Study On Influencing Factors Of Shaft Wall Stability Of Hard Brittle Shale

In recent years, the wellbore instability problem of the hard and brittle shale fromations has becoming prominent during drilling process. This problem has conditioned fast drilling and the considerable technical of the safe and high-efficient exploration for oil and gas resource in the development process of the deep and complicate petroleum. It should be overcome immediately. At this stage the research about the wellbore stability of brittle shale fromations is still not systemic, which couldn’t enough solve practical engineering problems. So in this paper, the study of wellbore instability mechanism is particularly important for oil and gas development.This paper takes a hard and brittle shale gas in China as objects of study. We study hard brittle shale composition, microstructure, physical and chemical properties and mechanical properties through physical experiment. Combined with theoretical analysis, we got the change rules of collapse pressure along with the drilling fluid, well track and geostress. Synthesized the physical experiment and the theoretical analysis, we sum up the main influencing factors and its laws of the hard brittle shale borehole wall stability. Some understanding of the hard and brittle shale fromations were got as follows:(1) The clay minerals content of the hard brittle shale volume is relatively low. It mainly consists of the swelling clay minerals illite, no swelling clay minerals montmorillonite, and a small amount of the slip layer, which led to the weak brittle shale hydration expansion.(2) The macro-cracks, micro-cracks, and bedding are development. The micro crack opening can reach more than5microns. The micro cracks and bedding of the formation are the main drilling mud invasion channel, which directly impacted on wellbore stability.(3) Under the axial loading, the pores and fissure of rock are not obvious in the compaction stage and directly into the elastic deformation stage. The rock has the nature of high compressive strength and strong elastic deformation characteristics. The rock quickly loses bearing capacity after the damage and shows the significant characteristics with elastic brittle damage(4) The drilling mud would cause formation collapse pressure rise. The density, sealing and inhibiting capacities of the drilling mud will have an effect on sidewall stability.(5) Collapse stress change rule is different under different stress state. When vertical stress is the maximum principal stress, the stability of vertical well is better than horizontal well. The wellbore stability of borehole wall, along the minimum horizontal principal stress direction, is better than that along the maximum horizontal principal stress direction during drilling. When vertical stress is intermediate and minimum principal stress, the stability of horizontal well is better than vertical well. The wellbore stability of borehole wall, along the maximum horizontal principal stress direction, is better than the minimum horizontal principal stress direction during drilling.