Underground salt deposits deform with time, leading to a structure known geologically as a salt diapir. Salt diapirs play an important part in hydrocarbon developments and are significant for petroleum exploration. Presence of geological inhomogeneities such as salt domes causes significant perturbation of in-situ stresses, which has serious implications for the stability of boreholes drilled in the vicinity of the diapirs. This work presents a modelling methodology to study stress distribution throughout a rock mass surrounding a diapir.
Salt diapirs of simplified axisymmetric geometry were modeled, and the effects of grid zone size, stress regime, rock mechanical properties and creep constitutive behaviour were analysed. The results showed good agreement with the published data. Also actual salt dome geometry from the North Sea was modeled. The results of the computations agreed well with those found in literature showing raised deviatoric stress in certain areas in the vicinity of the diapir, which results in greater potential for borehole instability. The model can be useful in borehole failure risk assessment with the potential to improve drilling efficiency, reduce downtime, by planning the boreholes so that their trajectories avoid areas of high risk where practicable.
Under high pressure, salt (evaporite rock) deforms as a viscous material and behaves as an intrusive body piercing or folding overlying sediments, leading to a structure known geologically as a salt diapir. Salt diapirs, or salt domes as they are often called, are characterised by their prominent shape. They play an important part in salt production, sulphur production, underground gas and toxic waste storage and hydrocarbon developments. Salt diapirs have no porosity or permeability and create a wide range of subtle traps such as stratigraphic and unconformity types. The traps are especially significant for petroleum exploration in highly mature areas where many of the fields around diapirs are economically marginal.
Diapirs come in a wide variety of shapes and sizes due to the diversity of salt mechanics, which embodies different means of movement initiation, growth dynamics and the eventual termination of growth (Seymour et al. 1993). The diverse and complex shapes of diapirs mean that these shapes are often difficult to describe.
Knowledge of the local in-situ stress state around a salt diapir is important if the region is to be developed as a petroleum province. The contemporary stress state has widespread influence on engineering a petroleum reservoir, as outlined by Hillis et al. (1995), which include the following:
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implications for wellbore stability and quality;
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implications for the orientation of induced hydraulic fractures and enhanced oil operations;
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implications for the orientation of open, natural fractures;
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implications for fault-trap integrity.
Figure 1 shows an example of stress related well bore problems associated with salt diapirism and faulting in the Mungo petroleum field, Central North Sea, UK (Holt et al. 2000).
The geomechanical model discussed in the current study will investigate stress perturbations caused by a salt diapir on the virgin stress state.
