Numerical Simulation Applications for Optimizing Fiber Optics Strain Monitoring During Injection in Fractured Reservoirs
Authors: Eyinla D. S. , Emadi H. , Navaid H. B. , Arora A. , Kebir A. 
Published: August 2024

ABSTRACT:
Addressing the challenges associated with fluid injection, such as CO2, into reservoirs containing fractures or faults is of paramount importance. An invaluable application of numerical simulations is the prediction of strain induced by stress changes during injection processes. This research aims to apply such an approach to quantitatively assess strain near fluid injection point and in other locations further away from the point of impact, while also simulating the effects of fault failures on caprock strain development. To achieve this, our study incorporates the monitoring approach of fiber optics Distributed Acoustic Sensing (DAS) into numerical simulation models using a case study of saline aquifer in Hardeman Basin, allowing us to analyze strain changes during hydromechanical simulations. Our observations reveal that the magnitude of strain is influenced by multiple factors, however, a notable observation is the sensitivity of the monitored strain in different directions, with significant changes observed in both lateral and vertical monitoring wells. Vertical strain (z-direction) exhibited larger magnitudes and showed positive strain within the reservoir layer as well as at the caprock and seal. Conversely, strain changes measured along the horizontal profiles (wells) in the x-direction displayed negative strain and with lower values. Results also show that there is an increase in the strain developed around the fault zone and within the reservoir section, while lower strain values are observed beneath the reservoir zone (injection zone). Overall, the strain magnitudes determined using this numerical method fall within the range detectable by fiber optic distributed acoustic sensing (DAS) tools. This affirms the applicability of these tools in optimizing the monitoring of injection-induced strain in proximity to the injection well and at greater distances, with a specific emphasis on monitoring using the vertical components.

1. INTRODUCTIONS
The optimization of fluid injection in fractured reservoirs is a critical component of various subsurface engineering applications, such as, carbon capture and storage (CCS), and wastewater disposal. Fractured reservoirs, characterized by their complex geological structure, coupled with the heterogeneous nature of geological systems, introduces additional complexities presenting some notable challenges for fluid injection processes (Eyinla et al., 2023a). Thus, injection operations often entail complex geological and geomechanical challenges, necessitating the need to monitor strain developed within and away from the reservoir, and particularly to prevent pressure-induced failure and leakage (Xue & Hashimoto, 2017).
To address these complex issues and ensure the safety of injection processes, in-situ monitoring becomes imperative. Reports revealed that the injection of large volumes of fluid in deep underground can induce changes in pressure, temperature, and stress, potentially leading to caprock integrity and leakage concerns (Chang et al., 2022; Gholami et al., 2021; Rutqvist et al., 2016; Schwab et al., 2017; Sun et al., 2017; Sun et al., 2021a). Various monitoring technologies, including geophysical methods, geodetic measurements, and downhole instruments, are therefore employed to capture these changes in real-time (Kim et al., 2015; Kumar Singh et al., 2023; Rutqvist, 2012; Sun et al., 2021b; Ullah et al., 2023). However, monitoring in complex underground environments remains a significant challenge.