ABSTRACT:

Sandia National Laboratories has long used the Munson-Dawson (M-D) model to predict the geomechanical behavior of salt caverns used to store oil at the Strategic Petroleum Reserve (SPR). Salt creep causes storage caverns to deform inward, thus losing volume. This loss of volume affects the salt above and around the caverns, puts stresses and strains on borehole casings, and creates surface subsidence which affects surface infrastructure. Therefore, accurate evaluation of salt creep behavior drives decisions about cavern operations. Parameters for the M-D model are typically fit against laboratory creep tests, but nearly all historic creep tests have been performed at equivalent stresses of 8 MPa or higher. Creep rates at lower equivalent stresses are very slow, such that tests take months or years to run, and the tests are sensitive to small temperature perturbations (<0.1°C). A recent collaboration between US and German researchers, however, recently characterized the creep behavior at low equivalent (deviatoric) stresses ((<8 MPa) of salt from the Waste Isolation Pilot Plant. In addition, the M-D model was recently extended to include a low stress creep "mechanism". This paper details new simulations of SPR caverns that use this extended M-D model, called the M-D Viscoplastic model. The results show that the inclusion of low stress creep significantly alters the prediction of steady-state cavern closure behavior and indicate that low stress creep is the dominant displacement mechanism at the dome scale.

1. Introduction

The U.S. Strategic Petroleum Reserve (SPR), operated by the U.S. Department of Energy (DOE), stores crude oil in solution-mined caverns in the salt dome formations of the Gulf Coast. There is a total of 60 storage caverns located at four different sites in Texas (Bryan Mound and Big Hill) and Louisiana (Bayou Choctaw and West Hackberry), along with several abandoned or decommissioned caverns. Each cavern is constructed and then operated using casings inserted through a wellbore or wellbores that are lined with steel casings cemented in place from the surface to near the top of the cavern. The West Hackberry salt dome is in the extreme southwestern corner of Louisiana, some 24 km from the Louisiana/ Texas border to the west and the Gulf of Mexico to the south. It has an oil storage capacity of about 35×106 m3(222×106 barrels) within 21 caverns, and has operated since 1980.

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