Evaluation of Seal Ability of a Multilayered Caprock System

Abstract:

Carbon dioxide capture and geological storage (CCS) is the most feasible method to mitigate the emissions of anthropogenic carbon dioxide. The injected CO2 can be efficiently stored in the underground formations for a very long time because of the seal integrity of caprock. Caprocks can be a heterogeneous multilayered system, in which the permeable shaly-sand layers are interbedded with very-low-permeability shales. When a thick (hundreds meters) single shale formation is not discovered, and instead a heterogeneous caprock system is considered, the issue of seal ability of the caprock system is critical.

The purpose of this study was to use an analytical method to study the seal ability of a multilayered caprock system where the shaly-sand and shale layers were interbedded. We modified the Buckley-Leverett frontal advanced equation, and derived the constant-pressure CO2-water two-phase analytical solution to estimate the CO2 front migration in a multilayered caprock system. The CO2 front breakthrough time and the amount of CO2 invaded into the caprock system changing with time were calculated, and the seal ability of the multilayered caprock system was evaluated.

Different multilayered caprock systems were designed to validate the accuracy of the proposed analytical method. We found that the relative permeabilities of shaly-sand and shale layers are critical parameters to the CO2 front breakthrough time. The positive correlation between the permeability and the CO2 migration velocity was observed. The heterogeneous multilayered caprock system can also provide a high seal ability to efficiently lower the risk of CO2 leakage.