Drillstring Vibrations: Classic Ideas and Modern Approaches - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 168

drillstring in order to free the string should it become immobilized during
drilling operations. Traditionally, jarring calculations are rough and
approximate; however, since expensive sensors are now incorporated in
downhole MWD tools, jarring actions must be sensitively designed to minimize
their potential damage. Drilling personnel are routinely involved in jar selection
and optimization. Typical operational parameters include free contraction
speed, hammer speed, impact force and stuck point placement. Since present-
day BHAs are becoming increasingly complex for analysis by closed form
solution methods, numerical methods have been developed by authors to model
what are essentially longitudinal stress waves of the type studied in this chapter.
Wang, Kalsi, Chapelle and Beasley (1987) and Kalsi, Wang and Chandra
(1988), for example, underscore the necessity for detailed numerical simulation.
Because the tools must be designed to survive the harsh vibration environment
during drilling, as well as the maximum shock transmitted during drilling, high
resolution simulations were needed. Thus, they used a general purpose
commercial finite element program to perform nonlinear transient dynamic
analyses, which tracked stress and displacement wave propagation throughout
the drillstring. The effects of jarring on stuck pipe depend on the level of
borehole friction acting on the drillpipe. “Spotting fluids” are often added to the
mud in the annulus in order to lubricate the string, that is, decrease the net
averaged apparent viscosity, to facilitate its movement. The resulting coefficient
of friction is an important input parameter for such jarring calculations.
The calculation of friction coefficient is complicated by the “eccentricity”
of the hole and the nonlinear mud rheology. Also, the preventive problem
concerned with avoiding stuck pipe due to poor hole cleaning indicates that
borehole friction should addressed in the drilling plan early on. These issues are
discussed in the author' s recent topic Managed Pressure Drilling: Modeling,
Strategy and Planning (Chin, 2012), which builds upon his earlier Borehole
Flow Modeling in Horizontal, Deviated, and Vertical Wells (Chin, 1992) and
Computational Rheology for Pipeline and Annular Flow (Chin, 2001). These
give solutions for eccentric holes and concentric ones with rotating pipe, axial
movement, for different mud types. The latest work also provides extensions to
multiphase flow, general pump schedules, changing pump rates, and so on,
important in managed pressure drilling and cementing in real-world operations.
4.2.11 Drillstring and formation imaging.
Why study waves traveling from downhole? The basic idea is central to all
imaging applications: incident waves, altered upon impact, reflect with different
amplitudes, phases and possibly shapes. This information, when properly
interpreted (or, “deconvolved”) using boundary condition models, describes the
target. For example, turn-of-the-century Compton scattering experiments helped
determine the structure of the atom. Ultrasonic imaging detects structural flaws