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dc.contributor.authorAtefi Monfared, Kamelia
dc.date.accessioned2015-08-27 15:21:29 (GMT)
dc.date.available2015-08-27 15:21:29 (GMT)
dc.date.issued2015-08-27
dc.date.submitted2015
dc.identifier.urihttp://hdl.handle.net/10012/9608
dc.description.abstractNumerous petroleum engineering, mining, and enhanced geothermal energy operations involve cyclic ‎injection of fluids into geological formations. Geomechanics of injection operations in weakly ‎consolidated or unconsolidated reservoirs is complex, and means for analyzing the involved physical ‎processes are limited. The key feature that must be considered is parting of the formation during ‎injection, which occurs at near zero effective stresses when strength and stiffness of the medium ‎become effectively zero. Even if peculiarities of the granular media behavior at near zero effective ‎stresses are disregarded and a highly idealized Mohr-Coulomb behavior coupled with constant ‎permeability Darcy flow is assumed, the injection problem is still highly challenging. This type of ‎poroplastic formulation remains analytically intractable even for simplest geometries. Numerical ‎computations are highly challenging as well, due to high fluid-solid matrix stiffness contrast. ‎ Much effort has been devoted thus far to understand soil-fluid interactions in geological reservoirs ‎triggered by borehole excavation and production operations. With regards to injection operations ‎however, practically no comprehensive study has been performed to access the fundamental ‎geomechanical processes involved. Previous attempts to evaluate injection operations mainly ‎concentrate on describing fracture growth in hard brittle formations. In principle, the geomechanical ‎processes prior to fracture initiation are particularly complicated in weakly consolidated strata. This ‎dissertation presents analytical solutions and numerical models to examine geomechanics of high ‎pressure fluid injection in conditions when flow rates are high enough to induce plasticity yet not ‎parting of the formation. The study considers injection through a fully-penetrating vertical wellbore ‎into an isotropic, homogeneous unconsolidated geological layer confined between impermeable seal ‎rock layers. Axisymmetric conditions are assumed. The main objective is to evaluate the time ‎dependent geomechanical response of the unconsolidated reservoir in such conditions focusing on ‎failure mechanisms and permanent changes in stress conditions around the injection area. Results of ‎this research makes it possible to address the issue of integrity of confining strata, facilitate assessments ‎of potential leakage areas, and offer aid for optimization of injection operations as well as in ‎formulating monitoring strategies.‎ First, rock-fluid interactions are evaluated prior to the state where limiting shear resistance is ‎reached during injection. Unlike previous studies, impacts of vertical confinement governed by the ‎stiffness of the overburden layer are incorporated. The Winkler spring model approximation is ‎implemented to describe the response of the confining strata in the plane perpendicular to the reservoir. ‎New poroelastic analytical solutions are derived to describe evolution of stress and strain components in ‎time as a function of induced pore pressures. Solutions are verified against fully-coupled numerical ‎models designed in this study. Next, novel insights into the geomechanics of parting in various stress ‎regimes is offered via a comprehensive assessment of stress perturbations surrounding vertical injection ‎wellbores. A thorough sensitivity analysis is conducted to examine the effect of vertical confinement ‎and rock-fluid characteristic parameters on the reservoir response in the wellbore vicinity. Results ‎demonstrate a notable impact of seal rock stiffness on the near wellbore rock behavior in formations ‎with high intrinsic permeability (typically exceeding 0.05 Darcy). The study shows that the key ‎parameter controlling the injection process in the poroelastic regime is the ratio of the overburden ‎Winkler stiffness to the reservoir’s bulk modulus, with the Winkler parameter reflecting the seal rock ‎stiffness. When this ratio approaches unity, practically no shear stress is induced in the reservoir while ‎for ratios exceeding unity, deviatoric stresses gradually increase. In situations when the stiffness ratio is ‎below unity, the porous formations can behave in a rather complicated manner depending on the initial ‎stress regime where redirection of the minimal principal stress occurs from a horizontal to a vertical ‎plane. Sensitivity analyses reveal that at the same injection rate rock failure occur more rapidly in ‎conditions of higher stress anisotropy, higher elastic moduli, lower permeability, higher degree of rock-‎fluid coupling, and a higher vertical confinement.‎ Next, rock-fluid interactions are evaluated in an unconsolidated reservoir formation confined ‎between two stiff seal rock layers subjected to injection pressures high enough to induce plasticity yet ‎not parting of the formation. The injection process is first examined numerically by constructing a ‎fluid-coupled poro-elasto-plastic model in which propagation of the significant influence zone ‎surrounding the injection borehole is quantified by the extent of the plastic domain. A comprehensive ‎assessment of stresses, pore pressures, as well as failure planes is carried out throughout an entire ‎transient state of an injection cycle, at steady state, and also during the shut-in period. The numerical ‎solution describes five distinct zones evolving with time around the injection well and corresponding to ‎different stress states: liquefaction at the wellbore followed by three inner plastic domains where ‎directions of major principal stress changes from vertical to radial and failure planes change accordingly. ‎The plastic domains are followed by a region where stress states remain in the elastic range. Failure ‎mechanisms at the wellbore is found to be in shear initially, followed by development of a state of zero ‎effective stress, i.e. liquefaction. Next, a novel methodology is proposed based on which new weakly-‎coupled poro-elasto-plastic analytical solutions are derived for the stress/strain components during ‎injection. Unlike previous studies, extension of the plastic zone is obtained as a function of injection ‎pressure, incorporating the plasticity effects around the injection well. The derived loosely-coupled ‎solutions are proven to be good approximations of fully-coupled numerical models. These solutions ‎offer a significant advantage over numerical computations as the run time of a fully-coupled numerical ‎model is exceedingly long (requiring about six months for 661 million time computational steps using ‎FLAC3D 3.0 code on Intel® i7 3.33 GHz CPU).‎ The final part of this dissertation includes a brief chapter on the post-injection behavior of ‎unconsolidated reservoir formations confined with stiff seal rock layers. Pore pressure dissipation, stress ‎variations, and the transition behavior of the plastic domain surrounding the injection wellbore to an ‎elastic state are numerically evaluated. Results offer an original insight into the permanent ‎geomechanical effects of injection operations in such formations. ‎en
dc.language.isoenen
dc.publisherUniversity of Waterlooen
dc.subjectfluid injectionen
dc.subjectporoelasticen
dc.subjectporoelastoplasticen
dc.subjectseal rock stiffnessen
dc.subjectcoupled soil-fluid behavioren
dc.subjectgeologocal reservoiren
dc.titleGeomechanics of Fluid Injection in Geological Reservoirsen
dc.typeDoctoral Thesisen
dc.pendingfalse
dc.subject.programCivil Engineeringen
uws-etd.degree.departmentCivil and Environmental Engineeringen
uws-etd.degreeDoctor of Philosophyen
uws.typeOfResourceTexten
uws.peerReviewStatusUnrevieweden
uws.scholarLevelGraduateen


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