The flowback behavior of salt in hydraulically fractured shale under multi-phase flow conditions: Modelling, simulation and application

Abstract

Mathematical models for characterizing hydrocarbon-water transport under flowback conditions have been intensively developed in past years. However, there have been few studies on salt transport. In this study, a mathematical model modelling the flowback behavior of salt in a multi-porous medium consisting of hydraulic fractures (HF), induced natural fractures (IF) and shale matrix (MP) under multi-phase flow conditions is proposed. The model highlights two mechanisms driving salt transport between two directly connected media: advection and diffusion. Capillary pressure and chemical osmosis, which affect multi-phase flow, are also considered. Finite difference method is applied to solve the mathematical model, and a numerical simulator has been developed by using Wolfram Mathematica. The flowback simulation of salt for a hydraulically fractured shale well in Bakken Formation, North Dakota under multi-phase flow conditions was conducted. Sensitivity analysis was conducted to quantify how total salt recovery was affected by different reservoir parameters: the salinity in shale matrix (Cm), the salt diffusion coefficient from IF to HF (D0), the salt diffusion coefficient from MP to IF (D1), the shape factor between IF and MP (α2), formation wettability and clay content (xc). Finally, the proposed model was used for fracture parameter inversion in comparison to a commercial software in terms of the procedures and indicators of history matching, and the estimated fracture parameters. The proposed model helps to understand the salt flowback behavior under multi-phase flow conditions by simulating the salt fluxes driven by advection and diffusion between the different media, and is proved to be efficiently practical by simultaneously matching flowback salinity and fluid rates for field application.