Study On The Mechanism Of Collapse And Instability Of Horizontal Wells In WY-CN Longmaxi Formation Shale

Horizontal wells have been one of the most important technologies during the exploration and development of shale gas, while the problem of wellbore instability always seriously restricts the requirement of developing shale gas with efficient and economic ways. Compared with the conventional sandstone, carbonate rock, shale has a hard brittle properties, easy expansion, easy to broken, stratigraphic principle, micro cracks and other weak surface development and occurrence variety, caused by drilling fluid filtrate invasion shale fonnation, weaken the shale rock particles and the cohesive force between bedding surface between effect, reduce the rock strength, easy happening well leakage, collapse and so on sidewall instability problem;combined with drilling process temperature field, seepage field and the field of shale rock strength and well circumferential stress change, make its wall rock spalling and collapse problem are more likely to happen.In the former study of wellbore stability has had certain development and application of the systematic analysis of shale wellbore instability in the early stage, the overall research strength is not enough, so we need further research to explore more under the mechanism of multi-field coupling in wellbore instability. Layer in this paper, based on the weak-plane failure criterion and double porous media theory, revealing the quantitative effects of micro crack seepage stability of wellbore,as well as the borehole trajectory, stratification, in-situ stress; under different mud system, combined with multi-field coupling theory, explore the weak surface under the influence of multiple field coupling mechanism of wellbore instability, collapse instability region calculation model is established; combining Mohr Coulomb, Mogi Coulomb, Drucker Parager, Hoek Brown rule of shale failure criteria adaptability analysis.and according to the actual situation, considering the breakout of the width of the collapsed pressure calculation model is set up. This article system comprehensive study of shale wellbore stability problem of WY-CN,enriching the basic theory of shale gas drilling and wellbore stability has a certain practical significance.