Abstract
Recent natural gas development by means of hydraulic fracturing requires a detailed risk analysis to eliminate or mitigate damage to the natural environment. Such geo-energy related subsurface activities involve complex engineering processes and uncertain data, making comprehensive, quantitative risk assessments a challenge to develop. This research seeks to develop a risk framework utilising data for quantitative numerical analysis and expert knowledge for qualitative analysis in the form of fuzzy logic, focusing on hydraulically fractured wells during the well stimulation stage applied to scenarios in the UK and Canada. New fault trees are developed for assessing cement failure in the vertical and horizontal directions, resulting in probabilities of failure of 3.42% and 0.84%, respectively. An overall probability of migration to groundwater during the well injection stage was determined as 0.0006%, compared with a Canadian case study which considered 0.13% of wells failed during any stage of the wells life cycle. It incorporates various data types to represent the complexity of hydraulic fracturing, encouraging a more complete and accurate analysis of risk failures which engineers can directly apply to old and new hydraulic fracturing sites without the necessity for extensive historic and probabilistic data. This framework can be extended to assess risk across all stages of well development, which would lead to a gap in the modelled and actual probabilities narrowing. The framework developed has relevance to other geo-energy related subsurface activities such as CO2 sequestration, geothermal, and waste fluid injection disposal.
Highlights
IntroductionIt incorporates various data types to represent the complexity of hydraulic fracturing, encouraging a more complete and accurate analysis of risk failures which engineers can directly apply to old and new hydraulic fracturing sites without the necessity for extensive historic and probabilistic data
Studies have disproved this connection of thermogenic methane linked directly to shale formations with papers criticizing the lack of baseline data to indicate the amount of thermogenic methane naturally present in groundwater, regardless of hydraulic fracturing (Davies, 2011; Molofsky et al, 2011; Siegel et al, 2015)
This paper presents a new framework for the quantitative assessment of risk of groundwater contamination from well integrity failure during hydraulic fracturing
Summary
It incorporates various data types to represent the complexity of hydraulic fracturing, encouraging a more complete and accurate analysis of risk failures which engineers can directly apply to old and new hydraulic fracturing sites without the necessity for extensive historic and probabilistic data This framework can be extended to assess risk across all stages of well development, which would lead to a gap in the modelled and actual probabilities narrowing. Unexpected levels of major ions and organic compounds in shallow drinking wells within close proximity to stimulation wells have been seen and indicate the migration of stimulation fluids into formations and the loss of zonal isolation during stimulation (Digiulio and Jackson, 2016; Sun et al, 2019) These casing and cement failures are most likely to occur during the well stimulation process under extreme pressures and compromising of these barriers during this stage has been the focus of this paper. There are very few risk assessment studies which take into account human experience which can be more valuable than a solely quantitative approach (Torres et al, 2016)
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