Abstract
Vertical and horizontal permeabilities and Kv/Kh ratios are presented as a function of Thalassinoides burrow volume fractions for a burrowed mudstone interval of a major Middle East carbonate reservoir, using a new workflow that integrates physical and digital core analysis processes.
Thalassinoides-burrows are predominantly vertical and the burrow fill material may present tortuous and high permeability pathways through the low permeability mudstone matrix. Therefore, the burrow-modified carbonate mudstone formations require proper characterization for waterflood reservoir simulation and performance forecasts. The typical T-burrow diameter is five to ten millimeters, hence core plugs and whole cores are at the limit of proper scales to formulate the composite reservoir properties. The mudstone matrix itself defines a relatively tight rather isotropic carbonate matrix background. Quantifying the T-burrowed and mudstone composite system requires a new workflow integrating standard laboratory procedures and digital core analysis. The Kv/Kh ratios for these layers are a very important input to waterflood reservoir simulation.
The workflow utilizes detailed investigations from whole core to nanometer scales. The whole core three-dimensional CT image, being the largest rock extracted from the reservoir, is the basis for all up-scaling. The matrix and burrows are examined by subsampling the whole core and core plugs. The kernel properties for the matrix and burrows, such as porosity, permeability, mineralogy, volumetric fractions, connectivity, and texture are defined using multi-scale imaging methods, which include macro-CT, micro-CT and SEM. Porosity permeability trends derived for the matrix and burrows are presented and used in the upscaling processes. Up-scaled plug and whole core digital rock models are built and explored numerically to define composite formation properties and trends as a function of burrow volume fraction.
In summary: matrix permeability of the mudstone appears isotropic; permeability of the burrow fraction is one to three orders of magnitude larger than the matrix permeability; preliminary indications support higher burrow permeability with higher dolomite fractions; the matrix adds to the burrow network permeability connecting flow between dead-end burrows and composite formation Kv/Kh ratios are above one. A preliminary trend for Kv/Kh ratio as a function of burrow volume fraction is obtained. Overall formation burrow volume fractions presented herein are within the range presented in the literature.
