Baseline Development of Rock Anisotropy Investigation Utilizing Empirical Relationships Between Oriented Physical and Mechanical Measurements and Drilling Performance

This paper describes a baseline investigation to confirm the isotropy of rocks material through physical and mechanical measurements followed by oriented drilling. This baseline is intended to evaluate drilling experiments in anisotropic rock materials to determine the significance of the anisotropy on drilling performance. The conducted tests include oriented measurements of compressional and shear wave velocities (Vp and Vs, respectively), density, Elastic Moduli, Point Load Strength Index (PLI), Indirect Tensile (IT) strength, and Unconfined Compressive Strength (UCS). The oriented laboratory drilling experiments were conducted under various pump flow rates and several weights on bit (WOB). In this work, an isotropic rock like material (RLM) was developed using Portland cement and fine-grained aggregate. The tested RLM specimens were of medium strength of ∼50 MPa. The RLM samples were cored in different orientations and then, tested and drilled according to these orientations. (e.g. 0°, 45° and 90°, representing horizontal, diagonal and vertical directions, respectively). Two main sets of lab tests were performed including pre-drilling and drilling tests. For the pre-drilling lab experiments, two main sets of tests were conducted to determine the physical and mechanical properties of samples (as outlined above) including PLI, IT, UCS, Vp, Vs, density and corresponding isotropic Dynamic Elastic Moduli. For the drilling tests, a vertical lab scale drilling rig was used with a 35 mm dual-cutter Polycrystalline Diamond Compact “PDC” bit. The drilling parameters involved were flow rates, nominal rotary speed of 300 rpm, and various WOB under atmospheric pressure. The relationships between the drilling data were analyzed including drilling rate of penetration (ROP), depth of cut (DOC), and corresponding effective WOB. The results of all mechanical, physical and drilling measurements and tests show consistent values indicating the isotropy of the tested rock material. This consistency verifies that the drilling tests are free of bias associated with drilling orientation.