Carbonate caprock-brine-CO2 interaction: Alteration of hydromechanical properties

Guijie Sang, Shimin Liu

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Caprocks play a crucial role in geological storage of CO2 by preventing the escape of CO2 and thus trapping CO2 into underlying porous reservoirs. An evaluation of interaction-induced alteration of hydromechanical properties of caprocks are essential to better assess the leaking risk and injection-induced rock instability, and thus ensuring a long-term viability of geological CO2 storage. We study the changes in nanopores, elastic velocities and mechanical responses of a carbonate caprock due to rock-water/brine-CO2 interaction (CO2 pressure ~ 12 MPa; 50 ?). Before the interaction, the total and accessible porosities are 1.6% and 0.6%, respectively, as characterized by the Small Angle Neutron Scattering (SANS) technique. SANS results show that the total porosity of the carbonate caprock increases apparently due to rock-brine-CO2 interaction and the increasing rate rises as brine concentration increases (2.2% for 0M NaCl, 2.6% for 1M NaCl, and 2.7% for 4M NaCl). The increase total porosity is due to the dissolution of calcite which tends to enlarge accessible pores (by 0.8%-1.2%) while slightly decrease the inaccessible pores (by 0.1%-0.2%). Under CO2-acidified water environment, P- and S-wave velocities (5536.7 m/s and 2699.7 m/s) of a core sample containing natural fractures decreases by 8.5% and 8.1% respectively, while both P- and S-wave velocities (6074.1 m/ s and 3858.8 m/s) for a intact sample show only ~0.5% decreases. The interaction also causes more than 50% degradation of the uniaxial compressive strength for the core sample with natural fractures. We also conduct simulations of the single-phase creeping flow and two-phase water-CO2 flow in micron-scale natural fractures, as extracted from X-ray Micro-CT images of the core sample. The simulated absolute permeability (2.0×10-12 m2) is much higher than the matrix permeability (6.7×10-20 m2before the interaction; 1.3×10-19 m2after the interaction), as calculated based on the Kozeny-Carman Equation. This indicates that natural fractures provide preferential flow paths for CO2 while flow through caprock matrix can be reasonably neglected. Simulation results also indicate that CO2 preferentially migrates in the natural fractures where there are more inter-connected and permeable channels. The study recommends that more attention should be addressed on interaction-induced alteration of fracture/faults permeability/stability, and its effect on the sealing integrity of carbonate caprocks.

Original languageEnglish (US)
Title of host publicationSociety of Petroleum Engineers - SPE Annual Technical Conference and Exhibition 2020, ATCE 2020
PublisherSociety of Petroleum Engineers (SPE)
ISBN (Electronic)9781613997239
StatePublished - 2020
EventSPE Annual Technical Conference and Exhibition 2020, ATCE 2020 - Virtual, Online
Duration: Oct 26 2020Oct 29 2020

Publication series

NameProceedings - SPE Annual Technical Conference and Exhibition
Volume2020-October

Conference

ConferenceSPE Annual Technical Conference and Exhibition 2020, ATCE 2020
CityVirtual, Online
Period10/26/2010/29/20

All Science Journal Classification (ASJC) codes

  • Fuel Technology
  • Energy Engineering and Power Technology

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