Study on formation rock characteristics and borehole collapse period in northern Jiangsu Basin
Huazhuang area in Gaoyou Sag of northern Jiangsu Basin activates a large amount of shale oil resources,but shale is prone to hydration,resulting in rock strength reduction and complex conditions such as borehole collapse.Therefore,it is necessary to determine the borehole collapse period in this block.The rock of Funing Formation in Huazhuang Block was taken as the research object,and the research on the microstructure of the rock,the strength characteristics of the formation,and the law of water absorption was systematically carried out.The collapse pressure prediction model was established on the basis of the experimental research,and then the safe density window of Well X in this block was established with this model.The influence of the well inclination angle and azimuthal angle on the window and the borehole collapse period was analyzed.The research findings indicate that:①The overall structure of the rock of Funing Formation in the Huazhuang area is loose,and the formation is rich in clay minerals,quartz,calcite,and other mineral elements.Brittle minerals constitute a high proportion,accounting for 31.8％-43.8％of the total content.Calcite-filled tensile cracks are developed,and clay minerals are predominantly illite-montmorillonite layer and illite,exhibiting high illite-montmorillonite layer content and significant hydration expansion properties.②Under a specific well inclination angle,as the azimuthal angle transitions from the maximum horizontal principal geostress azimuth to the minimum horizontal principal geostress azimuth,the formation rupture pressure gradually increases;collapse pressure gradually decreases,and the safe density window progressively widens.When the directional well azimuth is in alignment with the minimum horizontal principal geostress azimuth,the likelihood of collapse and well leakage diminishes,minimizing the drilling risk.③Based on the collapse pressure prediction model established in this study,it is recommended to employ a composite organic salt polymer drilling fluid with a density of 1.5 g/cm3 when drilling in the Funing Formation.The suggested collapse period is approximately 51 days.The research results hold significant guiding implications for investigating the formation's safe density window and borehole collapse period.
northern Jiangsu Basinrock characteristicmud shalesafe density windowcollapse period
万方数据
