Analysis of the Massive Salt Fall in Big Hill Cavern 103

Abstract

This report summarizes recent reviews, observations, and analyses believed to be imperative to our understanding of the recent two million cubic feet salt fall event in Big Hill Cavern 103, one of the caverns of the Strategic Petroleum Reserve (SPR). The fall was the result of one or more stress driven mechanical instabilities, the origins of which are discussed in the report.The work has lead to important conclusions concerning the engineering and operations of the caverns at Big Hill. Specifically, Big Hill, being the youngest SPR site, was subjected to state-of-the-art solutioning methods to develop nominally well-formed, right-circular cylindrical caverns. Examination of the pressure history records indicate that operationally all Big Hill SPR caverns have been treated similarly.Significantly, new three-dimensional (3-D) imaging methods, applied to old (original) and more recent sonar survey data, have provided much more detailed views of cavern walls, roofs, and floors. This has made possible documentation of the presence of localized deviations from "smooth" cylindrical cavern walls. These deviations are now recognized as isolated, linear and/or planar features in the original sonar data (circa early 1990s), which persist to the present time. These elements represent either sites of preferential leaching, localized spalling, or a combination of the two.Understanding the precise origin of these phenomena remains a challenge, especially considering, in a historical sense, the domal salt at Big Hill was believed to be well-4 characterized. However, significant inhomogeneities in the domal salt that may imply abnormalities in leaching were not noted. Indeed, any inhomogeneities were judged inconsequential to the solution-engineering methods at the time, and, by the same token, to the approaches to modeling the rock mass geomechanical response. The rock mass was treated as isotropic and homogeneous, which in retrospect, appears to have been an over simplification. This analysis shows there are possible new opportunities regarding completing an appropriate site characterization for existing operating cavern fields in the SPR, as well as expansion of current sites or development of new sites. Such characterization should first be consistent with needs identified by this report. Secondly, the characterization needs to satisfy the input requirements of the 3-D solutioning calculational methods being developed, together with 3-D geomechanical analyses techniques which address deformation of a salt rock mass that contains inhomogeneities. It seems apparent that focusing on these important areas could preclude occurrence of unexpected events that would adversely impact the operations of SPR. 5 ACKNOWLEDGEMENTS

Features of Bayou Choctaw SPR caverns and internal structure of the salt dome.

“…As with the earlier reports on the internal cavern geometry and structure of Big Hill [1] and West Hackberry [2] caverns that help to reveal the stratigraphy of the salt domes containing these caverns, this report brings together the same kinds of information for the Bayou Choctaw caverns and dome. As is well known, solution mined caverns in the salt domes of the Gulf Coast have been used over the decades for sources of industrial brine and commercial storage of crude oil, refined products, and industrial chemicals.…”

“…Most studies involve the determination of the top of salt and the extent of the dome flanks, i.e., the dome surfaces. As stated in the earlier report [1]: "The technical data comes from drilling into the dome and along the flanks or from geophysical surveys. Even here, because the domes are relatively large bodies, some several miles in diameter, the quantity of data per square foot of surface is frankly minimal.…”

Geomechanical testing of MRIG-9 core for the potential SPR siting at the Richton salt dome.

“…In addition, salt is also known to dilate (volume expansion) through a process of stress-induced microfracturing that creates new porosity. Microfracturing may also cause localized spalling of salt slabs from the cavern roof and walls (Munson et al, 2003) which could lead to damage of the hanging strings that provide access to the stored oil and ultimately to disruption of operations.…”

Laboratory evaluation of damage criteria and permeability of Big Hill salt.

“…Our concern, based on the recent massive salt fall (Munson et al, 2003) and gas regain rates in certain Big Hill caverns, is that we have underestimated the strength values used in past analyses. A previous report examines several creep tests on Big Hill salt and concludes that the salt is weak in comparison to other salts.…”

“…Generally, we have treated salt within a dome as an isotropic homogeneous medium in terms of its mechanical and dissolution behavior. Recent work (Munson et al, 2003) specific to the Big Hill site implies that this assumption may not have been correct.…”