Porothermoelasticity for swelling shales

Optimization of drilling fluid parameters such as mud weight, salt concentration, and temperature is essential to alleviating instability problems when drilling through shale sections, particularly in high-pressure and high-temperature environments. Under these conditions, selection of suitable mud parameters can benefit from analyses that consider significant thermal and chemo–mechanical processes involved in shale–drilling fluid interactions. A non-isothermal poroelastic theory suitable for shales is presented herein. Phenomena related to thermal and chemical osmosis are considered by extending the theory of porothermoelasticity to chemically active rocks. The modified pore pressure and stresses around a borehole in shale are obtained by solving the porothermoelastic field equations in generalized plane strain. Application of the solution to a typical field operational situation has demonstrated that thermal osmosis can significantly impact formation pore pressure, thereby reducing stability. Furthermore, analyses based on the new porothermoelastic formulation for shales suggest that mud temperature should be optimized in order to maximize the efficacy of chemical osmosis in stabilizing the borehole.