New Mechanism in Drillstring Vibration

Abstract

ABSTRACT In deep well drilling for oil and gas, it has been observed and recorded that severe vibrations build up and die down periodically. The beating phenomenon is a frequent occurrence, particularly, during all startup tests and on most of those exhibiting the low frequency torsional vibration. This paper interpreters the field data of beating like phenomenon and reveals a new mechanism, namely stick-slip vibration, which may initiate three-lobe pattern at the hole bottom. The possible self-excited vibration without sticking is also explored in this paper. INTRODUCTION The external excitation on the drill bit is usually regarded as the sole cause of the vibration of the drillstrinq. Paslay and Bogy considered the vibration due to the intermittent contact of the bit and the hole bottom in 1963 [1]. Huang and Dareing discussed the buckling and lateral vibration induced by the drill fluid in 1968 [2]. Bogdanoff and Goldberg presented a random model to explain the breakage of drill pipe in 1961 [3]. However severe vibration was usually attributed to the bit's running over the three-lobe bottom pattern at a constant speed, although the cause of the formation of the three-lobe bottom pattern was not understood [4,5,6]. Based on this attribution, Dareing and Live say successfully explained the longitudinal and torsional vibrations with triple rotary frequency in 1968 [5]. Dareing further indicated the possibility of eliminating the three-lobe bottom pattern by controlling the rotary speed in 1983 [7]. He also explored the self-excited vibration initiated by a small vertical harmonic motion of the bit [8]. Eronini et al investigated the interaction between the bit and rock and presented a dynamic vibration model in 1982 [9]. In analyzing field data, Dawson, Lin and Spanos postulated in 1987 that the drillstring vibration was a stick slip motion [10J; this was followed in a study by Kyllinstad and Halsey in 1987 [11]. By examining the beating phenomenon in oil well drilling, Lin and Wang demonstrated that the vibratory phenomenon of beating was a stick-slip motion. The effects of viscous damping, rotary speed and natural frequency of the drillstring on stick-slip vibration were also discussed [12]. This paper re-iteratesthat beating in drilling is induced by the nonlinearity of the total resistance to the drillstring (dry friction, viscous damping and torque on bit). This is done by further examining Denison's data and theory [13]. The formation of three-lobe pattern at the hal e bottom and self-excited vibration without the bit being stuck are also explored. Summary OF PREVIOUS WORK Our previous work was first concentrated on the large amplitude torsional vibration of the drillstring with bit off the bottom. The drill string was modeled as a one-degree-of-freedom torsional pendulum which has the following equation of motion (MATHEMATICAL EQUATION IS AVAILABLE IN FULL PAPER)