PoroChemoElastic Modeling of Wellbore Drilling in Fractured Shale with Solute Transport
Younane Abousleiman, Vinh Nguyen, 2004. "PoroChemoElastic Modeling of Wellbore Drilling in Fractured Shale with Solute Transport", Depositional Processes and Reservoir Characteristics of Siltstones, Mudstones and Shales, Erik D. Scott, Arnold H. Bouma
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Overcoming wellbore drilling instability in shale formations will always present one of the major challenges to drilling engineers. It is well established that the pore pressure differential between the drilling mud and the drilled rock formation is governed by the diffusion process which is a time-dependent phenomenon. In addition, shales are highly chemically active and they swell when brought in contact with aqueous drilling fluids. Chemical effects arise from imbalance in chemical potentials of the species in formation pore fluid and wellbore drilling fluid. Thus, the downhole chemical reactivity (mud/shale) is also a time-dependent variable. This work presents the development of an analytical formulation to analyze the wellbore stability in chemically active sound and fractured shale under the framework of Biot’s theory of poroelasticity. The majority of the existing wellbore stability prediction models are based on the solutions obtained using single-porosity poromechanics theory which considers the rock formation to be homogenous non-fractured and with one degree of porosity distribution. However, they are not applicable when the rock formation is fractured. The dominant characteristics of fractured rock formations, i.e., the coupling between various induced hydro-physico-mechanical processes and inter-porosity flow can be well represented under the realm of dual-porosity poromechanics.
In this work, the approach to modeling the porochemoelastic behavior of wellbore drilled in fractured shale is formulated as an extension of Biot’s poromechanics theory that fully couples the perturbation in stress, pore pressure and solute (salt) concentration in a consistent form. The shale is modeled as an imperfect semi-permeable membrane which can allows partial transport of the solutes. The pore fluid is assumed to be a binary solution that contains only a solvent (water) and a solute (salt). The transport equations for both solute and solvent take into account the hydraulic convection, chemical osmotic, advection and solute diffusion (Fick’s law). The magnitude of the chemical interaction is controlled by the membrane efficiency. The model is applied to derive a solution for an inclined/horizontal wellbore in isotropic sound and fractured shale formation subjected to the three dimensional in-situ state of stress. Mud weight window analyses are also illustrated in this work.
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Siltstones, mudstones and shales have been studied mainly with regard to general transportdeposition processes and clay mineralogy. A small group of investigators, with differing backgrounds, have worked on these fine-grained deposits. Recent studies on deepwater deposits from cores and outcrops indicate that the presence of finer-grained deposits greatly affect the fluid flow properties of deepwater reservoirs. Characteristics and rock properties of these deposits, which resulted from a variety of depositional processes, are just beginning to be understood.