Hydraulics Optimization
1) HSI
• Hydraulics horsepower should be maximized when drilling in applications where balling may be a concern. Such applications will include deep holes, the use of high mud weights, and reactive formations with water based muds,
• Maximum HSI is obtained when the nozzle pressure drop is equal to 65% of the total standpipe pressure. This can be modeled in the Hydraulics program by modifying the TFA of the bit.
• Optimal HSI for drill bits is in the range of 3 to 7, with the higher values increasingly beneficial to drilling in balling conditions.
• The optimal HSI may not always be reachable due to other hydraulic limitations.
Thus, additional factors that will assist (asides from mud additives) in resisting bit balling will be the actual design of the bit itself. Maximizing face volume, low blade count (PDC), and good cutter spacing, will all help.
2) JIF
• Jet impact force should be maximized in applications where both cuttings returns and bit balling are potential problems.
• General applications are shallow holes, drilling at high penetration rates through reactive formations.
• High JIF will also assist in removal of formation by jetting. Be aware that this may cause issues with hole washout in certain situations
• Maximum JIF is obtained when the bit pressure drop is equal to 50% of the total pump pressure
• Note: If using a center jet on a roller cone bit, the flow area of that nozzle should not exceed 18% for the total TFA
3) Flow Rate
• The flow rate should exceed the critical transport rate for efficient removal of cuttings, taking into account, hole geometry and trajectory
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• If using high flow rates, ensure that the bit to be used has the capacity for large size nozzles, as these will be required to avoid excessive standpipe pressure.
Multiple jets are also beneficial so that a good balance of HSI and JIF can also be obtained
• Flow rates should ideally be approximately 30 to 50 gpm, per inch of the bit diameter for small diameter bits. Flow rates for larger diameter bits should be in the range of 40 to 70 gpm per inch diameter. See table below for flow rate guidelines, though note that these are NOT limitations.
Bit Size (") Flow Rate (gpm)
3 1/8 80 - 160
6 1/8 175 - 300
8 1/2 350 - 550
9 7/8 400 - 700
12 1/4 550 - 900
14 3/4 700 - 1050
17 1/2 750 - 1200
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Drill Bit Hydraulics
Tool Considerations
a) PowerDrive. This tool requires a pressure drop in the region of 500 to 800 psi, though ideally 650 to 700 psi. This is constant across the range of tool sizes and is generally attained via the pressure drop across the bit, which is reliant on the correct nozzling. In some applications, nozzling of the bit to attain this required pressure drop is not possible.
In these instances, you should implement an internal flow restrictor that sits within the PowerDrive tool (basically, this is an internal centre jet nozzle). Total pressure drop is calculated via the addition of pressure drop across the bit plus that across the centre jet, as these nozzles are in series.
b) Motor bearings. In order to effectively divert mud across the radial and axial bearings of the motor, a minimum pressure drop across the bit of 250 psi is required. For low bit pressure applications, specific bearings exist that only require a minimum of 100 psi pressure drop. Though in most situations, these pressure drops are easily attainable, there may be issues when bits with large TFA’s are run. The maximum pressure drop for the bearings is 1500 psi
c) Nozzle blanking. In certain applications it may be desirable or necessary to blank one or more of the nozzles (i.e. effectively seal the nozzle closed using nozzle blanks).
Blanking nozzles will provide cross flow, but will also leave a segment of the bit with low hydraulic energy. When considering which nozzle to blank, on PDC bits you should select a nozzle that is at the greatest radial distance from the centre of the bit and one that does not clean a primary blade. With roller cone designs, you should blank the nozzle that cleans the cone with the least gauge cutter elements.
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Bit Hydraulics
Roller Cone Hydraulics
Roller cone bit hydraulics are restricted in terms, primarily, of nozzle quantity; There is typically only one nozzle per cone, thus three nozzles per bit. However, there is also the option, particularly on large bit sizes (>17 ½”) of a center jet. This enables good flow to reach the center of the bit, which is often starved in large bits due to the diameter
affecting the proximity of the standard nozzles from the center. Another important factor is that these large sized bits commonly drill soft sticky formations in top hole sections, thus the center jet provides useful add ional cleaning. There is also the option to ‘multiple port’ the central nozzle so that you can further optimize jet force and flow direction.
There are a number of common features specific to roller cone nozzle arrangements:
• Extended nozzles: The nozzle exit is extended so that it is closer to formation, improving the jet efficiency for cone cleaning and removing formation. High flow tubes are a similar concept that will accommodate a larger range in jet sizes.
• Asymmetric nozzles: Large variation in selected nozzles sizes on the bit in order to create cross flow across the face of the bit. An extreme variation would be to actually blank off one of the nozzles to generate this.
Centre Jet
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• Commercial arrangements: There are a number of nozzle design concepts
marketed by the bit manufacturers. These primarily relate to the orientation of the nozzles and their impingement points on the bit. For example:
o Mudpick – Developed by Reed. The nozzles are angled to focus the mud flow across the gauge teeth
o Clean Sweep – The mud flow is directed backwards across the cutting structure. Hughes
Note: The nozzles used are replaceable and interchangeable in the field to enable hydraulic optimization. They are generally retained in the nozzle housing via a circlip.
The nozzle types used by bit manufacturers vary and are often not interchangeable with those used by a separate manufacturer. Ensure sufficient, appropriate, nozzles are available at the rig site.
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Bit Hydraulics
PDC Hydraulics
A) Nozzles: Nozzle placement is ideally such to provide efficient cutter cooling and cuttings removal from the face of the bit, achieved via adequate radial distribution and quantity of nozzles. The impingement points of the nozzle on the bit profile must also be set so as to avoid body erosion and stagnant flow areas.
There are two primary forms of nozzles:
• Interchangeable: One or two-piece designs with varied jet size that screw into a nozzle housing located on the bit. Key interchangeable nozzle types and their associated jet ranges can be viewed in the table below.
• Fixed: Often referred to as ‘ports’. Generally unchangeable jet size in the field.
Some bit suppliers do have the option to seal, or close, the fixed nozzle.
Unlike roller cone designs, both quantity and combination will vary significantly from bit to bit, and thus it is important to clarify nozzle type, quantity, and range of jet sizes when planning bit hydraulics so as you do not derive a TFA which is unobtainable with that specific bit. You should also check that suitable jet sizes and appropriate wrenches are available at the rig site.