Flow Distribution in a Roller Jet Bit Determined From Hot-Wire Anemometry Measurements
- A.A. Gravignet (Sedco Forex) | L.J. Bradbury (Schlumberger Cambridge Research) | F.P. Quetier (Sedco Forex)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1987
- Document Type
- Journal Paper
- 19 - 26
- 1987. Society of Petroleum Engineers
- 1.5 Drill Bits, 1.6.9 Coring, Fishing, 1.11 Drilling Fluids and Materials, 1.11.5 Drilling Hydraulics, 1.6 Drilling Operations, 4.3.4 Scale, 1.10 Drilling Equipment
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In current practice, the optimization of drilling hydraulics consists of the selection of nozzle sizes that maximize either jet impact or hydraulic power at the nozzle. But what is required for a real optimization is the knowledge of the hydraulic forces available for cleaning at the rock face, not at the nozzle. This paper shows the results of hot-wire anemometry experiments that provide insight into the flow distribution in a jet bit. Direct measurements of the flow field, including turbulence levels, are reported and discussed.
As soon as the oil industry recognized that jets in roller bits increase the rate of penetration of roller bits, researchers set out to understand the scouring action of the jets.1 They were helped by the work done in aeronautical research on the nature of submerged jets, which are very important to aircraft and rocket propulsion. The literature in this field is considerable (see Ref. 9 for an extensive bibliography).
Overall, aeronauticists - through an interplay of theory and observation - concluded that the velocity distribution in a jet and hence the impact pressures are determined by the rate of momentum (alias thrust or jet impact force) at the nozzle and by the distance from the nozzle. This is true after the jet has been able to evolve from a potential core (i.e., a flat-flow profile) to a smoother distribution whose shape remains identical as the jet spreads downstream.
If this were true in rock bits, then the distribution of pressure at the bottom of the hole, the crossflow velocities, shear forces, and turbulent stresses would not depend explicitly on flow rates, nozzle diameter, or bit dimensions. It would be describable in terms of a few dimensionless groups. In particular, the number of nozzles should have no effect on the flow distribution near the impact of a jet.
Experimenters in the oil industry have attempted to check whether the above results were relevant to the case of a roller bit. Most of Feenstra and van Leeuwan's1 study concentrated on the effect of jet velocity on drilling efficiency with 21.6-cm [8 1/2-in.] bits; however, they did notice that jet velocities in bits confirmed the predictions for unconfined jets. McLean2 measured the flow distribution under 16.5-cm [6 1/2-in.] roller jet bits; the results indicated that the "equations derived from current jet technology provide a means of predicting the distribution of the impact pressure generated on a flat surface normal to an impinging jet." McLean3 showed that the properties of crossflow that may contribute to hole cleaning are proportional to the jet impact force at the nozzle, thus the recommendation for maximizing this quantity in hydraulic optimization programs.
Sutko and Myers4 studied in greater detail the pressure distribution under a 16.5-cm [6 1/2-in.] roller jet bit. The study included the effect of nozzle size, number, and extension. The experimental results, when replotted in dimensionless form by Warren and Winters,5 show that the flow distribution in their case cannot be described in dimensionless form as predicted by the results obtained with unconfined jets. Both nozzle diameter and number have an effect on the values of the dimensionless constants of turbulent jets. In a further study, Sutko6 investigated the forces acting on a simulated chip under a jet and concluded that shear stresses are less than inertial forces on the cutting.
Warren and Winters5 conducted studies with a 21.6-cm [8 1/2-in.] roller bit to determine the area and magnitude of the jet impact in real-size experiments with water. In addition, the study included pressure measurements at the bottom of a simulated borehole. This work involved extensive comparisons with results predicted for unconfined jets. The conclusion was that there is destructive interference between the jets and the fluids returning from under the bit. This interference would increase as the nozzle diameter increases because of a higher turbulence level at the jet boundary that causes a higher rate of decay of the jet velocity. Note that Warren and Winters' results do not fully agree with those of Sutko and Myers. The latter show no influence of nozzle diameter on the dimensionless pressure coefficient in the three-nozzle configuration, while the former do.
This review points out some difficulties one would encounter when trying to apply the available results to hydraulic optimization programs. There is no reliable method yet for calculating the impact pressures at the bottom of the hole; this is unfortunate because impact pressures appear to be directly related to the rate of penetration (see Fig. 1 in Ref. 5).
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