Task Description – This research program focused on the integrity and vulnerability of wells and caprocks associated with underground gas storage (UGS) facilities. Large uncertainties and challenges exist when assessing the impact of seismic ground motion (shaking) and direct fault shear displacements on UGS facilities, as such events are rare. This means that there are very limited field data from which to derive empirical relations for predicting damage from future events. The research aimed to reduce the uncertainties associated with assessing the impact of seismic events on California UGS facilities and provide usable input to the OpenSRA software.
The challenges in assessing the seismic impact on UGS facilities were approached top-down, starting with an historic assessment from published literature, a review of relevant characteristics of California UGS facilities and wells and an assessment of potential changes in fault permeability caused by fault displacement. These provide the basis for the full-physics numerical modeling. Numerical sensitivity studies were first performed to determine the most significant parameters for assessing the impact of; (1) direct fault shear across wells, (2) seismic ground motion (shaking) impact on wells, and (3) potential caprock leakage along an activated fault.
In the case of direct fault shear across wells, the approach was to derive the relationship between fault shear displacement and strain in vital well components, whereas for ground shaking impact on wells the maximum bending moments from prescribed ground motions were evaluated. These analyses were conducted for typical California UGS well designs as identified from public records.
The assessment of potential caprock leakage along an activated fault is perhaps the most challenging task. This task requires an assessment of how a fault might dilate and transmit fluids after being activated. This requires an estimate of the volume of gas that could leak out through the dilated fault. The approach was taken to provide a conservative worst-case scenario of the potential opening of the fault assuming a high fault fluid transmissivity and to estimate the potential gas leakage for such a scenario. As the probability of dilation and gas leakage is so small, the conservatism is not expected to significantly influence the overall risk assessment.
In summary this team:
Performed a historic assessment of published literature
Reviewed relevant characteristics of California UGS facilities and wells
Assessed potential changes of fault permeability caused by fault displacement (in conjunction with the Fault Displacement team)
Developed a numerical model for assessing the impact of dynamic shaking on wells
Developed a numerical model for assessing the impact of fault shear through a well
Developed a numerical model to assess fault shear through a caprock
Compiled all of the analyses and developed fragility curves for wells and caprocks (see Fragility Development
Lead Investigator: Jonny Rutqvist (Lawrence Berkeley National Laboratory, LBNL)
Team Members: Jens Birkholzer (LBNL), William Foxall (LBNL), Preston Jordan (LBNL), Yingqi Zhang (LBNL), Keurfon Luu (LBNL), Tsubasa Sasaki (LBNL), Kenichi Soga (University of California, Berkeley)