Abstract
Surface deformation is commonly observed phenomenon in various geothermal fields and reflects subsurface volume change due to shrinkage or dilation. However, faulting and fault reactivation induced by fluid injection (seismic and aseismic faulting) can also be a significant source of surface deformation. Monitoring subsurface deformation is useful in understanding reservoir behavior such as fluid mass and energy transfer and the evolution of transport characteristics. Moreover, detection of slip may allow a more precise understanding of injection-induced seismicity. We assess surface deformations (vertical displacement, surface tilt and horizontal strain) as signatures in two different modalities: (i) isotropic volume change (Mogi model) and (ii) injection induced shear offset (Okada model) and compare the results with both the resolution of current geodetic tools and existing observations of surface deformation. Comparison of predicted deformations with instrumental resolutions confirms that geodetic signals, especially tilt and strain, are indeed sufficiently large to describe reservoir evolution in detail and comparison to field data suggests probability of significant contribution of slip on surface deformation.
1. INTRODUCTION
Surface deformations of significant magnitude have been observed by both interferometric methods (InSAR) [1~9] and by direct measurement of surface tilt [10] in a number of geothermal fields. Subsurface deformations induced by cold water injection conform to two different modalities: (i) isotropic volume change and (ii) injection induced shear offset. Isotropic volume change can be induced by either thermal contraction (volume decrease) or pressure dilation (volume increase) in the reservoir with shear slip similarly resulting from changes in effective stress induced by changes in fluid pressures or temperature.
Linking the observed deformation signal with a causative subsurface mechanism is useful in defining active processes in reservoir evolution. Such models may be used to constrain magnitudes of heat energy transfer from rock to fluid, fluid leakage and the evolution of transport characteristics of the reservoir. The detection of slip processes in turn constrains fluid flow and the evolution of major flow paths. Moreover, the detection of slip may allow the precursors to injection induced seismicity to be defined and monitored [11, 12]
