The Effect of Microporosity on Transport Properties in Tight Reservoirs

Abstract

Diagenetic changes (e.g. cementation, compaction) in tight gas sandstones (TGSS) often disconnect the original, inter-granular pore space and further create microporosity within the original grains (e.g. by dissolution) or by filling the inter-granular porosity with clay. A petrophysically rigorous fundamental model of TGSS that accounts for microporosity would make the evaluation, development and stimulation of tight gas sandstone development more robust. The reduced connectivity of matrix pores has a profound effect on transport properties such as absolute and relative permeability, resistivity and capillary pressure - saturation relationships. To address this, we construct networks that incorporate both inter-granular (primary) porosity and microporosity, and use a network model to estimate flow properties. We present algorithms to geometrically match pore throat networks from two separate length scales that can be extracted directly from 3D rock images, or be constructed to match the relevant measured properties. Microporosity and its spatial distribution have a profound effect on the relative permeability curve. When inter-granular network is disconnected (but the microporous region is not), we provide capillary pressure - saturation curves in cases thus far unresolved with any other type of modeling. The results from model media reasonably match published experimental data. While we are motivated by TGSS, with suitable characterization the model is applicable to other reservoirs with dominant microporosity component (shale, carbonates).

A Darcy‐Brinkman‐Biot Approach to Modeling the Hydrology and Mechanics of Porous Media Containing Macropores and Deformable Microporous Regions

“…Volume averaging requires special care; in particular, it requires the existence of a well‐defined REV, significantly larger than any of the subvolume features present in any phase, within which averaging can be carried out (Hassanizadeh, 1986; Hassanizadeh & Gray, 1979; Ochoa‐Tapia & Whitaker, 1995; Whitaker, 2013). In the case of DRP studies of fine‐grained sedimentary rocks, the typical voxel volume is on the order of 10 μm 3 (Mehmani & Prodanović, 2011). In the microporous clay matrix, this volume contains ~10 3 to 10 5 clay particles and constitutes an appropriate REV.…”

Thermal Maturation of Gas Shale Systems

“…The burial diagenesis and transformation of organic-rich muds to highly heterogeneous organicrich shales form a complex process that is a function of many variables, including original mineralogy, fabric, texture, organic content, fluids, hydrology, and burial rate and depth. Diagenesis may significantly affect shale porosity and permeability (Katsube & Williamson 1994, Kim et al 1999, Ross & Bustin 2009, Yang & Aplin 2010, Schneider et al 2011, Emmanuel & Day-Stirrat 2012, Milliken et al 2012, which can range over more than an order of magnitude (e.g., Mondol et al 2008, Mehmani et al 2011. The pores associated with the mineral matrix include interparticle pores and intramineral pores (Loucks et al 2010(Loucks et al , 2012.…”

New Classification of Carbonate Rocks for Process-Based Pore-Scale Modeling

“…The approach of building two-scale networks could be applied to imaged media as well-details on image-based network con-struction can be found in Mehmani et al (2011). As shown in that paper, microporosity and its spatial distribution can have major influence on two-phase flow properties.…”

“…Numerical approaches to modeling the microporosity inside grains are available in the literature (Worthington and Pallatt 1992;Fordham et al 1999;Toumelin 2006;Mehmani et al 2011). Coupled microporous grains with intragranular microporosity, which are connected to intergranular pore space, were used in these approaches (Figs.…”

“…20 for schematic; detailed description of the procedure is beyond the scope of this publication and is available in Mehmani et al (2011). We chose fractions f ¼ 0.2, 0.5, and 0.8 of the original packing to be microporous (spheres were randomly chosen with the constraint that the set of partially dissolved grains in case f ¼ 0.2 is a subset of the case of 0.5 and of 0.8).…”