Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 118

large dynamic forces appear to be more damaging than static loads.
Polycrystalline diamond compact (PDC) bit damage results from backward
whirling. When a bit whirls, its instantaneous center moves about the bit face,
rather than around its true center as with smooth rotation. Cutter paths are not
circular: they strike rock at off-design angles, leading to bit failure in the form of
spalling and chipping.
Formation damage. Wave propagation figures importantly in formation
damage. For example, swab-surge may cause borehole instability in the
annulus, precipitated by sudden drillpipe movement. Also, formations can be
fractured by improper drilling practices; high dynamic bit loads may fracture
rock and result in lost circulation.
Directional drilling. Directional control and bit walk are crucial to
directional drilling. In the past, drillers used static loads only, but industry
consensus has taken the position that dynamic forces can play important roles.
Calculations should be directed at moments, torques, and forces at the bit;
empirical information on doglegs and rock-bit interaction should be related to
boundary condition modeling as it affects directional control. Modern interest in
rotary-steerable systems, and in particular, self-steering capabilities, may require
a stronger understanding of drillstring dynamics and real-time processing.
Increasing rate of penetration. Rate-of-penetration is related to
drillstring dynamics, e.g., identical bits drilling identical formations will perform
differently with different BHAs. Early workers noted the potential of drillstring
vibrations in increasing penetration rate, studying force versus displacement
phase relations at the rock-bit interface (Dareing, 1984, 1985). Using an energy
balance, we will show that power input at the bit is controlled by AE u x u t (0,t)
where A is the cross-sectional area, E is Young' s modulus and u(x,t) is the axial
displacement. Calculated examples using postulated rock-bit mechanical
impedance conditions are discussed.
Improved MWD tools and mud motors. MWD tools and mud motors are
now designed to operate at higher temperatures and pressures in more corrosive
environments. Because they run in longer and deeper wells, and necessarily
undergo reduced maintenance, they need to be more reliable and sturdy.
Operating in deviated and horizontal wells, these smaller tools undergo higher
build rates and experience higher torques and forces; also, the advent of
steerable drilling systems means increased bending stresses and bearing loads.
Advances in materials must accompany our improving knowledge of the
vibration environment. While building angle, drill bits will slip, bounce, jolt and
clog. These uncontrollable events, plus shock sub noise, will cause pressure
surges that create signal synchronization problems with received MWD signals
at the surface. Understanding bit and mud column dynamics will assist in
obtaining uninterrupted real-time MWD information.