Geodetic Imaging of Production and thermal Depletion in Geothermal Reservoirs

Open Access
Author:
Im, Kyungjae
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 23, 2015
Committee Members:
  • Derek Elsworth, Thesis Advisor
Keywords:
  • Geodesy
  • Geothermal
  • Fault
Abstract:
Surface deformation is a commonly observed phenomenon in various geothermal fields. This reflects subsurface volume change due to shrinkage and dilation or faulting and fault reactivation induced by fluid injection. Here 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 slip model) and compare the results with the resolution of current geodetic tools. To obtain precise volumetric deformation in a constrained reservoir, we assume a spherical fractured reservoir within an infinite medium. In this we allow tensional decoupling to estimate the magnitude of the maximum detectable deformation recorded when thermal contraction ensues. Comparison of predicted deformations with instrumental resolutions confirms that geodetic signals, especially tilt and strain, are indeed sufficiently large to describe reservoir evolution. Volume changes of 103m3 (induced by thermal contraction of a few tens of days of geothermal operation) or, 1mm slip on a reactivated fault patch of 1000m×500m at a depth of 2500m induces tilts of ~10 nano-radians and strains of ~10 nano-strains which are ~10 times larger than the resolution of current surface geodetic tools. Observed surface deformations and field operation data in various geothermal projects suggest the necessary involvement of both mechanisms in contributing the large deformations. Although the two different modes would induce surface deformation concurrently, we expect they can be identified both by the timing of events and the form of the surface deformation