Abstract This paper will address some important factors that should be considered when designing drillstrings for horizontal and extended reach wells (ERW). A second paper will look into the issues of casing design for the same type of wells and present some practical field cases and examples of drillstring and casing design for ERW. Buckling of the string and its influence on reach capability, fatigue and directional control will be emphasized.
Trang 1Copyright 2002, SPE/PS-CIM/CHOA International Thermal Operations and Heavy Oil
Symposium and International Horizontal Well Technology Conference
This paper was prepared for presentation at the 2002 SPE International Thermal Operations
and Heavy Oil Symposium and International Horizontal Well Technology Conference held in
Calgary, Alberta, Canada, 4–7 November 2002
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Abstract
This paper will address some important factors that should be
considered when designing drill-strings for horizontal and
extended reach wells (ERW) A second paper will look into
the issues of casing design for the same type of wells and
present some practical field cases and examples of drill-string
and casing design for ERW
Buckling of the string and its influence on reach capability,
fatigue and directional control will be emphasized
Introduction
Drill-string design is of utmost importance for operations in
highly deviated, horizontal and extended reach wells It is a
well known fact that drill-string failure represents one of the
major causes for fishing operations which may lead to millions
of dollars in losses for the Industry1, 2 This problem will be
intensified when the string is submitted to the more rigorous
conditions present in highly deviated wellbores
Besides that, use of an inappropriate string will have
influence in the operation performance since it may impede
the use of the optimized mechanical and hydraulic parameters
In extended reach wells, hydraulics plays a major role since
long high-inclined sections are very difficult to clean and there
is a tendency to accumulation of cuttings in the low side of the
wellbore High flow rates may be necessary to provide an
efficient cuttings transport mechanism, which may result in
pump pressures higher than the ones the rig pumps can handle
Among the factors that should be considered when
designing drill-strings, it may be mentioned:
• maximum expected loads;
• accumulated fatigue;
• buckling;
• hydraulics;
• equipment availability
There are so many variables involved in drill-string design that it is difficult to obtain a completely optimized string However, careful consideration of the above mentioned factors will allow the operator to obtain a design that will successfully carry on the job in a cost-effective way
The Buckling Factor
Drill-string buckling prediction will be very important while drilling extended reach wells The behavior of the string in a long, high inclined slant or in a horizontal section of the well will sometimes be determinant in terms of maximum reach and steering capability
When drilling an ERW, the trajectory of the well may need adjustments according to the lithology being encountered A body of shale, for example, may intercalate a sandstone oil reservoir Since the shale should be avoided in order to prevent low productivity and completion problems, the well must be deviated in this point However, in a long reach well,
to deviate from this shale can be a difficult task due to the high friction forces generated by the contact between the wellbore and the helically buckled string
A helically buckled string will cause the friction force along the pipe to increase and, therefore, less force will be transferred to the bit making difficult further advances
Nowadays, with the use of rotary steering tool systems, this problem can be minimized, however, there a number of wells that are still drilled using the regular steering tools
Critical Buckling Force
As stated in Ref 3, buckling occurs when the effective compressive load exceeds some critical value There are a number of articles3,4,5,6,7,8,9 dealing with models for prediction
of the critical buckling force Those models simulated buckling for different wellbore configuration such as vertical, inclined, curved and horizontal Also, some of those models presented results that were apparently conflictants An interpretation10 of those results suggests that they were derived from different situations or, as better explained in Ref 3, different loading stages Table I and II summarize the conclusions from Ref 10 and Ref 3, respectively, in terms of the axial force applied to the pipe and the shape it will assume
SPE/Petroleum Society of CIM/CHOA 79001
Drill-String and Casing Design for Horizontal and Extended Reach Wells – Part I
J C Cunha, SPE, Petrobras
Trang 22 SPE/PS-CIM/CHOA 79001
This will be a good guide in terms of buckling prediction for
inclined/horizontal wells
Table I – Axial Load x Pipe Configuration 10
Load Configuration
r
EIwsin
2
r
2EIwsin
F r
EIwsin
2
r
2EIwsin
F r
2EIwsin
4
2 ≤ < Sinusoidal or Helical
F r
2EIwsin
α ≤
Table II – Axial Load x Pipe Configuration 11
Axial Compressive Force Configuration
r
EIwsin
2
r
EIwsin
F r
EIwsin
75 , 3
r
2EIwsin
F r
EIwsin
4
3 ≤ < Unstable sinusoidal
F r
2EIwsin
α ≤
As it can be seen, the interpretations in Ref 10 and 3 are
similar, although the limits for sinusoidal buckling in Table I
and II are numerically different The value used in Table II as
the critical sinusoidal buckling force is very close to the one
obtained in Ref 8 for the critical helical buckling force This
value was also mentioned in Ref 10 as capable of causing
unstable sinusoidal buckling
Influence of Torque
Normally the influence of torque is not considered in the
calculations for critical buckling forces It was proved9,10 that
this influence, although very small for vertical wells, maybe of
significance for certain extended reach wells, reducing the
string buckling resistance As noticed in Ref 11, in a typical
ERW, torsion loads will be higher than for a vertical well of
the same measured depth, then, each particular case should be
analyzed in order to decide if the influence of torque should or
should not be considered
A model considering the influence of torque on the critical helical buckling force was derived12 and the following equations resulted:
p
T r
w p p
EI
π
α
8
2
2 2
2
− +
p
T p
EI
F 16π 6π
2
2
−
Equations 1 and 2 form a system which solution gives the values for the critical force F and pitch p for the helix formed
by the pipe inside a wellbore under the action of force F and torque T
Note that, if in equations 1 and 2, torque is set to zero, then the expression for critical buckling force previously presented
can be recovered substituting in equation 1 the value for p 2
obtained from equation 2 This will result in equation 3, presented in Tables 1 and 2 as the critical helical buckling force without considering torque
r
2EIwsin
4
In order to verify how torque can affect the buckling resistance of pipes, a few calculations were performed using equations 1 and 2
Initially, torque was set to zero and critical buckling force was calculated for various drill pipes with diameters varying from 3 ½ to 6 5/8 in After that, the calculations were made again, this time considering torques of 15,000 and 25,000 lbf.ft
After that, the bending stiffness (EI) of each pipe was plotted against the reduction in critical buckling load caused
by the torque The results can be seen in Figures 1 to 4
For Figures 1 and 2 it was assumed a 12 ¼ in wellbore with a 30 degree inclination For Figures 3 and 4 it was assumed a horizontal well also with a diameter of 12 ¼ in
From the graphs, it can be implied that torque can cause significant reduction on drillpipes with small diameter On the other hand, for bigger pipes, that are the ones most used in extended reach wells, torque will have little influence on the buckling resistance
Torque and Drag Predictions
Normally a torque-drag computer program is used for estimation of tension and torsion for the string during drilling operations Once an estimation of maximum loads is obtained,
a safety factor should be applied over those values to account for extra loads resulted from inefficient hole cleaning, pipe stuck, wellbore instability, etc In Part II of this paper, an example of calculation for a ER well will be provided
Trang 3SPE/PS-CIM/CHOA 79001 3
Fatigue
The drill-string is submitted to great stress variation during
operations in ER wells Besides dynamic and static loads, also
temperature variations and corrosion will make the high stress
concentration areas susceptible to fatigue damage
The normal practice to avoid fatigue failure is to inspect
the drill-string after a certain period of time or after a certain
footage drilled Although inspection is a common practice in
the Industry, fatigue failure keeps plaguing drilling operations
causing heavy losses yearly
One solution that could minimize those failures would be
to individually track the efforts undergone by each element of
the drill-string
Since the elements in a drill-string are subjected to
different mechanical conditions, that will depend on its
position on the string ant the amount of time they are being
used, a single element tracking system, as proposed in Ref 13,
is a sound tool to minimize fatigue failure
Once each element of the string is identified and its history
of mechanical condition is tracked, calculation of the
accumulated fatigue can be done using a numerical method14
Hydraulics
As stated before, hydraulics will be very important when
drilling extended reach wells Besides the fact that an efficient
bottom hole cleaning will aid the rate of penetration, sufficient
energy must be provided to the mud to carry the cuttings
through the long high inclined sections
Turbulent flow is normally more efficient to clean the
wellbore than laminar flow However, the flow rate necessary
to provide turbulent flow may be so high that it will exceed
the rig pumps capability in terms of pump pressure This
situation will be more common when a mud motor is added to
the string
Use of large diameter drill pipes may minimize hydraulic
problems since it will imply in less friction loss inside the
string and a more constrained annular When these drill pipes
are not available, another solution will be the use of drilling
fluids specially designed for extended reach wells15 with
improved cuttings transport capability
Conclusions and Final Remarks
Drill-strings for extended reach wells should be designed
taking into account simultaneously the various parameters
involved The main objective of the first part of this paper was
to draw attention for those important points, emphasize
theoretical aspects of the buckling problem, recommend a
procedure to deal with fatigue accumulation and indicate the
fundamental literature used to establish the basics of our
design process
For practical purposes, when using high diameter
drillpipes (5 ½ in or bigger), torque can be disregarded for
calculation of critical buckling force
In the second part of the paper, besides emphasizing the
design of casing strings, actual field cases for two extended
reach wells will be described in detail
Nomenclature
F =Axial load acting on the pipe, lbf
E =Young’s modulus, psi
I =Moment of inertia, in 4
EI =Bending Stiffness, lbf.in 2
T =torque on the string, lbf.ft
w =unit weight of the pipe (immerse in fluid), lbf/ft
r =radial clearance between the pipe and the
wellbore, in
p =length of helix pitch,ft
α =well inclination, degree
Acknowledgements
The author would like to thank Petrobras for permission for publishing this paper
References
1 Dale, B A.: “An Experimental Investigation on Fatigue Crack Grouth in Drillstring Tubulars,” paper SPE 15559,
presented at the 61 st Annual Technical Conference and Exhibition, New Orleans, LA, October 5-8, 1986
2 Cunha, J C.: “Risk Analysis Theory Applied to Fishing Operations: A New Approach on the Decision-Making Problem,” paper SPE 28726, presented at the
International Petroleum Conference and Exhibition of Mexico, October 10-13, 1994
3 Miska, S., Qiu, W., Volk, L and Cunha, J C.: “An Improved Analysis of Axial Force Along Coil Tubing in Inclined Horizontal Wellbores,” paper SPE 37056
presented at the SPE International Horizontal Well
Technology, Calgary, Canada, November 18-20, 1996
4 Lubinski, A.: “A Study On The Buckling Of Rotary
Strings,” API Drilling Production Practice, pp 178-214
(1950)
5 Dawson, R and Paslay, P R.: "Drill Pipe Buckling in
Inclined Holes," paper presented at the 57th Annual Fall
Technical Conference of the SPE of AIME, New Orleans,
LA, September 1982
6 Mitchell, R F.: "Frictional Forces in Helical Buckling of
Tubing," Paper SPE 13064 presented at the 59th Annual
Fall Technical Conference of the SPE of AIME, Houston,
TX, 1984
7 Chen, Y C and Cheatham, J B.: "Wall Contact Forces on Helically Buckled Tubulars in Inclined Wells,"
Transactions of the ASME, Vol 112, June, 1990
(142-144)
8 Wu, J and Juvkam-Wold, H C.: “Study of Helical Buckling of Pipes in Horizontal Wells,” paper SPE 25503
presented at the Production Operations Symposium,
Oklahoma City, OK, March 1993
9 Miska, S and Cunha, J C.: “An Analysis of Helical Buckling of Tubulars Subjected to Axial and Torsional Loading in Inclined Wellbores,” paper SPE 29460
presented at the Production Operations Symposium,
Oklahoma City, OK, April 1995
Trang 44 SPE/PS-CIM/CHOA 79001
10 Cunha, J C.: “Experimental and Mathematical Analysis
of Buckling of Tubulars Subjected to Axial and Torsional
Loading in Inclined and Horizontal Wells,” paper
presented at the Drilling Symposium of the ASME ETCE
96, Houston, TX, January 1996
11 Hill, T H., Summers, M A and Guild, G J.: “Designing
and Qualifying Drillstrings for Extended-Reach Drilling,”
SPE Drilling and Completion, pp 111-117, June 1996
12 Cunha, J C.: “Buckling Behavior of Tubulars in Oil and
Gas Wells A Theoretical and Experimental Study with
Emphasis on the Torque Effect,” Ph D Dissertation, The
University of Tulsa, 1995
13 Sampaio Jr., J H B., Placido, J C R and Ferreira, S N.:
“Using Radio Frequency Identification Electronic Chips
to Effectively Control the Elements of Drillstring,” paper
SPE 49203 presented at the SPE Annual Technical
Conference and Exhibition, New Orleans, LA, September
27-30, 1998
14 Placido, J C R.: “Development of a Predictive Drillpipe
Fatigue Model and Experimental Verification,” Ph.D
dissertation, The University of Tulsa, 1994
15 Cunha, J C., Martins, A L, Sa, C H M and Fernandes,
P D.: “Planning Extended Reach Wells for Deep Water,”
paper SPE 74400, presented at the International
Petroleum Conference and Exhibition of Mexico,
February 10-12, 2002
SI Metric Conversion Factors
Ft x 3.048 E -01 = m
in x 2.54 E +00 = cm
psi x 6.894 757 E +00 = KPa
lbf x 4.448 222 E +00 = N
T=15000 lbf.ft - 30 Degree Wellbore
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0.00E+00 2.00E+08 4.00E+08 6.00E+08 8.00E+08 1.00E+09 1.20E+09
EI (lbf.in2)
Figure 1: Reduction in Critical Buckling Load x Bending
Stiffness – T=15000 lbf.ft – Wellbore Inclination 30 Degrees
T=25000 lbf.ft - 30 Degree Wellbore
0.00 5.00 10.00 15.00 20.00 25.00
0.00E+00 2.00E+08 4.00E+08 6.00E+08 8.00E+08 1.00E+09 1.20E+09
EI (lbf.in2)
Figure 2: Reduction in Critical Buckling Load x Bending Stiffness – T=25000 lbf.ft – Wellbore Inclination 30 Degrees
T=15000 lbf.ft - Horizontal Wellbore
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00
0.00E+00 2.00E+08 4.00E+08 6.00E+08 8.00E+08 1.00E+09 1.20E+09
EI (lbf.in2)
Figure 3: Reduction in Critical Buckling Load x Bending Stiffness – T=15000 lbf.ft – Horizontal Wellbore
T=25000 lbf.ft - Horizontal Wellbore
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00
0.00E+00 2.00E+08 4.00E+08 6.00E+08 8.00E+08 1.00E+09 1.20E+09
EI (lbf.in2)
Figure 4: Reduction in Critical Buckling Load x Bending Stiffness – T=25000 lbf.ft – Horizontal Wellbore