Investigating the Possibility of Using Tracer Tests for Early Identification of EGS Reservoirs Prone to Flow Channeling

摘要

Channelized flow due to inherent heterogeneity in fracture aperture fields and further flow channeling caused by thermo-mechanical effects can substantially reduce the effective heat exchange area of fractures and thereby cause inferior performance of EGS reservoirs. Our previous studies have identified that initially channelized flow fields tend to have greater additional channeling than initially diffuse flow fields do. The current work investigates the possibility of identifying EGS reservoirs that enable highly channelized flow fields so that we can optimize production strategy. Based on a coupled Thermo-Hydro-Mechanical model that we had developed to simulate flow channeling in EGS dominated by a single fracture, we developed a conservative tracer module that can be used to quantify the tracer signature of the EGS at any given time in its heat production life cycle. We simulated tracer tests performed on the initial flow fields of hundreds of randomly generated fracture aperture fields for which the heat production performance had been quantified in a previous study. By quantifying the correlations between various tracer response metrics and EGS heat production lives, we found that the highest peak concentration and mean residence time of tracer tests can be used to identify flow systems that will lead to very short production lives. However, these tracer response metrics seem to be unable to differentiate flow systems with very good performance from those with intermediate performance if only conservative tracer is used.