Analysis of collapse mechanism of multi-bedding mudstone wellbore in Sulige gas field
Wellbore instability is the primary technical problem restricting low-cost drilling of mudstone formations in Sulige gas field in-to horizontal wells in long well sections.Based on the evaluation of the mineral composition,bedding development and mechanical char-acteristics of mudstone,this paper combined theoretical analysis,experimental evaluation and numerical simulation to study the mechani-cal behavior and failure instability characteristics of multi-bedding mudstone under fluid action.Due to the existence of mudstone bed-ding and microfractures in Shanxi Formation of Sulige gas field,the strength of the rock can deteriorate after the drilling fluid intrusion.The diversion capacity of the weak surface of mudstone was much higher than that of rock matrix,and drilling fluid penetrated into the formation along the weak surface,which can directly lead to the decrease of the strength of the mudstone structural surface and prone to collapse of the wellbore.The collapse pressure of mudstone formation in the actual drilling process of Sulige gas field was calculated,and the well inclination,azimuth angle and drilling fluid immersion time were substituted into the rock mechanical parameter deterioration model,which gave the safe drilling fluid density of the target layer,which has important guiding significance for realizing the stability and safe drilling of multi-bedding mudstone wellbores in Sulige gas field.
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