Time delayed mechanical borehole stability is mostly depending on the pore pressure consolidation process. Establishment of pore pressure equilibrium in shale is a time dependent process which is characterized by shale intrinsic properties i.e., porosity, permeability, fluid and rock stiffness parameter etc. In shale, water movement is greatly restricted by the low permeability of shale which may cause pore pressure storage. The Influence of induced pore water pressure and its dissipation is critical for the evaluating of time delayed borehole stability.
This paper discusses and presents a sensitivity analysis of the impact of shale intrinsic properties on transient pore pressure and its impact on time delayed mechanical borehole instability. The aims are to establish pore pressure trend, material plasticity, and time delayed borehole collapse risk. In an attempt to minimize transient pore pressure related instability problems, detailed and careful analyses are highly dependent on the constitutive models adopted for the shale. In this study, a physical model was used to introduce hypotheses of time delayed stability and both analytical and numerical models were developed to verify the hypotheses. The analytical model is based on poroelastic constitutive model, coupled with pressure diffusivity formulation. A numerical material model is developed based on finite elements. The analytical model quantifies shale intrinsic properties vs time delayed stability; whereas, the numerical model diagnoses pore pressure storage effect during underbalanced drilling in shale and its impact on mechanical instability. The undrained condition (immediately after the wellbore is drilled) as well as the drained condition was analyzed. The integrated approach in this study may give a clear picture on shale complexicity and its adverse effect on time delayed stability. Wellbore stability models that include some aspects of coupled have already been developed. However, time delayed borehole instability in shales in UBD condition is a new research area; adjustments are required in existing model.
The analytical simulation results show that shale permeability higher than 40 ηD, the difference of the consolidation time for different finite characteristic time are insignificant and converge to zero while permeability 100 ηD. In addition to shale permeability, other parameters (i.e. pore fluid viscosity, porosity and fluid bulk modulus) are influencing the consolidation processes. This paper demonstrates the effects of such parameters also. The numerical results implies that a M-C elastic - plastic model is capable of evaluating plasticity and material deformation effects in UBD conditions along with accounted time delayed transient pressure trend in shale. It was shown that the shale behaviour during UBD is a transient problem, and can not be described without a fluid diffusion process. It is also noted that shale intrinsic parameters are playing a significant role in time delayed borehole stability analysis