Global Sensitivity Analysis of a Reactive Transport Model for Mineral Scale Formation during Hydraulic Fracturing

Injection of water-based hydraulic fracturing fluid (HFF) into tight shale gas/oil formations can increase formation permeability and enhance production rates, but this process frequently causes mineral scale formation that can occlude pore space and hinder flow. To identify the most important factors that control the formation of mineral scales, A novel global sensitivity analysis method, distance-based generalized sensitivity analysis (DGSA), was applied, to a reactive transport model (RTM) that was previously built and calibrated to simulate precipitation of barite [BaSO₄] and iron (hydr)oxide [Fe(OH)₃] in shale matrices and on fracture surfaces. Reactive transport simulations were run with model parameters randomly sampled based on assigned uncertainties. Modeling results for barite and Fe(OH)₃ formation were clustered using machine-learning algorithms. A list of ranked critical input parameters was obtained after statistical quantification of cumulative distribution functions of input parameters. Barite formation is most sensitive to the rate of sulfate ion generation, which is determined by the pyrite dissolution rate coefficient and oxidant availability. Also, barite formation is sensitive to the initial amounts of barite in HFF and shale, followed by barite thermodynamics/kinetics. For Fe(OH)₃ formation, the ranked factors are Fe(OH)₃ precipitation rate coefficients, initial HFF pH, initial Fe(OH)₃ amount in HFF, and oxidant availability.