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Geomechanical analyses 
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Bohloli et al. 2014

Mechanical laboratory testing and interpretation of injection tests of the Longyearbyen CO2 storage pilot were used 
to evaluate geomechanical conditions for safe CO2 storage. Laboratory testing program includes compressive and 
tensile strength tests of overburden and reservoir core samples, and the injection program consists of various types 
of injection tests at different depths from the shallow aquifer down to the targeted sandstone reservoir. Water in-
jection tests (leak-off, step rate and fracture tests) were analyzed to determine fracture pressure for cap rock and 
reservoir formations, and fracture closure pressure for some intervals. In addition, laboratory tests, well log data and 
empirical correlations were used to analyze compressive and tensile strength vs. depth. Laboratory tests showed 
that despite shallow depth of the reservoir, less than 700 m, the strength and stiffness of intact material is very high, 
and that there is a significant strength anisotropy in the shale units. The high tensile strength of intact formations in 
combination with presence of pre-existing fractures makes fracturing of the intact intervals very unlikely. Interpre-
tation of the injection tests indicate 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 interval is significantly less than the vertical stress, 
which suggests horizontal stress to be the minimum principal stress. Therefore, opening of pre-existing vertical to 
sub vertical fractures is considered the most likely fracturing mode in the reservoir, whereas in the overburden it is 
uncertain due to the marginal difference between vertical and horizontal stresses.

The interpreted fracture pressure (red triangles), possible fracture gradients (red dashed line) and fracture closure 
pressure representing the minimum in situ stress (purple dots). A lithostatic gradient is added for the range of den-
sities discussed in the paper. Measured pore pressures and a hydrostatic gradient are added for clarity. The middle 
column shows the possible fracture opening mode (red arrows) interpreted for critical pressure build-ups in the dif-
ferent formations. The stratigraphic succession in the right-hand column is based on borehole Dh4 (Braathen et al., 
2012).