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3D imaging and characterization of the pore space of carbona

The ability of a rock to store and flow fluids is depen- dent upon the pore volume, pore geometry and its con- nectivity. Carbonate rocks are inherently heterogeneous having been laid down in a range of depositional envi- ronments and having undergone significant diagenesis. They are particularly difficult to characterise as the pore sizes can vary over orders of magnitudes and connectiv- ity of pores of different scales can impact greatly on flow properties. For example, separate vuggy porosity in a underlying matrix pore system can increase the poros- ity, but not the permeability and lead to large residual oil saturations due to trapping in vugs. A touching vug net- work can have a dramatic effect on permeability and lead to higher recoveries.
In this paper we image a set of carbonate core material from outcrops and reservoirs in 3D via micro Computed Tomography (µCT). The morphology of the pore space from different core material exhibits a broad range of topology and connectivity. Images at lower resolution (larger sample size) allow one to deduce the size, shape and spatial distribution of the (disconnected) vuggy poros- ity. Higher resolution images (down to 2 micron resolu- tion) on subsets of the core allow one to probe the 3D intergranular porosity. The delineation of regions with submicron porosity is achieved via a differential con- trast technique in the µCT. Experimental MICP measure- ments performed on the imaged core material are in good agreement with image-based MICP simulations. These results indicate the quality of the imaging method allow- ing one to probe the spatial distribution of the vuggy / macro / micro porosity contributions across several or- ders of magnitude in scale.
High resolution numerical simulations of single phase flow and solute transport are undertaken on the resolved digital image data. A hybrid numerical scheme is devel- oped to include the contribution of microporosity to the overall core permeability. These results show in many cases, the dominance of a few flow paths in dictating the permeability of the core material. The role of mi- croporosity in the flow fields is illustrated via 3D visual- isation, measurement of local flow velocities and solute transport results.
Pore network models generated from the images illus- trate the large variations in topology and geometry ob- served in carbonate samples. Both the visual appearance and quantitative details of the pore network show dra- matic differences. Resultant two phase imbibition resid- ual saturations are shown to be strongly dependent on the different topological and structural properties of the pore network. Laboratory measured rate dependent residual saturations for clastic and carbonate cores are compared with numerical simulations with encouraging results. These results illustrate differences in the petrophysical characteristics of the different cores when classified by core descriptive parameters (Lucia, 1999), porosity - per- meability, MICP (Skalinski et al., 2005) and relative per- meability (Hamon, 2003). 3D imaging and analysis may assist in the integration of different rock-typing methods.
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