Abstract:
Finite element analyses were performed to estimate axial deformation of cavern wells due to gas storage operations in solution-mined salt caverns. Caverns shrink over time due to salt creep and the cavern roof subsides potentially threatening well integrity. Cavern deformation, deformation of salt surrounding the cavern and impacts on well integrity were quantified using a generic geomechanical finite element model that resembles gas storage operations in solution-mined caverns in the Netherlands. The analysis results show that the largest deformation occurs at the floor and the lowest section of the side wall of a cavern, while the roof deformation is typically one order of magnitude smaller than those of the floor and the wall. The predicted vertical tensile strains that can cause tensile fracturing of well cement develop in the lowest 40-100m-long section of the well. The last cemented casing, typically set at a few tens of meters above the cavern roof, can therefore be affected by the vertical tensile strains that could damage cement sheath and threaten well integrity. The actual borehole construction need to be included in numerical models to investigate in more detail the impact of salt creep on wells in site- and configuration-specific underground gas storage projects.
Introduction
Underground natural gas storage projects have operated successfully since the early 1940's in many parts of the world (Thoms and Gehle, 2000). Nowadays, thousands of salt caverns are being used to store hydrocarbons (Bérest and Brouard, 2003). Underground storage is regarded as the safest way to store large quantities of hydrocarbons because salt formations are almost impermeable and hydrocarbons are protected from fire, intentional damage, aircraft impact, etc. However, a small number of accidents have occurred in the past; most of these incidents (blow out, leakage of hydrocarbons) were due to well failure (Bérest and Brouard, 2003). Generally, the well integrity problems are the main cause of most gas leakage incidents in underground natural gas storage operations in depleted hydrocarbon reservoirs, aquifers and salt caverns. Only 10 of the approximately 600 storage reservoirs operated in the United States, Canada, and Europe have been identified to have experienced leakage due to geological factors and well integrity issues, typically poor cement jobs, casing corrosion, and improperly plugged wells (Perry, 2005).
Well failure was also the cause of the recent leak in the southern California's Aliso Canyon gas storage facility, the second largest storage facility in the U.S., built in a depleted hydrocarbon field (CalIEPA, 2016; CalOES, 2016). A well was leaking methane for nearly four months, from October 2015 to February 2016. One hundred thousand tons of methane was released into atmosphere, which is the largest gas leak in the U.S. history in terms of its environmental impact. Another recent example of a leaking well occurred at the Epe gas storage facility in Germany in 2014, where gas oil is stored in salt caverns (Economic Affairs, 2016). The leakage incident at Epe caused concerns related to the safety of other underground storage facilities not only in Germany but also in the Netherlands, as Epe is located close to the Dutch border and solution mined caverns are leached on either side of the border.
