Abstract Polycrystalline diamond compact (PDC) bits have gained wide popularity in the petroleum industry for drilling soft and moderate-strength formations. However, in hard-formation applications the PDC bit still has limitations even though continuing developments in PDC cutter designs and materials have steadily improved drilling performance. Resolution of the limitations of PDC bits for drilling hard formations will contribute significantly to the price competitiveness of (i) oil and gas recovered from deep, hot, hard-rock formations, and (ii) electricity generated from enhanced geothermal energy reservoirs. In this paper the cutting efficiency has been analyzed, based on a force model for a single PDC cutter. The cutting efficiency of a single PDC cutter is defined as the ratio of the rock volume removed by a cutter to the force required to remove that volume of rock. The cutting efficiency is found to be a function of the back-rake angle, depth of cut, and rock properties (e.g., the angle of internal friction). The highest cutting efficiency is found to occur at specific cutter back-rake angles, which depend on the material properties of the rock. In particular, the cutting efficiency is directly related to the internal angle of friction of the rock being cut. The results of this analysis can be applied to each PDC cutter on a given bit, then the contributions of the individual cutters can be integrated to model the overall bit performance. Conversely, this analysis can serve as a guideline for developing new PDC bit designs that are optimized for specific rock formations Introduction PDC bits have gained wide popularity in petroleum and gas drilling due to their long bit life and their ability to maintain a high rate of penetration (ROP). The shearing action induced by fixed drag cutters has proven to be more efficient for penetrating rock than the crushing effect of the teeth or inserts on the rolling cones of a roller bit1,2,3,4. However PDC bits have traditionally had limitations when encountering hard formations5; hence, they are not yet preferred for hard-rock mining, petroleum/gas, or geothermal energy applications. Enhanced geothermal energy (i.e., geothermal energy recovered from large, +3km, depths) has recently been identified by a MIT-led multidisciplinary expert panel as one of the most promising energy sources in the US6. The panel study shows that enhanced geothermal energy has the potential, even for conservative resource estimates, to satisfy the entire US demand for electricity. In order to obtain competitive electricity prices from enhanced geothermal energy, technological advancements are required that will allow cost-effective drilling in hard formations. The objective of this paper is to develop an analytical model for characterizing the cutting efficiency of PDC bits in hardrock formations, thereby enabling improvements in the design of future PDC bits. PDC Bit Most PDC bits are composed of a hard matrix body, which is milled out of a solid block of steel or cast from sintered tungsten carbide. The matrix body features blades where the actual PDC cutters are mounted, and open areas, or slots, where the cuttings and mud flow can escape to the annulus.