Evaluating the Present In-situ Stress-State For the Richton, MS, Strategic Petroleum Reserve Site Using Geomechanical Analyses

The United States Strategic Petroleum Reserve has selected the Richton Mississippi (MS) salt dome as an oil storage site suitable to increase its current capacity to one billion barrels. In order to confirm that the Richton MS site is adequate for storage of oil deep underground, an evaluation of the present in-situ stress-state was investigated using numerical techniques involving large scale geomechanical analyses. Several Finite Element Analysis (FEA) models were constructed to approximate the complex three-dimensional Richton salt dome formation and surrounding rock in two-dimensions. These FEA models elucidated the significance of a parental salt bed which feeds the Richton salt diapir formation. Using complex creep material models to treat the near-field Richton salt diapir and elastic material models to simulate the surrounding media, the structural response of the Richton salt dome was interrogated for several geometric configurations and initial conditions. Results of these FEA simulations showed a significant difference in vertical, horizontal, and von Mises in-situ stress states, which provide valuable information as to where the salt caverns might best be located within the dome to maximize structural integrity, etc.1. INTRODUCTION1.1. Background The United States (US) Strategic Petroleum Reserve (SPR) is a large complex for stockpiling government-owned emergency crude oil. It was established in the aftermath of the 1973-74 oil embargoes. The SPR currently consists of four sites with deep underground storage caverns created in salt domes along the Texas and Louisiana Gulf Coast that store these emergency supplies of crude oil. The Richton Mississippi site is one of the SPR expansion sites available for reaching the total 1.0 billion barrel SPR capacity, as outlined by the SPR Plan [1] established in June 2007. Knowledge of the stress-state in the vicinity of the Richton Mississippi (MS) salt dome, at present, is critical to understanding the pre-excavation oil storage conditions such that mining and construction operations can be conducted.1.2. Scope Sandia National Laboratories (SNL), as scientific advisor to the Department of Energy (DOE) for the SPR, is assessing the geomechanical stability in and around the Richton, MS salt dome formation. This dome is being evaluated for the potential development of oil storage caverns. The material immediately adjacent to the Richton salt dome perimeter in the salt/non-salt interface has experienced a significant disturbance in its stress-state as the salt diapir formed over time. Results from numerical analyses (e.g., large-scale finite element model simulations) can be used to predict the geomechanical response of the salt dome prior to cavern excavation activities and thereby provide useful insight into the state-of-stress in and around its vicinity at present day. In order to assess the extent of this disturbed regime, several two-dimensional axisymmetric finite element analyses (FEA) were performed using existing near-field material property data. These FEA simulations were completed using modern sophisticated material models and utilize the most current geomechanical material property data.1.3. Objective Results from these two-dimensional (2D) FEA simulations predict, among other things, the subsidence, vertical, horizontal, and von Mises stress state in the Richton.