Elasto-plastic fracturing model for wellbore stability using non-penetrating fluids

Abstract

An elasto-plastic fracturing model that fits fracturing experiments is presented in this paper. This model is valid for non-penetrating fluids used during drilling operations. The model assumes a plastic zone at the borehole wall. The final equation is based on linear elastic-perfectly plastic material properties. Furthermore, models are presented both for initial fracturing and for reopening of existing fractures. The linear elastic model underestimate fracture pressures measured on hollow concrete cores. This new model fits well to the measured data. It is found that the particles forming the barrier are key factors, and it is from experiments observed that the fracture pressure can be increased by changing the composition of the particles forming the fluid barrier. Experiments have shown that it is possible to design the mud to reduce the circulation loss problem. The models presented in this paper will be important tools for this work. In order to establish a barrier, some filtrate loss is preferable. The particle size distribution is therefore a key parameter. It is also found that in addition to good filtrate control, the mechanical strength of the barrier particles control the maximum magnitude of the fracturing pressure. Experiments indicate that it is possible to nearly re-establish the initial fracture strength of a failed borehole.