ABSTRACT:
For half a century, the oil and gas drilling industry has used compressive strength and friction angle to parameterize Mohr-Coulomb-like constitutive relations in attempts to model the drilling process. Rates of penetration based on these models underpredict the effect of pressure on drilling. This suggests that there must be other rock properties that govern drilling under pressure. Discrete Element Modeling (DEM) of high pressure rock cutting indicates that mechanical properties of crushed rock detritus are more important than the mechanical properties of the original elastic rock. The deformation and extrusion of crushed rock detritus consumes the bulk of the energy expended in rock cutting down hole. These results suggest that some measure of the inelastic behavior of rock under pressure, such as the area under the stress strain curve, may be a more appropriate measure of rock drillability in high pressure environments. The area under a stress strain curve is a measure of specific energy. Characterizing rock in terms of the area under the stress strain curve may open the door to more accurate ways to parameterize specific energy models of drilling.
1 HISTORY OF MODELING OF ROCK DRILLABILITY
During the early part of the twentieth century, the drilling community did not account for the strengthening effect of downhole pressure on rock. Kühne (1952) pointed out the effect of pressure and suggested that rock may be treated as a Mohr-Coulomb material. Research conducted at Rice University explored the ramifications of Kühne’s proposal. (Bredthauer 1955, Cunningham 1955, Galle 1959). Similar research spread rapidly through the industry. This early research showed that the most important factor governing drillability downhole is the differential pressure, defined as the difference between the pressure of the mud in the borehole (borehole pressure) and the pressure in the pores of the rock (pore pressure).
