The Geotechnical Characteristics of Weak North Sea Reservoir Rocks

  • M. E. Jones
  • M. J. Leddra
  • A. Goldsmith
  • O. P. Berget
  • I. Tappel

Abstract

Laboratory investigations of the deformation characterstics of chalks and weakly cemented sandstones from a number of North Sea reservoirs, and from outcrops, demonstrate that these materials are elastic under initial loading, but exhibit strongly non-linear deformation behaviour at higher stresses. This behaviour can be described by a characteristic failure surface in stress/pore volume coordinates. This paper presents a description of this behaviour. When reservoir rocks of this type are loaded, maintaining a uniaxial strain condition, the material, undergoes ductile yield at relatively low stress followed by significant porosity decrease at almost constant stress (pore collapse). The onset of pore collapse depends on the rock type, its pre-deformational porosity and the extent and nature of the cementing material. Eventually a hardening occurs that is a response to the increasing compaction and porosity decline in the sample. Reservoir rocks will mobilize maximum compaction drive if they yield in this manner, but may also suffer a permeability loss due to the decrease in porosity. Other detrimental aspects of reservoir compaction (well casing collapse and surface subsidence) may also occur. When subject to large shear stresses in the presence of high pore pressures, the materials investigated are observed to deform readily in shear. They may approach a liquefact stat. This aspect of the mechanical behaviour is interpreted as a possible mechanism for the rapid transfer of substantial volumes of reservoir solids in a well. This phenomenon has been reported to occur in a number of fields, generally, following rapid drawdown of previously “shut-in” well.

Keywords

Effective Stress Triaxial Test Stress Path Reservoir Rock Uniaxial Strain 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Norwegian Institute of Technology 1990

Authors and Affiliations

  • M. E. Jones
    • 1
  • M. J. Leddra
    • 1
  • A. Goldsmith
    • 1
  • O. P. Berget
    • 2
  • I. Tappel
    • 2
  1. 1.Department of Geological SciencesUniversity College LondonUK
  2. 2.OljedirektoratetStavangerNorway

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