深层页岩地层拉张破裂特征及破裂压力预测OA

Wellbore instability is induced by tensile fracture propagation during drilling in deep shale formations of the Sichuan Basin.Taking the deep shales of the Lower Silurian Longmaxi Formation in the Penglai gas field of this basin as an example,by integrating mineral composition analysis,mechanical property experiments,and theoretical modeling,the propagation mechanism of tensile fractures was systematically revealed,and a corresponding formation fracture pressure prediction model was established.Results indicate that(i)under large burial depth and strong in-situ stress conditions,the Longmaxi Formation shales exhibit significant heterogeneity in mineral composition and microstructure,with obvious presence of bedding structures.The fracture propagation paths are jointly controlled by bedding structures and mineral distribution;(ii)shale failure modes are categorized into three types:propagation along bedding planes,composite propagation along bedding planes and loading directions,and penetrating propagation along loading direction.The tensile fracture propagation process sequentially undergoes three stages:crack initiation,propagation,and coalescence,with low-angle bedding planes being more prone to induce crack initiation and propagation;(ii)the classical fracture pressure prediction model was modified by introducing a structural correction coefficient m and considering the heterogeneity of mineral distribution and the anisotropy of tensile strength.Field validation demonstrates that the revised model can more realistically reflect the properties of shale reservoirs in the study area,providing a reliable basis for determining the safety drilling fluid density window adaptable to deep shales.