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

December 5, 2012

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