Date of Award
Master of Science (MS)
The Coso geothermal field is located approximately 220 kilometers north of Los Angeles, CA. In 2002, a project began to develop the east flank of the Coso geothermal field into an enhanced geothermal system (EGS); in such a system water is injected via injection well(s) into hot dry basement rock through naturally occurring or stimulated fractures. The injected water gathers heat from the reservoir rock before being extracted for direct use or energy production. To develop such a reservoir, adequate understanding of the reservoir geomechanics is necessary. This thesis investigates the state of stress and rock fractures, the existing permeable fractures in the reservoir, and the effects of water injection into fractures at the Coso EGS.
A lower bound estimate of the magnitude of the maximum horizontal in-situ stress (SHmax) was obtained using a fracture mechanics approach incorporating thermal effects on drilling induced fractures in well 38C-9. The maximum principal stress was found to transition from horizontal ( σ1 = SHmax) to vertical ( σ1 = Sv) A fracture propagation study was applied to compare the estimate presented herein with other published estimates that utilized frictional faulting and rock strength theory. The results showed the lower bound estimate resulted in little or no fracture propagation away from the wellbore; published estimates predicted extensive fracture propagation away from the wellbore.
The state of the jointed rock mass was characterized based on formation micro scanner (FMS) data as they applied to the joint network fractures with significant aperture (Rose et al, 2004). The joint network supported the stress regime concluded from the state of stress estimation. A linear and non-linear failure criterion was applied to investigate critically and non-critically stressed joints, also the pore pressure increase required to critically stress non-critically stressed joints was found. At the proposed injection depth, critically oriented joints with friction angles 25º were critically stressed.
A plane strain mathematical model was developed to investigate induced effects of water injection into a permeable deformable fracture. Three fracture geometries were considered: (i) injection/extraction from a line fracture, (ii) injection into an infinite radial fracture, and (iii) injection into a joint. Expressions for the induced pressure and temperature in the fracture and reservoir rock were developed and used to develop expressions for the induced thermoelastic, poroelastic, and combined thermo- and poroelastic fracture width changes, and the resulting induced fracture pressure. Analytic solutions were derived utilizing constant injection and leak-off assumptions. It was found the poroelastic effects tend to close the fracture as a result of leak off, while the thermoelastic effects tend to open the fracture as a result of the cold water injection into hot rock. For conditions in the Coso EGS, the thermoelastic effects are dominant. At early times and high injection rates, the poroelastic effects cannot be ignored when considering the induced pressure even though the effects on the fracture width are relatively small. The fluid/solid coupling incorporated into model (iii) can alter the fracture width and pressure.
Nygren, Andrew J., "Geomechanics applied to reservoir development in the coso geothermal field" (2005). Theses and Dissertations. 213.