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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe stick-slip phenomenon has been identified in the industry as an inefficient and often damaging drilling vibrational condition. Numerous studies, predominantly focusing on measurements of drillstring torque, have detailed the nature of this phenomenon, and have shown its detrimental effects on drillstring components and especially on PDC bits. Using surface rotary motor current, drilling contractors and surface logging companies have attempted to monitor drillstring torque so as to recognize and correct for this torsional instability.Recent MWD tool developments using downhole drillstring rotational speed have established a superior method to identify stick-slip and establish its severity. Measuring actual rotational behavior of the lower BHA gives a better insight to the torsional movement of the drill bit during the drilling process, without the problems involved when inferring this behavior from the surface torque readings, obtained at the opposite end of the drillstring. This paper describes the method of determining the stickslip behavior downhole from magnetometer readings. The sensors are sampled at a high enough frequency to compute nearinstantaneous values of downhole drillstring rotation, and variations in the RPM are then used to establish the incidence and severity of stick-slip. This information is telemetered to the surface and provided to the driller in order to allow for real-time changes in drilling parameters to correct for this inefficiency. Also presented in the paper are several examples using actual field data that detail stick-slip behavior and demonstrate how real-time information about downhole RPM variations can help to reduce the problems associated with this torsional drilling dysfunction.

fax 01-972-952-9435. AbstractDrillstring dynamics and hole cleaning problems are some of the most important limiting factors in extended reach applications. Here, long sections of the drillstring lie on the low side of the wellbore while rotating. When the rotary speed exceeds a critical threshold the drillstring starts to "snake", sliding up and down the borehole wall. If rotated well beyond the threshold speed, the drillstring will eventually start to "whirl" which can cause severe damage to string components after only a short period of time.In this paper an analytical solution for the threshold rotary speed is derived and presented. It is shown to be in the range of the rotary speeds used in modern extended reach applications. The analytical results are verified using a versatile finite element formulation to model the drillstring in greater detail. Animated time domain simulations with this model provide deeper insight into the dynamic behavior of the drillstring.Conclusions on improved drilling practices in extended reach applications -especially with respect to hole cleaning problems -are drawn from the theoretical results.

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