In situ method for sealing undesirable transverse fractures under hydraulic pressure during lithological displacement of an evaporite deposit

Abstract

Methods for sealing undesirable transverse fractures enlarged and / or created during lithological displacement of an underground water-soluble evaporite stratum by hydraulic pressure greater than overburden pressure at an evaporite / non-evaporite strata interface, comprising injecting and maintaining a sealing agent into these undesirable fractures to form a solidified matter in situ and ultimately seal them, while forming a main free surface at the interface suitable for initiating solution mining of the evaporite stratum. The solidified matter may be crystallized, precipitated, compacted, agglomerated, cross-linked, coagulated, water-swollen, and / or cemented matter, or may include a wall-building matter with the mineral on fracture faces. The sealing agent may comprise at least one component of the mineral and / or the non-evaporite. The evaporite stratum is preferably a trona stratum overlying an oil shale stratum. The sealing agent may comprise dissolved and / or solid trona, trona tailings particles, and / or water-swelling particles.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]The present application claims priority benefit to U.S. provisional application No. 61 / 718,212 filed on Oct. 25, 2012, this application being herein incorporated by reference in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]Not applicable.TECHNICAL FIELD OF THE INVENTION[0003]The present invention relates to a sealing agent composition, methods for lithological displacement of an underground evaporite mineral deposit using such sealing agent to seal undesirable pre-exiting fractures and / or fractures created during application of hydraulic pressure, and methods for in situ solution mining of the lithologically-displaced evaporite mineral deposit. In particular, the present invention relates to a method for sealing undesirable pre-existing fractures and / or fractures created in a trona bed during application of hydraulic pressure at a trona / shale interface using such sealing agent in order to form a prefer...

AI Technical Summary

Benefits of technology

The patent text describes a method for sealing undesirable fractures in evaporite beds to dissolve the desired solute. The method involves applying hydraulic pressure to create a gap and expose the main mineral free-surface, where the fractures are formed. A sealing agent is then flowed into the fractures and maintained in a solidified state to form a solidified matter inside the fractures and optionally in the gap. The solidified matter can seal or plug the fractures, and the process can be carried out at a shallow depth of 2,500 feet or less. The method can be used to solution mine the evaporite stratum and produce sodium-based products. The patent also describes a manufacturing process for making sodium-based products from the evaporite mineral stratum.

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 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.

Implementing a solution mining technique 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.

This method however proved unsuccessful and currently there are two approaches to trona solution mining that are being pursued.

Even though solution mining of remnant mechanically mined trona is one of the preferred mining methods in terms of both safety and productivity, there are several problems to be addressed, not the least of which is the resource itself.

When current trona target beds will be completely mechanically mined, the operators will eventually be forced to move into thinner beds of lower quality and to endure more rigorous mining conditions while the preferred beds are depleting and finally become exhausted.

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 & 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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