The U.S. Strategic Petroleum Reserve (SPR) is facing significant challenges related to the storage and availability of its crude oil resources. Approved for construction by the 1975 Energy Policy and Conservation Act (EPCA), the storage sites were envisioned to be needed for 25 years and are subject to an estimated five drawdown cycles (Shages, 2014). In retrospect, the design has not matched actual use, and this has led to degradation of the SPR and impacted its ability to perform its function.
The SPR stores crude oil (either sweet or sour) in 62 underground salt caverns located at four different sites in Texas and Louisiana. The official storage capacity is 727 million barrels, based on sonic measurements. A 2010 study concluded there was a significant mismatch in design and use of the storage caverns. Instead of the initial estimated five large drawdown cycles, a large number of small drawdowns occurred over the previous 20 years. From 1996 through 2014, there were 14 instances of oil removals less than 10 million barrels. These multiple drawdowns have caused cavern deformation, salt falls and other damage to the cavern integrity. In addition, these underground salt caverns are shrinking due to tectonic stresses. The cavern shrinkage (aka closure) is estimated to be approximately two million barrels per year – but may be significantly higher.
Salt is a unique geologic material with complex mechanical properties. It is often modeled as a non-Newtonian fluid. At high temperatures and pressures salt behaves like a plastic. It will behave more like a liquid in the sense that it flows even under small deviatoric stresses. Salt domes make a perfect storage medium in that they do not react with the oil and are self-healing. The plastic behavior of salt will cause it to naturally close fractures or gaps and prevent any leakage. However, salt domes also are under constant geologic pressure (i.e. salt creep) and these stresses are causing the caverns to shrink (Shages, 2014). The SPR attempts to manage the shrinkage by leaving a bed of salt brine at the bottom of each cavern and by keeping their caverns under a pressure of approximately 800 psi (personal communication, SPR personnel). Salt leaching offsets some of the shrinkage. Crude oil is extracted from the cavern by injecting fresh water or under-saturated brine down one well and produced in a secondary well. The injection causes, for example, 15 barrels (2.38 m3) of salt to be dissolved for every 100 barrels of oil removed from a cavern.
The mismatch between original design and actual utilization of the SPR’s caverns has led to the development of significant negative impacts to cavern integrity, wellbore integrity and the ability to maintain optimal mission readiness. One operational limitation is that whenever work is done to the cavern or well, the caverns must be depressurized. Removing cavern pressure causes the rate of shrinkage to increase rapidly.
Thus, the repeated removal of small volumes of oil over the life of a cavern has led to serious consequences on the shape and integrity of the caverns:
- Dissolution of salt during a drawdown always begins at bottom of cavern.
- If cavern is only partially emptied,the shape of cavern will become distorted with a bulge at the bottom.
- Gravity puts stress on the overhanging salt formation.
- Massive falls can occur and damage hanging steel tubulars.
- Cavern deformation and shrinkage is damaging well casings and cement.
For more information regarding the Strategic Petroleum Reserve:
Park, Byoung Yoon. “Interbed Modeling to Predict Wellbore Damage for Big Hill Strategic Petroleum Reserve” Roch Mech Rock Eng 47 (2014): 1551–61.
Shages, John. “The Strategic Petroleum Reserve: Policy Challenges in Managing the Nation’s Strategic Oil Stock” July 11, 2014. http://eprinc.org/wp-content/uploads/2014/07/EPRINC-Shages-SPR-July-11-2014.pdf
Stein, J.S. and Rautman, C.A., Conversion of the Big Hill Geological Site Characterization Report to a Three-Dimensional Model. SAND2003-3554. http://prod.sandia.gov/techlib/access-control.cgi/2003/033554.pdf