Geomechanical analysis of the influence of CO2 injection location on fault stability

Large amounts of carbon dioxide(CO2) should be injected in deep saline formations to mitigate climate change,implying geomechanical challenges that require further understanding.Pressure build-up induced by CO2injection will decrease the effective stresses and may affect fault stability.Geomechanical effects of overpressure induced by CO2injection either in the hanging wall or in the foot wall on fault stability are investigated.CO2injection in the presence of a low-permeable fault induces pressurization of the storage formation between the injection well and the fault.The low permeability of the fault hinders fluid flow across it and leads to smaller overpressure on the other side of the fault.This variability in the fluid pressure distribution gives rise to differential total stress changes around the fault that reduce its stability.Despite a significant pressure build-up induced by the fault,caprock stability around the injection well is not compromised and thus,CO2leakage across the caprock is unlikely to happen.The decrease in fault stability is similar regardless of the side of the fault where CO2is injected.Simulation results show that fault core permeability has a significant effect on fault stability,becoming less affected for high-permeable faults.An appropriate pressure management will allow storing large quantities of CO2without inducing fault reactivation.

Fault activation and induced seismicity in geological carbon storage--Lessons learned from recent modeling studies

In the light of current concerns related to induced seismicity associated with geological carbon sequestration(GCS),this paper summarizes lessons learned from recent modeling studies on fault activation,induced seismicity,and potential for leakage associated with deep underground carbon dioxide(CO2) injection.Model simulations demonstrate that seismic events large enough to be felt by humans require brittle fault properties and continuous fault permeability allowing pressure to be distributed over a large fault patch to be ruptured at once.Heterogeneous fault properties,which are commonly encountered in faults intersecting multilayered shale/sandstone sequences,effectively reduce the likelihood of inducing felt seismicity and also effectively impede upward CO2leakage.A number of simulations show that even a sizable seismic event that could be felt may not be capable of opening a new flow path across the entire thickness of an overlying caprock and it is very unlikely to cross a system of multiple overlying caprock units.Site-specific model simulations of the In Salah CO2storage demonstration site showed that deep fractured zone responses and associated microseismicity occurred in the brittle fractured sandstone reservoir,but at a very substantial reservoir overpressure close to the magnitude of the least principal stress.We conclude by emphasizing the importance of site investigation to characterize rock properties and if at all possible to avoid brittle rock such as proximity of crystalline basement or sites in hard and brittle sedimentary sequences that are more prone to injection-induced seismicity and permanent damage.

Geomechanical effects of CO_2 storage in depleted gas reservoirs in the Netherlands:Inferences from feasibility studies and comparison with aquifer storage

In this paper,the geomechanical impact of large-scale carbon dioxide(CO) storage in depleted Dutch gas fields is compared with the impact of COstorage in saline aquifers.The geomechanical behaviour of four potential COstorage sites is examined using flow and geomechanical simulations.Many gas reservoirs in the Netherlands are found in fault blocks,one to a few kilometres wide,laterally bounded by sealing faults.Aquifer depletion or re-pressurization in the lateral direction is seldom an issue because of a lack of active aquifers.Reservoir pressure changes are therefore limited to a gas-bearing fault block,while the induced stress changes affect the gas reservoir and extend 1-3 km away into the surrounding rock.Arguments in favour of COstorage in depleted gas fields are:proven seal quality,availability of field data,no record of seal integrity failure by fault reactivation from the seismically active producing Dutch gas fields,and the potential benefits of restoring the virgin formation pressure and stress state to geomechanical stability.On the other hand,COinjection in saline aquifers causes pressure build-up that exceeds the virgin hydrostatic pressure.Stress perturbations resulting from pressure build-up affect large areas,extending tens of kilometres away from the injection wells.Induced stresses in top seals are.however,small and do not exceed a few tenths of megapascal for a pressure build-up of a few megapascals in the storage formation.Geomechanical effects on top seals are weak,but could be enhanced close to the injection zone by the thermal effects of injection.Uncertainties related to characterisation of large areas affected by pressure build-up are significant,and seal quality and continuity are more difficult to be demonstrated for aquifers than for depleted gas reservoirs that have held hydrocarbons for millions of years.