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Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain
Confocal Microscopy — A New Way to Model Carbonate Porosity in 3D
(1) Schlumberger, Dhahran, Saudi Arabia.
(2) Schlumberger, Cambridge, MA.
Carbonate rocks have complex pore systems, ranging in size from caverns to micropores. 3D models of fine-scale porosity (<1 mm) are generally made using X-ray micro-Computed Tomography (CT) scans, with resolution limits on the order of a few microns. Transmitted laser scanning confocal microscopy (LSCM) and multi-point statistics (MPS) provide an alternative, high-resolution (0.1 μ) method to build 3D digital rock models of appropriate size and shape for pore-network construction and flow modeling.
Confocal microscopy uses point illumination and a pinhole placed in front of a detector to eliminate out-of-focus information. Because each measurement is a single point, confocal devices perform scans along grids of parallel lines to provide 2D images of sequential planes at specified depths within a sample. In this study, LSCM is applied to rock samples impregnated with fluorescing epoxy. Reflected light intensity indicates the physical location of pore spaces. Samples are standard thin sections (30-μ thick), or rock chips of any thickness. Samples are composed of rock and epoxy, or they may be pore casts where the rock has been removed by acid.
Reflected light is absorbed and scattered by the material above the focal plane, therefore the depth of penetration of LSCM is limited to 10-250 μ in rocks, and 500 μ in pore casts. LSCM data stacks commonly have flat aspect ratios, for example, 20 μ thick by 210 x 210 μ or larger in area. To build valid 3D models of physical pore systems, the depth of penetration should be at least 2 typical grain diameters. Therefore, a grain-size limitation exists for LSCM imaging.
3D digital rock models constructed from stacked LSCM scans are used as training images for multi-point statistical (MPS) modeling. MPS creates conditional simulations that use known results as fixed or “hard” data. We use MPS to create thick (mm-scale), high-resolution (better than 1 μ) digital rock models, suitable for pore-network modeling and/or flow simulation. Enlarged models avoid boundary effects that compromise flow-modeling results. MPS models can be used to address the question: What model size is needed to capture heterogeneity within a given rock type? Because MPS models are unconstrained by size or shape, we can use them to test the concept of representative element volume (REV). REV is the smallest volume that can be modeled to yield consistent results, within acceptable limits of variance of the modeled property, for example, porosity.