Field Application of Diamond-bit Hydraulic-Lift Principles (includes associated papers 17470 and 17553 )
- Warren J. Winters (Amoco Production Co.) | T.M. Warren (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- August 1986
- Document Type
- Journal Paper
- 277 - 287
- 1986. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.12.6 Drilling Data Management and Standards, 1.6.6 Directional Drilling, 1.6 Drilling Operations, 1.6.2 Technical Limit Drilling, 1.5.1 Bit Design, 1.6.9 Coring, Fishing, 1.5.4 Bit hydraulics, 1.6.1 Drilling Operation Management, 1.11 Drilling Fluids and Materials, 1.10 Drilling Equipment, 1.5 Drill Bits
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Hydraulic lift can have a major effect on the operation of diamond drill bits. The actual weight on bit (WOB) is less than the measured weight by the amount of hydraulic lift, or pumpoff force, acting beneath the bit. Pumpoff forces in excess of 15,000 lbf [67 kN] have been recorded.
Historically, the hydraulic-lift effect has been recognized but not understood well enough to be accounted for reliably. Failure to account for hydraulic lift can adversely affect actual bit performance and interpretation of the results. It is now possible to determine hydraulic lift reliably with the practical techniques presented here.
Field tests show that pumpoff force tends to double as the bit dulls. Appropriate WOB adjustments, determined through pumpoff tests run on the rig, are required to maintain bit performance. The applications to diamond bits run on mud motors are particularly advantageous.
The total axial force (measured WOB) applied to a diamond bit during drilling consists of a mechanical component and a hydraulic-lift component. The mechanical component, referred to as true WOB, is the total contact force between the diamonds and the rock. The hydraulic-lift component, referred to as pumpoff, is a force acting on the bit body because of flulid pressure beneath the bit.
Hydraulic lift occurs because the pressure acting upward on the bit face is greater than the pressure acting upward on the bit face is greater than the pressure acting downward on top of the bit. Pressure is higher under the bit because of energy losses (primarily frictional) of the drilling mud flowing between the bit face and the rock. The force exerted on the bit face by this pressure differential is similar to the force exerted on a piston in a hydraulic cylinder.
Hydraulic lift affects all bits that develop a pressure drop in the fluid flowing between the bit face and the rock. Its magnitude varies in direct proportion to the bit pressure drop. Hydraulic lift is thus affected mainly by such factors as flow rate, fluid density, and flow geometry.
Failure to account for hydraulic lift can produce large measurement errors in WOB, which is one of the most important controllable drilling variables. Bit performance is often judged by the rate of penetration (ROP), but this is valid only when the ROP is referenced to the true WOB. Measurements of wear rates and bit life must also be referenced to the mechanical load (true WOB) at which the bit is operated. Thus any study of diamond-bit performance should include a pumpoff analysis.
The influence of hydraulic lift is seen by a comparison of the ROP behavior of two bits (pictured in Ref. 1) before and after pumpoff is accounted for. Fig. 1 shows that at 75 rev/min and 20,000-lbf [88.9-kN] applied WOB, the modified bit drilled 10 ft/hr [3 m/h] compared with 18 ft/hr [5.5 m/h] with the standard bit. The standard bit appears to be superior, but a different conclusion is drawn when the WOB is corrected for pumpoff. Fig. 2 shows that the same mechanical WOB, the two bits drill identically. The modified bit simply requires 10,000 lbf [44.5 kN] more applied weight than the standard bit to overcome its greater pumpoff force. Key operational factors like this cannot be determined without a pumpoff analysis.
A companion article1 presents a comprehensive laboratory study of hydraulic lift. It shows that the apparent nozzle area, KA, and the effective pumpoff area, Ae, can be used to quantify the magnitude of a diamond bit's hydraulic lift.
The apparent nozzle area combines the effects of bit geometry and the fluid-discharge coefficient into a single parameter for use in the basic nozzle flow equation to predict the pressure drop beneath a diamond bit.
where 12,031 becomes 82 951 when expressed in SI metric values.
The effective pumpoff area (defined mathematically in Ref. 1) reflects the radial pressure distribution beneath the bit, which governs the magnitude of the pumpoff effect. Hydraulic lift is the product of bit pressure drop and effective pumpoff area. The measured WOB is thus resolved into two components- true mechanical WOB and pumpoff force:
Fig. 3 gives an example of the relationships expressed in Eq. 2. Bit manufacturers are not equipped to provide reliable estimates of apparent nozzle area and effective pumpoff area. These key parameters must be determined from pumpoff tests conducted at the rig site.
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