Cutting experiments were carried out to quantify the dependence of the specific energy e (the energy required to cut a unit volume of rock) on the bottom-hole pressure pm and on the pore pressure po 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 pm, and not on the "differential" pressure pm-po in shales under conditions when the rock is shear-dilatant. In such cases, e can indeed be expressed as e=eo+mpm, where eo denotes the specific energy under atmospheric conditions and m is a coefficient which is a function of the cutter backrake angle ? and the internal friction angle ? of the shale.
Shales represent about 75% of the rocks (in terms of linear footage) encountered while drilling wells for oil and gas exploration and production. Significant rate of penetration can be achieved when drilling shales (up to 60 m/h) without hardly any wear on the cutters even after several thousands feet of drilling. At the same time, poor penetration rate (of order of 1 m/h) is often reported when drilling in shales.
The reasons behind such low rates of penetration seem to be dependent on whether soft ("gumbo") or hard shales are being drilled. In soft shales, low drilling rates are generally associated with bit balling; this problem can be mitigated by a combination of bit design (very large cutters), improved bit hydraulics and surface treatment of the bit (electrochemical effects). In hard shales, poor performance appears to be due to the very large specific energy e (the energy required to cut a unit volume of rock) that is encountered at great depth. As explained in the body of this paper, e for hard shales can be expressed under certain conditions as
where eo denotes the specific energy under atmospheric conditions, m is a coefficient expected to vary in the range 3 to 20 for typical cutter configurations, and pm is the bottom-hole pressure. Note that e does not depend on the virgin pore pressure. For example, Zijsling1 reports results of experiments with a single cutter tester performed on Mancos shale showing a specific energy of about 600 MPa (J/cm3) at a bottom-hole pressure of 60 MPa.