Basin Management of Geologic CO2 Storage: Effect of Well Spacing on CO2 Plume and Pressure Interference
Authors: Wijaya Nur , Vikara Derek , Morgan David , Grant Timothy , Remson Donald 
Published: April 2022

Abstract
Large-scale deployment of carbon capture and storage (CCS) is a key decarbonization approach to achieve drastic reductions in greenhouse gas emission levels. The United States Department of Energy's National Energy Technology Laboratory, through the Carbon Storage Assurance Facility Enterprise Initiative projects, defines a commercial-scale CO2 saline storage project as one in which at least 50 million tonnes of CO2 are injected over the course of 20 to 30 years. Large-scale decarbonization through CCS may likely involve many commercial-scale CO2 storage projects located in close proximity. Nearby injection operations may result in CO2 plume commingling and create pressure buildup over time, which could cause pressure interference and may require preventative strategies to avoid exceedance of fracture pressure threshold. This study employs numerical modeling to analyze the evolution of the extent of CO2 and pressure plumes in which the commercial-scale CO2 storage projects inject simultaneously into a common storage formation from multiple projects located in proximity. Injection operations target an extensive saline formation with formation top of 1 km below ground surface, thickness of 200 m, horizontal and vertical permeabilities of 50 and 15 mD, porosity of 10%, and all external boundaries closed to fluid flow (i.e., top, bottom and all sides). The injection occurs at 1 million tonnes/year per well for 30 years, followed by a 50-year post-injection period (PISC). The effect of well spacing and resulting pressure buildup and CO2 plume migration is explicitly evaluated. For the circumstances modeled, our analysis indicates that the radius of the CO2 plume extends 2-4 km from the CO2 injection well(s). Under the multi-project injections, CO2 plume commingling does not occur during injection; however, during PISC and pressure equilibration, CO2 plumes commingle under certain well spacings. However, the radius of the pressure-buildup plume is in the range of tens or a few hundreds of kilometers, depending on the amount of pressure increase used to define the plume edge. These findings indicate that a degree of pressure interference can occur between storage projects located near each other, particularly during the early stages of the project. Additionally, our analysis of pressure interference shows that required well spacing needed to avoid approaching or exceeding fracture pressure thresholds can be extensive for formations with low fracture gradients (i.e., in the range of 80–125 km for formations with a pressure gradient of 0.6 psi/ft), but significantly smaller for formations with higher fracture gradients (i.e., 5 km for 0.8 psi/ft). Given the potential pressure interference which may occur from multiple projects, this analysis shows the importance of coordination among storage operators and regulatory stakeholders. Because this study analyzes a very specific geologic situation, well configuration, CO2 injection rate and boundary conditions, this exploratory study bears further investigations across other geologic situations.