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| Titel |
Mechanical behaviour and transport properties of anhydrite - implications for caprock integrity during long-term storage of CO2 |
| VerfasserIn |
S. J. T. Hangx, C. J. Spiers, C. J. Peach, A. ten Hove, A. M. H. Pluymakers |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067070
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| Zusammenfassung |
| Geological storage of CO2 in depleted oil and gas reservoirs offer one of the most easily and
cheaply implemented options to reduce CO2 emissions. To maintain storage integrity,
mechanical damage of the caprock in the form of dilatation, fracturing, shear failure and
associated permeability development should be prevented. Damage can be caused by
deformation resulting from localised reservoir compaction or heave during fluid depletion or
injection.
We investigated the mechanical strength and damage behavior of anhydrite-rich caprock,
which seals many current and potential CO2 storage sites around the world. To this end, we
studied two types of stratigraphically equivalent, but texturally and compositionally different,
anhydrite obtained from the basal unit of the Permian Zechstein evaporite sequence,
which caps many potential CO2 storage sites in the Netherlands. Main differences
between the anhydrites were the grain size, grain shape and dolomite content. Our aim
was to investigate the effect of rock texture and composition on the mechanical
behaviour and damage characteristics of anhydrite, needed to assess the effect of lateral
textural/compositional variations on lateral strength variations and susceptibility to
mechanical damage.
Conventional triaxial compression experiments, dry and with fluids (CaSO4 solution ± CO2,
Pp = 10-15 MPa), were performed under a range of conditions (T = 20-80°C, Pc = 1.5-50
MPa, strain rate ~10-5 s-1). Hydrofracturing experiments were performed under fixed axial
load, increasing fluid pressure at a constant volumetric flow rate until failure occurred. The
effect of stress on the permeability of deforming Zechstein anhydrite, going from
mechanically stable, through dilatant conditions, into failure and the post-failure stage, was
determined by combining the compression experiments (T = 20°C, Pc = 3.5-25
MPa, strain rate ~10-6-10-7 s-1), with transient pulse argon gas permeametry
(Pp = 1-1.2 MPa). Our results were used to determine the failure envelopes for
dry anhydrite and the effect of fluids upon them, while the permeability data was
coupled with dilatancy envelopes, providing data on the effect of stress state on fault
permeability.
For both anhydrites, we observed a transition from brittle to semi-brittle behaviour over the
experimental range. Peak strength varied by 15-35% between the two anhydrites and could be
described by a Mogi-type failure envelope. No influence of fluid on mechanical strength was
observed, besides an effective pressure effect, implying that there was no chemical effect of
CaSO4 solution with or without high-pressure CO2. Dynamic permeability measurements
showed a change from "impermeable" ( |
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