Dependence of Drilling Specific Energy on Bottom-Hole Pressure in Shales

Cutting experiments were carried out to quantify the dependence of the specific energy σ (the energy required to cut a unit volume of rock) on the bottom-hole pressure p _m and on the pore pressure p _o in shales. The experiments were conducted with an instrumented cutting device specially designed to operate inside a triaxial cell. With this set-up, four longitudinal grooves could be cut simultaneously on epoxy-coated shale plugs, under confining pressure and initial pore pressure (which is different from confining pressure). The results of about 20 cutting experiments performed on Mancos and Pierre I shales and on Johnstone, an artificial shale, for various combinations of confining pressure (up to 50 MPa) and initial pore pressure are reported in this paper. This preliminary experimental investigation supports the theoretical findings that the specific energy e depends only on p _m, and not on the "differential" pressure p _m - p _o in shales under conditions when the rock is shear-dilatant. In such cases, σ can indeed be expressed as σ = σ _o+mp _m, where σ _o denotes the specific energy under atmospheric conditions and m is a coefficient which is a function of the cutter backrake angle θ and the internal friction angle φ of the shale.