Supercritical CO2 flow through fractured low permeability geological media: experimental investigation under varying mechanical and thermal conditions
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Date
28/11/2016Author
McCraw, Claire Aarti
Metadata
Abstract
To ensure secure geological storage of carbon dioxide it is necessary to establish the
integrity of the overlying sealing rock. Seal rock fractures are key potential leakage
pathways for storage systems; understanding their behaviour in the presence of CO2
under reservoir conditions is therefore of great importance. This thesis presents experimental
investigations into the hydraulic behaviour of discrete fractures within low
permeability seal rocks during single phase supercritical CO2 flow, under varying mechanical
and thermal conditions representative of in-situ conditions.
An experimental rig was designed and built to enable the controlled study of supercritical
CO2 flow through 38 mm diameter samples under high pressures and temperatures.
Samples are placed within a Hassler-type uniaxial pressure cell and CO2 flow is controlled
via high precision syringe pumps. Flow experiments with supercritical CO2
within the pressure range 10-50 MPa were undertaken at temperatures of 38°C and
58°C with confining pressures of 35-55 MPa. The effects of stress loading and temperature
change on the hydraulic properties of the fractured sample were studied; continuous
differential pressure measurement enabled analysis of hydraulic response.
Experiments were undertaken on a pre-existing Wissey field Zechstein Dolomite fracture
and three artificial fractures (two East Brae field Kimmeridge Clay samples and
one Cambrian shale quarry sample). Fracture permeabilities ranged from 8 X 10-14 m2
to 6 X 10-11 m2 with higher permeabilities observed within the harder rock samples.
A broadly linear flow regime, consistent with Darcy's law, was observed in the lowest
permeability sample (East Brae). A Forchheimer-type non-linear flow regime was
observed in the other samples.
Transmissivity variations during experiments were used to infer the mechanical impact
of stress and temperature changes. An increase in effective stress resulted in transmissivity
reduction, suggesting fracture aperture closure. During initial stress loading
cycles, and subsequent higher temperature stress loading, a component of this transmissivity
reduction was found to be inelastic, suggesting permanent modification of fracture
geometry during closure. Pre- and post-experiment fracture surface characterisation
provides further evidence for the occurrence of plastic deformation. Transmissivity-stress
relationships were elastic during subsequent external stress-loading cycles, suggesting
elastic closure and opening of fractures without additional permanent fracture
geometry changes.
The impact of fluid property variations on fracture hydraulic conductivity, Kfrac, was
also analysed. Under constant effective stress Kfrac was found to be higher within high
temperature and low fluid pressure scenarios, due to higher density/viscosity ratios.
However, under constant confining pressure, fluid pressure changes are coupled both to
mechanical effects (from effective stress alteration) and hydraulic effects (from viscosity
variation), with opposing impacts on fracture hydraulic conductivity. At lower effective
stresses mechanical effects were found to be dominant, with fluid pressure increase
resulting in a notable increase to Kfrac due to aperture opening. At higher effective
stresses, mechanical changes are much smaller due to increased contact area between
fracture surfaces, and thus increased stiffness of fractures. Under such conditions hydraulic
effects may be dominant and result in a small Kfrac reduction as fluid pressure
increases, due to a reduction in the density/viscosity ratio. These results highlight that
CO2 fluid property variation can have a notable influence on hydraulic conductivity
under certain in-situ conditions.
The single phase CO2 fracture flow experiments undertaken during this study were designed
to enable a study of hydraulic and mechanical processes in isolation, without the
influence of chemical processes. In-situ, the additional presence of brine and thus multiphase
fluid behaviour and associated chemical processes makes the hydraulic behaviour
of fractures considerably more complex. Coupled process modelling enables the relative
influence of these processes to be simulated, but relies on experiments for validation.
These unique experimental findings are of great value for enabling validation of such
models as well as for informing analyses of geological and field studies.