The Influence of Microporous Cements on the Pore Network Geometry of Natural Sedimentary Rocks

Abstract

We investigate the pore network geometry and permeability of six natural sandstones and carbonate rocks. Using 3D microtomographic images, we segment each rock sample into three phases: Solid matrix of grains, macropores containing void spaces, and a third microporous phase containing nanometer-sized pores beyond the resolution of the image. In the majority of our samples, the microporosity exists inside cements deposited as a secondary phase along the surface of grains. Within the macropores, the pore radius, coordination number, throat radius, and throat length display a power law relation with porosity. We also find that the permeability of the aggregate depends on the porosity following the relation k = k 0 φ 3.32 , where log k 0 = 5.52 (mD). The fraction of connected porosity shows a strongly non-linear reduction with an increase in the volume fraction of microporous cement.

Pore-scale assessment of subsurface carbon storage potential: implications for the UK Geoenergy Observatories project

“…Previous studies examining the porosity and permeability in suites of rock samples using DRP have revealed relationships between total and connected porosity that are important to constrain when targeting formations as CCS reservoirs (Thomson et al 2019(Thomson et al , 2020b. Similar relationships have been found in both sandstones (Thomson et al 2020b) and basalts (Zahasky et al 2018) with a clear separation between partially and fully connected pore networks at 10 and up to 25%, respectively.…”

“…We acquired a mini-plug (10 mm in length and 5 mm in diameter) of each sample for μCT imaging and performed digital image analysis using the commercial software package PerGeos version 1.7.0 (Thermo Fisher Scientific). The methodology employed here is similar to a number of recent studies from the Royal Holloway research group (Thomson et al 2018(Thomson et al , 2019(Thomson et al , 2020a. In addition, we acquired thin sections of each original larger core plug (c. 20 mm in diameter), which allowed for optical validation of different phases that are challenging to distinguish using μCT alone.…”

“…The equivalent is done in the throat sections, providing measurements of throat radii. For further discussion of the algorithms used in PerGeos for PNM generation we refer the reader to Youssef et al (2007) and Thomson et al (2018Thomson et al ( , 2019. The raw data outputs from generating PNMs of each sample are available in the dataset associated with this study (Payton et al 2020).…”

“…The assigned boundary conditions are discussed in depth in Thomson et al (2018). We used an error tolerance level of 10 −6 for the convergence of the L 2 norm of the residuals, as this is the optimal tolerance in terms of trade-off between computation time and accuracy of the result (Thomson et al 2019). The solution obtained is a pressure and velocity field over the pore geometry from which the permeability is calculated using volume averaging.…”

Geological Carbon Sequestration by Reactive Infiltration Instability

“…As we show in Figure 6, the highest concentration of precipitated CaCO 3 in our simulations is less than 0.5%. Recent microtomographic studies on permeability reduction by cementation show that porosity and permeability reduction by precipitates is negligible for cement volume fractions less than 3-5% (Thomson et al, 2019;Thomson et al, 2020). The assumption of constant porosity, therefore, is reasonable for the range of time considered in these simulations.…”