Hydrodynamic Mechanism of Bottom Water Invasion Into Oil Storage Within Strategic Petroleum Reserve – Weeks Island SPR Case
Authors: Wojtanowicz Andrew K. , Bourgoyne Adam T. 
Published: September 2023

Strategic Petroleum Reserves (SPR) are federally-owned oil stocks stored in 63 underground salt caverns in Louisiana and Texas — established after the 1973–74 oil embargo and holding estimated 90-day oil supply (700 million barrels) for the country in case of a disruption of oil supply. The Weeks Island SPR — a salt mine converted to oil storage — displayed detectable increase of brine water in the oil storage that could indicate either external leakage due loss of the mine’s integrity or an internal invasion of the brine present in the bottom part of the salt mine — subject of the project reported herein.

It was hypothesized that internal brine invasion into the oil storage would result from an upward movement of the brine during routine hydraulic testing of SPR that involved a closed-loop oil circulation: producing oil to surface from the oil sump located in the upper level of the mine and injecting it back through a completed fill-hole discharge pipe into the water room located in the bottom level of the mine. The resultant oil-water mixture would flow upwards via an inclined drift to the upper (oil storage) mine’s level where water would break out of the mixture and find its way to the oil sump.

In the project, the hypothesis is verified theoretically — with a mathematical model, and experimentally — using a physical analog of the brine movement process. The model combines three mechanisms of water movement: entrainment, separation and transport. The entrainment of water in the oil stream injected by the fill-hole discharge pipe into the stagnant water body in water room follows the theory of turbulent jets. The oil/water separation is quantified with empirical model using data from the bench-top mixing experiments based on hydrodynamic analogy between jet mixing and rod stirring. The brine transport mechanism draws on the theory of stratified flow and the concept of gravity separators in-series. The theoretical model is used for computer simulations of the SPR internal water movement process, and for designing a physical analog of the process.

The computer program calculates composition and stability of the oil/brine mixture, and the rate of the brine invasion to the oil storage by considering oil flowrate, level of brine in the water room, and geometry of the fill-hole discharge pipe completion. The simulation results show that a simple rearrangement of the fill-hole discharge geometry could give up to seven-fold reduction of the brine transport rates resulting from lower mixing energy at the fill-hole discharge.

The physical analog is a scaled-down model of the SPR testing system comprising the bottom level’s fill-hole/water room unit, the inclined drift, and the upper oil level of the salt mine. The actual system has been scaled down by collapsing prototype geometry with geometrical similitude scale of 1:10, preserving the same value of the oil jetting Froude’s number, and using the same oil and similar brine with the same density ratio. The testing involved monitoring and videotaping the submerged jet plume below the exit of the fill-hole completion into the water room and the interface between the lower stream of brine (flowing backwards) and the upper stream of water-in-oil mixture (flowing upwards) along the inclined drift. Considerable fraction of water (up to 40%) in the mixture discharged from the drift’s top verify the hypothesis that entrainment of water into the oil stream is an effective mechanism of water displacement from the water room to the upper level of the mine during the hydraulic testing of SPR. Moreover, if oil injection is conducted for an extended period of time, all bottom water could be displaced to the oil storage level.