 |
| Titel |
The mechanical behaviour of anhydrite and the effect of deformation on permeability development - implications for caprock integrity during geological storage of CO2 |
| VerfasserIn |
Suzanne Hangx, Christopher Spiers, Colin Peach |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038781
|
|
|
|
|
|
| Zusammenfassung |
| Depleted oil and gas reservoirs offer one of the most easily and cheaply implemented options
for geological storage of CO2. Most of the stored CO2 will mainly be present in the
supercritical fluid phase and dissolved in the formation fluid, as CO2 mineralisation reactions
are slow and the mineralisation potential of most reservoirs is low. Therefore, long-term
top-seal or caprock integrity is pre-requisite for guaranteeing the integrity of depleted
reservoir storage systems.
In the long term, caprock integrity may be affected by fluid-rock interaction, i.e. chemical
attack. However, as mentioned earlier, such reaction effects are slow and it is unlikely
that they alone are significant for typical shale, mudstone or anhydrite caprock
compositions and thicknesses. Probably more important is mechanical damage in the form
of dilatation, fracturing, shear failure and associated permeability development,
which can be caused by caprock deformation and the stress changes accompanying
localised reservoir compaction during depletion, or localised heave during CO2
injection.
One of the most widespread sealing formations topping hydrocarbon reservoirs around
the world is anhydrite rock. Anhydrite also forms the caprock at several trial CO2
injection sites currently under operation (e.g. Teapot Dome, USA; the Weyburn
and Zama Fields, Canada; the K12-B field, the Netherlands). Furthermore, in the
Netherlands and North Sea for example, many potential storage sites are overlain
by the basal anhydrite of the Permian Zechstein evaporate sequence. For these
reasons, there is accordingly much interest in quantifying damage development in
anhydrite. Recent work by Hangx et al. has delineated the stress conditions under
which anhydrite rock is mechanically stable, versus the conditions under which
dilatant damage and failure occur. However, the magnitude of the permeability change
accompanying dilatation and failure of anhydrite under reservoir conditions remains
unknown.
We determined the effect of stress and deformation on the failure behaviour and
permeability of Zechstein anhydrite under conditions ranging from mechanically stable
(intact, non-dilatant), through dilatant conditions, (semi-brittle) failure and into the
post-failure stage. To this end, we performed conventional triaxial (i.e. axi-symmetric)
compression experiments performed at room temperature, confining pressures of 3.5-25 MPa
and strain rates of ~10-6-10-7 s-1. At the same time, we measured the permeability of the
material to argon gas, using transient pulse permeametry (Pp = 1-1.2 MPa). We
used our results to complement our previous failure and dilatancy envelopes for
dry anhydrite with permeability data, as well as providing data on the effect of
stress state, notably mean stress, on gouge-filled fault permeability in anhydrite
caprock.
Overall, we observed a transition from brittle to semi-brittle behaviour over the
experimental range, and peak strength could be described by a Mogi-type failure envelope.
Dynamic permeability measurements showed a change from “impermeable” (<
10-21 m2) to permeable as a result of mechanical damage. During deformation,
permeability increased by -¥ 3-5 orders of magnitude, eventually reaching a constant,
post-failure value, which decreased with confining pressure from ~10-16 m2 at 3.5-5
MPa to 10-19 m2 at 25 MPa. The onset of measurable permeability was associated
with an increase in the rate of dilatation at low pressures (3.5-5 MPa), and with the
turning point from compaction to dilatation in the volumetric vs. axial strain curve
at higher pressures (10-25 MPa). Sample permeability was largely controlled by
the permeability of the shear faults developed. Static, post-failure permeability
decreased with increasing effective mean stress. Simple analytical calculations based on
the elastic flexure of a seal formation, combined with our failure and dilatancy
envelopes obtained in our studies, show that for realistic conditions caprock integrity
will not be compromised by mechanical damage and permeability development. |
|
|
|
|
|
|