Experimental investigation of the rock cutting process and derivation of the 3D spatial distribution of the formation strength using in-cutter sensing

Recent advances in downhole measurements allow to accurately measure drilling forces at the drill bit, and estimate the evolution of the rock strength along the well profile. This paper presents an experimental methodology that allows to measure drilling forces, at the cutter scale, with a sensor embedded behind a polycrystalline diamond compact (PDC) cutter, and to infer the 3D spatial distribution of the rock strength. Two experimental campaigns have been performed on a laboratory-scaled drilling rig and complemented with standard mechanical tests to validate rock strength estimations. In the first campaign, homogeneous synthetic rock samples have been prepared. The average rock strength of each sample derived from cutter force measurements and a cutter–rock interaction model, compares well with the one derived from mechanical tests. In the second campaign, heterogeneous synthetic rock samples have been prepared. They are made of two layers of gypsum mixtures of different strengths, separated by a slanted bedding plane. Based on the instrumented cutter measurements, the 3D spatial distribution of the rock strength has been reconstructed along its path. Rock strength estimations are consistent with results obtained from mechanical tests, and the reconstructed geometry of the bedding plane matches well its actual geometry. The experimental methodology and technology presented in this paper lay the foundations for estimating rock properties in 3D, at the drilling stage. It has the potential to provide geoscientists information about complex lithological structures at an early stage, reducing the need for expensive and time-consuming coring and logging operations.