Analyses of Wellbore Instability in Drilling Through Chemically Active Fractured-Rock Formations

Abstract

Summary Numerous time-dependent wellbore-instability problems have been reported while drilling through the chemically active fractured-shale formations in the Arabian Gulf. Very often, these shales are characterized by the abundance of not only macroscale bedding planes but also networks of microscale natural fractures. The presence of fractures weakens the shale mechanically and produces higher-permeability fluid-flow paths within the low-permeability rock formation. Because of different fluid-diffusion rates between the fractures and shale matrix, there are two distinct pore-pressure fields in saturated fractured shale. Additionally, in chemically active shale formations, osmotic pressure arises because of the imbalance in mud/shale chemical activity. Practically, it is extremely complex to isolate the fractures from the matrix for analysis or to identify fracture size and fracture density. However, a first-order approach in an attempt to understand the porous fractured-shale behavior is to use dual-porosity and dual-permeability theory of poromechanics in the analytical modeling. In this study, a poromechanical inclined-wellbore solution has been derived that incorporates time dependency, a primary porosity and permeability for the matrix, a secondary porosity and permeability for the fractures, and the chemical effect. The expressions for the stresses and pressure solutions are presented and detailed in Appendix A. These analytical solutions are used to simulate an inclined-wellbore-stability problem in a fractured-shale formation, accounting also for bedding planes in some instances in addition to the microfractures. Additionally, retrieved rock samples of fractured shale were tested by use of an innovative laboratory characterization device, the Inclined Direct Shear Testing Device (IDSTD) (PoroMechanics Institute; Norman, Oklahoma; 2008). This device tests tiny shale specimens (rock volume less than 0.14 in.3) and is capable of measuring cohesion and friction angle while the sample is subjected to in-situ stresses, varying mud pressures, and mud-circulation times.