Constraints on stress tensor from microseismicity at Decatur

Abstract. Induced microseismicity has been detected in the Decatur CO2 sequestration area, providing critical constraints on the stress state at the reservoir. We invert the full stress tensor with two subsets of source mechanisms from the induced microseismic events. To achieve this, we incorporate additional information on the vertical stress gradient and instantaneous shut-in pressure (ISIP) measured in the area. Additionally, our results demonstrate that constraining the intermediate stress tensor to a vertical orientation is essential to achieve a consistent stress inversion. The inverted stress is then used to estimate the minimum activation pressure required to trigger seismicity on fault planes identified by the source mechanisms. The comparison of the minimum activation pressure with injection pressure indicates one of three possibilities: the ISIP pressures are significantly lower than predicted (approximately 28–29 MPa), the maximum horizontal principal stress is extremely high (exceeding 120 MPa), or the coefficient of friction is significantly lower than 0.6 on a large number of activated faults. Our analysis also shows that poorly constrained source mechanisms do not lead to reasonable stress constraint estimates, even when considering alternative input parameters such as ISIP and vertical stress. We conclude that induced microseismicity can effectively be used to estimate the stress field when source mechanisms are also well constrained. For future CO2 sequestration projects, measuring and constraining ISIP pressure and maximum horizontal stress in the reservoir will ensure that more accurate estimates of stress state from moment tensor inversions can be obtained for improved prediction of the long-term reservoir response to injection.