Unlithified (‘plastic’) clays are considered excellent seals in waste storage. However, time-lapse seismic from the Sleipner CO2 injection site indicates vertical migration of injected CO2 through clay layers that were most likely continuous prior to injection and that would have been rated as effective seals. The objective of this project was to investigate mechanisms that create pathways for CO2 across such unlithified clay layers. Deformation and gas pathway creation were studied in experiments and geomechanical (soil mechanical) simulations. The expected results have relevance for seal appraisal in general, because similar mechanisms may deteriorate top seals of storage sites.

In a detailed experiment, a sample from the caprock formation at the Sleipner site was subjected to a series of hydraulic, consolidation, and gas injection tests. Data derived from this study were used in the construction of a preliminary critical state soil mechanics model in order to assess the current stability of the caprock and its possible response during gas storage.

On the numerical modelling side, a simplified heterogeneous model of the Sleipner site has been created in order to study the effect of CO2 buoyancy on the stability of the interbedded shale layers. Stress analysis showed that shear failure of the intrareservoir shale layers is only possible if the initial stress state is very close to failure already.

A conceptual finite-element model was built to further study the effect of buoyancy on the mechanical integrity of intra-reservoir shale layers. The results of simulations indicate that the changes in the effective stresses due to the presence of a CO2 accumulation underneath thin intra-reservoir shale layers are generally very low.

The effect of CO2 injection on the stress state in sand was found to be such as to move it further away from the failure, thus stabilizing the sand. The effect in shale was found to be the opposite, with the stress state moving closer to failure. However, the change in shale is very small - one to two orders of magnitude smaller than in sand, which implies that the shale must already be very close to failure prior to injection if it were to get unstable by the CO2 injection process. This conclusion is in line with the other simulations.

 

Triaxial testing device and supplement for pore pressure application as well as for the measurement of porosity and permeability.

 

For more information please contact: Alexandre Lavrov at SINTEF Petroleum Research