New formulation of mechanical specific energy (MSE) taking into account the hydraulic effects for PDC bits

Chevreau, Marc-Aurèle
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In today’s unstable economic environment, the cost of drilling continues to be the most important limitation factor to drill new wells. Thus, to reach the deepest depth at the lowest price, drilling needs to be optimized. To do so, drillers monitor two values: the rate of penetration (ROP) and the mechanical specific energy (MSE). However, MSE has been proven to be more valuable because it links ROP with drilling parameters as weight on bit (WOB) or torque. Moreover, it allows computation of mechanical efficiency (EFFM), which is directly linked to the drilling efficiency. The latest MSE formulations only consist of three components: a thrust one, a rotary one and a hydraulic one, which only represents the action of the jet impacts. The associated mechanical efficiency consists of the minimum MSE (MSEmin) divided by the actual MSE. This formulation has been proven to be inaccurate by several authors. In fact, they showed that as hydrostatic pressure increases, the mechanical efficiency decreases. This decrease has been explained by these authors by the fact that some important hydraulic phenomena are not considered. These phenomena are the shear dilatancy and change in failure mechanism. Shear dilatancy is the phenomenon that happens when the drill bit shears the rock: the rock dilates which causes a decrease in pore pressure and thus an increase in differential pressure which strengthens the rock. This strengthening has been quantified and MSE has been reformulated to take into account this phenomenon. At atmospheric conditions, the failure mechanism is usually brittle, creating chip like cutting. However, when applying hydrostatic pressure this failure mode switches towards the ductile mode creating ribbon like cutting. The formulation of the MSEmin has been adapted to take into consideration this phenomenon. In order to help an engineer to take into account these phenomena, a program was developed that shows graphically and quantitatively the influence of these mechanisms. The program allows the user to vary several parameters and is capable of extracting drilling data from an existing Excel sheet. This is important since the influence of these mechanisms can create differences of around 10% between the efficiencies calculated with and without considering these phenomena, and this difference can reach values over 20% in certain formations