Virtually all Petroleum geomechanical processes deal with rocks that are infiltrated by fluids. In view of the strong influence that pore pressure exerts on the deformation and failure of rocks (as it known since the 1923 seminal work of Terzaghi), it should therefore not be of any surprise that the Petroleum Industry has been one of the driving force behind basic and applied research in the mechanics of fluid-infiltrated solids.
In the last decade, there has been a strong research emphasis on evaluating the overall implication of the presence of pore fluid on the geomechanical process, through an analysis of initial/boundary value problems. This is also the main topic of the paper, which reviews the consequence of the mechanical interaction between the pore fluid and the rock on some petroleum engineering processes such as drilling, borehole stability, and hydraulic fracturing. First, however, some fundamental features of the response of fluid-infiltrated solids are outlined.
Drained and Undrained ResponseRate Effects
There are two basic mechanisms that cause pore pressure to evolve with time: diffusive mass transport of the pore fluid driven by non-equilibrated pore pressure perturbations on the one hand, and "mechanical" pore pressure variation associated with change in pore volume, on the other hand. Of fundamental importance, is the intrinsic rate-sensitivity (or time-dependency) introduced by the diffusion mechanism, as it is embodied in the dimension [L]2/[T] of the diffusivity coefficient. In contrast, the time-dependency associated with the "mechanical" pore pressure change is external, in the sense that it is forced by the boundary conditions. It is important to note that none of these two mechanisms relies on any particular constitutive description of the rock.
A related issue is the existence of two limiting deformation states, drained and undrained, which is one of the key features of the response of fluidinfiltrated materials. In undrained deformation, there is no variation of fluid content in a material element, and the pore pressure change with respecto some initial conditions is exclusively related to the variation of pore volume. Depending on the constitutive model of the rock (and the loading history), the pore volume change can be either elastic or elastoplastic; in the former case, Ap is proportional to change in the mean stress. (Note that the undrained pore pressure change is sometimes neglected, although this can strictly be justified only if the fluid is much more compressible than the solid.) In drained deformation, the pore pressure is completely determined by the current fluid boundary conditions; this is a situation where the flow of pore fluid has either vanished (if the pore pressure is hydrostatic) or has reached steady state conditions.