Safety impact of inter-layer fractures opening in CO₂ storage systems

In the CO2 geo-sequestration system, the barrier ability of caprock is an important issue for the CO2 storage safety. In the central Taiwan, the deep saline formation located at the southern Taihsi basin has the CO2 geo-sequestration potential and is nearby the largest CO2 emission source (e.g., Taichung Coal-fired Power Plant) of Taiwan. The storage system of the target site consists of the CS shale (as a caprock) and YTP-GYS sandstone formation (as a storage reservoir). The CS shale, a multilayered caprock, is a heterogeneous caprock, which composed of multilayers with extreme-low permeability shale layers and permeable thin shaly-sand layers. In this study, a numerical method is used to study the impact of the fractures opening in a CO2 caprock formation. Modified Barton-Bandis model is used to describe the fractures opening and recovering behavior in the caprock formation. A potential CO2 storage site in the central Taiwan is as a case study to study the barrier ability of caprock and the leakage risk of CO2. The simulation study of the CS caprock formation showed that the fractures opened when the effective stress was lower than the threshold value. The fracture in the bottom layer were created a CO2 leakage pathway into the upper layer. However, only a small amount of CO2 invaded into the bottom of the CS caprock. The multilayered caprock system is capable of sealing the stored CO2 to prevent the CO2 leakage into the overburdens. Based on the results of the scenarios analysis, the geomechanical properties of rock determined the performance of the fractures opening and leakage risk of CO2 in caprock formation. A high Young’s modulus of rocks increases the change of effective stress resulting in early fracture opening. The rock with low Young’s modulus can absorbs the increased pressure due to its flexibility, and thus the opening of fracture is retarding and the leakage of CO2 is lower. A high Poisson’s ratio of rocks increases the change of effective stress resulting in early fracture opening and longer opening time. That is, in the case with a lower Poisson’s ratio, the effective stress reaches the threshold value later. Besides, the scenario with high Young’s modulus, there is a lot of CO2 invaded into the bottom of the caprock formation due to the longer fracture opening in the bottom layer. The permeable thin shaly-sand layer provided a new migration path and prevented the upper layer from the fracture cracking. The intra-shale layers in multilayered caprock formation, acts as a seal, would increase the safety in CO2 geo-sequestration system. The multilayered caprock formation is capable of sealing the stored CO2 to prevent the CO2 leakage into the overburdens.