Drilling
1. If the stress fields in drilling remain characterized by high spatial gradients of stress; and if the stress gradients are known to interact with rock fracture properties; then, how come, will we be able to replicate such a complex drilling mechanics involving the stress and strain fields in the rock (correlating fracture strength with the stress gradient) at the laboratory-scale (even using triaxial tests)?
2. Feasible to measure the ‘true length’ of an initial crack towards estimating the ‘specific surface energy’ (estimated from the external energy required to propagate a stable tensile fracture through the rock; and from the area of the newly created fracture surface)?
3. While estimating the ‘specific fracture energy’ (fracture energy per unit of true new surface area), how do we ensure whether it includes the ‘energy used in crack branching during rock fracture’ as well, on top of including ‘surface energies’ and other energies?
4. Albeit, the ‘volume of rock removed per unit time’ remains linearly related with the ‘rate of penetration of the bit’; why does the ‘volume of rock removed per unit time’ (from the bottom) remain quadratically related with the hole diameter?
5. At any point of time, during drilling, whether the ‘energy required to break out a unit volume of rock’ becomes non-linearly related either with the ‘rate at which work is done by the bit on the rock at the bottom of the hole’; or, with the ‘rate of penetration of bit’?
6. In rotary drilling (rolling-cone/drag-bit), whether the ‘rate at which work is done by the bit on the rock at the bottom oh the hole’ tend to be non-linearly related – either, with the ‘torque resistance to rotation at the bottom of the hole’; or, with the ‘rate of rotation of the bit’?
7. Whether the energies expended in rotating the drill string against the frictional resistance of the walls of the hole; and against the viscous drag of the drilling fluid - could remain to be greater than ‘the energy required to break out a unit volume of rock’?
8. For elastic fractures, would it remain feasible to apply Griffith criterion for the initiation of crack propagation, if the incipient crack (a small crack or equivalent region of weakness that existed prior to loading) begins to propagate when the reduction in local strain energy produced by an infinitesimal extension of the crack becomes ‘equal’ or, ‘lesser’ in absolute value than the enhancement in the new surface energy required for this infinitesimal extension?
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