Eyvind Aker

Experimental investigation of CO2 breakthrough and flow mechanisms in shale

Supercritical CO2 breakthrough and flow mechanisms in shale have been investigated in laboratory experiments using a high pressure flow cell and cylindrical samples of shale from the Draupne formation in the North Sea. The main objective is to study the basic mechanisms involved in the breakthrough process and define the controlling parameters for supercritical CO2 flow in a low permeable shale. Experimental testing provides new insight into the CO2 breakthrough process through simultaneous measurements of deformation and ultrasonic velocities in the sample. A marked sample dilation associated with the CO2 breakthrough is identified accompanied with a pronounced drop in ultrasonic velocities. X-ray images of the sample using a high resolution 3D computer tomography (CT) scanner provide information on macroscopic fracture distribution inside the sample before and after testing. The CO2 breakthrough pressure for the Draupne material seems to depend on confining pressure and effective pressure rather than pore pressure difference across the sample. After breakthrough the effective CO2 permeability was found to follow a simple model for permeability in fractured rock. The drop in ultrasonic velocity was associated with mechanical changes and possible micro fracturing inside the sample. Based on our observations we conclude that pressure-induced opening of micro-fractures during the breakthrough process is an important mechanism for flow in addition to capillary displacement. Our findings may have important consequences for later testing and estimation of CO2 breakthrough pressure and flow in shale.

Experimental Methods for Characterization of Cap Rock Properties for CO 2 Storage

This paper presents laboratory methods utilized at Norwegian Geotechnical Institute for characterizing cap rock for CO2 storage reservoirs. The focus is on the physico-mechanical characterization of shale using standard rock physical-mechanical testing and some special designed set-ups for advanced experimental conditions. The Brazilian, uniaxial and triaxial tests are explained along with some examples from North Sea, Barents Sea and Svalbard. The CO2 core flood tests have been designed and carried out in the NGI laboratory for investigating rock-CO2 interaction. Monitoring techniques such as CT-scanning, acoustic measurement, acoustic emission and resistivity are used and described in the paper. Brief description of development/upgrading of laboratory instruments for conducting more advanced experiments is also included.

Impact of Tensile Strength Anisotropy on Fracturing Pressure of Svalbard Sandstone and Shale Cap Rocks

This paper presents results of the laboratory tests for determining the tensile strength of anisotropic shale samples cored from Aagardhfjell Formation in Longyearbyen, Svalbard. This formation is considered as seal for the suggested CO2 storage reservoir. Therefore, it is essential to characterize and investigate behavior of the shale under storage conditions. This study includes results and analysis of Brazilian indirect tensile strength of cores parallel and perpendicular to bedding. The results are analyzed to incorporate in the calculation of fracture pressure. Because of significant burial and uplift of the Barents Sea region, the shale exhibits very high tensile strength and strong anisotropy. Calculation and analysis of fracture pressure, based on the laboratory data and some assumptions, show that the orientation of possible fractures can be dictated by the tensile strength instead of in-situ stresses. An example of the impact of tensile strength anisotropy on the fracture pressure of shale formation has been assessed. The results of this study suggest that the tensile strength of rock has significant impact on fracture pressure and the orientation of fractures.