury capillary pressure curves, are in fact, quite insensitive to the vuggy portion of the pore space and mainly reflect the properties of matrix porosity. The NMR data clearly show a partitioning between vuggy and matrix porosity, and thus comprise a valuable tool for pore size characterization for such materials. INTRODUCTION Carbonate formations comprise more than fifty per cent of the worlds recognized petroleum reserves. Compared to sandstones, carbonate reservoirs show a larger variety when it comes to petrophysical properties both on the core scale level as well as on the reservoir scale level. In turn this makes measurements and subsequent interpretation of special core analysis data more challenging than for similar sandstone cases. One of the issues with the larger heterogeneity of carbonate samples is that the representative elementary volume to a large extent relies on the property of interest as well as the method of measurement. This is in particular true for dual porosity systems such as vuggy materials. Vuggy facies make up a significant part of the carbonate reserves, meaning that a better characterization of such is of great importance. Vuggy systems are often divided into two subcategories, labelled as separate and touching vugs respectively. The categorization depends on how the vugs are connected. In the first case vugs are thought to communicate only through the matrix whereas in the latter case the vugs can form a continuum throughout the media.