Method to minimize brine contamination and/or gas migration during in situ trona solution mining

Abstract

A method for in situ solution mining of trona in which an aqueous solvent dissolves trona and forms a brine, which comprises: applying a hydraulic pressure greater than the overburden pressure at an interface between trona roof and overburden to lithologically displace the overburden from the trona roof and form a gap; flowing a liquid settable and / or sealing composition into such interface gap and allowing such composition to solidify inside such gap to form a water-impermeable and optionally gas-impermeable barrier inside. This technique should limit contamination from the overburden; should seal or plug fractures transversing the trona roof; should prevent water infiltration from overburden; and / or should minimize gas migration into the overburden from the cavity. The lithological displacement whereby the interface gap is formed may be carried out at the same time as the composition is flowed inside the gap being formed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]Not applicable.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]Not applicable.TECHNICAL FIELD OF THE INVENTION[0003]The present invention relates to methods for in situ solution mining of an evaporite ore from an underground cavity which uses a solvent to dissolve ore to form a brine. More particularly, a first aspect relates to a method for preventing brine contamination from overburden for the solution mining of trona ore, where contaminants such as chloride, sulfate and water-soluble organics are present in one or more overlying strata in the overburden above the trona bed being mined; and a second aspect relates to a method for minimizing gas migration into the overburden from an underlying cavity which is solution mined in the trona ore.BACKGROUND OF THE INVENTION[0004]Sodium carbonate (Na2CO3), or soda ash, is one of the largest volume alkali commodities made world wide with a total production in 2008 of 48 mill...

AI Technical Summary

Benefits of technology

The patent describes a method to minimize brine contamination from evaporite mineral ores. The method prevents contact between the solvent and the roof of the ore and avoids contamination from inorganic and organic substances. It also prevents seepage of water through the overburden and avoids poisoning of the solvent and brine. This method can help develop a bottom-up solution mining approach for shallow-depth evaporite mineral ores.

Problems solved by technology

The cost of the mechanical mining methods for trona is high, representing as much as 40 percent of the production costs for soda ash.

Furthermore, recovering trona by these methods becomes more difficult as the thickest beds (more readily accessible reserves) of trona deposits with a high quality (less contaminants) were exploited first and are now being depleted.

Thus the production of sodium carbonate using the combination of mechanical mining techniques followed by the monohydrate process is becoming more expensive, as the higher quality trona deposits become depleted and labor and energy costs increase.

Furthermore, development of new reserves is expensive, requiring a capital investment of as much as hundreds of million dollars to sink new mining shafts and to install related mining and safety (ventilation) equipment.

These insoluble contaminants not only cost a great deal of money to mine, remove, and handle, they provide very little value back to the mine and refinery operator.

Owing to the complicated process of deposition of the trona beds, the roof shales tend to contain significant amounts of chloride-laden minerals, as well as other water-soluble contaminants (e.g., sulfates).

It is these upper shales that pose the greatest potential for chloride contamination.

Due to chloride's high solubility, once chloride is in solution in the brine or brine, it is economically not feasible to separate it from the desirable solutes.

For all in-situ trona solution mining processes, avoiding chloride contamination poses a more significant challenge, as the ‘chloride poisoning’ problem is derived from the environment of deposition of the trona beds.

If the roof shales, or waters from brackish aquifers above the trona stratum are allowed to come in contact with the solvent in significant volumes, they are quite likely to ‘poison’ the brine and render it unsuitable for refining.

Implementing a solution mining technique without the negative impact of increased mining hazards and increased costs to exploit sodium (bi)carbonate-containing ores like trona ore, especially those ores whose thin beds and / or deep beds of depth greater than 2,000 ft (610 m) which are currently not economically viable via mechanical mining techniques, has proven to be quite challenging.

The cost of drilling horizontal boreholes and / or of directional drilling can add up.

According to FMC's 1985 article though, the application of hydraulic fracturing for trona solution mining was found to be unreliable.

Fracture communication attempts failed in some cases and in other cases gained communication between pre-drilled wells but not in the desired manner.

These attempts of in situ solution mining of virgin trona in Wyoming were met with less than limited success and technologies using hydraulic fracturing to connect wells in a trona bed failed to mature.

In fracturing between spaced wells in dense underground formations, such as mineral formations and the like for the purpose of removing the mineral deposits and the like, by solution flowing between adjacent wells, the ‘fracking’ methods used in the oil and gas industry are not suitable to accomplish the desired results.

The application of hydraulic pressure (induced hydraulic fracturing) thus will develop both vertical and horizontal fractures.

If solvent flow in these transverse fractures is allowed to occur so as to reach contaminated overlying layers, this would allow contaminants from these overlying layers to contact the solvent, to dissolve into the solvent, and to “poison” the resulting brine rendering it useless or at least very expensive for further processing.

Such poisoning by sodium chloride from these minerals may occur during solution mining of trona, and it is suspected that the solution mining efforts by FMC in the 1980's in the Green River Basin were mothballed in the 1990's due to high NaCl contamination.

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