| Influenced by coal formation and tectonic movement, coal instability is common during drilling in China, which greatly restricted the efficiency of coal formation drilling and subsequent stimulation.Understanding the mechanism of borehole instability has important significance to efficient drilling coal formation and CBM development in deep. The coal and rock mechanism of borehole instability studies mostly based on shallow formation (800m above), and most of them ignored the effect of non-mechanical role. Coal rock mechanical properties and crack geological characteristics has become the main factors that leads to wellbore instability.Respectively, establishing a reasonable mathematical model and calculating none mechanical to rock mechanical and fracture extension,which could help to reveal wellbore instability mechanism and its influencing factors in deep.The geological object of this thesis is Xujiahe Group of Upper Triassic coal rock in Western Sichuan Basin, and the operating area is Xinchang Structure. The research model of this paper about deep formation’effect was established referring to fracture geological characteristics,stress field distribution and pore fluid pressure characteristics in operating area,mainly refers to the high stress impact.The compaction under high stress,adding the low permeability of top and floor formation.will make it easier to keep high fluid pressure in formatiom,which would speed up the rock’s instability and the crack’s extending more easily,which would accelerate the instability of rock around the pore.Coal rock stress sensitivity was evaluated by stress sensitivity efficient method to calculate the coal crack width of coal rock near the well by solving the mechanical model through programming based on Compound Trapezium Numerical Integration theory and B&S model. Based on infiltration differential theory, we will find out the invasion of fluid mud.By Trusdell formula, we can get the impact of gas saturation and pore fluid pressure to the maximum compressive of coal and rock.Using Fairhoust equation and Mohr-Coulomb theory, We could find out the impact of non-mechanical effect to the changes of safe density windows of drilling mud in deep; Using fracture mechanics equation and D-P theory, We also can find out the impact of non-mechanical effect to the changes of safe density windows of drilling mud in deep.Comparing the two metheods, we explore the main factor that controls the instability of borehole instability.And combined with previous date, we discussed the relationship between the instability of floor, roof rock and the instability of coal rock simplily.Conclusion shows that, no matter which factor leads instability, the presence of pore fluid pressure produced tremendous effect to calculate the safe density windows of mud in deeps, as the non-mechanical role get bigger the safe density windows of drilling fluid will reduced accordingly.Research shows that the non-mechanical get bigger with the pore fluid pressure be higher.Thus,during drilling in deep coal rock, we must pay much attention to the effects of non-mechanical role to wellbore instability. |
