September 6, 2008
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Home / Issue Archive / 2007 / June #6 / Aluminum vs. Steel: Preventing Drill String Buckling when Drilling Horizontally

№ 6 (June 2007)

Aluminum vs. Steel: Preventing Drill String Buckling when Drilling Horizontally

By M. Guelfgat, D. Basovich, I. Buyanovskii, A. Vakhrushev; Aquatic Company, Russia

Selection of efficient drill string (DS) assembly is critical for successful drilling of deep and horizontal wells. The basic limitations while drilling such wells are the drilling tool weight and related resistance forces interceptive to DS penetration and rotation resulting in pipe overstresses, and potential DS buckling while drilling horizontal wells, as well as the torque increase during thrust load and rotating torque transfer to a drill bit.  

Light Alloy Drill Pipes of Improved Dependaility (LAIDP) can reduce the weight overall of the DS when compared with Steel Drill Pipes (SDP). This reduction in weight can considerably decrease stress on the DS and prevent buckling in many cases.
Besides, LAIDP have a set of advantageous physical and mechanical properties if compared with other Steel Drill Pipes (SDP):
- high resistance to corrosion environment (hydrogen sulfide and carbon dioxide);
- non-magnetic properties providing for efficient application of these pipes as lightweight MWD housing;
- low rigidity that is advantageous for sidetracking with low deviation radius.
The key requirements to LAIDP design are specified in ISO 15546 Aluminum Alloy Drill Pipes for the Oil and Gas Industry, which is in force since 2002.
LAIDP includes the pipe fabricated of aluminum alloy and a steel tool joint with pin and box connected to the pipe end with the help of a TT-type tapered buttress thread.  

Subject to size and purpose, manufacturers provide the following types of LAIDP tube stock:
- with internal and external upset ends;
- with increased protective thickness for DS better aligning in borehole, pipe body wear protection as well as for increase DS buckling stability;
- heavy walled to be used as non-magnetic pipes and MWD casings for slant and horizontal drilling;
- with outside spiral rib to enhance drilling cuttings transport from horizontal borehole sections.
Physical and mechanical properties of four groups of aluminum alloys used for LAIDP manufacturing  are shown in Table 1.
All aluminum alloys for drill pipe fabrication have similar 2,780 kg/cu. m density and 0.71*105 MPa Young modulus.

Comparative Analysis of LAIDP and SDP Properties

Comparative analysis of efficiency of drill pipes fabricated of different materials should not be limited with individual comparison of weight or strength properties only because DS strain state is typically a function of combination of these parameters. So, it is reasonable to compare drill pipe performance specifications by three following criteria:

Criterion 1, specific strength of drill pipe material, is an integral parameter specified as a relationship between tensile load in a pipe body the acceptable in accordance with strength conditions and its weight in drilling mud, such as:
 
        

(1)
    

 

where: Pmax is acceptable load, which creates st strain equal to minimum yield of pipe material in the top cross-section of a uniform-sized DS composed of drill pipes of one size range and material; Gtube and Ltube are, respectively, pipe weight on the surface and pipe length, and K is pipe lightning factor in drilling mud.
Specific strength L calculated by Formula (1) has length dimensions and physically determines the maximum possible length of suspended one-sized DS section, at which the extension strain caused by the drill string dead weight in drilling mud achieves the yield value.
Fig. 1 provides comparison of specific strength L for 5-in. to 57/8-in. with 7-in. joints SDPs made of high-strength G-105, S-135 and V-150 steel grades and LAIDPs of the close size range: 57/8 -in. to 7-in. with joints of similar size range, made of aluminum alloys of Groups I (D16Т) and II(1953Т1); all pipes are used for drilling with 81/2-in. drill bits.
The available data demonstrate that drill pipe specific strength is mainly a function of pipe weight and material strength, and to a smaller extend from pipe size range, i.e. outside diameter and wall thickness. Besides, LAIDP of Group 1953T1 is the second to SDP in terms of acceptable load, however it has higher specific strength within entire range of drilling mud density, and this effect grows with drilling mud density increase.

Criterion 2 that shall be taken into account while comparing LAIDP and SDP for horizontal drilling characterizes drill pipe capability to transfer thrust load to the bit without any loss of DS buckling stability.

It is evident that for preservation of DS buckling stability the maximum compressive thrust load applied to the lower horizontal DS section shall not exceed critical load Fкр, whereas thrust load that can be applied to a drill bit via this section can be identified from the formula:

(2)

 

where: Gд is the maximum thrust load that may be applied to a drill bit via the pipe horizontal section without any loss of DS buckling stability;
Fкр is a critical sinusoidal buckling load that may be calculated for the horizontal borehole as follows:
          

(3):

 

EI is a bending stiffness of pipe material;
w is a pipe linear weight in liquid;
d  is an equivalent radial gap between the borehole walls and the pipe;
L is DS length within the limits of the horizontal borehole under consideration;
fо is an extended resistance (friction) factor of pipe axial movement within the borehole that is a function of pipe position on the borehole bottom wall, on volume of cuttings inside the borehole, as well as on relationship between momentary values of progressive and rotary components of velocity vector of pipe exterior surface points [1].

Fig. 2 shows comparative charts of maximum thrust load Gд calculated by Formulae (2) and (3) that can be delivered via DS to a drilling bit in 81/2-in. open hole during rotary drilling and at drilling mud density 1,200 kg/cu. m (10.05 ppg) with regard to the length of LAIDP 147 x 13 - 1953T1 horizontal section having OD = 7-in. pipe joints with FH thread, and SDP 5 in. х 19,50#, S-135 with welded joints of the same size range.

Fig. 2 shows that subject to length L there are three areas with the following characteristics: in Area 1 higher load on a drill bit can be achieved via SDP to compare with LAIDP without any loss of buckling stability; on the opposite, in Area 2 LAIDP is more advantageous; and in Area 3 LAIDP only still can deliver load to the drill bit.

The dimensions of the above areas are the function of maximum potential LG length of horizontal pipe section that can be respectively calculated for LAIDP and SDP by the formula:
   
        

(4):
    

 

LG parameter can be considered as Criterion 2 for comparison of SDP and LAIDP during horizontal drilling.

Criterion 3 of LAIDP and SDP comparison during horizontal drilling characterizes drill pipe capability to deliver torque to a drill bit.
Maximum torque Мд that can be delivered to BHA via drill pipe horizontal section is described by the formula:         

 

(5),

 

where: fr is a generalized resistance (friction) factor against the borehole walls during DS rotation;
[M] is a maximum permissible torque at the beginning of horizontal section.

Fig. 3 provides maximum torques Мд calculated for conditions of the above example that can be delivered to BHA via horizontal SDP or LAIDP section depending on its length.

The chart in Fig. 3, like the one in Fig. 2 contains three specific areas with various relations between the torques delivered to a drill bit via horizontal SDP and LAIDP sections.

The dimensions of the above areas are the function of ratio of maximum potential length Lм of pipe horizontal section that can be respectively calculated for LAIDP or SDP by formula:

(6),

 

Lм parameter can be considered as Criterion 3 for comparison of SDP and LAIDP during horizontal drilling.

It should be noted that comparison of drilling pipes of other size range of materials may result in configuration of load and torque charts different from Fig. 2 and 3.

In particular, if both Fкр ([M]) and LG (Lм) for one pipe exceed respective parameters of the compared pipes, it means that application of such a pipe may result in drilling of a longer horizontal well and at higher thrust load (torque) delivered to a drill bit.

Data on Field Practice of LAIDP Application for Deep Drilling

The most remarkable example of the effective LAIDP application is Kolskaya SG-3 super-deep well, the drilling of which to a record depth of 12,262 m (40,230 ft) would be actually impractical without lightweight pipes [2]. At borehole bottom the slope angle was 12° and temperature was 220°C (428°F).
DS included the following LAIDP sections (bottom - top):
- 147 х 11 and 147 х 13 Group III (АК4Т1) alloy, section length 3,805 m (12,484 ft);
- 147 х 11 Group I (D16Т) alloy, section length 1,205 m (3,953 ft);
- 147 х 11, 147 х 13, 147 х 15 and 147 х 17 Group II (1953Т1) alloy, section length 5,150 m (16,896 ft).

Drilling rig of load capacity 4,000 kN (899 kips) was used for combined drilling technique with simultaneous drill bit rotation with the help of a downhole motor and rotary table.

Recently in Western Siberia, due to a considerable increase in the amount of horizontal and multilateral wells, LAIDP application turned out to be very effective and currently over 230,000 meters of pipes of this kind are used in the area.

Foreign companies also use LAIDP while drilling offshore.
Below are provided some drilling data and comparative analysis for Well 834R, Well Pad 78 drilled in Novo-Pokurovskaya area by Megion UBR company with application of LAIDP 147 x 13-D16T. The well is typical for the region. All calculations shown in the report have been made based on Aquatic-manufactured special software 3-DDTBHC (Drillstring-Drag-Torque-Buckling-Hydraulic-Calculation) [3].

The below data correlate well with changes of drilling parameters for Well 834R/78.
Well profile and design are shown in Fig. 4: measured depth is 4,264 m; TVD is 2,884 m; horizontal interval with 89° slope angle is 1,003 m; maximum deviation intensity is 2°/10 m, temperature in the design depth is 110°C.

The following drilling method was used: combined technique utilizing hydraulic downhole motor, with simultaneous DS rotation with a rotary table. Drilling mode used in the design depth: weight on the bit 80 kN, rotary speed 60 RPM, penetration rate 9 m/hour, drill mud density 1,050 kg/cu. m, drilling mud circulation rate 25 liter/sec.

Comparison of key design parameters of the DS performance at measured depth of 4,264 m is shown in Table 2.
The provided resluts of comparative calculation and analysis of field data on LAIDP and SDP application in Well 834R/78 reveal that at the design depth of 4,264 m, due to considerable DS weight and friction reduction, aluminum alloy drill pipes provide for decrease of the following parameters:
- DS own weight in drilling mud by 2.08 times;
- rotary table torque by 1.86 times;
- hook load during tool POOH by 2.22 times;
- friction pressure in well circulation systems by 3.5 MPa.

As a result, LAIDP application at this well and at other similar wells made it possible to use 125-ton drilling rigs instead of 200-ton rigs, which were used with SDP.

The analysis of data for 31 LAIDP sets used in Western Siberia revealed that service life of one LAIDP set identified by pipe body and joint wear, as well as by thread connections endurance at cyclic loads, comprised 70,000 to 90,000 m of borehole at minimum 400 make-up/breakdown operations.

References:

1. Alexandrov M.M. Friction Forces During Drill String Movement in Borehole. Nedra, 1978
2. В. Basovich V.S., Guelfgat M.Y. Aluminum drill pipes for super-deep projects. Eurasia Offshore, April 2007
3. Basovich V.S., Guelfgat M.Y., Buyanovski I.N., Basovich D.V. Technical and technological limitations of drill string assembly for horizontal wells of supreme length. Oil and Gas Wells Construction Magazine, #4, 2007, Moscow, VNIIOENG.

Copyright © 2007 Eurasia Press, Inc. (USA). All rights reserved.
Copyright © 2007 Eurasia Press (www.eurasiapress.com)