Mechanical stability analyses are mandatory to identify suitable candidates for openhole completions. These analyses should comprehend the effects of production drawdown and reservoir depletion state anticipated by the production plan. The understanding and modeling of the physico-chemical interactions between rock and flown fluids and their impacts on the rock mechanical properties have been presented in a companion paper.
This paper introduces the three methodologies intended for the wellbore stability analyses, namely the i) elastic analytical; the ii) empirical solids production and the iii) 2D elastoplastic finite element simulator (FE) and describes the reasons two of them have failed to incorporate the dissolution-induced rock weakening effects on the wellbore stability. The resulting numerical model has been implemented as a standalone specialist 2D FE Simulator. The simulator provides the well design engineers a user-friendly interface upon which they can evaluate the effects of the wellbore and reservoir pressure changes, as well as the aforementioned rock weakening effects from an acidizing job, on the wellbore stability.
The mechanical wellbore response presented by the simulator elucidates how the stress and strain patterns change after an acid well stimulation job. Wellbores under virgin rock conditions tend to fail by localized plastified zones, also referred to as breakouts, whereas wells subjected to acidizing jobs present extensive straining due to rock weakening. The boundaries of the acidized zone encloses a much less rigid rock prone to deform and compact. On one side this compliant zone provides a gradual shear distribution along the well radius inside the formation and mechanical confinement to the surrounding intact rock and therefore helps this rock in supporting the stresses as if the wellbore were much larger. However, on the other side this significant weakened rock volume poses uncertainties about its own stability or solids production risks. The installation of a perforated or slotted liner/casing acting as a bearing element tends to hold this compliant material back and accounts for the material uncertainties and lack of model representativeness in terms of eventual preferential dissolution patterns.
The unprecedented results presented by the developed simulator are assisting the company on the standalone completion decisions and increasing the robustness of well completion designs, therefore assuring the perennity of the CAPEX.