Generation Mechanism and LWD-Based Identification Methods for Highly Fractured Shale: A Case Study of Wufeng–Longmaxi Formations, Sichuan Basin

During the development of shale gas fields, horizontal wells drilling through highly fractured shale formations are prone to drilling abnormal conditions such as borehole collapse and lost circulation, which significantly increase drilling risks and operational complexity. Therefore, this study investigates the shale of the Wufeng–Longmaxi Formations in the Sichuan Basin to identify the specific intervals where highly fractured shale develops and their development characteristics by analyzing data from drilling, logging while drilling (LWD), wireline logging, and core analysis. Furthermore, it analyzes the generation mechanism of highly fractured shale and the induced causes of drilling abnormal conditions, and develops a real-time drilling identification method based on LWD technology. The study shows that the top interval of the Wufeng Formation belongs to highly fractured shale, characterized by frequent abnormal conditions and a low gas production contribution rate. Moreover, bedding fractures at the top of the Wufeng Formation are more developed than those in the overlying Longmaxi Formation, and the Wufeng Formation exhibits stronger heterogeneity relative to the Longmaxi Formation. This leads to discontinuous or uneven rock strength where fractures develop or lithology changes drastically, making the rock mass susceptible to fracturing when the external environment changes, thereby forming highly fractured shale. Based on the LWD response characteristics of highly fractured shale, a mechanical specific energy (MSE) evaluation model can be established using drilling engineering theory. Combining this model with near-bit and far-bit γ ray (GR) and gas show parameters can effectively identify highly fractured shale, and the feasibility of this method is verified through field cases. The study results can meet the needs of field engineers for real-time evaluation, providing a new method and technical approach for the evaluation of highly fractured formations.