Chimneys, channels, pathway flow or water conducting features - an explanation from numerical modelling and implications for CO2 storage

Abstract

Large amounts of CO2 have to be captured, transported, injected and safely stored in the subsurface in order to counteract increasing atmospheric CO2 concentrations due to steadily high consumption of energy from fossil fuels. The injection of gigatons of carbon dioxide poses challenges for pressure management and the mechanical integrity of the storage complex. Optimizing the injection strategy, controlling deformation, preventing leakage and assuring safe long term containment requires reliable and predictive hydromechanical modelling. Based on a newly derived thermodynamically consistent set of equations we have developed fully coupled codes in one, two and three dimensions. In accordance with field observations and laboratory measurements, stress dependent poro- viscoelastoplastic deformation is taken into account. Our simulations predict the spontaneous formation of self- localizing high-porosity channels (or pathway flow) under conditions applicable to CO2 storage in reservoirs. These channels form due to mechanical instabilities that occur as a natural outcome of buoyancy driven flow in viscoplastically deforming rocks. Our results indicate that viscous deformation may explain the formation of chimneys such as those observed at the Sleipner pilot, and that non-linear effects have a major impact on the velocity and distribution of the fluid as well as on stress and deformation of the rock matrix. Thus, complex rheology and non-linear coupling between porosity, permeability, pressures and stresses should be included in state of the art reservoir simulation software in order to determine safe injection and storage conditions and correctly model observations and monitoring results.