Traveling undercut solution mining systems and methods

Abstract

In-situ solution mining method of an ore bed, particularly containing trona, which comprises exposing to a solvent an ore region inside a borehole drilled in the ore, and dissolving a desired solute within the exposed region to provide a liquor and create a voided ‘undercut’, such undercutting making the ore susceptible to gravitational loading and crushing. Unexposed ore falls into the undercut by gravity without breaking the ore roof resulting in exposure of fresh ore to the solvent and in preventing solvent exposure to contaminating material near the roof. The desired solute is eventually dissolved away in the entire bed from its floor up to its roof. Solvent injection may be delivered through a conduit positioned inside the borehole, and may be moved by retracting or perforating the conduit. The method may employ an advancing undercut initiated up-dip and traveling down-dip, or a retreating undercut initiated down-dip and traveling up-dip.

1 . A method for in situ undercut solution mining of a subterranean ore bed comprising trona, the ore bed containing a solute selected from the group consisting of sodium sesquicarbonate, sodium carbonate, and sodium bicarbonate, the ore bed comprising a floor and a roof, the method comprising the following steps:
 f) passing a solvent through a conduit positioned into an unlined borehole portion, the unlined borehole portion having a downhole end positioned within the ore bed, the conduit having a downhole injection zone positioned in the unlined borehole portion at a predetermined distance from the borehole downhole end;   g) injecting the solvent via the downhole injection zone in order to expose, to the solvent, an ore region adjacent to the downhole injection zone;   h) dissolving the solute from the solvent-exposed ore region in a manner effective to form a liquor comprising dissolved solute, the dissolving being effective in forming an undercut above the ore bed floor;   i) repeating steps (a)-(c) to enlarge the undercut by more dissolution of solute from the solvent-exposed ore and to trigger the fracture of unexposed ore located above the undercut and the downward movement by gravity of fractured ore rubble into the undercut; and   j) flowing the liquor down-dip by gravity towards a subterranean collection zone in order to pass the liquor to a terranean location.   
     
     
         2 . The method according to  claim 1 , wherein the downward movement of fractured ore rubble by gravity into the undercut allows the ore roof to sag but not to break. 
     
     
         3 . The method according to  claim 2 , wherein the ore roof sagging but not breaking prevents exposure of solvent to chloride-containing material located at or above the ore roof to minimize contamination of the liquor by dissolved chloride. 
     
     
         4 . The method according to  claim 1 , wherein the unlined borehole portion is horizontal. 
     
     
         5 . The method according to  claim 1 , wherein the unlined borehole portion is slanted with one of its ends being at a higher elevation than the other end, and the unlined borehole portion is not vertical. 
     
     
         6 . The method according to  claim 1 , wherein the ore bed has a dip gradient and wherein the undercut is initially formed up-dip at the floor of the ore bed, and continues to be formed in the down-dip direction. 
     
     
         7 . The method according to  claim 6 , wherein a borehole comprising said unlined borehole portion is drilled vertically to penetrate the trona ore bed within an up-dip region of the trona bed, and the borehole is further extended with the unlined portion directionally drilled horizontal within the trona bed above the bed floor. 
     
     
         8 . The method according to  claim 6 , wherein a return drillhole is drilled vertically to penetrate the trona bed within a down-dip region of the trona bed, and wherein the drillhole is further extended vertically with an unlined portion past the trona bed floor to form a sump as the downhole collection area in which a downhole pump is installed. 
     
     
         9 . The method according to  claim 8 , wherein another portion of the drillhole is directionally drilled through the trona bed to meet the downhole end of the borehole to hydraulically connect the borehole end with the sump as the downhole collection area where the return downhole pump is located, and wherein the liquor comprising dissolved solute flows towards the borehole end, passes through the drillhole portion and collects into the sump. 
     
     
         10 . The method according to  claim 1 , wherein the downhole injection zone is a downhole conduit extremity and/or a series of perforations on the conduit body. 
     
     
         11 . The method according to  claim 1 , wherein the downhole injection zone of the conduit comprises along the portion of its length that is surrounded by the unlined borehole portion, apertures for spraying the solvent, and wherein the solvent exiting the apertures is sprayed onto the ore within the unlined borehole portion to dissolve away solute from the solvent-exposed ore region around the apertures. 
     
     
         12 . The method according to  claim 11 , wherein the ore bed has a dip gradient, and wherein the apertures spray the solvent in a lateral and down-dip direction. 
     
     
         13 . The method according to  claim 1 , wherein the ore bed has a dip gradient, and wherein the undercut is initially formed down-dip at the floor of the ore bed, and continues to be formed in the up-dip direction. 
     
     
         14 . The method according to  claim 1  wherein the conduit is concentrically positioned inside at least a part of the unlined borehole portion. 
     
     
         15 . The method according to  claim 14 , further comprising
 (g) moving said injection zone of the conduit to another location within said unlined borehole portion.   
     
     
         16 . The method according to  claim 15 , wherein said injection zone of the conduit is a downhole conduit extremity, and wherein the step (g) of moving said injection zone of the conduit is carried out to expose fresh ore to the solvent by at least one of the following steps:
 g1) retracting the conduit within the unlined borehole portion thereby increasing the distance between the downhole conduit extremity and the downhole end of the unlined borehole portion;   g2) perforating the conduit body along a pre-selected length moving upstream from said conduit extremity.   
     
     
         17 . The method according to  claim 1 , wherein the conduit injection zone is designed to laterally inject the solvent in order to avoid injection of solvent in a vertical direction. 
     
     
         18 . The method according to  claim 1 , wherein the solvent injection is carried out in a manner effective to initially favor the lateral widening of the undercut and thereafter favor the upward widening of the undercut.