Multiporosity/Multipermeability Inclined-Wellbore Solutions With Mudcake Effects

Abstract

Summary This paper investigates the effects of mudcake and formation multiporosity/multipermeability on the evolution of a safe-drilling mudweight window. The analytical poroelastic solutions for an inclined wellbore drilled through a multiporosity/multipermeability porous medium were derived, taking into account the mudcake buildup on the wellbore wall. A multiporosity/multipermeability porous medium consists of an overlapping of N distinct porous-continuum networks, each of which has its own geomechanical and petrophysical properties. Wellbore collapse and fracturing are investigated by studying a dual-porosity naturally fractured weak sandstone. The first porosity is matrix porosity, and the second porosity is the fractures. In addition, a triple-porosity source shale with multiple scales of natural fractures was modeled. The first porosity corresponds to the shale matrix one, and two scales of fracture distribution account for the other two sets of porosity. The Drucker and Prager (1952) criterion is applied to analyze wellbore collapse and shear failure, whereas the tensile strengths of both formations are considered, conservatively, as negligible. The safe-drilling mud-weight window is calculated to illustrate the time-dependent effects of formation N-porosity/N-permeability nature and the wellbore-wall boundary conditions (with/without mudcake). On the one hand, natural fractures narrow the safe-drilling mud-weight window by degrading rock strength and facilitating hydraulic-pressure invasion. On the other hand, the mudcake leads to a wider safe-drilling mud-weight window by generating compressive effective radial stress on the wellbore wall and impeding hydraulic-pressure invasion. Mudcake thickness and mudcake permeability are found to have significant effects. The drilling-mud design to build a mudcake on the wellbore wall is essential when drilling through difficult naturally fractured formations.