Fuzzy fault tree analysis of hydraulic fracturing flowback water storage failure

Unintended release of flowback water as a result of above-ground walled storage system (AGWSS) failure was studied using a fuzzy fault tree analysis (FFTA). A fault tree comprising 45 basic events was constructed, and knowledge gathered through expert elicitation was used to estimate the occurrence possibilities of basic events. Fuzzy logic was introduced to reduce the epistemic uncertainties in expert judgments. Consistency analysis and grey pairwise comparison techniques were used to weight the judgments from different experts. The result of a case study shows that the failure probability of AGWSS was estimated to be 5.75E-04, indicating a relatively low level of failure possibility comparing to other systems used for oil and gas production. Importance analysis of basic events indicates that loss of containment integrity, water loading accidents, and external catastrophes are critical causes responsible for AGWSS failure. The developed FFTA methodology can be used by the unconventional gas industry for mitigation of flowback water spill risk.