The Effects of Back Rake and Side Rake Angles on Mechanical Specific Energy of Single PDC Cutters with Selected Rocks at Varying Depth of Cuts and Confining Pressures

Abstract

Abstract One of the key objectives within the drilling industry is optimizing rate of penetration (ROP) and a major contributor to obtaining this objective is the PDC bit design. Whilst previous papers have proven that the PDC cutting structure geometry, particularly back rake and side rake angles, affect PDC bit performance when tested at atmospheric conditions, no information in the SPE literature exists for similar tests at confining pressures. The effect of side rake angle on cutter aggresiveness and cutter interaction at depths of cut (DOC) in excess of 0.04” are particularly unknown under confined pressure. The results of more than 150 tests show that back rake and side rake angles have substantial effects on Mechanical Specific Energy (MSE) and the aggressiveness of PDC cutters. Experiments with three different rock types; Carthage marble, Mancos shale, and Torrey Buff sandstone, revealed that at both atmospheric and elevated confining pressures, PDC cutters with 10 deg back rake angles require half the energy to cut the same volume of rock and produce higher cutting efficiency compared with cutters having 40 deg back rake angles. Possible reasons for this behavior are explained through the analysis of the cutting process. Results show that a cutter with low back rake requires less horizontal cutting force in order to cut the same volume of rock. This observation indicates that not only will a PDC bit with lower back rake angles, drill more efficiently, but it will also require less torque in order to drill at the same ROP. Other factors such as reduced durability of cutters at low back rake angles should also be considered while applying these results to PDC bit designs. Test results at both atmospheric and confining pressures revealed that MSE decreases with increasing DOC up to 0.08” on all three rock types. However, the tests also showed that MSE starts to increase slightly at DOCs above 0.08”, possibly suggesting an optimal minimum DOC. Experimental results also show that, whilst Mancos shale and Carthage marble have about the same compressive strength, Mancos shale requires three times less energy to cut compared to Carthage marble. This indicates that, compressive strength of some rocks such as shales cannot be used alone as a reference rock property for accurately evaluating and comparing drilling efficiency. A new 3D mechanistic PDC cutter-rock interaction model was also developed which incorporates the effects of both back rake and side rake angles, along with rock specific coefficient of friction. The results from this single-cutter model are encouraging as they are consistent with the experimental data.