Porous sedimentary rocks are elastic at low stresses, but with increasing deviatoric stress they yield and exhibit behaviour dependent upon the stress path. Under hydrostatic and KO loading, these rocks respond to increased stress by either, an increase in pore fluid pressure or a reduction in pore volume, depending upon the permeability. This is equivalent to consolidation or compression and is responsible for compaction in hydrocarbon reservoirs. Under KO loading the equilibrium stress ratio (KO) varies between the elasticity and normal compaction. Triaxial loading under drained and undrained conditions can lead to steady state flow. This failure has identical properties to the critical state. Failures adjacent to oil wells subjected to large drawdown pressures are most probably due to the rock locally attaining the critical state. This is a cause of solids production. Similar deformation mechanisms are operative during natural (diagenetic and tectonic) deformation of sedimentary rocks.
Deformations of porous sedimentary rocks occur in response to both natural and man-made changes in stress. The former include compaction during sediment burial (Addis and Jones 1985; Rieke and Chilangarian 1974) and shear deformation during tectonism (Yassir 1989a; Jones and Addis 1986), whilst the latter are generally associated with surface engineering works (for example, Lambe and Whitman 1979) and sub-surface fluid extraction (Jones, Leddra and Addis 1987; Potts, Jones and Berget 1988; Rentsch and Mes 1988). In this paper, discussion will be restricted to consider only the behaviour of porous sediments at depth, that is, in environments where they are subject to geological process, and man-made fluid extraction, but not to other engineering activities. The deformations under consideration here are those associated with volume change (compaction) and shape change (shear). Under natural loading, compaction either typically occurs as a porous sediment is buried, or is resisted by cementation and pore pressure generation. Shear deformations are frequently a consequence of tectonism and are associated with faulting, folding and texture development in the rock. Generally, compaction is associated with the mobilisation of shear stress (Jones and Addis 1986), but natural shear may occur under conditions where the normal stresses remain constant. Fluid extraction from hydrocarbon and water reservoirs will give rise to changes in the stresses acting upon the reservoir rock which in turn may mobilise compactional and/or shear deformations. Reservoir compaction has been known to cause major shear deformations (e.g. over the Wilmigton Field, see Jones, Leddra and Addis 1987) when deformation becomes localised on discontinuities. This is unusual however, generally shear deformations associated with fluid extraction are restricted to, or are perceived to be restricted to, the near wellbore region. These deformations normally involve damage to well casing and/or the production of significant quantities of the reservoir rock (solids production) (Ruddy et al 1988).
Porous sedimentary rocks are multi-phase systems, consisting of a skeleton of mineral grains (which generally is rigid at low stress due to the presence of a mineral cement) and pore spaces filled with various pore fluids.