Drill fluid and method for tunneling

A method for drilling a tunnel through a formation, the method comprising:

• providing a mixed metal-viscosified drilling fluid including a mixed metal viscosifier which is a mixed metal layered hydroxide corn pound of the following empirical formula:
• M′mM″n(OH)(2m+3n+qa+br)(Aq)a(Br)b. xH2O,
• where M′ represents at least one divalent metal cation and m is an amount of from greater than zero to 8;
• where M″ represents at least one trivalent metal cation and n is an amount of from greater than zero to 6;
• where A is an anion or negative-valence radical, and a is an amount of A ions of valence q, provided that if A is monovalent, a is from greater than zero to 8, and if A is polyvalent, a is from greater than zero to 4;
• where B is a second anion or negative-valence radical, and where b is an amount of B ions of valence r and is from zero to 4;
• provided (m+n) is greater than or equal to 1;
• further provided qa+br cannot be greater than 2m+3n;
• provided that qa cannot equal 2m+3n;
• and still further provided that (2m+3n+qa+br) is less than 3;
• and where xH2O represents excess waters of hydration, with x being zero or more;
• circulating the drilling fluid through the tunnel while drilling into the formation;
• monitoring the drilling fluid for a condition of drilling indicative of a problematic increase in viscosity of the drilling fluid;
• adding to the drilling fluid at least 1% w/v potassium salt;
• and adding a non-toxic anionic thinner to the drilling fluid to adjust the viscosity of the drilling fluid.

A method for drilling a tunnel through a formation, the method comprising:

• providing a mixed metal-viscosified drilling fluid including a mixed metal viscosifier which is a mixed metal layered hydroxide corn pound of the following empirical formula:
• M′mM″n(OH)(2m+3n+qa+br)(Aq)a(Br)b.xH2O,
• where M′ represents at least one divalent metal cation and m is an amount of from greater than zero to 8;
• where M″ represents at least one trivalent metal cation and n is an amount of from greater than zero to 6;
• where A is an anion or negative-valence radical, and a is an amount of A ions of valence q, provided that if A is monovalent, a is from greater than zero to 8, and if A is polyvalent, a is from greater than zero to 4;
• where B is a second anion or negative-valence radical, and where b is an amount of B ions of valence r and is from zero to 4;
• provided (m+n) is greater than or equal to 1;
• further provided qa+br cannot be greater than 2m+3n;
• provided that qa cannot equal 2m+3n;
• and still further provided that (2m+3n+qa+br) is less than 3;
• and where xH2O represents excess waters of hydration, with x being greater than zero;
• circulating the drilling fluid through the tunnel while drilling into the formation;
• monitoring the drilling fluid for a condition of drilling indicative of a problematic increase in viscosity of the drilling fluid;
• adding calcium sulfate to the drilling fluid to bring the concentration to at least 0.05% w/v calcium sulfate in the drilling fluid;
• and
• adding a non-toxic anionic thinner to the drilling fluid to adjust the viscosity of the drilling fluid.

A mixed metal-viscosified drilling fluid corn prising:

• water;
• 15 to 45 kg/m3 of bentonite;
• a mixed metal viscosifier at a weight ratio of 1:3 to 1:30 relative to the weight of the bentonite, the mixed metal viscosifier being a mixed metal layered hydroxide corn pound of the following empirical formula:
• M′mM″n(OH)(2m+3n+qa+br)(Aq)a(Br)b. xH2O,
• where M′ represents at least one divalent metal cation and m is an amount of from greater than zero to 8;
• where M″ represents at least one trivalent metal cation and n is an amount of from greater than zero to 6;
• where A is an anion or negative-valence radical, and a is an amount of A ions of valence q, provided that if A is monovalent, a is from greater than zero to 8, and if A is polyvalent, a is from greater than zero to 4;
• where B is a second anion or negative-valence radical, and where b is an amount of B ions of valence r and is from zero to 4;
• provided (m+n) is greater than or equal to 1;
• further provided qa+br cannot be greater than 2m+3n;
• provided that qa cannot equal 2m+3n;
• and still further provided that (2m+3n+qa+br) is less than 3;
• and where xH2O represents excess waters of hydration, with x being zero or more;
• at least one of:
• at least 0.05% w/v calcium sulfate or at least 1% w/v potassium salt;
• a polyacrylate anionic thinner;
• and
• a base to maintain the pH above 7.0.