Evaluation of reservoir and cap-rock integrity for the Longyearbyen CO2 storage pilot based on laboratory experiments and injection tests
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- NGI-artikler 
Original versionNorsk Geologisk Tidsskrift 2014, 94(2-3):171-187
Mechanical laboratory testing and interpretation of injection tests of the Longyearbyen CO2 storage pilot are used to evaluate geomechanicalconditions for safe CO2 storage. The laboratory testing program includes compressive and tensile strength tests of overburden and reservoir coresamples, and the injection program consists of various types of injection tests at different depths from the shallow aquifer down to the targetedsandstone reservoir. Water injection tests (leak-off, step rate and fracture tests) were analysed to determine fracture pressure for cap-rock and reservoirformations, and fracture closure pressure for some intervals. In addition, laboratory tests, well-log data and empirical correlations were usedto analyse compressive and tensile strength vs. depth. Laboratory tests showed that despite the shallow depth of the reservoir, less than 700 m, thestrength and stiffness of intact material is very high, and that there is significant strength anisotropy in the shale units. The high tensile strength ofintact formations in combination with the presence of pre-existing fractures makes fracturing of the intact intervals very unlikely. Interpretation ofthe injection tests indicates that fracture pressure has a higher magnitude and gradient in the overburden than in the reservoir. In the overburden,fracture closure stress representing the minor horizontal stress is slightly lower than the vertical stress. Fracture pressure in the reservoir intervalis significantly less than the vertical stress, which suggests horizontal stress to be the minimum principal stress. Therefore, opening of pre-existingvertical to subvertical fractures is considered the most likely fracturing mode in the reservoir, whereas in the overburden it is uncertain due to themarginal difference between vertical and horizontal stresses.