Salt Characteristics As They Affect Storage of Hydrocarbons
Authors: Aufricht W.R. , Howard K.C. 
Published: December 2007

Results of laboratory tests, field tests and field observation of salt characteristics pertinent to either the storage of hydrocarbons or the disposal of industrial wastes are given. Of particular significance is the indication that under many conditions dry salt may be sufficiently permeable to allow appreciable flow of non-aqueous fluids. This flow appears to occur along crystal boundaries and cleavage planes and through bands of impurities, including dehydrated shales or mudstones. Present data indicate that water, even in very small quantities, materially affects these characteristics. The variation and the presence or absence of permeability appear to be a function ofthe type and amount of impurities (including shale),the crystalline structure and cleavage planes,the confining or overburden pressure andwater content. Additional study of these properties will be required before an accurate evaluation of the feasibility of storage or disposal under various conditions can be made.

The amount and effect of moisture content are particularly important, and additional study of both the amount and effect of in situ moisture is needed.

Introduction

Storage of hydrocarbons, particularly LPG, in solution-formed salt cavities has been practiced successfully for several years. The disposal of industrial wastes, including certain hazardous materials, in both solution-formed and mined salt cavities has been under active study recently. Investigators of the properties of salt relative to its use for these purposes have generally assumed salt to be impermeable. This assumption appears to be based upon the satisfactory performance of present hydrocarbon storage and upon the fact that salt is plastic to some degree and, under pressure, any pores or cracks should be closed. However, results of laboratory and field tests submitted in this report indicate that salt, when dry and at relatively shallow depths, is sufficiently permeable to allow the flow of significant quantities of fluid. This flow appears to occur along crystal boundaries, cleavage planes and through bands of impurities and dehydrated shale. Contacting of the salt with water reduces permeability drastically. The water, combined with external (overburden) pressure, reduces permeability along crystal boundaries and cleavage planes to zero - apparently due to the greatly increased plasticity of the salt when contacted with water. The water should also hydrate the shale, at least temporarily, reducing permeability to essentially zero. Some reduction of flow through the impurities is also indicated in the presence of water due to the re-deposition of the impurities, particularly calcium sulfate. These data indicate that water may be an essential ingredient in the satisfactory use of salt deposits for storage or waste disposal. In view of the significant effects of the water, additional study of the amount of in situ moisture in naturally occurring salt is needed. Most salt in place is reported to be essentially dry. However, small but measurable quantities of free water are present. A portion of this free water is held in the crystals themselves, and certain data submitted in this report indicate that all of the water may not be removed until a temperature approaching the fusion point of the salt is reached.

General Characteristics of Salt

In general, both bedded and dome salt show similar characteristics and a significant lack of homogeneity. The crystalline structure of the salt varies from place to place in any individual deposit. Numerous impurities are present, the most apparent of which are streaks with high concentrations of calcium sulfate. As might be expected, many salt deposits show numerous shale lenses. A number of these gross characteristics have been observed in various salt mines. Those particularly pertinent to storage in salt are illustrated in Figs. 1 through 8 and are discussed in the following paragraphs.

Impurities

Impurities, particularly many of the various metal salts, are present in all naturally occurring salt. Bands of salt highly contaminated with calcium sulfate (anhydrite) are common and are well illustrated in the accompanying Figs. 1 and 2. In several instances, water and oil have been found to flow in the form of seeps through these bands. Excellent illustrations of this are shown in Figs. 3, 4 and 5. Most seeps of this type are quite small and are confined to relatively small areas.