Multi-well solution mining exploitation of an evaporite mineral stratum

Abstract

A method for in situ solution mining of a mineral from an underground evaporite stratum using a set of wells in fluid communication with at least one mineral cavity with some wells operated in solvent injection mode and other wells operated in brine production mode and optionally with some inactive wells, comprising switching the operation mode of one or more wells. The evaporite mineral preferably comprises trona. The at least one cavity is preferably formed by lithological displacement of the evaporite stratum at a weak interface with an underlying insoluble stratum by application of a lifting hydraulic pressure. The extracted brine can be processed to make valuable products such as soda ash and/or any derivatives thereof, rock salt, or potash. This method can provide more uniform dissolution of mineral in the cavity, can minimize flow channeling, can minimize sodium bicarbonate blinding for solution mining of incongruent trona ore, and/or may avoid uneven deposit of insolubles.

What we claimed is: 
     
         1 . In an underground formation comprising an evaporite mineral stratum comprising trona, nahcolite, wegscheiderite, or combinations thereof, a method for solution mining an evaporite mineral from at least one cavity having a mineral free face, said method comprising:
 a) providing a set of wells in fluid communication with at least one cavity, said set comprising a first subset of wells being operated in injection mode and a second subset of separate wells operated in production mode;   b) injecting a solvent into the at least one cavity through the first subset operated in injection mode for the solvent to contact the mineral free face as the solvent flows through the at least one cavity and to dissolve in situ at least a portion of the mineral from the free face into the solvent to form a brine;   c) extracting at least a portion of said brine to the ground surface through the second subset of wells operated in production mode;   d) switching the operation mode of at least one well from the set after a suitable period of time; and   (e) repeating the steps (a) to (d),   
       wherein the at least one cavity is initially formed by one or more borehole horizontal sections drilled through the mineral stratum or initially formed by a lithological displacement of the mineral stratum, said lithological displacement being performed when said mineral stratum is lying immediately above a water-insoluble stratum of a different composition with a weak parting interface being defined between the two strata and above which is defined an overburden up to the ground, said lithological displacement comprising injecting a fluid at the parting interface to lift the evaporite stratum at a lifting hydraulic pressure greater than the overburden pressure, thereby forming an interface gap which is a nascent mineral cavity at the interface and creating said mineral free-surface. 
     
     
         2 . The method according to  claim 1 , wherein the set of wells comprises a number ‘n’ of wells with n equal to or greater than 4, and wherein a number of wells which is less than ‘n’ are arranged in one or more patterns centered around at least one center well. 
     
     
         3 . The method according to  claim 2 , wherein the pattern is in the shape of at least one polygon with from 3 to up to 16 sides, a honeycomb shape, or at least one ovoid shape, preferably a circle, an oval, or a polygon with 4 to 6 sides. 
     
     
         4 . The method according to  claim 2 , wherein the wells in the set are paired, and wherein cross-over valves are provided and controlled so that the two paired wells serve alternatively as injection and production wells. 
     
     
         5 . The method according to  claim 1 , wherein the set of wells comprises from 4 to 100 wells. 
     
     
         6 . The method according to  claim 1 , wherein, when one of the wells switches operation mode in step (d), the solvent injection and brine production for this well are carried out by a same pump, preferably by a same surface pump. 
     
     
         7 . The method according to  claim 1 , wherein step (d) comprises switching the operation mode of at least one well from the first subset and also switching the operation mode of at least one well from the second subset after the suitable period of time. 
     
     
         8 . The method according to  claim 1 , wherein step (d) comprises switching the operation mode of two or more wells from the first subset from injection to production and also switching the operation mode of two or more wells from the second subset from production to injection after the given period of time. 
     
     
         9 . The method according to  claim 1 , further comprising:
 carrying out step (e): switching at least one well from the first or second subset from an injection or production mode to an inactive mode; or   carrying out step (e′): switching at least one well from the set from an inactive mode to an injection or production mode; or   carrying out step (e) and (e′) simultaneously on at least two different wells from the set.   
     
     
         10 . The method according to  claim 1 , wherein the suitable period of time for switching operation mode in step (d) is from 1 hour to 1 week. 
     
     
         11 . The method according to  claim 1 , wherein the set of wells is in fluid communication with at least two cavities, and wherein at least one of said cavities is formed by lithological displacement of the mineral stratum. 
     
     
         12 . The method according to  claim 1 , further comprising forming the at least one cavity by lithological displacement of the mineral stratum, said cavity formation comprising applying a lifting hydraulic pressure characterized by a fracture gradient between 0.9 psi/ft (20.4 kPa/m) and 1.5 psi/ft (34 kPa/m). 
     
     
         13 . The method according to  claim 11 , wherein the lifting hydraulic pressure is from 0.01% to 50% greater than the overburden pressure at the depth of the interface. 
     
     
         14 . The method according to  claim 11 , wherein the parting interface is horizontal or near-horizontal with a dip of 5 degrees or less. 
     
     
         15 . The method according to  claim 11 , wherein at least one other cavity is initially formed by one or more borehole horizontal sections drilled through the mineral stratum. 
     
     
         16 . The method according to  claim 11 , wherein the fluid injection for lithological displacement for the at least one cavity which is formed by lithological displacement is carried out via an initial vertical well which is drilled from the ground surface past the depth of the interface, said initial vertical well being part of the set of wells, and wherein said initial vertical well is cased and cemented through its entire length, but comprises an in situ injection zone being in fluid communication with the strata interface, said in situ injection zone of said initial vertical well comprising a downhole end opening and/or casing perforations. 
     
     
         17 . The method according to  claim 11 , wherein the fluid injection lithological displacement is carried out in an initial directionally drilled well which is cemented and cased, said initial directionally drilled well being part of the set of wells; wherein said directionally drilled well comprises at least one horizontal borehole section comprising an in situ injection zone being in fluid communication with the strata interface; and wherein the fluid injected through the initial directionally drilled well exits through the in situ injection zone of the horizontal borehole section, thereby lifting the overlying evaporite stratum at the interface so that the gap created at the interface is an extension of the horizontal borehole section. 
     
     
         18 . The method according to  claim 1 , wherein the injected solvent in step (b) comprises an aqueous alkaline solution. 
     
     
         19 . The method according to  claim 1 , wherein the injected solvent in step (b) comprises an unsaturated aqueous solution comprising sodium carbonate, sodium bicarbonate, sodium hydroxide, calcium hydroxide, or combinations thereof. 
     
     
         20 . The method according to  claim 1 , further comprising, before switching in step (d), stopping liquid flow in the at least one well for which the operation mode is switched. 
     
     
         21 . A manufacturing process for making one or more sodium-based products from an evaporite mineral stratum comprises a water-soluble mineral ore selected from the group consisting of trona, nahcolite, wegscheiderite, and combinations thereof, which comprises:
 carrying out the method according to  claim 1  to dissolve said ore from a cavity in said evaporite mineral stratum to obtain a brine comprising sodium carbonate and/or sodium bicarbonate, and   passing at least a portion of said brine through one or more units selected from the group consisting a crystallizer, a reactor, and an electrodialysis unit, to form at least one sodium-based product.