Effects of Bit Hydraulics on Full-Scale Laboratory Drilled Shale

Abstract

Summary The effects of bit hydraulics while drilling shale with a standard three-cone bit are examined in this paper. Tests were conducted by drilling into large-diameter, intact shale samples at simulated downhole conditions in Drilling Research Laboratory's wellbore simulator. The shale samples were recovered from massive surface outcroppings and preserved for laboratory use. The effects of hydraulic horsepower from 20 to 400 hhp and bit weights from 20,000 to 50,000 lbm on rate of penetration are presented. Introduction Most deep shales and some intermediate shales found in the U.S. are typically "slow" drilling formations. Efforts to improve rate of penetration in shale have been performed in both field tests and small-scale laboratory investigations. Until recently, laboratory drilling studies on shale have been confined to microbit1 or single-cutter2 studies because the ability to obtain and preserve large, intact shale samples had not been developed. In addition, laboratory facilities where full-scale drilling tests could be conducted at simulated deep-well conditions did not exist. Massive surface shale formations have been located and techniques have been developed to extract and preserve large-diameter, intact samples. With these samples and the ability to simulate full-scale drilling conditions, a systematic, technical approach was taken where the effects of drill bit hydraulics were examined while drilling shale at simulated downhole conditions. Background and Definitions The in-situ conditions of a typical deep wellbore and surrounding rock formation are illustrated in Fig. 1. The formation is subjected to overburden stress, confining stress, wellbore pressure, and formation pressure. As previously demonstrated,3–5 rate of penetration is influenced strongly by the bottomhole conditions and appears to be most sensitive to the differential pressure between the wellbore and formation. These effects, however, can vary widely depending on rock properties such as rock type, strength, density, permeability, and mud properties such as composition, filtration rate, viscosity, solids content, and particle size.6 Bit hydraulics - i.e., the means of removing cuttings from the hole bottom and cleaning the bit with the drilling fluid - are a key factor in improved bit performance. Bottomhole cleaning theories,7 microbit studies,8 and full-scale laboratory drilling experiments in hard, impermeable rock9 have shown the need for adequate bit hydraulics to maximize rate of penetration and avoid bit balling. Field tests with extended nozzles10 also have demonstrated great potential for increasing rate of penetration in certain formations with improved bit hydraulics. Some nondrilling laboratory studies have shown the effects of nozzle size on pressure distribution at the hole bottom.11