Digital core laboratory: Petrophysical analysis from 3D imaging of reservoir core fragments

Machined fragments of a number of core plugs have been analyzed using a high resolution X-ray micro-computed tomography (micro-CT) facility. The facility includes a system capable of acquiring 3D images made up of 2000 ³ voxels on core plugs up to 6 cm in diameter with resolutions down to 2 μm. The cores analyzed include a range of plug fragments from two reservoir sands. The cores exhibit a range of pore and grain sizes, porosity and permeability. Computational results made directly on the digitized tomographic images are presented for permeability, formation factor, resistivity index and drainage capillary pressure across a range of porosity. We show that data over a range of porosity can be computed from a single fragment. Where available, we compare the computations of petrophysical data on fragments to conventional laboratory measurements on the full plug. Permeability predictions from digital and conventional core analysis are consistent. It is shown that a characteristic length scale can be defined as a quality-control parameter for the estimation of permeability. Results for formation factor, drainage capillary pressure and resistivity index are encouraging. The results demonstrate a potential to pre dict petrophysical properties from core material not suited to laboratory testing (e.g., sidewall or damaged core and drill cuttings) and the feasibility of combining digitized images with numerical calculations to predict properties and derive correlations for specific rock lithologies. The small sample size required for analysis makes it possible to produce multiple measurements on a single plug. This represents a potential multiplier on the quantity of core data allowing distributions of petrophysical properties to be estimated meaningfully. We discuss the current limitations of the methodology and suggest improvements, in particular the need to obtain higher resolutions for tighter cores and to calibrate the simulated data to parallel laboratory core measurements. We also describe the potential to extend the methodology to a wider range of petrophysical properties. This development could lead to a more systematic study of the assumptions, interpretations and analysis methods commonly applied within the industry and lead to better correlations between petrophysical properties and log measurements.