Different Shales Dictate Fundamentally Different Strategies in Hydraulics, Bit Selection, and Operating Practices

Abstract

Full-scale simulator tests under hydrostatic pressure in Wellington, Catoosa and Mancos shales reveal fundamental differences in balling characteristics and drillability. These differences call for distinctly different strategies in hydraulics, bit selection and the optimization of mechanical drilling parameters. This paper presents test data and photographs to illustrate the unique behavior of these three shales, which differ widely in composition and mechanical properties. Introduction New insights into drilling under hydrostatic pressure are accumulating rapidly with the widespread use of full-scale simulators. Correspondingly, opportunities for improvements in hydraulics, bit selection and drilling practices can be identified quickly and accurately. One area of paramount interest is the drilling of shales, which encompasses the majority of the drilling in petroleum exploration and development programs. Historically, shale poses particular problems for conventional roller cone bits, which, in most instances, are markedly inefficient in drilling shales under hydrostatic pressure. Even shear-cutting polycrystalline diamond compact (PDC) bits can be rendered ineffective at high bottom hole pressures. In separate and unrelated simulator testing, it was recognized that balling characteristics change significantly with shale type. This observation prompted the more extensive study reported in this paper. Simulator Testing All tests were run on the full-scale simulator, the details of which have been described in the literature. The majority of the tests were run with an 8 1/2" IADC 517 Class roller cone bit at 3000 psi bottom hole pressure, 3500 psi confining pressure and 4000 psi overburden. The formation pressure at the base of the 15.5" diameter and 36" long impermeable shale cores was kept equal to the confining pressure. A 9.4 lbs/gal water base lignosulfonate mud was used as the circulation fluid. The make-up and rheological properties of the mud were also given by Ledgerwood, et al. For the standard test procedure, rotational speed (rpm) and bit hydraulics (HSI) were set and held constant for a drilling interval of 15". The rate of penetration (ROP) was increased every five inches in increments of 10, ranging from 10-30 ft/hr. The dependent variables, weight on bit (WOB) and torque were measured at 20 samples per second. Fig. 1 shows the plot of a typical test, in which the data was subjected to an 11-point averaging routine. As illustrated, it takes about two to three inches for the drilling process to reach equilibrium conditions, leaving about two to three inches of steady state data. In most tests, that was sufficient. However, in a few cases with severe cutting structure balling, both WOB and torque were still on an upward slope at the end of the 5" step. P. 307^