Reservoir characterization of carbonate pore systems with both movable and non-movable phases of hydrocarbon (HC) was improved by use of a modified, multi-stage core cleaning procedure. Analyses were completed on core plugs collected from a 60 m carbonate-dominated core with highly variable depositional textures and pore networks. Rock fabrics included grain-dominated carbonates of variable size and sorting, framework-supported carbonates with finer interstitial matrix, and mixed carbonate-siliciclastic sands with a higher degree of grain sorting.
A modified workflow for sample cleaning was developed in collaboration with Weatherford Laboratories. The multi-stage workflow describes more accurately in-situ reservoir quality in reservoirs with heavy HC fractions, and it is applicable to carbonate and siliciclastic reservoirs. For this study, samples were first cleaned with a lighter solvent to remove only movable fluids from pore networks. Following measurement of effective porosity and permeability, the samples then were cleaned by standard solvents to remove all soluble fluids. After final cleaning, total porosity and permeability was measured.
When cleaned for movable fluids only, core plug porosity is comparable with NMR wireline data that estimate porosity related to movable fluid phases. The standard cleaning workflow yielded total porosity that resembled more conventional log-derived total porosity from density and neutron porosity log data. The difference in the permeability measured between these two cleaning methods often varied by an order of magnitude.
This cleaning methodology resulted in a more accurate depiction of reservoir quality and improved validation of wireline log data. When integrated with other datasets to better document the controls on reservoir quality and porosity distribution, these results provided valuable insight into the differentiation between in-place and recoverable hydrocarbons in this heavy oil-dominated petroleum system.