American Oil and Gas Reporter - August 2015 - 49

SpecialReport: Horizontal Well Operations
Given its light weight and strengthto-weight characteristics, ADP is well
suited to long deviated or extended-reach
wells, where reducing torque and drag
loads is critical to drilling performance.
The most common alloy used in manufacturing ADP has a maximum yield
strength of 58,000 psi. While this is considerably lower than common steel pipe
grades, the strength-to-weight ratio of
ADP is equivalent to or better than steel
pipe because of its lighter material density
and greater buoyancy effect. ADP typically
has a greater wall thickness than equivalent
SDP, and is generally at least 40 percent
lighter than equivalent-sized steel pipe.
Table 1 compares the mechanical
properties of steel and aluminum grades
in terms of density, Young's modulus,
yield and tensile strength, and the strengthto-weight ratio for 5-inch SDP and 57⁄8inch ADP. The strength-to-weight ratio
represents the ultimate length of a single
size drill string that can be suspended
without exceeding the yield strength of
the material.
ADP's high strength-to-weight ratio
decreases torque and drag loads while
maintaining resistance to axial, torsional
and lateral loads. Since aluminum alloys
are more flexible than steel, ADP buckles
at a lower given compression than SDP.
However, the increased flexibility is balanced by the fact that the lighter weight
reduces contact side forces to minimize
friction between the drill string and borehole. Minimizing friction decreases hook
load and torque at the surface, and importantly, reduces compression while
running an ADP string in the hole to
potentially extend horizontal reach.
Extending Lateral Reach

The favorable results in the Eagle
Ford test indicate that using ADP in as-

sociation with a steerable mud motor
could be an economically and technically
viable alternative to rotary steerable systems. The principal advantage of using a
rotary steerable system is the ability to
control the direction of the well in the
rotary mode, overcoming drag friction to
extend the lateral section further into the
target formation.
Horizontal drilling with a steerable
mud motor consists of a sequence of
sliding phases (generally 10-20 percent
of the total footage drilled) and rotary
phases (80-90 percent of the footage).
Pipe rotation at surface is stopped during
the sliding mode to adjust tool face orientation to steer the bit in a given direction,
and weight is then applied on the mud
motor-rotated bit.
In the slide mode, drag friction generates compression all along the drill string,
with a risk of buckling and poor weight
transfer to the bit. When a steerable mud
motor is in slide mode, it creates a situation
for maximum compression along the drill
string, and the excessive friction limits
the extent of the horizontal section. In
the rotary mode, drill pipe rotation at
surface resumes, overcoming the drag
friction (rotary mode ROP can be 10
times greater than in sliding mode), but
the direction of the well is not fully controlled.
Oscillation, where the drill pipe is
rocked alternatively to the left and right
at surface at low rotation speed to reduce
axial drag, is one solution to limitations
while drilling in the sliding mode with a
steerable mud motor. Instead of generating
axial drag while sliding, friction occurs
along the tangential direction (torque)
because of the oscillation from the surface
on a given portion along the drill string
(the drill string is twisted). This technique
has proven effective, leading to better

weight transfer to the bit and higher
sliding mode ROP.
Another option is an axial oscillation
tool, which generates pressure pulses
along the drill string to produce an axial
motion to partially reduce drag friction
around the tool.
The solution applied in the Eagle Ford
project consisted of simply replacing
joints of standard SDP with lighter ADP
while keeping the same BHA and well
design to reduce the amount of drag friction while sliding to increase ROP and
extend the lateral section.
ADP Technology

ADP is manufactured in an extrusion
process, and can be produced with varying
wall thicknesses along a seamless tube
pipe body. This tapering allows for increased
thickness and greater strength around the
tool joint interface, and an upset wear band
can be extruded integrally in the center of
the pipe body (ADP can be fitted with any
type of API or premium steel tool joint
through a thermal shrink fit process). The
tapering and center wear band both play a
role in increasing the pipe's resistance to
bending fatigue, and to an extent, help
reduce helical buckling tendencies. They
also reduce pipe body wear and mitigate
fatigue risk caused by bending stress.
Given the design and geometrical properties of ADP (steel tool joints, aluminum
alloy body, and larger-outside diameter
wear band on the middle of the pipe), advanced numerical modeling is required
to fully predict the occurrence of buckling
and calculate the state of stress along the
pipe to avoid possible damage. In the
Eagle Ford wells, torque and drag were
calculated using advanced numerical modeling software (Buckling Severity Index)
that considers the effects of friction, tool
joint, rotation and well bore geometry in

TABLE 1
Characteristics of Steel and Aluminum Drill Pipe
Specific
Weight

Young's
Modulus (Msi)

Yield Strength
(ksi)

OD Pipe (in)

Tensile
Strength (lbf)

Linear Weight
(lbf/ft)

Strength-to-Weight
Ratio (ft)*

Steel S135

7.85

30

135

5

712,000

23.6

35,613

Steel E75

7.85

30

75

5

395,600

22.1

21,130

Aluminum 2014-T6

2.78

10

58

5.743

505,000

16.6

53,237

Material

*Mud weight = 10 ppg

AUGUST 2015 49
American Oil and Gas Reporter - August 2015

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