Analysis of Stick-Slip Effects During Oil-Well Drilling
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The drilling operation is a costly portion of the upstream petroleum industry. It is worth making efforts to reduce unwanted non-productive time (NPT), which is often induced by downhole problems during drilling. In this thesis, dynamic models of the drillstring to account for both axial and lateral motions have been developed. Based on these models, problems related to surge and swab pressures and equipment failures are addressed. Downhole pressure variations are often caused by surge or swab pressure, from the axial movement of drillstring during tripping in or out of the well, and also from heave motion of the drillstring suspended from a heaving drilling vessel. In this thesis, the Stribeck friction model is used to analyze the friction-induced selfexcitation vibration (stick-slip) through four regimes of contact: sticking, boundary lubrication, partial fluid lubrication, and full lubrication. The results indicate a significant increase in the peak velocity of the drillstring compared to the heave velocity of the vessel. In addition, axial tension induced geometrical stiffness has been taken into account, which results in a time dependent normal force between the drillstring and the borehole in the curved section of the borehole. Simulations show that the elastic wave form of the downhole drillstring motion is different from the surface motion. Due to the occurrences of abnormal downhole pressure, the conventional method might lead to substantially over/underestimated pressure. Comparing with field data, the theoretical results are matching the observed downhole pressure. Moreover, based on this study, some recommendations are further proposed for improving the model, experimental validation, and mitigating of downhole pressure variations. Another important study in this thesis is related to the potential failure of drill collar connections, which is attributed to cumulative fatigue due to bending vibration. An important class of bending vibration is whirl, which is formed by the eccentricity of the rotational drill collar. The contact between the drill collar and the borehole causes an harmful backward whirl, even a chaotic whirl. A two-degrees-of-freedom nonlinear lumped parameter model is utilized for representing the whirl of the drill collar in this thesis. Different from other studies, the stick slip vibration causing fluctuation of rotary speed is taken into account. It is shown that the chaotic whirl happens at a lower rotation speed. In this lumped element model, the contact forces obey the Hertzian contact law, which leads to lateral bounce of the drill collar and impact borehole wall chaotically. The modified Karnopp friction model is adopted to simulate the stick slip rotary vibration of the bottom hole assemble (BHA). Based on the time domain responses of whirl, the continuous bending stress history is broken down into individual stress ranges with an associated number of stress cycles using the rainflow counting method. The cumulative fatigue damage is estimated using Miner’s rule. Based on the study, some of recommendations are introduced to further improve model and extend the model application (such as snake motion of drillstring). In addition, some mitigation solutions are proposed to expand fatigue life of drill collar. The third contribution in this thesis is related to the axial vibration assisted drilling. This technique has recently started to be implemented in gas/oil well drilling. The application of the technology is validated to improve the drilling efficiency (increase the rate of penetration) and performance (mitigation of the stick slip). In this thesis, the mechanism underlying axial vibration assisted drilling is analyzed using a cutting experiment with a single cutter. The cutter is connected to a microvibration based piezoelectric actuator, which generates cyclic loads perpendicular (normal) to the cutting path. The experimental results indicate that the cyclic loads lead to decreasing both the shear force (30% reduction) and the normal force (50% reduction)significantly. Simultaneously, the lateral vibration of the cutter, which can be imaged as the drill bit stick slip results from cutter-rock interaction, is clearly mitigated during the cutting process. Observations with a confocal microscope show that the abrasion at the front of the cutter is increased due to the greater surface roughness when rupture damage of rock occurs (increasing 7.5% area and 50% depth). Based on the analysis of cutting mechanism, the intrinsic specific energy (or strength) of the rock sample is found with obvious deterioration when increasing the frequency of the cyclic loads. According to the S-N curve of fatigue life of Berea sandstone, it is concluded that this deterioration evolution process of the rock sample is because the cyclic loads induced cumulative damage (up to 42% in maximum). Finally, in order to make this technology applicable, some potential improvements in further work were listed including high temperature high pressure influence, abrasion analysis, and 3D scan etc. The main body of the thesis is given in chapter 3-5, which is fully based on the publicized articles (I, II and III) 1. Some additional publications (IV, V and VI) are presented in Appendix B (4, 5 and 6) as an expansion of my study.