MSE and ROP optimisation, worked through on a drilled section

Rate of penetration tells you how fast the hole is getting deeper. It does not tell you whether the bit is working well: a slow metre in hard sandstone and a slow metre on a foundered bit look the same on the ROP track. Mechanical specific energy (MSE) separates them. This article works through the method on an 8½″ section in hard, abrasive sandstone: the equations, a hand check, drill-off tests that find the founder point, and the MSE log that shows when a bit has stopped paying its way.

What MSE measures

MSE is the mechanical energy spent to remove one unit volume of rock. Teale defined it in 1965 as the work done by the weight on bit and the rotary torque, divided by the volume drilled. Energy per volume has the units of pressure, so MSE is quoted in psi and can be set beside the strength of the rock.

That comparison is the point. A bit that cuts cleanly spends energy roughly in proportion to the rock's strength: MSE rises in a hard sandstone and falls in a soft shale, and the ratio stays about the same. When something goes wrong at the bit, energy goes into vibration, into regrinding cuttings or into rubbing a worn cutter, and MSE rises while the rock stays the same. The signal to watch is the ratio of MSE to rock strength, and above all a change in it.

The strength to compare against is the confined compressive strength (CCS), the rock's strength under downhole confining pressure, usually estimated from offset sonic logs. Because it is an estimate, read the ratio for changes rather than for its absolute value.

The equations

Teale's MSE, in oilfield units, has a weight term and a rotary term:

W

weight on bit, lbf

A, d

bit area, in², and bit diameter, in

N, N_bit

surface RPM, and the bit's RPM (surface plus the motor's, where there is one)

T

surface torque, ft·lbf; T_bit is the torque at the bit

R

rate of penetration, ft/h

μ

the bit's sliding friction coefficient (Pessier and Fear), taken as 0.5 when nothing better is known

The rotary term is most of the total. Surface torque includes the string rubbing against the hole: a few kft·lbf in a vertical well, most of the reading in a long lateral. The usual fixes are to subtract the off-bottom torque taken at the last connection, or to estimate the bit's own torque from the weight, as Pessier and Fear did. The second gives the downhole MSE, used for efficiency in this article because it judges the bit and leaves the string out.

The founder point and the drill-off test

A sharp PDC bit drilling cleanly takes a depth of cut that grows with the weight on it. At a fixed RPM, ROP rises in a straight line with WOB and MSE stays flat. At some weight the line bends over: that weight is the founder point. Above it the extra weight goes into a dysfunction instead of into cutting. In soft shale it is often bit balling or poor cleaning under the bit; in hard rock it is usually stick-slip, where the bit stalls and then spins free, or whirl. ROP stops rising or falls, and MSE climbs.

The founder is found by a drill-off test, or step test. Hold RPM and flow, step the weight up in even increments, hold each level for a few minutes, and read ROP from the hole made over each hold rather than from the lagging ROP channel. Repeat at two or three RPMs, because the founder usually moves with speed. An hour or two of testing replaces a guess with a measurement for the rest of the run.

The example well

The example is a vertical gas well set in conditions typical of the Cooper Basin, Australia. The section is the 8½″ hole from the 9⅝″ shoe at 1,050 m to TD at 3,150 m: a soft Cretaceous and Jurassic sequence, Triassic mudstones, then the Permian coal measures, with thin coals, shales and silica-cemented quartz sandstones that are hard and abrasive.

Bit 1 drilled 1,509.5 m in 54.3 hours on bottom, 27.8 m/h, and came out at the top of the Permian for the harder-wearing seven-blade design. The rest of the article follows bits 2 and 3.

A hand calculation for one sample

Take one sample in the sandstone at 2,680 m: 24.9 klbf, 150 rpm, 17.6 m/h and 5.75 kft·lbf of surface torque.

1. Bit area. π / 4 × 8.5² = 56.75 in². The weight term is 24,900 / 56.75 = 439 psi.
2. ROP in feet. 17.6 × 3.281 = 57.7 ft/h.
3. Surface MSE. 480 × 150 × 5,750 / (8.5² × 57.7) = 99,230 psi, plus 439: 99.7 kpsi.
4. Bit torque. 0.5 × 8.5 × 24,900 / 36 = 2,940 ft·lbf. The other 2.81 kft·lbf of the surface reading is the string turning in the hole.
5. Downhole MSE. 480 × 150 × 2,940 / (8.5² × 57.7) = 50,730 psi, plus 439: 51.2 kpsi.
6. Efficiency. The sandstone's CCS is 24.5 kpsi: 24.5 / 51.2 = 48 %.

DrillIQ's engine, from the unrounded sample, gives 99.9 kpsi surface and 51.3 kpsi downhole, within 0.2 % of the hand figures. Almost half the torque at surface never reaches the bit, so surface MSE would call this bit 25 % efficient when it is cutting at 48 %.

Reading MSE against rock strength

Above 2,560 m, bit 1 in the Triassic mudstone runs at about twice the rock's strength and the two lines move together. That is efficient drilling on this plot: MSE follows the rock at a steady multiple of it.

Bit 2 went in at 2,560 m and was run at 28 klbf and 110 rpm, more weight than bit 1 had used, for the harder rock. In the first Permian sandstone the gap between the lines opened at once: from 2,600 to 2,640 m MSE sat at 76.1 kpsi against a CCS of 23.5, 31 % efficient, at 9.6 m/h. Surface torque scattered four times as much as it did later in the run (a coefficient of variation of 8.0 % against 2.0 %), the surface sign of stick-slip. The rock was no harder; the bit was being driven past its founder.

At 2,642 m the crew ran three drill-off tests and reset to 24 klbf at 150 rpm. In the next 38 m of the same sandstone MSE fell to 53.3 kpsi, 46 % efficient, and ROP rose to 16.3 m/h: 70 % faster on less weight. The bit held that ratio through the next sandstones. The coals show as sharp dips in CCS. There the engine reads 28 % efficient, because ROP runs into the 50 m/h hole-cleaning limit. That is not a bit problem, so leave the coals out when judging the bit.

From about 2,850 m the picture changed slowly. The thick sandstone from 2,845 to 3,040 m has one strength, 25.5 kpsi, and the parameters never moved, yet MSE crept up to 60.5 kpsi by 2,900 m and 83.3 kpsi over the last 40 m before the pull, with ROP down to 10.5 m/h. A steady rise at steady parameters in uniform rock is the signature of a dulling bit. Bit 2 came out at 2,985 m for penetration rate. Bit 3, the same design, went straight back to 56.9 kpsi and 15.3 m/h in the same sandstone, which proves the rise belonged to the bit.

The efficiencies look low, and they are normal. Dupriest and Koederitz found bits at peak performance putting only about a third of their energy into breaking rock, and scaled surface MSE by 0.35 before comparing it with rock strength. Downhole MSE reads higher because the string's torque is already out of it. What matters is that 46 % fell to 31 % twice, for two different reasons, and each time the log showed it within a few metres.

The drill-off tests

The tests were run in the sandstone between 2,642 and 2,657 m. At 90, 120 and 150 rpm the driller held 500 gpm and stepped the weight from 12 to 30 klbf in 3 klbf steps, four minutes a level. The three tests took 1.7 hours and drilled 14.6 m.

At 90 rpm ROP climbs from 4.8 m/h at 12 klbf to 7.9 m/h at 21 klbf and then falls away; 30 klbf drills slower than 18. The engine fits two straight lines through the levels and puts the break, the founder, at 21.0 klbf. At 120 and 150 rpm the line stays straight longer and breaks at 24.0 klbf. Below the founder the 150 rpm line gains 0.70 m/h per klbf; above it the slope turns slightly negative. At its founder the 150 rpm test made 16.2 m/h, twice the 90 rpm test's best.

The MSE view shows why the founder is the weight to run. Below it MSE is flat at 53–55 kpsi at every speed: the bit removes rock at the same cost per cubic inch however hard it is pushed. Above it MSE turns up. At 150 rpm, 27 klbf still bought 0.4 m/h more than 24 klbf, but at 58.6 kpsi instead of 53.5: 10 % more energy for 3 % more ROP, the rest going into the cutters and the BHA. At 90 rpm, 30 klbf cost 104 kpsi, nearly twice the efficient value.

Higher RPM raised the founder because the limiter here is stick-slip, and a string turning faster resists stalling better. That is a property of this bit, BHA and rock, not a rule. In abrasive sandstone the other side of the trade is cutter wear: more revolutions mean more rubbing and heat at each cutter. The crew ran 24 klbf at 150 rpm and watched MSE for the first sign of wear, which came about 200 m later.

From MSE to an ROP optimisation routine

Used as a routine, MSE is how a rig keeps raising its drilling rate.

• Find the limiter. If ROP is below what the rock and bit allow, something limits it: the founder, hole cleaning, a pressure or torque rating, vibration, or a rule written for an older bit. If MSE is efficient and ROP is still low, the bit is not the limiter; look at weight transfer, hydraulics or the rig.
• Test, don't guess. Run a drill-off test whenever the bit, BHA or rock changes. Two tests in the same rock that agree beat any fit to the section's history, which mixes formations, bits and speeds.
• Change one thing and watch MSE. It should drop or hold within a stand. If it rises, the change found a new limiter.
• Decide the pull on evidence. Weigh the dull bit's slower ROP against the trip with a cost-per-metre calculation, and grade the bit when it comes out; the article on bit dull grading does that for the bits in this section.
• Record by formation. The founder and the efficient MSE ratio in each formation and hole size are what the next well inherits.

Hydraulics belongs in the same routine: below a certain flow the cuttings are not cleared from under the bit and MSE rises for reasons no weight change will fix. The article on hydraulics and hole cleaning covers that side.

A checklist for the rig team

• Plot downhole MSE against a CCS estimate for the section, not ROP alone.
• Use downhole MSE or subtract off-bottom torque; raw surface MSE overstates the bit's energy.
• Run a drill-off test at two or three RPMs whenever the bit, BHA or formation changes.
• Read ROP from the hole made over each hold, and hold each level for three to four minutes.
• Drill at the founder, not above it; weight above it buys little ROP for a lot of energy.
• Treat a steady MSE rise at constant parameters in uniform rock as bit wear, and plan the pull.
• Leave out soft streaks where ROP is held by a hole-cleaning limit.
• Record the founder and the efficient MSE ratio by formation for the next well.

How DrillIQ handles it

DrillIQ's ROP Optimisation module computes surface and downhole MSE on every on-bottom sample, with the bit's RPM from the motor or the downhole tool where there is one, and judges efficiency against each section's rock strength. It finds WOB step tests in the time log, decides whether each was a planned test, reads the founder from a two-line fit, and uses that measured founder in place of a curve fitted to the section's history. From the drilled samples it builds corridors for WOB, RPM, flow, torque and pressure, an ROP target and a recommended setting with a confidence grade. On this section it graded its own recommendation weak, because the weight barely moved outside the tests. It also measures each bit run's revolutions, work and energy for Bit Performance & Grading. Every number above came from that engine.

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References

1. Teale, R. The Concept of Specific Energy in Rock Drilling. International Journal of Rock Mechanics and Mining Sciences 2 (1965) 57–73.
2. Pessier, R.C. and Fear, M.J. Quantifying Common Drilling Problems With Mechanical Specific Energy and a Bit-Specific Coefficient of Sliding Friction. SPE 24584, 1992.
3. Dupriest, F.E. and Koederitz, W.L. Maximizing Drill Rates with Real-Time Surveillance of Mechanical Specific Energy. SPE/IADC 92194, 2005.
4. Dupriest, F.E. Comprehensive Drill-Rate Management Process To Maximize Rate of Penetration. SPE 102210, 2006.
5. Bourgoyne, A.T., Millheim, K.K., Chenevert, M.E. and Young, F.S. Applied Drilling Engineering. SPE Textbook Series Vol. 2, 1986.

About this example. The well in this article is a representative scenario set in typical Cooper Basin, Australia conditions and worked through with DrillIQ's engines. It is not a client's well, and its figures are not field results.