The CO₂ Sealing Efficiency of Caprocks

Andreas Busch¹, Sascha Alles¹, David Dewhurst², and Bernhard M. Krooss^1
1Institute of Geology and Geochemistry of Petroleum and Coal, Lochnerstr. 4-20, RWTH Aachen University, 52056 Aachen, Germany
²CSIRO Petroleum, Australian Resources Research Centre, Australia

Shale lithotypes of various compositions occur abundantly in sedimentary basins and act as natural seals for petroleum and natural gas reservoirs over extended geologic periods. Based on to their mechanical, petrophysical and chemical/mineralogical properties, shales are becoming increasingly of interest in the context of long-term isolation of anthropogenic (e.g. radioactive) waste and subsurface storage of fluids. The efficiency and long-term integrity of seal formations (caprocks) is also one of the central issues for CO₂ storage in saline aquifers, depleted oil and gas reservoirs and coals. Due to its chemical reactivity and physico-chemical properties, CO2 is expected to differ substantially from other natural gas components in terms of transport behaviour and interaction with the mineral/water system.

An experimental procedure has been developed to measure molecular diffusion of CO₂in water-saturated shales. This non-steady state method provides information on the effective diffusion coefficients and the CO₂storage capacity of the shales. Effective diffusion coefficients for CO₂were found to range between 10^-9and 10^-11m²/s. Storage capacities were found to vary significantly but can be as high as 0.14 mmol CO₂/g sample. Single-phase (water) flow tests prior to the breakthrough experiments yielded absolute permeability coefficients in the range from 10^-18to 10^-22m².

Volumetric sorption experiments with CO₂at pressures up to 20 MPa have been performed on dry and moist shale samples to verify the findings noted above. The results of these experiments revealed unexpectedly high storage potentials of the same order of magnitude as those from the diffusion measurements. The CO₂storage capacities are not only related to organic carbon content. In further experimental studies it was shown that the sorptive CO₂storage potential of clay minerals (montmorillonite, kaolinite, etc.) can be significantly high. Comparison of x-ray diffraction patterns of post-experiment samples and original samples did not reveal any detectable mineralogical differences.

These findings may provide a new view on the issue of caprock integrity. In addition to their sealing properties, natural shale sequences could represent a significant sink for carbon dioxide deposited in the subsurface by fixing and immobilising it and hence reduce the risk of leakage to the surface.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands