Stress is a measure of the amount of force exerted per unit area. There are 9 stress components: three normal stress components and six shear components. Normal stresses (either tensile or compressive) are due to the forces acting at the right angle to a plane, while shear stresses result from parallel forces. There are three planes, termed principal planes, where there are no sheer stresses and only normal stresses, called principal stresses. In tectonically inactive regions, the vertical stress, due to the weight of the column of overburdenRocks and sediments above any particular stratum, is often the maximum principle stress. With increasing depth, the ratio of the horizontal stresses to the vertical stress approaches to unity (van Golf-Racht, 19821982 - T. D. van Golf-RachtFundamentals of Fractured Reservoir Engineering,see more).
During hydrocarbon production, pore pressure depletion leads to corresponding changes in the stress field, not only the effective stresses but also the total stresses. The relationship between total stress and effective stress is defined by effective stress law, where the effective stress is the difference between total stress and the pore pressure times the poro-elastic (Biot) constant. Both deformation/strain and the yield/failure of a rock are controlled by the effective stress. Fluid injectionThe process of using pressure to force fluids down wells/production-induced pore pressure changes may lead to surface heave/subsidence, and sometimes seismicityThe episodic occurrence of natural or man-induced earthquakes. The reservoirA subsurface body of rock with sufficient porosity and permeability to store and transmit fluids properties (e.g. permeabilityAbility to flow or transmit fluids through a porous solid such as rock) may also be affected. The coupling between pore pressure and stress, the ratio of the induced change in the (total) minimum horizontal stress and the pore pressure change is referred to as the stress path.
InjectionThe process of using pressure to force fluids down wells of CO2Carbon dioxide into a subsurface formationA body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it affects the in situ stress field mainly by the following processes:
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hydraulic fracturing,
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shear parting,
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expansion of the host rock(geology) The rock formationA body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it that contains a foreign material, and
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fault(geology) A surface at which strata are no longer continuous, but are found displaced slipThe extent to which a fault(geology) A surface at which strata are no longer continuous, but are found displaced has slipped in past times (reactivation).
During CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells, a reservoirA subsurface body of rock with sufficient porosity and permeability to store and transmit fluids may develop plastic behaviour (stress path is not reversible), and pre-existing faults may reactivate or even new faults may be generated. The type of stress regime affects the potential for mechanical failure and the type of failure. Therefore, careful estimation of the stress field is essential for design and performance assessment of an industrial CO2Carbon dioxide-injectionThe process of using pressure to force fluids down wells operation. It has been reported that at an injectionThe process of using pressure to force fluids down wells site, shear failure along pre-existing fractures would probably occur earlier than tensile failure (Rutqvist et al., 20082008 - J. Rutqvist, J. T. Birkholzer, Chin-Fu TsangCoupled reservoir–geomechanical analysis of the potential for tensile and shear failure associated with CO2 injection in multilayered reservoir–caprock systemssee more).
The Mohr-Coulomb failure criterion is widely used for the analysis of shear failure in rocks subject to deviatoric stress loading. The onset of shear failure at a weak plane is affected by its orientation relative to the in situ principal stresses. The knowledge of which stresses are the major and minor principal stress is essential for the assessment of shear failure. For example, under a strike-slip fault(geology) A surface at which strata are no longer continuous, but are found displaced stress regime (where horizontal stress is larger than vertical stress), the deviatoric stresses acting on the reservoirA subsurface body of rock with sufficient porosity and permeability to store and transmit fluids are largely maintained during hydrocarbon production. On the other hand, there tends to be an immediate increase in the deviatoric stresses under a normal fault(geology) A surface at which strata are no longer continuous, but are found displaced stress regime.
During CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells into a depleted(hydrocarbon reservoir) one where production is significantly reduced hydrocarbon reservoirA subsurface body of rock with sufficient porosity and permeability to store and transmit fluids, the initial stress state would be recovered as the reservoirA subsurface body of rock with sufficient porosity and permeability to store and transmit fluids pressure is brought back to its initial value, provided that the stress path is reversible during CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells. Further increase in the reservoirA subsurface body of rock with sufficient porosity and permeability to store and transmit fluids pressure would generally result in an increase in the deviatoric stresses, as wellManmade hole drilled into the earth to produce liquids or gases, or to allow the injection of fluids as a general reductionThe gain of one or more electrons by an atom, molecule, or ion in the effective minor stress, thus leading to an increasing riskConcept that denotes the product of the probability of a hazard and the subsequent consequence of the associated event of shear failure in the strike-slip fault(geology) A surface at which strata are no longer continuous, but are found displaced stress region. This leads to the concept of maximum sustainable pore pressure (for safe CO2Carbon dioxide storage(CO2) A process for retaining captured CO2, so that it does not reach the atmosphere), above which rock shear failure or fault(geology) A surface at which strata are no longer continuous, but are found displaced re-activation might occur (Shi and Durucan, 20092009 - J.-Q. Shi and S. DurucanA coupled reservoir-geomechanical simulation study of CO2 storage in a nearly depleted natural gas reservoirsee more).
InjectionThe process of using pressure to force fluids down wells-induced pressure increase, if sufficiently large, could lead to a compromise of the caprockRock of very low permeability that acts as an upper seal to prevent fluid flow out of a reservoir sealAn impermeable rock that forms a barrier above and around a reservoir such that fluids are held in the reservoir, and thus potential geomechanical consequences should be assessed prior to commencing CO2Carbon dioxide injectionThe process of using pressure to force fluids down wells. Two main effects need to be considered: fractureAny break in rock along which no significant movement has occurred dilation due to increased pore pressures and induced shear slip due either to raised pore pressures or a reductionThe gain of one or more electrons by an atom, molecule, or ion in normal stress due to buoyancyTendency of a fluid or solid to rise through a fluid of higher density forces exerted by the CO2 plumeDispersing volume of CO2-rich phase contained in target formation. FractureAny break in rock along which no significant movement has occurred orientations that are likely to be conducive to fluid flow or susceptible to seismic slip can be determined relative to the principal stress axes, if the in situ stress is known (Chadwick et al., 2008).