Numerical modelling of stresses in the rock mass around a diapir was conducted using the finite difference code by the Itasca Consulting Group, Inc. A salt diapir of typical simplified (generalised) axisymmetric geometry was modeled, and the effects of grid zone size (discretization), stress regime, salt and surrounding rock mechanical properties were analysed. The results showed that stress perturbations around the diapir were in good agreement with the published data. Also an actual salt dome geometry from the Mungo field, Central North Sea was modeled, input data for the diapir geometry was obtained using the seismic survey data. These original ASCII data were pre-processed using the Surfer software package by applying a square surface gridding algorithm. This was necessary allowing sharp spikes on the original surface of the diapir to be smoothed. The Mungo model was run using the same mechanical properties for the salt and the surrounding rock mass as were used in the generalized model. The results of the computations agree well with those found in literature showing a lowering of hoop stress and raised radial stress in the vicinity of the diapir, which results in higher deviatoric stress and hence greater potential for borehole instability. The model can be useful in borehole failure risk assessment with a potential to improve drilling efficiency, reduce downtime, etc. by planning the boreholes so that their trajectories avoid the areas of high risk as much as practicable.
1. INTRODUCTION
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 as illustrated in Figures 1.1 and 1.2. 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 [1]. The diverse and complex shapes of diapirs mean that these shapes are often difficult to describe.
implications for wellbore stability and quality;
implications for the orientation of induced hydraulic fractures and enhanced oil operations;
implications for the orientation of open, natural fractures;
implications for fault-trap integrity.
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 [2], which include the following: Figure 1 shows an example of stress related wellbore problems associated with salt diapirism and faulting in the Mungo petroleum field, Central North Sea, UK [3].
