The New Insight of Microseismic Activity in Fault Zones for Geological Carbon Storage: A Case Study of the Decatur Project in the United States

Based on the U.S. Decatur CO₂ storage project, the interrelationships between the formation thickness, tectonic stress state, and microseismic activity induced by CO₂ injection during the carbon storage process are investigated in this paper, providing scientific basis for site selection and safety assessment of carbon storage. By analyzing the characteristics of the reservoir (Mt. Simon sandstone) and caprock (Argenta formation) thickness at the Decatur project, the impact of these factors on pressure diffusion during injection is investigated. Combining the distribution of tectonic stress and historical accumulation data, the sensitivity of fault slip in the Precambrian basement is analyzed. The Argenta formation near the CO₂ injection point is relatively thin or absent, lacking sufficient thickness to prevent pressure migration towards the basement fault, which results in microseismic events predominantly occurring in the Precambrian basement. After CO₂ injection into the Mt. Simon sandstone reservoir, the pore pressure initially increases near the injection point, and then diffuses over a larger area. Simple CO₂ injection alone is not enough to fully trigger large-scale microseismic activity. The Precambrian basement is more sensitive to stress release caused by fluid injection due to the accumulation of historical tectonic stresses. In the Decatur project, when the CO₂ injection rate reached 1.25～1.4 million tons per year, fault slip and microseismic activity were induced. The selection of CO₂ storage sites should be comprehensively evaluated based on formation thickness, tectonic stress state, and the injection-induced mechanism to ensure the safety and long-term stability of the injection.