Study On Evaluation Of Fuzzy Fault Tree Analysis For Gas Drilling Safety

Gas drilling has been widely used in various oilfields at home and abroad because of its remarkable advantages in improving the drilling speed,solving the malignant leakage,protecting the oil and gas layer and reducing the drilling cost.However,some safety risk factors of gas drilling have also become the bottleneck restricting the development of gas drilling,such as formation gas,formation water production,downhole explosion,drill string failure etc.It is necessary to analyze and study the safety risk of gas drilling.Based on a great deal of literature investigation and practical combination,the main research achievements are drawn:(1)The types of risk safety during gas drilling and their corresponding consequences were summarized,and downhole explosion and drill string failure were analyzed emphatically.It was found that in the use of air as circulating medium for drilling operations,it was possible to have downhole explosion and drill string failure when drilling into both oil-gas and shallow gas reservoirs.(2)Through investigation,theoretical model and practical application,the fuzzy fault tree was proposed based on the fuzzy mathematics and fault tree.The risk factors of downhole explosion and drill string failure were analyzed,and their corresponding fuzzy fault trees were established.In order to study comprehensively these two kinds of accidents,successful tree models of downhole explosion and drill string failure were constructed.(3)Qualitative analysis of downhole explosion fault tree and drill string failure fault tree in gas drilling were performed in this study.The minimum cut set,the least-path set,and the structural importance coefficients of both downhole explosion and drill string failure fault trees were calculated.The effective ways to prevent downhole explosion and drill string failure occurred were recommended.(4)The fault trees of downhole explosion and drill string failure in gas drilling failure were applied to the field practice.The fuzzy probabilities of downhole explosion and drill string failure accident by using triangular fuzzy number were calculated,providing the fuzzy importance degree of each basic event in the failure tree.(5)The minimum cut set of downhole explosion fault tree had 48 groups,and the least-path set had 7 groups.The probability of downhole explosion occurrence was between 0 and 2.407%.The probability of occurrence was 0.168%;The minimum cut set of the drill string failure fault tree had 65 groups,and the least-path set had 7 groups,The probability range of drill string failure was within 0-44.694%,the probability of the most likely occurrence was 3.513%.According to the evaluation results,the prevention measures of downhole explosion and drill string failure in gas drilling were put forward.The application of fuzzy fault tree evaluation model in gas drilling can provide guidance for comprehensive evaluation of downhole explosion and drill string failure risk.