Anatomy of Pore Networks in Caprock Relevant to Geologic CO2 Sequestration

Cole, David R.; Sheets, Julie; Swift, Alex; Murphy, Michael; Welch, Sue; Anovitz, Lawrence; Rother, Gernot; Vlcek, Lukas

A number of factors dictate how CO2-bearing fluids, and with them reactants and products of intrapore transformations, migrate into and through both reservoir rocks and caprocks, wet and ultimately adsorb and react with the solid surfaces. These include the size, shape, distribution and interconnectivity of the porous matrix, the mineralogy and surface chemistry of the pore network, the chemistry of the fluids and their physical properties. While there may be a general tendency for more clay-rich lithologies to have lower porosity and permeability, the link between pore size distribution and connectivity and pore-wall mineralogy is still poorly constrained. Further, the distribution, size and chemistry of pores may influence the nature of secondary mineral formation. In order to more accurately predict the reactive-transport behavior of CO2-bearing brines in both reservoir rocks and caprocks, we have begun testing the following key hypotheses:

i) connected nano- to microscale pores and fractures constitute a non-trivial contribution to total rock porosity, ii) the mineralogy of the pore and fracture network - i.e., potential reactive surface area - is markedly different compared to bulk mineralogy, iii) reactions by scCO2-brine fluids with caprocks will alter both the porosity and permeability, thus impacting caprock integrity. Efforts have focused primarily on testing the first two hypotheses for caprocks. Detailed characterization studies using neutron scattering, electron microscopy and conventional petrophysics of representative examples of regionally extensive mid-continent mudstone and shale caprock to the Mt. Simon Formation (a sandstone target for CO2 injection) indicate the following:

(a) total porosity exhibiting bimodality may be typical of shale and mudstones,

(b) connected porosity exhibiting bimodal tendencies may not be uncommon in shale and mudstone caprocks,

(c) as expected, fissile shale contains far greater abundance of nanopores than do mudstones,

(d) connected porosity also mimics the bimodal total porosity trends with connected nanopores observed below about 400 nm and connected micropores between 50 and 100 µm, and

(e) pore mineralogy (hence potential reactive surface area) is generally very different than the bulk mineralogy, especially for mudstones where phases present in minor abundances in the bulk may contribute more to the connected pore network.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013