Drill String Impact And Wellbore Internal Pressure On The Stability Of Shaft

Wellbore instability problems include blowout, lost circulation and wellbore collapse et al.. The accidents caused by wellbore instability give rise to serious drilling quality and safety problems. According to statistics, more than six billion dollars are spent in controlling wellbore instability in the global petroleum industry every year and wellbore failures lead to significant lost time accounting for over40%of all drilling related non-productive time. So it is vital to understand the mechanisms of wellbore instability and analyze the reason of wellbore instability for insuring the safety during drilling, improving the rate of penetration and efficiency. In view of the problem of wellbore instability or enlargement, many researchers have done lots of the studies; however, most of these studies are focus on the physic-chemical interaction between rock (especially shales) and drilling fluid, ignoring the effect of drill string impaction. Actually, in many cases, drill string impaction which caused by drill string vibration has the vital effect on wellbore instability; besides, the research of the effect of wellbore pressure on wellbore stability is on the premise of isotropic rock, it is different from the actually. Thus, the research on the effect of drilling string vibration and wellbore pressure is very necessary. The two factors of drilling string vibration and wellbore pressure which cause wellbore instability are researched based on the theory of experimental study, rock mechanics, elastic-plastic mechanics and finite element theory and so on. The analysis of the specific process and results are as follows:1. The effect of drilling string impact wellbore caused by drilling string vibration are researched through the investigation of the related literature at home and abroad and the research of the down tool vibration characteristic;2. In this paper, the law of the rock fragmentation in mud drilling while drill string impact is researched through the simulation, then the relationships between rock fragmentation volume and drill string impact are achieved under different conditions (different contact length of drill string, different impact speed, different rotation speed, different depth of well and the consideration of rock softening). What is more, author has derived the formulas of the drill string critical speed and rock fragmentation volume when wellbore rock fails based on the laboratory experiment and simulation.3. The instability law of wellbore while drill string impact was researched through numerical simulation, then the relationships between wellbore rock’s fragmentation volume and drill string impact were achieved under different conditions (different contact lengths of drill string, different impact speeds, different rotation speeds, different depths of well and different in-situ stress directions). What is more, authors have researched the fragmentation mechanism of wellbore rock and derived the formulas of maximum pressure wellbore rock suffered when it begins to fail4. The mud pressure plays a significant role in wellbore stability, so this paper also analyzed the mud density for isotropic formation and the impact of cleat formation borehole wall stability, for isotropic strata borehole wall stability analysis, through the research on the borehole wall mud density of radial stress and tangential stress to illustrate the influence of the wall shear failure and tensile failure mechanism; Cleat formation on sidewall stability analysis, established for the cleat formation of mathematical model of discrete element by writing computer program analyzes the various factors influencing the cleat strata instability.The conclusions of this paper can help drilling workers to understand the relationship between drill string vibration, mud density and wellbore instability correctly.