Geomechanical characterization of shale caprock of the Longyearbyen CO2 storage pilot

This study is a small part of ongoing research for CO2 sequestration at Longyearbyen CO2 Storage Lab, Svalbard, Norway. The primary aim of this study is to determine geomechanical properties of two important caprock units to assess the integrity of sealing for safe and successful CO2 storage in the future. Core materials for this study are provided by UNIS CO2 Lab that previously collected and stored by the Norwegian Geotechnical Institute (NGI). Results from this study are compared with previous work performed by NGI in 2010, 2012 and 2014. Six core samples were selected from well Dh-6 targeting cap rock unit of Longyearbyen CO2 storage lab, consists mainly of two geological formations; Rurikfjellet and Agardhfjellet in depth range 308.00-428.28 m. All the samples were well preserved in paraffin sealing and kept in the controlled temperature room at NGI. Samples were unsealed and inspected for any pre-existing defects e.g. fractures or any damage. These cores were cut to make plugs according to the ASTM standard for Brazilian and UCS tests. The diameter and thickness of disc shaped plugs for Brazilian test are 40-40.5mm and 19-21mm respectively. The diameters and heights of cylindrical plugs for UCS tests are 40.4 and 28 mm and 82 and 60 mm respectively. Two geomechanical laboratory tests; Brazilian and Uniaxial compressive strength tests were performed on the prepared plugs to determine tensile and uniaxial compressive strength respectively. A detailed study of tensile strength was executed on a set of ten disc shaped plugs from two different geological units; Rurikfjellet and Agardhfjellet formations at certain depth ranges 308.00 to 386.89 m and 425.28 to 428.28 m respectively. Before performing laboratory tests on these plugs, density was measured and also a detailed analysis of bulk mineralogy and mineral identification plus micro-structure (only three thin section were made) study was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. All these plugs were loaded perpendicular to the bedding planes and that is why the calculated strength was called vertical tensile strength. Almost all the plugs were failed in expected time according to ISRM standard and the pattern of produced fractures after the test is also as expected. Variations based on tensile strength are higher in Agardhfjellet formation (2.29-10.35 MPa) as compared to Rurikfjellet formation (3.96-5.53 MPa). Estimation of tensile strength for a cap rock can be very important aspect, especially when it is subjected to a fluid injection into a subsurface reservoir. The information of tensile strength can be used to avoid hydraulic fracturing caused by fluid injection, also fracture propagation and its nature can be determined. Uniaxial compressive strength test is also performed on a set of six plugs. Four of the plugs were tested from Rurikfjellet Formation (depth range 308.00 to 369.53 m) and two plugs were tested from Agardhfjellet Formation (depth range 425.28 to 428.28 m). Most of the tested plugs were failed within the time frame mentioned in ASTM standards for UCS test and few of the plugs show very clear shear failure. Mainly geomechanical parameters [uniaxial compressive strength (σ), Young s modulus (E) for axial deformation] and P- and S-wave velocities are measured from this test. Variations of these all three measurements are analyzed within each formation and also compared with each other. Agardhfjellet Formation has highest values in these measurements as compared to Rurikfjellet Formation. Index test from UCS can be utilized to estimate overburden pressure of rocks and behavior of material when it is subjected to compression in uniaxial directions. Furthermore this data can be used for various modelling of the caprock/seal evaluation. Finally results from Brazilian and uniaxial compressive strength test are compared with previous work performed at NGI. The previous work of NGI was done on core samples from wells Dh-2, Dh-4 and Dh-6 from different formations, also including Rurikfjellet and Agardhfjellet formations at different depths. Results from this study from similar depth are matching with previous results. The overall rock strengths (UCS and tensile strength) are higher at deeper intervals. Reasons can be the compaction, cementation, etc. For more accurate and better understandings of caprock integrity to ensure safe CO2 storage, further research is required than this study. For instance, more core samples should be tested from Agardhfjellet formation because in this study only two were tested and both are very different in all reported parameters. Therefore, evaluation of an entire formation would not be appropriate on the basis of few tested samples. Fracture analysis could be done for both macro and micro levels but it was not considered here due to limited data sources, laboratory facilities and time constraint, though it is highly recommended and can be very useful for the leakage risk assessment in the caprock