Fundamental controls on flow in carbonates: an introduction

The challenges posed by carbonate reservoirs today have been recognized for decades. ‘Old’ problems include the prediction of heterogeneous carbonate rock properties in the subsurface, the variable wettability of carbonate rocks, and the uneven sweep and early water breakthrough that commonly result from the presence of fractures, karst and/or other heterogeneities providing high-permeability flow paths (Bulnes & Fitting 1945; Craze 1950). While recent shifts in the industry have diverted attention to unconventional resources, conventional carbonate reservoirs remain the largest class of oil resources for the planet. Their recoveries, however, remain relatively low overall (Montaron 2008; Burchette 2012). While new technologies offer a deeper understanding of near-wellbore properties and processes, there remain large uncertainties in the distributions of flow velocities and the nature of flow paths between wells. These uncertainties arise from our inability to delineate geological features over a wide range of length scales, to represent them appropriately in flow simulations, to quantify their impacts on flow, as well as their impacts on evolving multiphase fluid distributions. While we wrestle with suitable proxies for geological heterogeneities that cannot be deterministically resolved in subsurface data or explicitly represented in field-scale reservoir models, many in the industry are forced to rely on long-established simulation technologies that have not kept pace with developments in modelling. In the absence of fully deterministic characterization and representation of the reservoir in our flow-simulation models, we need to mitigate the risks associated with uncertainty. Experiments designed to improve our ability to predict in the subsurface and to explore the range of possible responses can help. Clearly, there is an opportunity to pursue carefully designed subsurface experiments to test hypotheses for flow behaviour. Shallow subsurface experiments have already shown that, even between very closely spaced wells, it can be extremely difficult to define precise flow paths …