Pressure induced deformation and flow using CO2 field analogues, Utah
Skurtveit, Elin1, Braathen, Alvar2, Larsen, Eivind B2, Sauvin, Guillaume1
1
Norwegian Geotechnical Institute
2
University of Oslo
Geological sequestration of CO2 is steadily maturing towards a stage where it may provide a
significant means for mitigating global emissions. A key concern of the general public, potential
investors, insurance companies and sequestration site operators is the risk of leakage of injected
CO2 from the reservoirs into nearby/overlying groundwater aquifers or hydrocarbon pools, or even
to the surface. Existing sequestration sites are generally small and have not been operational for
sufficient time to fully forecast and assess leakage scenarios of laterally extensive reservoirs on the
time scales relevant for subsurface CO2 storage. Thus the study of exhumed reservoirs showing
evidence of CO2 accumulation in geological history offer a unique possibility to supplement our
knowledge on leakage processes observed along faults and fractures, and to understand the relevant
spatial and temporal scales of CO2 leakage in order to better constrain the failure potential due to
injection induced pressure buildup.
Natural subsurface CO2 plumes of Utah, USA, are of particular relevance to CO2 storage studies
[e.g. Kampman et al., 2013; Ogata et al., 2014] and provide detailed insight into flow of CO2 in
subsurface reservoirs. The Little Grand Wash Fault has undergone 400.000 years of CO2 expulsion.
A research well drilled by Shell into the damage zone of the fault in 2012 retrieved a complete core
of a multi-storied succession of reservoirs and caprocks. The observed patterns of bleaching suggest
reducing fluid migrating through tight/low permeability host rock along interconnected fractures,
diffusing into the surrounding host rock according to local variations in permeability. The core is
cut by several sub-vertical fractures (i.e. fracture corridors) that provided pathways for fluid flow
through both reservoir and seals. Fractured core samples from low permeable facies of the Entrada
Formation have been selected for further studies on fluid flow along the fracture and interaction
with the host rock during fracture controlled leakage. A second case study, the Humbug Flats of the
NE San Rafael Swell, is an exhumed stacked reservoir that basically offers an identical stratigraphic
and structural setting as the Little Grand Wash Fault. An oblique view into a fault and footwall
anticline shows bleaching both along certain layers and up along faults and fractures. In this area,
intact samples from selected facies of the Entrada Formation have been sampled for geomechanical
characterization of failure criterion.
The geomechanical characterization includes uniaxial and indirect tensile strength measurements
for four characteristic facies L1-L4 within the Entrada Formation at Humbug Flat field location.
Facies L1 and L4 is fluvial plain/overbank deposits with porosity ranging from 8-14 %, L2 is a
fluvial sandstone with porosity around 20 % and L3 is a bleached eolian sandstone with 30 %
porosity. The tensile strength is in the order of 0.5 to 3.5 MPa, whereas the uniaxial compressive
strength (UCS) is 30-40 MPa for L4 and 60-70 MPa for L1. UCS was not measured for L2 whereas
for the weak L3 facies, the measured UCS is around 2 MPa with tensile fractures splitting the plug
vertically. The results show a good exponential correlation between tensile strength and porosity,
with the highest porosity corresponding to the lower strength.
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From the actively CO2 leaking Little Grand Wash Fault, fractures are sampled with the purpose of
measuring the fracture stiffness and stress depended flow potential. Both fractures with a clear zone
of bleaching along the fracture and fractures with very limited signs of bleaching were sampled and
characterized. Preliminary results from the fracture characterization using computer tomography
(CT) show that there is a distinct difference between fractures within extensive bleaching and the
non-bleached fractures of the Entrada formation (Figure 1). The unbleached fracture show a small
fracture aperture, with several contact points along the fracture and limited mineral growth, whereas
the bleached fracture has a wider fracture aperture, more complex fracture pattern and extensive
precipitation of mainly calcite and some heavy minerals.
For fractures, distribution of contact areas versus aperture depends on the stress acting on the
fracture. This is fundamental for fluid flow along the fractures and the fluid rock interaction. Work
in progress includes the quantification of contact areas versus aperture distribution for varying
stress conditions together with measurements of fracture stiffness and flow properties.
Fig 1 Example of bleached and unbleached fractures in core from the Entrada Formation, Utah.
Computer tomography (CT) images from plug cored along the fracture show fracture aperture
(white), heavy minerals as fracture fill (black) and contact points along the fracture where there is
no aperture.
Acknowledgement:
Thanks to Shell for access to core samples and funding from Research Council of Norway (RCN)
for the CO2 seal bypass project (no 244049).
References:
Kampman, N., A. Maskell, M. Bickle, J. Evans, M. Schaller, G. Purser, Z. Zhou, J. Gattacceca, E.
Peitre, and C. Rochelle (2013), Scientific drilling and downhole fluid sampling of a natural CO2
reservoir, Green River, Utah, Scientific Drilling, 16, 33-43.
Ogata, K., K. Senger, A. Braathen, and J. Tveranger (2014), Fracture corridors as seal-bypass
systems in siliciclastic reservoir-cap rock successions: Field-based insights from the Jurassic
Entrada Formation (SE Utah, USA), Journal of Structural Geology, 66(0), 162-187.
2