In-situ stresses favor tensile stress development in drag bit cutting, but possible benefits to penetration rate are eliminated by the highly negative rake angles of PDC bits. Experiments confirm that in-situ stresses do not influence bit penetration rate. A simple Mohr-Coulomb criterion is found to provide a good qualitative explanation of the decline of penetration rate with increased mud pressure observed both in laboratory tests and in field drilling.
It is well known that in-situ rock stresses increase with depth in the earth crust and that these stresses, can cause collapse of the walls if the hole is not sufficiently supported internally as it drilled. In-situ stresses also produce stress concentrations across the bottom of the hole and questions are sometimes raised as to the influence of these concentrations on the drilling rate. Drilling muds, used to provide the internal support pressure needed to stabilize the bore-hole wall, also cause an associate substantial reduction in drilling rates compared to those obtained when mud pressure is absent or low. This paper describes computational and experimental studies carried out to examine the influence of in-situ stresses and mud pressure on the penetration rate polycrystalline Diamond Compact (PDC) bits when cutting Buxy limestone - a hard, fined-grained, essentially impermeable rock.
It is assumed that the circular well-bore is vertical (i.e. parallel to the z axis in Figure 1), and that the principal in-situ stresses, all compressive, are parallel and normal to the borehole axis i.e. σx=σy=Q, and σz=R. The elastic stress concentrations produced in the vicinity of a drag bit cutting tool (of which the PDC tool is an example) at the bottom of the hole were computed using the finite element code VIPLEF developed at Ecole des Mines de Paris (ENSMP/CGES). The problem was analysed in two stages, as illustrated in Figure 2, viz, a) determination of the elastic stress concentrations at the bottom (flat) surface of the hole due to the in-situ stresses Q, R [Figure 2a), b) determination of the additional (local) stress concentrating effect in the immediate vicinity of the cutting tool due to the inclined slope change in geometry produced by the cutting action [Figure 2b]. Stresses imposed by the and by mud pressure in the not considered in the above cutting tool borehole are two stages.
The chipping action of a drag bit produces, more or less continuously, an inclined surface or slope ahead of the cutting edge of the bit. For the purpose of this study, it is assumed that the slope is inclined at 45" upwards from the (flat) cut surface. Since the depth of cut, h, is small compared to the diameter of the wellbore, it will be assumed that the stress σro acting across the bottom of the hole is constant over the depth (taken here to be 8h) affecting the stress concentration in the slope region.
