Feasibility Report on Alternative Methods for Cooling Cavern Oils at the U.S. Strategic Petroleum Reserve

Oil caverns at the U.S. Strategic Petroleum Reserve (SPR) are subjected to geothermal heating from the surrounding domal salt. This process raises the temperature of the crude oil from around 75 F upon delivery to SPR to as high as 130 F after decades of storage. While this temperature regime is adequate for long-term storage, it poses challenges for offsite delivery, with warm oil evolving gases that pose handling and safety problems. SPR installed high-capacity oil coolers in the mid-1990's to mitigate the emissions problem by lowering the oil delivery temperature. These heat exchanger units use incoming raw water as the cooling fluid, and operate only during a drawdown event where incoming water displaces the outgoing oil. The design criteria for the heat exchangers are to deliver oil at 100 F or less under all drawdown conditions. Increasing crude oil vapor pressures due in part to methane intrusion in the caverns is threatening to produce sufficient emissions at or near 100 F to cause the cooled oil to violate delivery requirements. This impending problem has initiated discussion and analysis of alternative cooling methods to bring the oil temperature even lower than the original design basis of 100 F. For the study described in this report, two alternative cooling methods were explored: (1) cooling during a limited drawdown, and (2) cooling during a degas operation. Both methods employ the heat exchangers currently in place, and do not require extra equipment. An analysis was run using two heat transfer models, HEATEX, and CaveMan, both developed at Sandia National Laboratories. For cooling during a limited drawdown, the cooling water flowrate through the coolers was varied from 1:1 water:oil to about 3:1, with an increased cooling capacity of about 3-7 F for the test cavern Bryan Mound 108 depending upon seasonal temperature effects. For cooling in conjunction with a degas operation in the winter, cavern oil temperatures for the test cavern Big Hill 102 were cooled sufficiently that the cavern required about 9 years to return to the temperature prior to degas. Upon reviewing these results, the authors recommended to the U.S. Department of Energy that a broader study of the cooling during degas be pursued in order to examine the potential benefits of cooling on all caverns in the current degasification schedule.