Risk Assessment of Simultaneous Dual-Well Drilling Operations on the Same Shale Gas Well Pad Based on a BT-BN Model

Shale gas is an important unconventional energy resource in China with considerable development potential. However, the coupling of hazards during simultaneous dual-well drilling on multi-well pads has not been adequately quantified. This study examines a scenario in which two drilling rigs operate concurrently on the same shale gas well pad and develops an integrated Bowtie–Bayesian network (BT–BN) model to compare single-well drilling with simultaneous dual-well drilling. Bowtie analysis identified 63 risk factors, preventive barriers, and events for simultaneous dual-well drilling and 35 for single-well drilling across human, equipment, environmental, and management dimensions. An improved fuzzy evaluation method was used to quantify the prior probabilities of 15 root nodes, and the conditional probability tables of the remaining non-root nodes were calculated using the extended recursive noisy-OR (ERNOR) procedure with an independent background cause. Bayesian inference was used to calculate the relative composite risk value at the target node in each scenario. The target event was loss of well control in the single-well scenario and simultaneous loss of well control in both wells in the dual-well scenario. Ratio-of-variation analysis was used to identify influential factors. For comparable basic risk factors, the single-well and dual-well models used the same seven-level expert-evaluation scale and probability-calculation procedure; the dual-well model additionally included factors arising specifically from simultaneous cross-operations. Under the same evaluation conditions, the relative composite risk value for simultaneous dual-well drilling, calculated through the Bayesian network using expert-assessed risk values, was 47.85% higher than that for single-well drilling. The results provide expert-knowledge-based quantitative support for comparative risk assessment and safety-management decision-making for simultaneous dual-well drilling on shale gas well pads.