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Home / Issue Archive / 2009 / January - December #1 / Basic Indexes Of Efficient Application of Light Alloy Drill Pipes of Improved Reliability (LAIDP)

№ 1 (January - December 2009)

Basic Indexes Of Efficient Application of Light Alloy Drill Pipes of Improved Reliability (LAIDP)

When drilling deep, super-deep, and especially horizontal boreholes it is extremely important to provide the high performance reliability of Drill String, reduce its stressed-deformed state and provide trouble-free service under extreme loads and at high temperatures.

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Drill srtring assembly and weight materially impact on technical-and-economic indexes of wellbore drilling process, formation of resistance forces and set the level of loads on drilling rig elements.
The use of LAIDP made of special-purpose aluminum alloys, which possess a number of valuable physical and mechanical properties advantageously distinguishing them from steels, a principal material for making drill pipes. It is one of the most promising ways for handling these tasks. Major properties, which shall be attributed to these ones, are as follows:
- low specific weight;
- high buoyancy in muds of different density;
- high specific strength;
- reduced value of Young’s and shear modulus;
- dampening properties;
- corrosion resistance in aggressive environment and, primarily in Н2S and СО2;
- non-magnetic properties; 
- easy drillability (destruction with bits and milling cutters).
These properties of aluminum alloys establish the basis for the efficient application of LAIDP in Drill Strings for drilling oil and gas wellbores.
1. Low Specific Weight
This is the main parameter determining weight of pipe linear meter, overall dead weight of Drill String and its stressed-deformed state.
Tensile loads on pulling Drill String from bottomhole and drilling torque on its rotation are principal limitations on drilling deep and long wellbores. Tensile loads are formed by dead weight of Drill String and resistance forces on its movement (drag) String rotation torque is formed only by resistance forces (drag) The listed parameters depend directly on specific weight of pipes material determining dead weight of Drill String. Resistance forces (drag) also depend on dead weight of Drill String, which forms pressing forces to wellbore wall and determines friction forces along with friction factor.
Considering weight of steel tool joint, LAIDP in the air is almost 2.5 times lighter than steel pipe of similar standard size while reducing strength properties by 1.5-1.8 times. This difference determines lower values of Drill String stressed state on application of  LAIDP in its assembly.
Design parameters on drilling a deep wellbore at TMD of 8,000 m, steel and combined string with LAIDP being used, can be given as example. At the design TMD the steel string rotation torque, RPM being equal to 80, made 23.2 kNm and rotation torque for aluminum string made 12.2 kNm. Hook load on pulling out from bottomhole at steel string made 2605 kN, and 1,104 кN for aluminum one, i.e. main load indexes for aluminum string are almost two times lower.
It is a common mistake when considering the higher strength properties of steel pipes, that we shall be able to bring higher tension load and torque to the stuck sections on their elimination by overpull, but it is not quite so. As was shown above, the major strength margin of pipes is spent on loads formed by dead weight of string and resistance forces. In addition to this, Drill String on sticking can be regarded as a fixed-ended beam and, meanwhile, increase of resistance forces, which also depends on the dead weight, will be considerably higher. Calculations show that to the depths (lengths) of 3,000-3,500 m a steel string has real advantages on bringing tensile load and torque to a stuck section, and at big depths (lengths) this advantage passes to LAIDP string.
2. Weight reduction in Drilling Mud.
As shown above, principal effect of LAIDP application is associated with low specific weight of aluminum alloys, which determines weight values of Drill String. Index of String dead weight also becomes considerably lower due to increased buoyancy of alloys (buoyant force), which is determined by ratio of drilling mud density to the density of pipes material. So, with a density of drilling mud equal to 1,200 kg/m3, the weight reduction factor of LAIDP Drill String makes  it 0.56, whereas, the one for a steel string in the same conditions makes it only 0.85. In Figure 1 is given dependence diagram of buoyancy factor for aluminum and steel pipes at different density of drilling mud.
3. High Specific Strength
Strength properties of steel drill pipes are higher than the ones of aluminum, and considering that the principal strength margin is spent on the loads formed by dead weight of String, then application of  LAIDP becomes more efficient.
To evaluate this we introduced a concept of strength-to-weight ratio characterizing the ratio of yield point parameter to linear meter weight subject to buoyancy in drilling mud. strength-to-weight ratio physically characterizes length of one-size Drill String in drilling mud of different density, at which tensile stresses reach the yield point in its upper section due to String dead weight without consideration of resistance forces.
In Figure 2 is given diagram of specific strength for strings made of 9 m. pipes of different steel grades and types of aluminum alloys, which shows that aluminum alloys are almost two times better by this parameter .
4. Young’s and shear modulus
Specified parameters of physical and mechanical properties of material provide general characteristics of its plasticity, level of active stresses and resistance of metal to action of sign-variable bending loads.
Considering that values of aluminum alloys for alternating these parameters are two times lower than the ones of steel, stresses arising in LAIDP are accordingly lower than the ones occurring in steel pipes and operating parameters are, therefore, higher.
This is confirmed with the principal formulae characterizing impact of Young’s and shear modulus values on loading conditions of Drill String.
So, on rotation of String variable component of bend stresses () depends on Young’s modulus (E):

where: d – diameter of pipes;

  – bending deflection

D – borehole diameter;
 L0 – length of bending line semi-waves
Young’s modulus sets stresses on movement () of Drill String over wellbore bent section:
 ,
where: R – borehole curvature radius.
String dynamic () stresses on pulling-and-running operations shall be determined as:
 ,
where: U– tripping rate;
     – specific weight of material;
    – accelereation of gravity.
Torsional vibrations on jams depend on shear
modulus (G):

where  – angular velocity.
5. Dampening Properties
It is known that longitudinal, torsional and transverse vibrations generated by BH service of bits propagate over Drill String as waves. Meanwhile, part of them can be transferred to wellhead and wellbore area, partially disseminate, as well as reflect from different reflection sources and go back to the bit, where they are summed up with direct (initial) vibrations. In general, ratio of direct and reflected vibration phases can vary from phase match (resonance) to antiphase (antiresonance.)
Aluminum drill pipes, as elements of wave screens, differ from steel pipes with increased damping capability. For example, with walls thickness equal to 10mm, the dissipation of vibration energy by inner friction in heavy wall LAIDP material is 20% higher, than the one of steel pipes. Damping properties display at maximum in heavy LAIDP – to 50% as compared to steel pipes, which allows to decrease excessive amplitudes of all types of vibrations and eliminate their negative impact on the process of drilling.
6. Resistance to Corrosion
Completed package of laboratory and wellbore tests determined high corrosion resistance of aluminum alloys in different aggressive environments with different рН indexes of drilling mud. To date we have somehow expanded range of allowable LAIDP service at different рН values of mud. If this range earlier made 6.5-9.5, it is now extended to 4.5-11.0. Corrosive damage of aluminum alloys is insignificant and practically doesn't influence strength properties of the pipes.
Total absence of corrosive damage on service in the environment fully saturated with hydrogen sulfide and carbon dioxide is advantageous quality of aluminum alloys.
A question, which may arise, is as follows: what happens in this environment to the steel tool joint, which aluminum pipe is equipped with? Steel tool joint in H2S and carbon dioxide environment is certainly subjected to corrosion, but, as shown by application practice of LAIDP in such conditions, steel tool joint corrodes considerably less, than the one on a steel pipe. It is difficult to justify such phenomenon today, but it is possible to assume that aluminum pipe acts as anode protection (protector) of a steel lock, which cuts rate of corrosion down.
Results of aluminum alloys corrosion tests in drilling environments are summarized in Table 1.
7. Non-Magnetic Properties
Non-magnetic properties of aluminum alloys allow to carry out all possible types of magnet logging of wellbores inside Drill String, which is especially important for control of telesystems operation while correcting profile of horizontal borehole.
These properties allow to use special integral joint aluminum drill pipes as non-magnetic housings of geosteering systems. Replacement of steel non-magnetic rosings (manells) for aluminum pipes considerably cuts costs and increases performance reliability of systems, especially on drilling horizontal wellbores.
8. Drilling Ability
On carrying out drilling operations it is required to set cement plugs and run in casing string liners on drill pipes. It is a frequent case that during these operations cement slurry rises above Drill String shoe and sticks it on setting. Operation of releasing Drill String from set cement is rather unpleasant, durable and is qualified as emergency situation. It is usually eliminated by drilling out stuck steel pipes with special milling cutters. Application of integral joint aluminum drill pipes in bottom part of Drill String in the course of cementation considerably simplifies the task, since aluminum pipes are easily drilled out with ordinary cone bit at the rate of 15-20 m/hour.
Application of LAIDP showed that their maximum efficiency is displayed at the depth of vertical borehole more than 3,000m and for all lengths of horizontal wellbores.

LAIDP should be applied on drilling wellbores, which are known for at least one of below listed limitations:
1. Limitation on hoisting capacity of Drill Rig.
2. Necessity of cutting down values of tensile loads, torque, and complexity in bringing axial load to the bit in the course of drilling horizontal wellbores.
3. Increased corrosive aggressiveness of mud, especially with presence of hydrogen sulfide and carbon dioxide.
4. Difficulty of drill pipes transportation to distant drilling sites.

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