Investigating the Effect of Fault Presence on the Feasibility and Limitation of Above Cap Rock Pressure Monitoring in the Inyan Kara, North Dakota
Authors: Hichem A. K. Chellal , Mohamed Lamine Malki , Egenhoff Sven 
Published: August 2024

ABSTRACT:
In this paper we investigate the feasibility and limitations of a novel CO2 monitoring technique that relies on pore pressure monitoring in above injection zone/caprock permeable layers combined with accurate two-way coupled reservoir simulations. We investigated the effect of the presence of faults on this monitoring technique and we found that when accurately implemented, it has the potential of identifying the existence of leaky faults in a very small-time frame (five to 10 days for high to moderately leaky faults and up to two hundred days for low transmissibility faults). When compared to direct monitoring techniques that rely on measuring CO2 saturation in the monitoring layer, we found that Above Injection Zone pressure monitoring has the advantage of early detection of CO2 leakage through medium to high conductivity pathways but would be limited when the hydraulic conductivity of the leakage pathway is low and would perform better when combined with a direct monitoring technique.

1. INTRODUCTION
Recently, CO2 storage has gained a lot of interest in the central region of North Dakota where more than five projects are already in the pilot stage and others are already operating (Merzoug et al.,2024). To assure the safety of this industrial application, state and federal governments have put in place multiple requirements that need to be met in order to obtain the permit for such operations (Mao et al., 2024). Among The many requirements is the monitoring of the CO2 throughout the injection period and after shut-in to make sure that CO2 is stored safely in its designated reservoir and if a leak were to occur, detection is made in timely manner to assure remedial jobs are performed efficiently (Bakelli et al., 2023; H. A.K. Chellal et al., 2022,). In this study, we investigate the feasibility and limitations of Above injection zone (AIZ) pressure monitoring in CO2 geological storage in the presence of faults of deferent kinds ranging from sealing faults to high conductivity faults. This is mainly motivated by the the economic efficiency of implementing AIZ pressure monitoring as a mean to detect CO2 leakage and the fact that small faults can go undetected in seismic imaging but can still serve as a preferential leakage pathway for CO2 endangering the integrity of the storage complex (Hamadi et al., 2023). Many previous studies have proven the importance and cost efficiency of this monitoring technique in different locations (Ennis-King et al., 2017; Wiese et al., 2013; Zheng et al., 2022; Chellal et al., 2023b; Kim & Hosseini, 2014). To briefly describe it, this monitoring technique relies on accurate coupled reservoir-geomechanics simulations that can accurately capture the interchangeable effects of fluid flow and rock deformation and the implementation of high accuracy pressure gauges in a permeable formation that is above the cap rock of storage formation or at least though to be hydraulically isolated form the storage interval if both the monitoring and the storage intervals lie within the same geological unit. The injection of CO2 storage causes an immediate deformation in the monitoring layer due to Poroelastic deformation, any deviation from the pressure profile of the base case of simulation with a fully covering cap rock can be indicator of hydraulic communication with the storage interval, more details on the theory of this monitoring approach can be found in (S. Kim & Hosseini, 2014; Zheng et al., 2022).