Experimental investigation of the Heletz shale caprocks sealing capacity: implication for CO2 geological storage integrity.
Résumé
Using a combination of core flooding experiments and wettability measurements, we evaluate the sealing
efficiency of Heletz caprock under CO2 sequestration conditions. The flow through experiments consisted of
flowing CO2 enriched fluid into two micro-fractured cylindrical cores (15 mm length - 9 mm diameter, with
hydraulic aperture: 2.7 µm for the sample named H18A and 13 µm for sample named H18B) and monitoring the
permeability changes, the evolution of the chemistry from the inlet and outlet fluid. The changes in microstructures
and mineralogy were also studied using an environmental scanning electrons microscope (ESEM) and X-ray
micro-tomography (XRMT) images.
The fracture permeability was found to decrease significantly in the two experiments from 14.1×10-12 m2 to
5.0×10-12 m2 for experiment H18B and from 6.5×10-13 m2 to 2.8×10-13 m2 for experiment H18A. Calcite
dissolution and reconversion of k-feldspar to illite and kaolinite were the main reaction on sample H18B while
“calcite precipitation” in batch condition was the dominant reaction on sample H18A. Accordingly, the decrease
in permeability was induced by the dispersion of dissolution products and the re-organization of clay particles
within the fracture for sample H18B as shown by micro-tomography and ESEM images. The fracture healing due
to the calcite and clay mineral precipitation along the fracture was attested by ESEM image for sample H18A.
The results of capillary pressure breakthrough calculated by applying the Washburn equation and the reservoir
scaling method from intrusion of mercury are approximately 380 kPa and 310 kPa for H18B and H18A respectively. Although, these values are sensibly different but close to each other and in good agreement to indicate the
weak storage capacity of the heletz caprock. Subsequently less than 90 m of CO2 column height can be efficiently
stored in the Heletz reservoir. Thus the self-mitigation of the CO2 leakage is expected only when few quantity of
CO2 will be injected.