Salt Solution Caverns for Petroleum Industry Wastes
Authors: Davidson Brett C. , Dusseault Maurice B. 
Published: December 2007

A safe, practical and economically feasible alternative to current waste management practices for large volumes of hazardous petroleum industry wastes can be achieved through geological disposal in salt solution caverns. Wastes are placed in as dense a slurry as possible, and undergo a separation process with the solids settling to the bottom of the cavern while the lighter brine and hydrocarbons rise to the top where they can be removed and recycled. Once filled with solids, the cavern is sealed and slow cavern closure begins, compacting the wastes to a dense "plug" entombed in salt of extremely low permeability. The exceptional security offered by salt solution cavern placement suggests that it is a viable approach for highly toxic wastes, given that it is possible to design and place an engineered slurry which will compact to a waste pod of low porosity and permeability.

The article is conceptual in nature, but the conclusions and observations are based firmly on substantial laboratory, field, and modeling experience in salt rocks applied to caverns and underground mines.

Introduction

The petroleum industry requires economic, secure waste management methods to permanently dispose of various types of wastes generated during exploration, production, workovers, refining, product manufacture, and contaminated site remediation. Permanent disposal realistically means final geological disposal where wastes are placed on or in the ground. Examples of geological disposal include:Deep placement in old or newly constructed mines;Release of liquids into rivers, lakes and oceans (dilution);Injection of fluids into deep permeable strata;Landfills, land farming (ploughing into fields), or other surface disposal approaches; and,Hydraulic fracturing of a solid-liquid slurry into deep strata (slurry fracture injection);Solution cavern disposal.

Refs. 1 through 4 address general aspects of the first five techniques. With the exception of slurry fracture injection, all of the above techniques are either too costly to use systematically, cannot guarantee long-term waste isolation, or do not carry the acceptance of society. Slurry fracture injection is a technology which has been commercialized for large-volume, low-toxicity wastes, and has proven economical and environmentally secure; it will not be discussed here other than to note that it has recently gained substantial acceptance as a preferred approach to disposal of produced sand and dirty liquids in Alberta and Saskatchewan. Table 1 provides a comparison of methods and relative costs of various waste disposal practices.

A short list of desirable characteristics for permanent geologic disposal would include:Geological conditions providing extremely low probability of negative interaction with the biosphere for long times;Well regulated and safe procedures for transporting, handling and disposing of waste streams;A straightforward, economic, and flexible technology to handle and permanently place the waste materials; and,A permanent site where current and future land use is not impaired and where no permanent, long-term post-maintenance is required.

Oil field waste streams suitable for cavern disposal include:Produced sands and solids (heavy oil operations);Contaminated soils from produced oil and water spills;Tank bottoms from treaters and other oil field facilities;Ecology pit solids and sludges with heavy metals;NORMs from pipe scale and other sources;Refinery catalysts and noxious solids streams; and,Site remediation solids (e.g. refinery site clean-up).

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