Present-day stress analysis of the St. Lawrence Lowlands sedimentary basin (Canada) and implications for caprock integrity during CO2 injection operations

Abstract

A geomechanical analysis of the St. Lawrence Lowlands sedimentary basin is important to reliably estimate the maximum sustainable fluid pressures for CO2 injection that will not reactivate pre-existing faults in the caprock thereby inducing a breeched CO2 reservoir. This requires the determination of prevailing stresses (orientations and magnitudes), fault and fractures geometries and rock strengths. The average maximum horizontal stress orientation (SHmax) is estimated N59°E±20° in the St. Lawrence Lowlands. The stress orientations were obtained from stress-induced wellbore breakouts inferred from four-arm dipmeter caliper data in 17 wells. These wellbore failure features are confined to Paleozoic lithological units of the St. Lawrence Platform succession and frontal thrusts of the Quebec Appalachians at depths from 250m to 4km. Our results are consistent with the regional NE–SW SHmax stress orientation trend that is generally observed in eastern Canada and the U.S. The stresses/pressure gradients estimated for the St. Lawrence Lowlands (depths<4km) are: Shmin 20.5±3kPa/m, Sv 25.6kPa/m, SHmax 40±7.5kPa/m, pore pressure Pp 9.8kPa/m indicating a strike-slip stress regime Shmin<Sv<SHmax. The high-angle NE–SW regional faults and fractures in the Paleozoic sedimentary succession and the Grenvillian basement are oblique to the SHmax stress orientations (10° to 36°) and could be reactivated (slip tendency 0.34 to 0.58) under the present-day stress field if fluid pressures exceeded the critical threshold. Further refinement of regional geomechanical model is required to estimate the maximum sustainable injection pressure necessary for shear reactivation along the regional faults. The regional pore pressure–stress coupling ratio under assumed parameters is about 0.5–0.65 and may contribute to reduce the risk of shear reactivation of faults and fractures. The maximum sustainable Pp that would not cause opening of vertical tensile fractures during CO2 operations is about 18.5–20MPa for the depth of 1km.