Deformation Behavior of Chalk Studied Close to In Situ Reservoir Conditions |
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Authors: | E Omdal M V Madland T G Kristiansen N B Nagel R I Korsnes A Hiorth |
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Institution: | (1) Department of Petroleum Technology, University of Stavanger, Stavanger, Norway;(2) BP Norge, Stavanger, Norway;(3) Itasca Houston Inc., Houston, TX, USA;(4) International Research Institute of Stavanger, IRIS, Stavanger, Norway |
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Abstract: | A laboratory test program, which simulated reservoir conditions of pressure and temperature, was conducted on outcrop and
reservoir chalk samples of various porosities. All the samples experienced a stress path following uniaxial strain condition
K
0 that led to compaction failure, i.e. pore collapse. The experiments were loaded by depletion of pore pressure conducted under
load controlled conditions. This depletion phase was followed by a creep period, where time-dependent deformation was monitored.
The intention of creating such reservoir condition in these laboratory experiments was to gain knowledge of the nature of
chalk compaction. Chalk is an important reservoir rock for the oil and gas industry with unique storage capability with porosities
up toward 50%. However, this rock is also very weak which has resulted in significant reservoir compaction and in turn severe
seabed subsidence and casing failure. Mapping of the mechanical behavior of chalk in terms of deformation is thus decisive
for a proper understanding of these reservoirs. The results of this study show that chalk is indeed a rate-dependent material
under laboratory loading conditions as time effects were revealed as the loading rate was varied. However, the results raise
uncertainty about the importance of rate dependency for chalk under completely drained conditions. Further, such high-porosity
chalk suffers for substantial plastic strains and obvious strain hardening. Indeed, a relation between deformation/porosity
and hardening is proposed by the introduction of real-time modulus values. Time-dependent deformation, also called creep was
influenced by the depletion phase, as consolidation or transient creep influenced the deformation response for as much as
175 h after a change in load. This indicates that transient creep is dependent on the stress history. However, observations
suggest the existence of a universal mechanism for steady state creep, governed by neither the initial porosity nor the stress
history or chalk type, which thus seems to be an independent strain contributor. Finally, time dependence is found on the
K
0 development for chalk tested at typically laboratory rates, which has been discussed as a reflection of the nature of the
grain re-arrangement during failure and plastic deformation. Ultimately, such time dependence of the K
0 may contribute to the understanding of stress path data deduced from field data. |
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