Analytical and Numerical Study of Consolidation Effect on Time Delayed Borehole Stability During Underbalanced Drilling in Shale

Abstract

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 hD, the difference of the consolidation time for different finite characteristic time are insignificant and converge to zero while permeability 100 hD. 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 cannot 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.

An Experimental Investigation of Shale Mechanical Properties Through Drained and Undrained Test Mechanisms

“…Because shale is expected to be anisotropic, it is reasonable to expect the permeability to be anisotropic and to expect the largest values parallel to the bedding. Islam et al (2009) reported that the magnitude of the consolidation time for the finite diffusive length adjacent to the borehole wall was only in the range of hours and days for samples with a permeability of 100 gD. Therefore, drained tests were necessary for time-delayed material failure analysis.…”

“…For the drained tests, the rate was approximately 2 9 10 -8 s -1 . In addition, the size of the test sample and the time required to perform a test depended highly on the permeability of the shale (Nes et al 1998;Islam et al 2009;Loret et al 2001) and the fluid viscosity.…”

“…The bedding plane is, thus, a key issue in this study. In general, the critical parameters that cause instability are the planes of weakness, anisotropy, the relative normal stress values on the borehole, the well trajectory, and the relative angle between the borehole and the bedding plane (Bradley 1979;Aadnøy and Chenevert 1987;Aadnøy et al 2009;Crook et al 2002;Fjaer et al 2008;Søreide et al 2009;Islam et al 2009). …”

Coupled THM Modelling of Wellbore Stability with Drilling Unloading, Fluid Flow, and Thermal Effects Considered

“…The causes of wellbore instability can be classified into either mechanical (i.e., failure of the rock around the borehole because of high stresses, low rock strength, or inappropriate drilling practice) or chemical effects which arise from damaging interaction between the rock and the drilling fluid [4][5][6]. In recent years, wellbore stability analysis has mainly focused on the chemical interaction between rock and drilling fluid.…”

Comparative Study on Calculation Methods of Mud-Weight Window for Subsalt Wells: 1-D, 3-D, and Integrated 3-D Methods

“…The 3-D method has had more and more applications in recent years (Islam, Skalle, Faruk, and Pierre 2009;Shen 2009;Shen, Bai, andSmith 2010, Shen 2013). Details of the geostructure, including syncline or anticline, will be taken into account in its calculation.…”