Research Paper GEOSPHERE Origin and heterogeneity of pore sizes in the Mount Simon Sandstone and Eau Claire Formation: Implications for multiphase GEOSPHERE; v. 12, no. 4 fluid flow doi:10.1130/GES01245.1 Peter S. Mozley1, Jason E. Heath2, Thomas A. Dewers2, and Stephen J. Bauer2 16 figures; 3 tables 1Department of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA 2Department of Geomechanics, Sandia National Laboratories, P.O. Box 5800, Mail Stop 0750, Albuquerque, New Mexico 87185-0750, USA CORRESPONDENCE: peter .mozley@ nmt .edu ABSTRACT energy storage (CAES) operations (Medina and Rupp, 2012; Heath et al., 2013). CITATION: Mozley, P.S., Heath, J.E., Dewers, T.A., The Mount Simon Sandstone is overlain by the upper Cambrian Eau Claire For- and Bauer, S.J., 2016, Origin and heterogeneity of pore sizes in the Mount Simon Sandstone and The Mount Simon Sandstone and Eau Claire Formation represent a poten- mation, a generally low-permeability mudstone and regional seal (Neufelder Eau Claire Formation: Implications for multiphase tial reservoir-caprock system for wastewater disposal, geologic CO2 storage, et al., 2012; Lahann et al., 2014). Of primary importance are reservoir-caprock fluid flow: Geosphere, v. 12, no. 4, p. 1341–1361, and compressed air energy storage (CAES) in the Midwestern United States. properties that govern multiphase flow, because injectivity, sweep efficiency, doi:10.1130/GES01245.1. A primary concern to site performance is heterogeneity in rock properties that and capillary trapping greatly affect site performance. Geologic controls on could lead to nonideal injectivity and distribution of injected fluids (e.g., poor heterogeneity of pore structure and flow properties can be complex and diffi- Received 6 August 2015 Revision received 11 April 2016 sweep efficiency). Using core samples from the Dallas Center domal structure, cult to characterize due to the interplay of textures from primary depositional Accepted 26 May 2016 Iowa, we investigate pore characteristics that govern flow properties of major environments and a variety of potential postdepositional processes, includ- Published online 23 June 2016 lithofacies of these formations. Methods include gas porosimetry and perme- ing precipitation-dissolution with a range of textures, mechanical compaction, ametry, mercury intrusion porosimetry, thin section petrography, and X-ray pressure solution, and fracture porosity or mineralization (Hoholick et al., 1984; diffraction. The lithofacies exhibit highly variable intraformational and inter- Bowen et al., 2011). Despite the regional extent and storage potential of the formational distributions of pore throat and body sizes. Based on pore-throat Mount Simon–Eau Claire system, few conventional cores are available that size, there are four distinct sample groups. Micropore-throat–dominated sam- cut the entire thickness of the Mount Simon Sandstone and the Eau Claire– ples are from the Eau Claire Formation, whereas the macropore-dominated, Mount Simon contact (Bowen et al., 2011), and thus detailed studies of the mesopore-dominated, and uniform-dominated samples are from the Mount major lithofacies and their associated multiphase flow properties have been Simon Sandstone. Complex paragenesis governs the high degree of pore heretofore limited. and pore-throat size hetero geneity, due to an interplay of precipitation, non- In this study we use mercury intrusion porosimetry (MIP) to quantify pore uniform compaction, and later dissolution of cements. The cement dissolution size distribution and evaluate the impact of porosity heterogeneity on multi- event probably accounts for much of the current porosity in the unit. Mercury phase flow. MIP is a readily available tool for examining capillarity and pore intrusion porosimetry data demonstrate that the heterogeneous nature of structure of porous media. The technique has been applied extensively for the pore networks in the Mount Simon Sandstone results in a greater than assessing sealing capacity of caprock in order to understand hydro carbon normal opportunity for reservoir capillary trapping of nonwetting fluids, as traps (Almon et al., 2005). It is also applied, but less commonly, to the study quantified by CO2 and air column heights that vary over three orders of mag- of sandstones and siltstones, including argillaceous sandstones and tight- nitude, which should be taken into account when assessing the potential of gas sandstones. Wardlaw and Cassan (1979) examined 27 samples of coarse- the reservoir-caprock system for waste disposal (CO2 or produced water) and grained siltstone to medium-grained sandstone from a wide variety of units of resource storage (natural gas and compressed air). Our study quantitatively variable ages and a wide range of permeability; they examined relationships demonstrates the significant impact of millimeter-scale to micron-scale poros- among pore-throat aperture, grain size, pore size, and mercury recovery effi- ity heterogeneity on flow and transport in reservoir sandstones. ciency, which is a proxy for residual trapping. They noted that the presence of carbonate cement increases heterogeneity in pore-throat aperture along INTRODUCTION with a decreased recovery efficiency. Wardlaw and Cassan (1979) also found that high mercury recovery efficiency is found in samples that have high The Cambrian Mount Simon Sandstone, and its stratigraphic equivalents, porosity, small pore to throat size ratios, and small mean particle sizes, the For permission to copy, contact Copyright occur throughout the Midwestern United States, where it is a target injection latter being somewhat counterintuitive. Pittman (1992) used an unpublished Permissions, GSA, or [email protected]. horizon for wastewater disposal, geologic CO2 storage, and compressed air industry data set of 196 sandstone samples for which con ventional porosity © 2016 Geological Society of America GEOSPHERE | Volume 12 | Number 4 Mozley et al. | Porosity heterogeneity in sandstone Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1341/3337069/1341.pdf 1341 by guest on 28 September 2021 Research Paper and permeability data as well as mercury intrusion data were available; he 94.0° W used these data to develop empirical relationships to allow determination of pore-aperture size parameters. Nelson (2009) provided a summary of a number of MIP studies on sandstone. In this study we build upon this prior IOWA, USA –1960 work by using MIP data to evaluate the potential impact of milli meter-scale Rinehart “A” #1 to micron-scale pore-aperture heterogeneity on multiphase flow in reservoir Keith #1 sandstone. N We examined a conventional core from the Dallas Center domal structure (area of Dallas Center, Iowa) that was obtained for site evaluation of a planned 41.7° CAES project (Heath et al., 2013; Dewers et al., 2014). This continuously cut core Mortimer #2 includes both a complete section of the Mount Simon Sandstone (~28.3 m) as well as a portion (~36.1 m) of the overlying Eau Claire Formation, including the reservoir-caprock interface. Using gas porosimetry and permeametry, MIP, thin Keith #1 –1980 section petrography, and X-ray diffraction (XRD), we characterize pore types –1960 and rock textures and quantify the range of pore-throat sizes in major litho- –1920 facies of the Mount Simon Sandstone and Eau Claire Formation. These data then allow us to make direct comparisons of pore characteristics of the reser- –1900 voir-caprock system. The large number of MIP samples (n = 30) and detailed Mortimer-1980 #1 petrographic observations provide a unique data set for characterizing the nature and origin of the pore-size heterogeneity within a sandstone reservoir (saline aquifer) and across and into the overlying caprock. Because the petro- –2180 physical characteristics and sealing capacity of the Eau Claire Formation cap- rock were discussed in detail elsewhere (Neufelder et al., 2012; Lahann et al., –2080 1 mile 2014; Swift et al., 2014), we mainly provide detailed analysis for the Mount Simon Sandstone. 1 kilometer Figure 1. Structural elevation map of the top of the Mount Simon Sandstone at the Dallas Center GEOLOGIC SETTING domal structure. Contours are in feet below sea level (modified from Heath et al., 2013). Regional and site-specific geologic information are presented to facilitate comparison between properties of the Mount Simon Sandstone and the over- and heterolithic sandstone-mudstone (Saeed and Evans, 2012). Bowen et al. lying Eau Claire Formation at the Dallas Center structure (Fig. 1), where the (2011), focusing on the Illinois Basin, stated that depositional environments core was collected, with other locations in the Midwestern United States. For may include shallow-marine, deltaic, fluvial, eolian, and possibly sabkha set- example, Decatur, Illinois, is the site of the Illinois Basin–Decatur Project by tings, with lithofacies including cobble conglomerate, stratified conglomer- the Midwest Geological Sequestration Consortium, where CO2 is injected at ate, poorly to well-sorted sandstone, and interstratified sandstone and shale; a rate of 1000 t/day in the Mount Simon Sandstone (Finley, 2014). The Cam- and shale. The Mount Simon Sandstone thus exhibits strong heterogeneity brian Mount Simon Sandstone, a major regional aquifer, extends broadly in lithofacies (Bowen et al., 2011); such heterogeneity motivates an under- throughout the Midwestern United States and