Initiation of Stimulation Treatments Depends on Wellbore Mechanical Responses During Drilling

A stimulation treatment follows from, and is affected by, the set of processes that occur during well construction. The creation of the opening causes a major change in stress state, and may involve permanent rock deformations affecting the wall of the wellbore. Drilling practices seek to avoid such failures because of the immediate consequences associated with rock debris loading the well, but also because deformations provide pathways for fluids to invade the rocks where they may lead to further de-stabilisation via chemical interactions. A hydraulic stimulation treatment uses the pressures of injected fluids to cause rock breakage over a short period of time, creating new flow pathways. The mechanics of the stimulation process, at the moment of its initiation, is very like the condition of mud overbalance while drilling, where the wellbore is inflated to a larger-diameter condition. In stimulation treatments, this wellbore inflation and the resulting rock deformation responses, occur in a situation that already has been modified by the drilling actions.
We describe numerical simulations of these near-well geomechanical processes, relating both to drilling and to stimulation activities, using a method that is based on a combined finite-discrete element formulation. The 2D numerical method treats the model domain initially as a continuum, but allows fractures to develop along any of the faces of the finite elements, with composite fractures able to grow into large and complex configurations. We show that an important factor is the removal of the rock mass from the drilled hole, which leads to a very different stress state around the opening compared to mathematical models that have a pre-existing hole in them. Depending on the far-field stress state and the material properties, the simulations of drilling often develop fracture arrays that are associated with borehole elongation in the far-field minimum stress direction (which is compatible with standard knowledge). Those arrays alter the near-well stress state in major ways. The numerical simulations also reveal the characteristics of deformation processes, involving fracturing and consequent strain changes, and thus their stress changes, which affect the part of the wellbore wall that is broadly aligned with the maximum far field stress. Except with the strongest and stiffest rocks, the fractures in this region initiate and develop mainly as shears, which contrasts sharply with the received wisdom for what is expected. This response also applies when we impose a stimulation loading inside the wellbore (fluid injection).