“…Injection of fluids into naturally or hydraulically fractured formations has been an important research topic due to its relevance in a vast number of engineering applications: waste disposal (Cornaton et al., 2008; McCarthy & Zachara, 1989; Witherspoon et al., 1981), carbon sequestration (Iding & Ringrose, 2010), contaminant transport (Sahimi, 2011), enhanced oil recovery (Jiménez‐Martínez et al., 2016), and tracer surveillance (Hu & Moran, 2005; Rugh & Burbey, 2008; Warner et al., 2014). In many applications, a key objective of the numerical model built to simulate the processes is to quantify the mixing between the injected and resident fluids and quantify the extent of the associated mixing zone (Bonazzi et al., 2020, 2021; Cirpka & Valocchi, 2007; Dentz et al., 2011; Jha et al., 2011a; Z. Zhao et al., 2011). In cyclic well operations, where the well alternates or cycles through injection and production/withdrawal stages, the evolution of fluid mixing is complicated by short time scale variations in the pore pressure field.…”