A dynamic and non-linear risk evaluation methodology for high-pressure manifold in shale gas fracturing

• An index system is established to evaluate failure risk for high-pressure manifold.
• This paper presents a weight optimization model to express index weight.
• A dynamic and non-linear model – value function model is proposed in this paper.
• A comprehensive methodology is proposed to evaluate overall manifold failure risk.

In the process of shale gas fracturing, high-pressure manifold is always subjected to high horsepower, large displacement and fluid-solid coupled impact, resulting in serious failures such as erosion defect, stress corrosion and fatigue crack. Some kinds of manifold failures are correlated to each other, and the relationship between these failures and real-time monitoring parameters – pressure, displacement and sand ratio is non-linear in most cases. There is a study gap on the comprehensive risk evaluation for high-pressure manifold in existing research, which leads to inadequate or inaccurate safety decision. In this paper, manifold failures, failure characteristics as well as real-time monitoring parameters are considered respectively as the first-level, second-level and third-level indexes, and a set of index system is established for the failure risk evaluation. The correlation among indexes at the same hierarchy level is expressed as index weight by a weight optimization model. And a value function model is also presented to deal with the dynamic and non-linear index relationship among different hierarchy levels. Finally this comprehensive methodology is proposed to evaluate the overall risk, failure bucket and the risk level of high-pressure manifold in a real-time and accurate way. Results from a study of the DPT-115 well in Daniudi gas field indicate that the overall risk of manifold is mostly at levelⅡ and erosion defect acts as the main failure bucket.