Influence of micro-computed tomography image resolution on the predictions of petrophysical properties

Micro-CT scanning is a non-destructive technique that can provide three-dimensional images of rock pore space at a resolution of a few microns. However, these greyscale images cannot be directly input into simulators to predict flow properties; they require image processing to segment the solid and void space in the rock. Dynamic and static single phase properties can then be computed using the images directly or on extracted equivalent network models. In this paper, we study the effect of imaging resolution (five different voxel sizes ranging from 6-20 μm) of Clashach and Doddington sandstone on predicted single phase properties (porosity and absolute permeability) and network properties. Experimental data is used to validate the predictions. The results suggest that the computed porosity was largely independent of resolution and in good agreement with the measured value, while image resolutions of a few microns are sufficient to determine the permeability of a highpermeability rock such as Doddington but may not be sufficient for lower permeability samples. The topologically representative networks are sensitive to resolution, adding additional smaller pores and throats as the resolution is increased. This latter reason was confirmed by a network extraction analysis that indicated the average throat radius was 6 m, similar to the highest resolution used and insufficient to image all important features of the pore space properly. Introduction Advances in three-dimensional digital imaging have allowed direct visualization of the pore space of many rocks (Dunsmuir et al., 1991; Coles et al., 1994; Lindquist and Venkatarangan, 1999; Wildenschild et al., 2002; Coenen et al., 2004; Wildenschild et al., 2005). These digital images can be obtained with micro-x-ray computerized tomography (micro-CT) without destroying the interior structure of the rock (Wildenschild et al., 2005). Pore-scale simulators require an accurate quantitative description of the microstructure (Arns et al., 2002); the level of detail that is observed, however, depends on the machine resolution (Wildenschild et al., 2005). This paper will study how image resolution affects predicted flow properties. There are two approaches for computing flow and transport based on a three-dimensional representation of the pore space. The first is to compute the flow field directly on images using finite difference or particle-based methods (see, for instance (Boek and Venturoli, 2010; Mostaghimi et al., 2010). The second approach to extract a topologically representative network of pores and throats, and to compute flow through the network (Silin et al., 2003; Al-Kharusi and Blunt, 2007; Dong and Blunt, 2009). While this second approach simplifies the geometry of the pore space, it enables displacement in each network element to be computed semi-analytically. It is very effcient for the determination of multiphase flow properties where capillary forces dominate [(Blunt and King, 1992; Øren and Bakke, 2003; Valvatne and Blunt, 2004; Piri and Blunt, 2005a,b)]. In this paper we will also study the impact of image resolution on the properties of the extracted networks. Experimental Methodology We obtained micro-CT scans of dry cylindrical cores of Clashach and Doddington sandstone (diameter 4.95 mm, length 10 mm) with a v/tome/x Phoenix instrument at five different voxel sizes that we will assume is the resolution (5.789 μm, 8.946 μm, 11.972 μm, 14.998 μm and 19.997 μm) for Clashach and (5.999 μm, 8.996 μm, 11.972 μm, 14.998 μm and 19.997 μm) for Doddington. For convenience we will label these resolutions as 6 μm, 9 μm, 12 μm, 15 μm, and 20 μm for both samples. Clashach sandstone is from northeast Scotland, UK. This sandstone is of Permian age and aeolian in origin, consisting of about 90% quartz and K-feldspar (Ngwenya et al., 1995). Doddington sandstone is from the Doddington quarry at Wooler, UK. This IPTC-16600-MS Influence of Micro-Computed Tomography Image Resolution on the Predictions of Petrophysical Properties Nayef Alyafei, Imperial College (IC), Oussama Gharbi, IC, Ali Qaseminejad Raeini, IC, Jianhui Yang, IC, Stefan Iglauer, Curtin University, and Martin J. Blunt, IC Copyright 2013, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Beijing, China, 26–28 March 2013. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s).