Analysis of Geomechanical Properties in Well Design and Well Optimization
Main aspect in wellbore instability is the selection of an appropriate rock failure criterion. This criterion involves only the maximum and minimum principal stresses 1  and 3  and therefore assumes that the intermediate stress 2  has no influence on the rock strength. When Mohr-Coulomb criterion was developed it justified by experimental evidence from conventional triaxial test ( 1  > 3 2    ). Based on triaxal failure machine the Mohr-Coulomb criterion has been extensively used to represent rock failure under the polyaxial stress state ( 1  > 3 2    ). In contrast to the prediction of redistribution Mohr-Coulomb much evidence has been accumulating to suggest that 2  thus indeed have a strengthening effect. This research shown that Mohr-Coulomb failure criterion only represent, that triaxial stress state ( 1  = 1 2 3     or ) which only occasionally encountered in-situ.The linear failure criterion has been justified by experimental evidence from triaxial tests as well as polyaxial test. It is natural extension of the classical Mohr-Coulomb criterion into three dimensions. As the Mohr-Coulomb criterion only represents rock failure under triaxial stress state, it is expected to be conservative in predicting well bore instability. The 3D analytical model employed by MogiCoulomb has been achieved by using linear elasticity theory to calculate the stresses. A model was developed used in analyzing shale intrusion, at depth 6100ft the well bore pressure of the formation is found to be 0.9180psi/ft and decreases to 0.420psi/ft at depth 7300ft. The membrane efficiency of the formation increases from 0.698 to 0.719 at the depth considered above.
The influence of chemical potential on wellbore stability
An experimental investigation was begun to understand fundamental borehole stability mechanisms. This laboratory study was directed toward understanding why and how drilling-fluid chemistry affects borehole stability. The ability of drilling-fluid chemistry to alter shale mechanical behavior and the borehole stress state as a result of shale water content alteration was investigated. Experimental results demonstrate the influence of various drilling-fluid parameters (e. g., salt concentration, salt type, and diesel/water ratio) in oil-based drilling fluids on the water content, compressive strength, and mechanical properties of five shales. The influences of temperature and drilling-fluid exposure time on shale strength alteration also are addressed. The results are explained on the basis of chemical potential differences between oil-based drilling fluid and shale. The change in the shale water content caused by these differences is identified as the predominant factor leading to alteration of shale mechanical behavior and hence borehole stability.
On the use of the Hardening Soil Small Strain model in geotechnical practice
This article highlights the importance of using advanced constitutive soil models in numerical modelling as daily geotechnical practice. In this context, the Hardening Soil (HS) model is presented as the most advanced soil model implemented in the ZSoil finite element code. The article recalls the basic features of the HS model and it explains their role in numerical simulations. The role of pre-failure non-linear behaviour of soils is also discussed. An application of the HS model is illustrated on typical geotechnical problems such as retaining wall excavation, tunnel excavation and undrained loading followed by consolidation.