Mechanisms of formation damage have been evaluated in order to explain the size and depth of the “altered zone” close to a borehole, as observed from advanced well logs. It is found that one possible mechanism for formation alteration is tensile failure of the formation caused by radial stress release, pore pressure increase by mud-fluid flowing into the formation close to the borehole tip and temperature reduction due to colder mud-fluid. In this paper numerical modelling is performed at two scales based on data from two field cases. First a global model for calculation of average (continuum) stress changes during radial stress unloading and mud flow into the formation and temperature changes. Then in a second stage the stress changes from the global model are applied at the boundaries of a micro FEM model of a realistic micro-scale grain structure. The 2D micro FEM model is closer to explain the observed potential stiffness reduction based on the difference between the mud temperature and the formation temperature. However, there are limitations in the 2D model and extension to 3D model and dedicated laboratory tests are required to more accurately quantify formation damage and further investigate the physical processes of formation damage.


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