The Effect of Nozzle Diameter on Jet Impact for a Tricone Bit

Conventional techniques used to optimize the hydraulic conditions run with tricone bits usually are aimed at maximizing either jet hydraulic horsepower or jet impact force. A basic assumption underlying these optimization methods is that increased energy at the nozzle produces increased energy at the bottom of the hole. Conventional tricone bit nozzles are aimed at the "corner" of the borehole. The jets exist in a crossflow due to the return path of the fluid from under the bit. Many studies document the deflection and diffusion of a submerged jet stream in a crossflow. Studies were conducted with an 8 1/2-in. [21.59-cm] Intl. Assn. of Drilling Contractors (IADC) Series 6–1-7 bit to determine the area and magnitude of the jet impact under a tricone bit. Flow rates up to 565 gal/min [35.6 dm /s] and jet velocities up to 400 ft/sec [122 m/s] were used. The impact area was determined by (1) observing the location of an eroded zone beneath the bit in a block of Indiana limestone and (2) measuring the dynamic pressure beneath the bit in a test cell. Both tests indicate that a larger fraction of the jet energy reaches the hole bottom with small jets than with large jets. The tests also indicate that a larger fraction of the jet energy reaches the hole bottom when two jets are used instead of three. Introduction Flow rate, fluid velocity, jet nozzle geometry and drilling fluid properties are well recognized as factors that affect the penetration rates of tricone bits. Parameters such as jet impact force, hydraulic horsepower, jet velocity, and jet Reynolds number have been used in attempts to quantify the effect of bit hydraulics on penetration rate. All these parameters refer to fluid properties at the nozzle. It is assumed that these parameters also reflect the hydraulic energy at the bottom of the hole. There is no consensus on which parameter most directly governs bit hydraulic effects on rate of penetration (ROP) or on the exact mechanism by which hydraulics affects penetration rate. The parameters most commonly used to quantify the effect of hydraulics on ROP are jet impact force and hydraulic horsepower per square inch of bit area. Given a maximum allowable surface pump horsepower, standpipe pressure, and drillstring geometry, there is a unique set of nozzles that maximize either hydraulic horsepower or jet impact force at the bit. Maximum horsepower is obtained with smaller nozzles than are required to maximize impact force. The optimal hydraulic condition usually is considered to be that which gives either maximum impact or horsepower at the bit while keeping the annular velocity within limits established by borehole erosion and cuttings transport considerations. It is assumed that the optimal nozzle diameter is the diameter required to maximize either bit hydraulic horsepower or impact force. This assumption is valid only if the horsepower or impact force equations completely account for the effect of nozzle diameter on bit cleaning, Over the years, laboratory test data obtained with the drilling rig described in Ref. 1 have indicated that ROP is a function of nozzle diameter at constant bit hydraulic horsepower conditions. The drilling rate tests reported here were conducted specifically to determine the influence of nozzle diameter on ROP. An 8 1/2-in. [21.59-cm] IADC Series 6–1-7 bit was used to drill Indiana limestone with a 9.1 -lbm/gal [1090-kg/m ] claybase mud. The tests were run with a constant weight on bit (WOB) of 42,000 lbf [187 kN], a rotary speed of 75 rpm [1.25 rev/s] and a borehole pressure of 100 psi [689 kPa]. Fig. 1 shows the results of these tests for nozzle diameters of 9/32 in. [0.71 cm], 13/32 in. [1.03 cm], and 15/32 in. [1.19 cm]. The figure clearly shows both a large effect of impact force and an influence of nozzle diameter independent of impact force on ROP. For example, 500 lbf [2.22 kN] of impact force with three 9/32 in. [0.71-cm] nozzles gives the same ROP as 800 lbf [3.56 kN] of impact with three 15/32-in. [1.19-cm] nozzles. A similar effect is observed when ROP is plotted vs. hydraulic horsepower. Rock erosion tests and pressure measurements beneath the bit were used to evaluate the cause of these differences. Literature Review The hydraulic environment under a conventional tricone bit is quite complicated. Several factors that are likely to affect the fluid flow and cuttings removal from under a bit are discussed in the following. The jets on the bit are aimed toward the corner of the borehole bottom with an inclination of 10 to 15 deg. as shown in Fig. 2. The flow field of the jet may be influenced by the wellbore wall, bottom of the hole, cones on the bit, and returning fluid from under the bit, as well as stagnation zones under the bit due to the effect of adjacent jets. The approach taken in this literature survey is to review the available information pertaining to each of these parameters individually. SPEJ P. 9^