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.

1 . In an underground formation containing a trona stratum comprising sodium sesquicarbonate lying above one or more substantially-insoluble strata containing water-soluble contaminants selected from the group consisting of chloride, sulfate, and water-soluble organics, said trona stratum comprising an ore roof with a parting upper interface above which is defined an overburden up to the ground and below which an aqueous solvent is to injected in a cavity to dissolve trona and to form a brine which is recovered at least in part at the ground surface,
 a method for minimizing brine contamination from overburden during in situ trona solution mining of a cavity formed in the trona stratum, comprising:   (a) applying a hydraulic pressure which is greater than the overburden pressure at the upper interface to lithologically displace the overburden from the trona ore roof, thereby forming an interface gap;   (b) flowing a composition selected from the group consisting of a liquid settable composition, a sealing agent, and combinations thereof, into the upper interface gap; and   (c) allowing such composition to stay for a time sufficient to form a barrier inside said upper interface gap, said barrier being water-impermeable and optionally gas-impermeable.   
     
     
         2 . The method according to any of the preceding claims, wherein the steps (a) and (b) are performed at the same time by injecting said composition to apply said hydraulic pressure at the upper interface and for its flowing into said upper interface gap. 
     
     
         3 . The method according to  claim 1 , wherein step (b) is carried out by injection of said composition via a vertical well which is drilled from the ground surface through the trona stratum and past the floor of the trona bed, and wherein the vertical well is cased and cemented through its entire length, but comprises an in situ injection zone being in fluid communication with the upper interface, said in situ injection zone of said vertical well comprising a downhole end opening and/or casing perforations. 
     
     
         4 . The method according to  claim 1 , wherein the hydraulic pressure is applied in step (a) by using a fracture gradient between 0.95 psi/ft and 1.5 psi/ft, preferably between 1 psi/ft and 1.3 psi/ft, more preferably between 1.05 psi/ft and 1.15 psi/ft; and wherein the hydraulic pressure in step (b) is maintained to the hydraulic pressure used in step (a) when steps (a) and (b) are not carried out simultaneously. 
     
     
         5 . The method according to  claim 1 , wherein said composition comprises water-insoluble particles, optionally water-soluble particles, a binder, water, optionally one or more additives for controlling viscosity and/or setting time, density, or a catalyzing agent; wherein said water-insoluble particles comprise a calcium compound; fused or colloidal silica; silica flour; tailings recovered from a trona surface refinery; biological solid matter; agricultural solid matter; sand; cementing compositions; bentonite; fly ash; slag; aggregate; plastic; rubber; at least one cementing material used in well completion or construction activities; one or more polymer resins; or combinations thereof. 
     
     
         6 . The method according to  claim 5 , wherein said composition is a liquid settable cementing composition. 
     
     
         7 . The method according to  claim 1 , wherein said upper interface of said trona stratum is at a shallow depth of 2,500 feet or less. 
     
     
         8 . The method according to  claim 1 , wherein said defined upper interface is horizontal or near-horizontal. 
     
     
         9 . The method according to  claim 1 , wherein step (c) is carried out for a time sufficient to permit said barrier to achieve a compressive strength of at least 2500 psi. 
     
     
         10 . The method according to  claim 1 , wherein step (c) is carried out for a setting time period of at least 24 hours to permit said barrier to set completely. 
     
     
         11 . The method according to  claim 1 , wherein said barrier formed in step (c) is also gas-impermeable. 
     
     
         12 . The method according to  claim 11 , further comprising step (i): injecting a blanket gas into the cavity, wherein said formed gas-impermeable barrier prevents said injected blanket gas to migrate out of the cavity into the overburden. 
     
     
         13 . The method according to  claim 11 , further comprising step (j): releasing methane during trona solution mining into the cavity and extracting at least some of the methane from the cavity to the ground surface, and wherein said formed gas-impermeable barrier prevents the released methane to migrate out of the cavity into the overburden. 
     
     
         14 . The method according to  claim 1 , further comprising:
 (d) releasing the hydraulic pressure after said barrier is formed in said upper interface gap in order to form a tight seal between said barrier and said immediately-above stratum and between said barrier and said trona ore immediately located underneath said barrier.   
     
     
         15 . The method according to  claim 1 , further comprising:
 e) forming the cavity in the trona stratum at or above a floor interface between said trona stratum and an underlying substantially water-insoluble stratum, after steps (a)-(d) are performed;   said cavity formation step (e) comprising:
 lithological displacement of said trona stratum at said floor interface by injecting a lifting fluid at said floor interface through the same well through which said composition is injected in step (b) or through a different well; or 
 forming at least one uncased horizontal borehole from a directionally drilled well, said at least one uncased horizontal borehole being in fluid communication with said well through which said composition is injected in step (b). 
   
     
     
         16 . The method according to  claim 1 , wherein said trona stratum is immediately above a substantially water-insoluble stratum and comprising a defined parting floor interface between the two strata, and wherein said method further comprises:
 (a′) applying a hydraulic pressure which is greater than the overburden pressure at said floor interface to lithologically displace said trona stratum, thereby forming a lower interface gap between said strata and exposing a main trona free-surface, wherein said application of hydraulic pressure further induces formation of new undesirable transverse fractures and/or intersects natural undesirable transverse fractures in said trona stratum, thereby exposing minor trona free-surfaces in said undesirable fractures;   (b′) flowing a sealing agent into said lower interface gap and into said transverse fractures; and   (c′) maintaining such sealing agent in said lower interface gap and said transverse fractures to form a solidified matter inside said transverse fractures and optionally in said lower interface gap.   
     
     
         17 . The method according to  claim 16 , wherein step (b) and (b′) are carried out from the same well. 
     
     
         18 . The method according to  claim 16 , wherein said sealing agent comprises water-insoluble particles, said water-insoluble particles comprising at least one water-insoluble calcium compound, fused or colloidal silica, bentonite, water-insoluble matter recovered from a mechanically-mined trona after its dissolution in water or aqueous medium, tailings recovered from a mineral surface refinery, biological solid matter, agricultural solid matter, sand, cement, or combinations thereof. 
     
     
         19 . The method according to  claim 16 , wherein step (b) includes,
 before injecting said composition into the cavity via a well, forming a drillable plug whose top edge does not block the flow of said composition to the upper interface and whose top edge is located inside the well near and below the upper interface to prevent said composition to flow down in the well; and wherein after the barrier is set in said upper interface gap, the method further comprises: removing said plug by drilling said plug out.   
     
     
         20 . The method according to  claim 1 , further comprising:
 (f) injecting an aqueous solvent into the cavity formed in said trona stratum and which is located underneath said barrier to dissolve some of the trona ore and to form a brine comprising sodium carbonate and/or bicarbonate in the cavity; and   (g) extracting at least a portion of the resulting brine to the ground surface; wherein said barrier formed at said trona upper interface minimizes contact and dissolution of at least one water-soluble contaminant from an overlying stratum with said aqueous solvent and resulting brine, and/or reduces leakage of contaminant-laden water percolating from overburden into the cavity which is being mined.   
     
     
         21 . The method according to  claim 20 , wherein step (b) and (f) are carried out from the same well, and step (g) is carried out from one or more different wells. 
     
     
         22 . The method according to  claim 20 , wherein step (b) and (g) are carried out from the same well, and step (f) is carried out from one or more different wells. 
     
     
         23 . The method according to  claim 1 , wherein step (b) is carried out by injection via a well with a casing, and wherein prior to applying the hydraulic pressure, the method further comprises:
 placing a drillable plug in said casing slightly below said upper interface; and   perforating or cutting said casing at said upper interface to allow fluid communication between said upper interface gap and the inside of said casing.   
     
     
         24 . The method according to  claim 23 , wherein after performing said maintaining step, the method further comprises:
 drilling the drillable plug positioned slightly below the upper interface and optionally any possible excess matter which is solidified from said composition in a casing region immediately above said plug.   
     
     
         25 . The method according to  claim 24 , wherein, after drilling said drillable plug and optionally any excess solidified matter, the method further comprises:
 perforating or cutting said casing to provide casing opening(s) at a lower interface between said trona stratum and an underlying stratum to allow fluid communication between said lower interface and the inside of said casing; and   after this cutting step, a lifting fluid is applied to said lower interface to lift said trona stratum from said underlying stratum, said lifting fluid comprising a solvent being suitable for dissolving trona and to form a brine.   
     
     
         26 . The method according to  claim 1 , wherein the recovered brine has a chloride content being equal to or less than 0.5 wt %. 
     
     
         27 . In an underground formation containing a plurality of trona ore strata comprising sodium sesquicarbonate and being of various heights and located at different depths, said trona strata being separated by a series of substantially-insoluble interburdens containing water-soluble contaminants selected from the group consisting of chloride, sulfate, and water-soluble organics, each trona stratum comprising an ore roof with an upper interface with the immediately-overlying interburden,
 a method for minimizing brine contamination from interburden during in situ trona solution mining of two or more trona ore strata from said plurality, comprising:   selecting two or more trona ore strata to be mined from the top down of said plurality based on selection criteria comprising a minimum of 60 wt % sodium sesquicarbonate in said trona ore and a minimum stratum height of at least one meter;   carrying out sequentially in the selected trona ore strata to be mined from the top down, the following steps (a) to (h) as follows:   steps (a) to (c) of the method according to  claim 1  on a selected trona ore stratum using a well drilled through said selected trona ore stratum;   step (d): releasing the hydraulic pressure after the barrier is set in the upper interface gap of the selected trona ore stratum;   step (e): forming a cavity at or near and above the floor interface with the underlying interburden comprising a technique selected from the group consisting of lithological displacement of the trona ore stratum at the floor interface; and forming at least one uncased horizontal borehole;   step (f): injecting a solvent in the formed cavity to dissolved trona and thereby enlarging the cavity and to form a brine;   step (g): extracting at least a portion of the resulting brine to the ground surface via one or more of production wells;   step (h): stopping steps (f) and (g) when the brine extracted from the cavity has a level of said contaminant exceeding a threshold content above which is not acceptable for make a salable product or when the cavity is enlarged by dissolution to reach the roof of said cavity; and
 repeating steps (a) through (h) on a selected trona stratum meeting said criteria and being the next one to-be-mined from the top down of said plurality located underneath the previously-mined trona stratum, preferably using the same vertical well used in steps (a) to (c) with the previously-mined trona stratum, optionally drilling said vertical well further down if necessary past the floor of the to-be-mined trona stratum, and using one or more of the same production wells used in step (g) during mining of the previously-mined trona stratum, optionally drilling said one or more production wells further down if necessary past the floor of the to-be-mined trona stratum. 
   
     
     
         28 . In an underground formation containing a trona stratum comprising sodium sesquicarbonate lying above one or more substantially-insoluble strata containing water-soluble contaminants selected from the group consisting of chloride, sulfate, and water-soluble organics, said trona stratum comprising an ore roof with a parting upper interface above which is defined an overburden up to the ground and below which an aqueous solvent is to be injected in a cavity to dissolve trona and to form a brine which is recovered at least in part at the ground surface,
 a method for minimizing brine contamination from overburden during in situ trona solution mining of a cavity formed in said trona stratum, this method comprises the following steps: 
 (A) applying a hydraulic pressure greater than the overburden pressure at said upper interface to lithologically displace (lift) said overlying overburden from saidunderlying trona stratum, thereby forming an interface gap at said upper interface, wherein said application of hydraulic pressure further induces formation of new undesirable transverse fractures and/or intersects pre-existing undesirable transverse fractures in saidtrona stratum; 
 (B) flowing a sealing agent into said upper interface gap and into the transverse fractures; and 
 (C) maintaining the sealing agent in said upper interface gap and in said undesirable transverse fractures to form a solidified matter inside said fractures and said upper interface gap.