Drilling through shale is an inevitable endeavor for both conventional and unconventional oil and gas reservoirs, where for the latter acts as a source and trap rock – substantiating its position in operational difficulty. Shale formations tend to be anisotropic and typically characterized by high in-situ stresses. In certain shales, geomechanics is very important in understanding how to stabilize the wellbore. Overall, it can prescribe a correct mud weight (or rather the overbalance) to provide the mechanical stability (stress-induced). However, this over-balance can be rapidly destroyed due to the rise in shale pore pressure associated with the different potentials (hydro-, chemo-, thermo-) acting at the shale-fluid interface. These ‘potentials’ can have an effect on the borehole stability, where in several cases, create phenomena that are just recently being wholly understood.
From a chemo-mechanical perspective, designing the proper fluid that will maintain the stability of shale formation, over time, requires the knowledge of membrane efficiency that is crucial for the geomechanical effects on the wellbore. However, in current industry practices there are limited studies that integrate geomechanics with comprehensive drilling fluid properties. Typically, inputs are arbitrary assumptions when utilizing geomechanical software. Classic workflow during geomechanics design includes selecting suitable mud-weight window which do not exceed the fracture gradient (to prevent losses) or remain well above the collapse pressure (or pore pressure) whichever is more suitable to prevent kicks or blowout.
The work presented herein takes the workflow a step further, by introducing key parameters of relevant shale characteristics/behavior to current geomechanical software to provide more accurate wellbore stability simulations as a function of fluid properties (i.e. membrane efficiency). Two different shale types, Mancos and Pierre shale II, which significantly vary in their physical, chemical, mineralogical, and mechanical properties are selected for wellbore stability analysis. The methodology consisted of three phases, (1) shale characterization, (2) pore pressure transmission and membrane efficiency calculations, (3) wellbore stability simulations via geomechanics software. The software is an integrated software used to assess stress and wellbore stability by calculating a "borehole stress and failure orientation" module to perform concurrent simulations to determine stability parameters (mud weight, fluid activity, membrane efficiency) needed for drilling through complex shale.