Economics and Application
Regardless of how well designed or manufactured a bit, the situation in which it is used, and the applied practices, ultimately determine the success or failure of the run.
When to use a diamond bit • When economics dictate
• Generally, the rate of penetration ultimately determines the economics of the bit run • When on-bottom times are important
• When oil-phase mud systems are used (preferred)
• When water-phase systems are used in non-hydrating formations (preferred) • When rotating at high speeds (turbine or PDM)
• When high bottom hole temperatures are encountered (approx. 300° F and higher) • When drilling in deviated hole section requiring light bit weight -When drilling significantly
overbalanced Why use a diamond bit • Economics
• To reduce the number of trips in order to :
- minimise running through dangerous hole sections - minimise rig wear
• To avoid tripping in bad weather
Where to use a diamond bit (Formations in which diamond bits are normally beneficial) • Polycrystalline Diamond Compact (PDC) Bits
- Very weak, poorly consolidated, brittle, shallow sediments (e.g. Miocene sands, silts, clays) - Low strength, poorly compacted, brittle, non-abrasive, relatively shallow sediments,
precipitates and evaporites (e.g. salt, anhydrite, marls, chalk – Devonian/Muschelkalk) - Moderately strong, somewhat abrasive and ductile, indurated medium-depth sediments,
precipitates and evaporites (e.g. silty claystone, siliceous shales, porous carbonates, anhydrite — Eocene)
• Natural/Thermally Stable Diamond Bits
- Moderately strong, somewhat abrasive and ductile, indurated medium-depth sediments, precipitates and evaporites (e.g. siliceous shales, porous carbonates, anhydrite, silty claystone— deep Miocenes)
- Strong and abrasive indurated, very ductile deep sediments, precipitates and evaporites (e.g. sandy shales, calcareous sandstones, dolomites, limestone — Pennsylvanian/Mississippian) - Very strong and abrasive, indurated ductile and non-ductile sediments, precipitates and
evaporites (e.g. Bunter sandstone, bromides, etc.) Formations detrimental to diamond bits
• Polycrystalline Diamond Compact (PDC) Bits
- Hard, cemented abrasive sandstone (e.g. sedimentary quartzite) - Hard dolomites (sedimentary or metamorphic)
- Iron (e.g. pyrite — metamorphic or igneous) - Chert (metamorphic or sedimentary) - Granite and basalt (igneous) • Natural/Thermally Stable Diamond Bits
- Hard, cemented quartzitic sands that are highly fractured and abrasive How to determine the application
• Use geological information to determine which offset wells in the area are likely to be representative for the well to be drilled.
• Review the bit records from the offset wells, in particular their condition when pulled and the calculated economics.
• Review the wireline logging data from the offset wells
DRILLING PRACTICES
DRILLING PRACTICES
DIAMOND BITS - FIELD OPERATIONS
Prior to running the bit
• Reach an agreement with the operator/contractor on what is expected of the bit including, if appropriate, its suitability for drilling float equipment
• Reach an agreement on what mechanical and hydraulic requirements are available and/or necessary to achieve optimum or expected performance
• Before running the diamond bit into the hole, have a junk basket run on the previous bit • After the previous bit is pulled, inspect it for junk damage and other wear, then gauge it • If the previous bit appears OK, the bit may be readied to run into the hole
• Check 0-ring and install nozzles, if appropriate • Check for cutter damage
• Check that the bit is within tolerance on diameter and that there is no foreign material inside it • Recommend the use of drill pipe screens
Running the bit (rotary assembly)
• Handle the diamond bit with care. DO NOT set the bit down without placing wood or a rubber pad beneath the diamond cutters
• A correct bit breaker should be used and the bit should be made up to the correct torque as determined by the pin connection size
• Care should be taken in running the bit through the rotary table and through any known tight spots. Hitting ledges or running through tight spots carelessly may damage the bit gauge • Reaming is not recommended, however, if necessary, pick up the kelly and run the maximum
fluid possible. Rotate at about 60 RPM. Advance bit through tight spot with no more than 4000 pounds weight on bit (WOB) at any time
• As hole bottom is approached, the last three joints should be washed down slowly at full flow and with 40 to 60 RPM to avoid plugging the bit with fill
• The bottom is found by observing the rotary torque indicator as well as the weight indicator. The first on bottom indication is usually an increase in rotary torque
• Once the bottom is located, the bit should be lifted just off bottom (0 to 1 foot if possible) and full volume circulated while slowly rotating for about 5 to 10 minutes
• After circulating, ease back to bottom and be patient in establishing the bottom hole pattern • When ready to start drilling, increase the rotary speed to about 100 RPM and start cutting a new
bottom hole pattern with approx. 1000 to 4000 pounds WOB
• Cut at least one foot in this manner before determining optimum bit weight and RPM for drilling • Determine optimum ROP through a drill-off test
Running the bit (PDM & turbine)
• Start the pumps and increase to the desired flow rate when approaching bottom • After a short cleaning period, lower the bit to bottom and increase WOB slowly • After establishing a bottom hole pattern, additional weight may be slowly added
• As weight is increased, pump pressure will increase, so the differential pressure and WOB must be kept within the recommended downhole motor specifications
• Drill pipe should be slowly rotated to prevent differential sticking • All other operating practices are as per standard practices Pull the bit when
• The bit stops drilling
• The bit ceases to be economical as shown by cost/foot calculations
• When very high on-bottom torque with little WOB and a decrease in ROP occurs - the bit may be undergauge in a tough formation
• There is a dramatic decrease in ROP and on-bottom torque • If there are changes in stand pipe pressure
- if it goes up there is probably a cutter structure failure
DRILLING PRACTICES
DIAMOND BITS - COMMON PROBLEMS
Difficulty going to bottom Low pressure differential across nozzles or bit face High pressure differential across nozzles or bit face Fluctuating standpipe pressure
Bit won't drill
Slow rate of penetration
Excessive torque
Bit bouncing
Previous bit undergauge New bottom hole assembly Collapsed casing Out of drift Bit oversized Stabiliser oversized Flow area too large Flow area too small
Different drilling parameters than designed for
Washout in drill string Flow area too small Excessive flow rate
Diamonds too small for formation Bit partially plugged
{formation impaction} Formation change Ring out
Downhole motor stalled
Drilling through fractured formation Formation breaking up beneath bit Stabilisers hanging up
Equipment failure Bottom not reached
Stabilisers hanging up or too large Formation too plastic
Establishing bottom hole pattern Core stump left
Bit balled
Not enough weight on bit: hydraulic lift
RPM too low/high Plastic formation Change in formation Overbalanced Diamonds flattened off Cutters flattened Pressure drop too low Wrong bit selection Excessive weight on bit Slow rotary speed Stabilisers too large Collars packing off Bit undergauge Slip-stick action Broken formation Pump off force
Ream with roller cone bit
When reaming to bottom, pick up and ream section again. If difficulty remains, check stabilisers. Roll casing with smaller bit
Use bi centre bit or reduce bit size
Gauge bit with API gauge; if not in tolerance, replace bit Replace with correct size stabiliser
Increase flow rate and correct on next bit Increase flow rate/strokes
Change liners
Attempt to optimise flow area on next bit change Check bit pressure drop, drop softline, trip to check pipe and collars
Reduce flow rate, change flow area on next bit Reduce flow rate
If ROP acceptable, change on next bit
If ROP unacceptable, pull bit and use bit with correct diamond size
Check off bottom standpipe pressure
Let bit drill off, circulate full volume for 10 minutes while rotating. Check off bottom pressure again
Pick up, circulate, resume drilling at higher RPM, reset, drill off test
On and off bottom pressure test, pull bit Refer to manufacturer's handbook If ROP acceptable, continue If ROP acceptable, continue Check equipment
Try combination of lighter weight and higher RPM Check overpull
Check stabilisers on next trip Repair equipment
Check tally
Check torque, overpull
Check pressure – increase flow rate, decrease/increase bit weight, RPM
Can take up to an hour
Attempt to carefully drill ahead with low bit weight Back off and increase flow rate, then slug with detergent or oil
Increase weight on bit Increase/decrease RPM Reset drill off
Reset weight Reset drill off Accept ROP Pull bit
Compare beginning and present pressure drops – new bit may be required
Increase weight Pull bit
Increase flow rate – new bit may be required Pull bit
Reduce weight and RPM Increase RPM Decrease weight
Check bottom hole assembly, stabilisers should be 1/32" to
1/16" under hole size
Increase flow rate and work up and down Pull bit
Change RPM/weight combination Reduce RPM and weight Increase weight Decrease volume