1. Rate of Penetration Limitations in High Angle Wells
Richard Jachnik
Applied Fluid Systems Ltd
ST16-XXX
2. Mining Old Data
Data from several MSc research Theses into Hole Cleaning
parameters originating from TUDRP were re-examined.
Simple Flow properties expressed as Pv/Yp for fluids have been
re-interpreted with rheology and curve fitting to derive a flow
profile which is then run in a complex numerical program to
derive the average viscosity and eccentric pressure loss during
pipe rotation.
There are many factors that can limit ROP. Effective cuttings removal
is just one major factor but a very important one.
3. Flow Loop Data Analysis
Using bed height where recorded, we can calculate Packing
Fraction (Pf)
Tomren: (Carbopol) gave 0.43 average Pf for 80°/60° & 0.5 for
40°
Duan: (Water & thin Pac fluid)@ 90° gave 0.45 Pf average of 60
tests with sand sizes from 0.45 to 3.3 mm diam : 1.4mm sand.
0.33 to 0.4 Pf no rotation; 0.51-0.58 Pf @ 40-80 rpm.
Erroneous high or low packing fraction values indicate the
difficulty in measuring bed height accurately - as acknowledged
by some researchers. Variability in the data.
Spreadsheet built using weights derived from recorded cuttings
vol % (Cv%), known cuttings sg and an average packing fraction
for hole angle.
4. Spreadsheet Example. Tomren
Data 60°Derived from CV%
Cuttings with same sg as a fluid will float. Fluid Density is treated as a
linear relationship from water to where Cuttings sg=Fluid sg (a
maximum) & Transport Efficiency adjusted accordingly to 100% at this
point.
5. Transport Efficiency
Historically was only used for Vertical wells & is just one way of
looking at complex inter-related hole cleaning parameters with
a simple equation that contains ROP
The TE% was usually defined as Annular Velocity - Slip Velocity
(A simplification but ignored by many!)
The cuttings volume % depends on several parameters not
seen in the equation- i.e. hole angle, pipe RPM, Mud density,
viscosity, cuttings size
Transport Efficiencies are quite low in high angle inclinations
but can increase exponentially towards 100% as particular
conditions are met
Another parameter- Removal Efficiency is also useful in
understanding hole cleaning ability in flow loop studies.
ROP (ft/hr)*(Hole Diam)^2 (in)*100
14.71* Flow rate (gpm)*Cuttings Volume %
6. Calculated Wt of cuttings (lb) in
flow/at moving bed surface Reported moving (lb)
5.7 5.6
4.7 5.0
4.0 4.5
3.5 4.2
3.2 3.7
Tomren Carbopol Data 80°
Reported moving weights derived from
Tomren’s reported Cv% moving - (accuracy in
determination?)
Spreadsheet relationships appear valid to given CV% values from reported data
7. Typical Transport Efficiencies
Tomren top, Bassal Bottom
Tomren Carbopol 54 ft/hr 14/14 Mud, 50 rpm 0.5 eccentricity,
5” hole 1.9” pipe
Bassal: 35 ft/hr, 20/20 Mud, concentric, 150 rpm, 8” hole
4”pipe. . Both are low weight muds
AV
ft/min
0 20 40 60 80 90
229 64.3 39.6 24.9 5.5 4.8 4.7
229 No data -- -- 13-18 14-18
(65°)
-- -- 9-16
The spread in Bassal values are due to the range in Particle size from ±8 mm to
±6.5 mm. The larger sizes have the higher TE%. Tomren used 0.25 in actual chips.
8. Removal Efficiency
Defined as 100*(weight of rock to drill in loop length-weight cuttings in loop at
steady state)/(wt of rock to drill in loop length)
9. Rotational Effects:
Bassal Gravel 90°. AV 255 ft/min
PV20, YP20 mud, 8.6ppg, 35 ft/hr ROP. Concentric pipe
RPM TE% RE%
0 3.2 93
50 5.7 96
100 9.6 97.6
150 10.6 97.8
NB: increasing the rpm from 100 to 150 rpm results in minimal improvement in
Removal Efficiency
For any combination of annular dimensions/fluid viscosity there will be a point where
increasing RPM does not significantly increase stress & where more rpm is pointless
10. Thick Mud Example
Hole Angle
Data
58 Viscometer
RPM
Dial value
Hole Size in 12.25 600 94
Pipe size 6 5/8 300 78
RPM 0 200 70
Eccentricity 0 100 58
Cuttings size 4.32 mm 6 43
Cuttings SG 2.6 3 40
ROP ft/hr 206 Mud weight 9.5 ppg
MTV ft/min 229 Pv/Yp 16/62
GPM 992
Cv% zero rpm 6.8 TE% 31.3
Cv% 120 rpm 6.5 TE% 32.6
18. Effect of Tool Joint 120 rpm
The picture show s a 6⅝” TJ fully eccentric in 8 ½” open hole
19. Conclusions (1)
Flow Loop data from various TUDRP studies was re-
evaluated to determine packing fraction, Transport and
Removal efficiencies.
Limitations in the TUDRP data make extrapolation to high
rates of penetration difficult.
The Transport Efficiency equation can be used at various
hole angles and can determine ROP limits for fixed CV%
values.
Analysis of the field case history shows that during
Instantaneous ROP the CV% was excessive.
The DD did well to get the hole down without major
incident which also serves to illustrate how important
drilling competency is in these high ROP situations.
20. Conclusions (2)
Backreaming was limited to stands in the zone from
2907 (72°) to 4625 (66°) but sometimes required two
passes. In total there were 9 stands backreamed. (all due
to cuttings build up due to the high ROP).
The desire for a record meant that risks were being
taken with pack-off’s that were likely ignored in
planning.
The large increase in ECD near TD shows that the
formations could tolerate these significant ECD’s (this is
not always the case).
It is doubtful that cuttings removal risks were properly
quantified before start and with lack of other field data it
is not possible to investigate these further.
21. Conclusions (3)
Drilling Wells Ever Faster May Not be the Measure of
Success
JPT December 2015, Pages 43-47.
Editor's Notes
1). Good morning Ladies & Gentlemen.
2). Headline: The presentation is divided into two parts. The first part deals with historical flow loop data originating from work carried out at Tulsa University under the auspices of Drilling Projects acronym (TUDRP) where several oil and service companies were involved in MSc thesis direction and peer review.
Some of the early work goes back to the late 1980’s and some is less than 15 years old. Consistency in what was measured and how it was reported is lacking from many of the studies.
3). One important factor in high angle cuttings removal is the packing fraction of the cuttings. A paper by Cho at al published in 2001 (SPE 71374) details their experimental results based on static and shaking settling of various rock types in an 8% KCl solution. We have derived Packing fractions from Tomren data (Carbopol polymer) and Duan data for water and low viscosity Pac’s in eccentric annuli with up to 50 rpm rotation. Tomren values tend to be slightly lower than the values obtained by Cho et all for similar type cuttings. The Pf values are derived from the reported bed heights which are in them selves not exact measurements and so some variability of the Pf is expected.
4). A spreadsheet was constructed from first principles using the basic flow loop data of cuttings volume %
This example is Tomren Carbopol at 60 degrees. The flow loop length was 40 ft in this instance. Curve fitting software (either Table Curve 2D or in some cases 3D) was used to fill in data gaps and extrapolate where required. An exponential equation was used to smooth his raw data in this example although the differences between actual and smoothed data are typically <1%. Curve fitting can be laborious especially when a large number of equations are available. A trial and error approach is sometimes required. Use of the Transport Efficiency equation can help to indicate where a curve fit is inappropriate. Many flow loop tests were conducted at one ROP and only later were tests run with increasing ROP (data of Larsen and Jalukar) but even here data is limited. The cuttings weight is derived from the Cv% and the used packing fraction. It is the weight in the flow loop length or a specific distance of hole or casing. The stationary bed height is derived from the weight remaining in the flow loop length less the weight moving. If this height is less than one average particle diameter then zero is assumed. The turqoise column is the Cv% at 100 % Transport Efficiency which is assumed to be where fluid density = cuttings density. The TE % and RE% and wt of cuttings in flow/moving bed will be explained shortly.
5).One can see the equation contains an ROP quantity, the flow rate and volume of cuttings as a % and will change for different hole sizes. It was usually defined for vertical wells as the annular velocity less the cuttings slip velocity which has been shown to be inaccurate in many cases and conveniently ignored. It is possible to generate a true slip velocity and a rise velocity for vertical data in eccentric condition with rotation and a slide of this is included in the stick presentation but not included here for brevity.
Transport efficiency is low at high angles (above 50°), since the role of suspension in retarding settling velocity is also greatly reduced as distances for particles to fall to the low side also become minimal. By juggling around this equation with estimates of 2 of the 3 unknowns (Transport Efficiency and CV%) we can obtain reasonable inferences to ROP limitations. CV% maximum for vertical wells has typically been recommended at 5% based on experience. TUDRP researchers showed that often a much lower CV% was required to achieve critical flow velocity (where all cuttings are moving forwards so that no bed exists in high angle inclinations while flow is ongoing.- (This value is usually between 1 and 3% CV.) The weakness in their idea is that continuous circulation is usually not possible so cuttings beds will exist even at the flow rates required for critical velocity unless full re-suspension is attained quickly when flow restarts.
6). To place these values in perspective: At an Annular velocity of 229 ft/min the mud will exit a 40 ft long flow loop in 0.175 minutes (this corresponds to the lowest row). Since 74 lb remained in the loop out of a total rock weight to be cut of 891 lb, the difference (891-74)=817 lb has been shifted out of the loop in the equivalent time taken to drill (45 minutes) even though mud suspension is minimal. The Transport Efficiency for this row is 4.7% and the Removal Efficiency 91.7%.
7). The low Transport Efficiency values at higher inclinations are probably why Transport Efficiency has not historically been used in High angle wells. It is a reflection of the suspending abilities of the fluid (which is low at high angles). There are other complex mechanism’s in cuttings removal (rolling,piggy backing, skipping etc) and Removal Efficiency is a value of the quantity of drill solids that have been removed by the flow (by whatever means). Particles may be anywhere in the annulus especially during turbulent flow. During laminar flow they may be affected by the plug zone of yield stress fluids and so may exit quicker than average Annular velocity but they can also migrate from the plug zone to lower viscosity regions and vice-versa. The symbol after the TE% value for Tomren 90° indicates a Matrix calculated value not a measured one. In the Bassal study some orbital pipe motion was allowed but was not quantified.
8).Removal Efficiency can be more valid than Transport Efficiency for understanding general hole cleaning effectiveness at any angle. Removal Efficiency is a value of the quantity of drill solids that have been removed by the flow (by whatever means). It is a reflection of a variety of parameters not just suspension. Particles may be anywhere in the annulus especially during turbulent flow. During laminar flow they may be affected by the plug zone of yield stress fluids and so may exit quicker than average Annular velocity but they can also migrate from the plug zone to lower viscosity regions and vice-versa. Later TUDRP data actually reported cuttings weights during studies. To derive the cuttings weight in flow loops requires the CV% being accurate in the first instance along with a realistic packing fraction. Even though the Transport Efficiency is only 10% in this example from Tomren Carbopol data at 80 degrees, Removal Efficiency can reach 98%. Use of both efficiencies allow better interpretation of hole cleaning effectiveness.( The second last data points are at a flow rate of 229 ft/min)
9).In this example 4 in pipe was concentric in 8 in Hole and allowed to move in a random unrestrained fashion so we are dealing with more than a pure rotation effect. We can see that at the flow rate used even 150 rpm did not significantly increase transport efficiency. Removal efficiency improved greatest as the rpm increased from zero to 100 but little gain was seen at 150 rpm. Any modelling of motions outside of central axis rotation would not be always applicable to different annular dimensions and eccentricity where pipe may have different motions. It is also quite complex though some recent papers have tried to address the subject.
10).I chose this very thick mud with field ROP to illustrate problems in extrapolation: Data was taken from SPE 26217 where the authors list their recommended flow rates for minimum transport velocity based on their software program - which was based on their own small diameter flow loop data. The MTV stands for Minimum Transport Velocity and is considered as that flow which will keep all cuttings in suspension. The mud is so thick it meet the 100% Transport Efficiency criteria for vertical wells and up to 20 degrees at the flow rate used. However all other data including slurry flow in pipes indicate that cuttings will still settle in high angles at the recommended flow rate.
Flow loop data show that in the absence of rotation thicker muds lead to increased cuttings concentration and thicker cuttings beds. One just would not drill without any rotation at 60 degrees with this ROP – so in some respects it is really academic.
There will always be some high degree of eccentricity at 60 degree as most of the pipe will lie on the low side. Knowledge of a level of eccentricity is required for all high angle wells. This can be obtained in several ways, the most accurate is through a Torque and Drag analysis. (See SPE 102565). So So while the Transport Efficiency equation and the Removal Efficiency equation are coarse measurement yardsticks they can give reasonable idea’s as to what is happening in field cases.
11). These two clips show drill pipe in an eccentric condition of (0.82) where tool joints are assumed to be fully eccentric and the pipe body stiff. (7 5/8” Tool joint sizes were assumed for the 6 5/8” drill pipe). The red represents the highest flow velocity the blue the lowest. One can see why in the absence of rotation a cuttings bed could form. The flow modelling shows stagnant area’s that are not being swept even with 120 rpm rotation with this very thick fluid. While it is possible that a higher RPM may eventually sweep the low side adequately, excessive RPM of 180+ can lead to string issues with tools due to excessive vibration.
12). This flow modelling example relates to our field case about which I will discuss in a moment. The mud flow properties are very similar only the density of the flow loop mud is slightly lower. At zero rpm we see a similar picture to the thick mud in the previous slide. However at 120 rpm we see that all the dark blue low velocity area has now disappeared as rotation is now sweeping all of the low side. As a result the cuttings concentration is almost within the TUDRP criteria of Critical flow velocity.
13). In this slide we have curve fitted flow loop data to give the same annular velocity as our field case (either in open hole or casing) – (see the y value). The ROP for the Critical Concentration is given as the x value where the CV% will be <4. (The Jalukar cuttings size was expressed as “Large” with a D50 of 0.2 in.) Equation 1 was not used as gives an asymptote result at the high ROP levels. By inputting various CV% into the spreadsheet we obtain a maximum value of 3.34% for the higher annular velocity in the open hole and 2% for the lower annular velocity in the casing which complies with the critical flow rate (CFR) criteria. Fitting analysis shows that inaccuracies exist in the Jalukar reported CFR values as these are subjective. A better criteria such as CV% or cuttings weights should have been used.
This angle of 75 degrees is the only one where tests were conducted with mud thicker than 7 Pv/7 Yp at 100 rpm! Preconceived idea’s by the researchers? and their belief that these thin flow properties were optimum illustrate some of the data gaps that exist in the large body of TUDRP data gathered over 30 years! The viscosities of both Jalukar flow loop mud and the field mud are essentially identical. and can be represented as 14 Pv/14 Yp.
14). This slide shows the instantaneous ROP which in some cases exceeds 2000 ft/hr. The dots reflect the survey points. The well was drilled with a motor so ROP’s below 500 ft/hr reflect the small portion of slide drilling while orienting. The much lower average ROP (purple) which is also based around the survey points and then the simple fixed hourly average (blue). The latter two ROP’s include backreaming, circulation and connection time.
15). Note the high CV% which increases with extreme ROP. These high ROP’s mean that there is a high cuttings concentration within a certain volume of mud which will only reduce slowly as circulation continues should ROP subside. If we limit CV% to 5% the ROP is limited to 814 ft/hr (or less if one applies the critical concentration limit – since this is always <4 Cv%). Continuous circulation was not possible in this well so when pumps are stopped for connections, cuttings will fall into beds especially from the high ROP drilling slugs. So we can expect variable bed heights. Constant bed height inside casing at angles of 75-65 degrees is calculated at 1.5 in with 120 rpm with an ROP of 380 ft/hr.
16). Using Jalukar large flow loop data and the derived spreadsheet we have been able to calculate the Transport Efficiency in the field case with the aid of numerical modelling- it is about 80% for flow in the open hole. At 400 ft/hr the spreadsheet indicates no bed height in the open hole, But the cv% starts to become excessive at value above 600 ft/hr. The situation is somewhat worse inside casing due to the lower annular velocities But the overall situation is made considerably worse due to the high instantaneous ROP’s and resulting variable bed height.
17). With Numerical modelling we can calculate the eccentric pressure drop. The velocity vectors show again that in the absence of rotation the low side has low flow velocities which is conducive to cuttings bed build up especially on connections. With 120 rpm even at the flow rates used (800 gpm in 8.5” hole) the fluid sweeps all the hole reasonably well which re-inforces the view of no bed height at 400 ft/hr. Only the purely rotational effect about a central axis can be modelled with this approach. (The program uses a bipolar co-ordinate system type)
18). This simulation shows a fully eccentric tool joint inside casing. Large diameter Tool joints can aid break up of cutting beds but may also give rise to greater overpull if pipe is pulled through significant cuttings beds opposite pipe body. Minimising bed height opposite the pipe body is always the best approach in high angle wells. It was not possible to simulate drill string dynamics with the obtained data which may have improved hole cleaning slightly. A fully eccentric condition for the tool joints is not expected to have been a constant parameter as the well was deepened and due to the well profile.
19). As stated.
20).
The Direction Driller’s field notes show ECD’s of 11.2 ppg down to approx 5900 ft. (whereas the calculated mud wt using the BP model is 11 ppg) It then increases significantly to 12.58 ppg at 6440 ft.- Which indicates cuttings bed build up and/or lack of cuttings removal in the less inclined regions. The mud is too thin for a high Transport Efficiency at low angle. Many wells in other area’s would not be able to tolerate the high ECD so each case has to be evaluated on it’s merits, but these ROP’s indicate that drilling limits have been reached.
