Stability Analyses of Differently Shaped Salt Caverns for Underground Natural Gas Storage

Open Access
Author:
Onal, Erol
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 02, 2013
Committee Members:
  • Luis F Ayala H, Thesis Advisor
  • Jamal Rostami, Thesis Advisor
  • Yilin Wang, Thesis Advisor
Keywords:
  • natural gas
  • underground storage
  • salt caverns
  • cavern stability
  • parametric study
  • cavern design
Abstract:
The primary purpose of underground storage for natural gas is to balance the variable demand for gas in high consumption seasons against the constant supply by production. There are three basic types of underground storage including depleted reservoirs, aquifers and salt caverns. Construction and use of salt cavern storage has expanded significantly during the past decades because it has a rapid cycling ability with to meet daily even hourly variations in customer needs. Cavern stability is the most important consideration for storage cavern design. Main factors for the structural stability of caverns in salt formation are local geology, rock properties, cavern depth, cavern geometry and cavern locations. In this study, impact of different design parameters on cavern stability and possible deformation characteristics such as displacement, shear and volumetric strain are investigated. For this purpose, numerical modeling using finite element method (FEM) was used. The FEM modeling program Phases© version 7.0 by RocScience was utilized for stability analyses. This program provides useful data on stress distribution, ground displacement and strains as a function of parameters such as in-situ stress ratio, cavern depth and salt properties. A parametric study was performed to evaluate the sensitivity of the program output and results to rock properties, in-situ stresses, and cavern design parameters. The effect of each design factor on the cavern stability with different shapes was studied. The objective of the modeling was to provide optimum cavern design for the given geological conditions. Three specified cavern shapes including cylindrical, tapered cylinder and teardrop was considered in this parametric study. The modeling was performed for three different scenarios. First, single cavern design was studied and behavior of different shaped single caverns was investigated. This effort provided a basis for selection of the most suitable cavern design for underground natural gas storage. The result of modeling showed that the cylindrical design with domed ceiling and floor is the most stable shape and the deformations were below 2% diameter, which is considered to be a reasonable rate for prevention of failure in underground structures. The 2nd step was to study the impact of excavating multiple caverns and the resulting stress, deformation, and stability issues. For multiple caverns, the cavern interaction was the main consideration to evaluate the effects of the spatial configuration of the caverns on deformation of neighboring caverns. The distance of two times the diameter of larger caverns is determined to eliminate mutual interactions between caverns. The third step was to assess the influence of depth of cavern within the salt formation or salt thickness on cavern design. Sufficient salt should be left above the cavern to provide the safe redistribution of stresses. The optimum distance between cavern top and salt formation is investigated and distance of at least one diameter seem to be sufficient to prevent negative interference from the overburden or cap rock.