Effect of quartz overgrowth precipitation on the multiscale porosity of sandstone: A (U)SANS and imaging analysis

Abstract

We have performed a series of experiments to understand the effects of quartz overgrowths on nanometer to centimeter scale pore structures of sandstones. Blocks from two samples of St. Peter Sandstone with different initial porosities (5.8 and 18.3 %) were reacted from 3 days to 7.5 months at 100 and 200°C in aqueous solutions supersaturated with respect to quartz by reaction with amorphous silica. Porosity in the resultant samples was analyzed using small and ultrasmall angle neutron scattering and scanning electron microscope/backscattered electron (SEM/BSE)based image-scale processing techniques. Significant changes were observed in the multiscale pore structures. By three days much of the overgrowth in the low-porosity sample dissolved away. The reason for this is uncertain, but the overgrowths can be clearly distinguished from the original core grains in the BSE images. At longer times the larger pores are observed to fill with plate-like precipitates. As with the unreacted sandstones, porosity is a step function of size. Grain boundaries are typically fractal, but no evidence of mass fractal or fuzzy interface behavior was observed suggesting a structural difference between chemical and clastic sediments. After the initial loss of the overgrowths, image scale porosity (> ~ 1 cm) decreases with time. Submicron porosity (typically ~25 % of the total) is relatively constant or slightly decreasing in absolute terms, but the percent change is significant. Fractal dimensions decrease at larger scales, and increase at smaller scales with increased precipitation.

Characterization and Analysis of Porosity and Pore Structures

“…In discussing these three approaches, however, note that the regularization approach does not necessarily guarantee non-negative results for each bin. A similar multi-distribution pattern has been observed in sandstones (Anovitz et al 2013a(Anovitz et al , 2015a suggesting that modeling sandstones as a continuous fractal distribution is inappropriate.…”

“…In the mid-Q range the data can be fi tted to a power-law slope. While we have shown that, in many cases, there are actually signifi cant details in this region (Anovitz et al 2013a(Anovitz et al , 2015a, to a fi rst approximation the log-log slope represents the fractal properties of the sample. Several scattering studies suggest that the length correlations of pore-grain interfaces can often be described by self-similar fractals with nonuniversal dimensions (2 < D < 3) (cf.…”

“…Schmidt 1989;Radlinski 2006), including: coals and hydrocarbon source rocks (Bale and Schmidt 1984;Reich et al 1990;Haenel 1992;Radlinski et al 1996Radlinski et al , 2000aRadlinski et al ,b, 2004Sastry et al 2000;Sen et al 2001Sen et al , 2002aMcMahon et al 2002;Prinz et al 2004;Avdeev et al 2006;Connolly et al 2006;Radlinski 2006;Mares et al 2009;Melnichenko et al 2009Melnichenko et al , 2012Sakurovs et al 2009;Mastalerz et al 2012Mastalerz et al , 2013Cai et al 2014;Thomas et al 2014), sandstones, shales, and carbonates (Hall et al 1983Wong and Howard 1986;Triolo et al 2000Triolo et al , 2006Sen et al 2002b;Lebedev et al 2004Lebedev et al , 2006Lebedev et al , 2007Giordano et al 2007;Anovitz et al 2009Anovitz et al , 2011Anovitz et al , 2013aAnovitz et al ,b, 2014Anovitz et al , 2015aFavvas et al 2009;Jin et al 2011Jin et al , 2013Clarkson et al 2012a…”

“…Ranges shown for data obtained from SEM/BSE imagery, USANS and SANS measurements are approximate as the data overlap. Connolly et al 2006;Anovitz et al 2009Anovitz et al , 2011Anovitz et al , 2013aAnovitz et al ,b, 2014Anovitz et al , 2015aJin et al 2011;Mastalerz et al 2012;Melnichenko et al 2012;Navarre-Sitchler et al 2013;Wang et al 2013;Swift et al 2014). This leads to a non-integer power-law as a function of the scattering given by I(Q) = I 0 Q -x + B where B is the incoherent background.…”

“…An alternative approach for a sample with asterisms is to mask out the directional scattering, and radially average the remainder. Figure 32 shows an example of an integrated scattering curve (Anovitz et al 2015a) for a sample of St. Peter sandstone with experimentally-generated quartz overgrowths. This presentation, log(I(Q)) plotted as a function of log(Q) is sometimes referred to as a Porod plot.…”