Abstract
The presented experimental study focuses on the hydro-mechanical characterisation of a shale caprock (Opalinus Clay) in contact with carbon dioxide. The objective of this paper, consists in the evaluation of the material's sealing capacity in terms of entry-pressure, mechanical behaviour and sensitivity of the transport properties to chemo-mechanical effects induced by gaseous and liquid CO2 injection. Two types of Opalinus Clay core samples are tested; shaly and carbonate-rich. The sealing capacity has been evaluated on the shaly OPA according to the stepwise and the residual methods and compared to the results from mercury intrusion porosimetry. The obtained results and the differences associated to the different involved physical processes are discussed and compared with literature data. Injection tests carried out in saturated and unsaturated conditions have revealed that sub-critical CO2 propagation in a water saturated material is not associated with generation of fractures. On the other hand, the generation of capillary forces is affecting the mechanical behaviour beside the sealing capacity. The impact of chemical effects on the permeability of both types of OPA is analysed with long-term CO2 injection tests, where no significant variations of permeability are measured during the exposure time investigated. The challenges related to this type of analysis with laboratory scale experiments are illustrated and new insights on the behaviour of Opalinus Clay when subjected to injection of a non-wetting fluid are highlighted.
Highlights
Carbon capture and storage (CCS) is widely recognised to have great potential for sustainable and environmentally friendly energy produc tion
The sealing capacity of the Opalinus Clay is first evaluated with the short-term injection test on the shaly OPA in terms of capillary entrypressure
Several limi tations related to the detection of macro pores have to be considered when running Mercury Intrusion Porosimetry (MIP) on shales, such as the Opalinus Clay, and they are mainly related to the presence of fissures and surface effects; an exten sive review of these aspects can be found in Minardi (2018)
Summary
Carbon capture and storage (CCS) is widely recognised to have great potential for sustainable and environmentally friendly energy produc tion. The low permeability (in the order of nano Darcy), the low porosity (lower than 20%) and modal pore size (nano meter range), and the high clay content make the Opalinus Clay a good formation to be employed as a barrier material (Marschall et al, 2005) for the sequestration of CO2 These features are essential for a high sealing capacity and low sensitivity of the hydro-mechanical properties to chemical effects. The possible chemical effect induced by the exposure to an acidic fluid is analysed by means of pore size distribution and grain density analysis To achieve these objectives, CO2 injection tests are performed at the laboratory scale under different levels of injection pressure and different durations. The obtained results are presented in the second part of the article
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