A multi-mechanism multi-pore coupled salt flowback model and field application for hydraulically fractured shale wells

Abstract

Flowback records matching has been increasingly used for reservoir and fracture parameters inversion for hydraulically fractured unconventional oil and gas wells. However, previous publications mainly focus on flowback water transients but flowback salinity transient analyses are few. In this study, a salt flowback model characterizing salt transport in a hydraulically fractured shale reservoir has been proposed. The physical model treats the hydraulically fractured shale reservoir as a multi-pore medium, which includes wellbore, primary hydraulic fracture, secondary hydraulic fracture, clay-dominant pore and other matrix pore. The salt transport accounts for three mechanisms, i.e. advection, diffusion and clay-dominant adsorption. We apply semi-implicit algorithm for mathematical model solution and numerical simulator development. The simulation results indicate that advection predominates in the salt flowback. Salts and water flow into shale matrix from secondary hydraulic fractures at the beginning of flowback and then the flowing direction reverses. The cumulative flowback salinity of well exhibits an increase with the flowback time. However, the instant flowback salinity of well first increases and then decreases along with the increase in shut-in time. Finally, 2 wells from field in X shale formation, have been applied to match the flowback salinity and water records. The parameter combinations of the conductivity and half-length of hydraulic primary fractures, as well as the number of secondary fractures are recovered based on the good match of flowback salinity records and water load recovery. This study provides another way for post-stimulation evaluation and supplements existing flowback model.