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| Titel |
The effect of aging, temperature and brine composition on the mechanical strength of chalk |
| VerfasserIn |
Reidar Inge Korsnes, Anders Nermoen, Trond Stødle, Eirik Vika Storm, Merete Vadla Madland |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099209
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| Publikation (Nr.) |
 EGU/EGU2014-14964.pdf |
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| Zusammenfassung |
| Chalk strength has been of great focus for several research communities since the 1980s
when the Ekofisk subsidence problem was discovered. Sea water injection was initiated
in 1987 to improve the oil production and to re-pressurize the reservoirs to halt
the subsidence. The oil production was improved significantly, but the reservoir
compaction in the water saturated regions continued, in contrast to the regions with no
water breakthrough. This observation indicates a water weakening effect of the
chalk.
Extensive studies have been performed during the last decades to enlighten how the brine
chemistry alters the rock mechanical properties. These studies have shown that the
elastic bulk modulus, yield strength, creep and the deformation rate at constant
stress conditions depend on the pore fluid composition. In general, the injected
brine is in non-equilibrium with the rock surface inducing alteration of the rock
mineralogy.
In this study we examined two aspects of the mechanical strength, namely the bulk modulus
and the onset of yield during hydrostatic stress loading with 0.7 MPa pore pressure. The test
program consisted of aged and un-aged cores, ambient and 130°C test temperature, and four
brine compositions: MgCl2, NaCl, Na2SO4, and synthetic sea water (SSW) at ion strengths
of 0.657 M. The aging was performed by submerging saturated cores in a closed
container with the respective test brine for three weeks at 130°C. Un-aged cores
were saturated the same day as they were tested. For each brine composition we
present four test setups; (a) aged and tested at 130°C, (b) aged and tested at ambient
temperature, (c) un-aged and tested at 130°C, and (d) un-aged and tested at ambient
conditions.
The main results from our study are:
1. By using NaCl and MgCl2 as saturating brines, neither the test temperature nor the aging
procedure affected the yield stress and bulk modulus significantly.
2. Using Na2SO4, the yield point and bulk moduli were reduced if the core experienced
130°C either during the hydrostatic loading phase or during core aging. As such,
weakening was observed also at ambient test temperature, given the plug experienced
130°C during aging. These results imply that the plug was permanently weakened
by the aging. In addition, un-aged cores were weakened at 130°C, indicating that
weakening by Na2SO4 at elevated temperatures is an immediate process which could be
explained in terms of adsorption of surface active ions creating inter-granular disjoining
pressures.
3. Using SSW, the bulk moduli and yield strength were reduced at 130°C independent of
aging or no-aging. Aged and un-aged cores tested at ambient temperature show no alteration
to the mechanical properties, with results similar to the ones obtained for the NaCl and
MgCl2 saturated cores. This observation indicates that, in these cases, the mechanical
properties are in-sensitive to the brine chemistry. |
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