# Patent

## Title
Treatment of subterranean formations

## Abstract

A method of treating a subterranean formation by contacting the formation with the following: (a) ammonium sulfamate; (b) an oxidizing agent selected from a perchlorate or a nitrite or combinations thereof; and (c) an acid.

## Background
TECHNICAL FIELD
This invention relates to treatment of subterranean formations, for example to fracture formations and/or stimulate hydrocarbon, for example oil and/or gas, production.
BACKGROUND
Oil and gas reserves trapped within low permeability reservoirs such as shale and tight-gas formations are difficult and expensive to recover using traditional methods. To maximise the production from such formations, an extensive and complex fracture network must be created. One of the most commonly employed methods is hydraulic fracturing. Whilst hydraulic fracturing does create fractures, the extent and complexity of the fracture patterns may be insufficient to maximise oil recovery and furthermore using high volumes of fracturing fluids is costly and can damage formations. Thus, there is a need for additional methods to further increase the productivity of an oil producing well, for example a hydraulically fractured well. A suitable method of a secondary fracturing operation generates new (micro)fractures, extends existing (micro)fractures or opens up naturally occurring fractures. This secondary fracturing method increases the complexity of the fracture network and stimulated reservoir volume (the total volume of reservoir rock that has been fractured).
A wide range of fracturing methods and formulations has been proposed. However, known methods may be costly and/or use corrosive chemicals. Additionally, some known methods involve introduction and/or production of carbon dioxide within a formation. Production of large quantities of carbon dioxide in a formation may be undesirable because it may absorb into oil and/or disadvantageously produce carbonic acid which may be undesirable.
There is an on-going need to develop improved fracturing methods and chemicals.
It is known to generate heat and gas in downhole operations for use in a secondary fracturing operation. However, known methods produce a limited amount of gas. The pressures experienced within the formation means that a large amount of gas needs to be generated to produce a pressure sufficient to overcome the confining pressure within the wellbore.
The present invention is based, in preferred embodiments, on treatment, for example fracturing, of subterranean formations by use of a mixture of chemicals which are arranged to undergo an exothermic reaction and/or produce large quantities of gas underground. The combination of heat and gas pressure can be used to treat, for example fracture, the formation. The combination of heat and gas may create new fractures, extend existing fractures or create microfractures within a hydraulic fracture. In preferred embodiments, the mixture of chemicals generates large gas volumes per mole of reactants in the mixture and releases non-toxic by-products. In addition, in preferred embodiments, a relatively low level or suitably substantially no carbon dioxide is produced by reaction of chemicals used in the treatment.
SUMMARY
It is an object of the present invention to address problems associated with fracturing and/or stimulation of formations.
DETAILED DESCRIPTION
According to a first aspect of the invention, there is provided a method of treating a subterranean formation, the method comprising contacting the formation with the following:
(a) ammonium sulfamate; and
(b) an oxidizing agent selected from a perchlorate or a nitrite or combinations thereof; and/or
(c) an acid (e.g. acid (AA) referred to herein).
In one embodiment, said method may comprise contact with the compounds referred to in (a) and (b); or in (a) and (c). Preferably, however, the method comprises treatment of the subterranean formation with the compounds referred to in (a), (b) and (c).
A preferred perchlorate is an alkali metal perchlorate with sodium perchlorate being especially preferred.
Preferably, said oxidizing agent is selected from a perchlorate or a nitrite; and, more preferably, said oxidizing agent comprises a perchlorate or a nitrite, but not both.
Preferably, as between a perchlorate and nitrite, a nitrite is preferred. Said nitrite is preferably arranged to provide nitrite ions in aqueous solution.
Said nitrite may be selected from alkali metal nitrites, alkaline earth metal nitrites, ammonium nitrite or organic nitrites. Said nitrite is preferably selected from lithium nitrite, sodium nitrite, potassium nitrite, calcium nitrite, magnesium nitrite, ammonium nitrite and combinations thereof. Said nitrite is preferably sodium nitrite.
Said ammonium sulfamate and said oxidizing agent, especially said nitrite, are preferably contacted so that they react and nitrogen is generated in the formation. In one embodiment, no carbon dioxide may be generated in the formation using the method.
A ratio (A) is defined as the number of moles of ammonium sulfamate divided by the number of moles of said oxidizing agent contacted with the formation. There is no minimum or maximum amount of nitrite required for the invention and so ratio (A) may be any value greater than 0. Ratio (A) may be from 0.05 to 2.0, for example 0.1 to 0.8; and preferably ratio (A) is 0.2 to 0.6.
The method preferably comprises contacting the formation with said ammonium sulfamate, said oxidizing agent (e.g. said nitrite) and an acid (AA). Said acid (AA) may be a monoprotic acid, a salt of a monoprotic acid, a polyprotic acid, a partial salt of a polyprotic acid, or a salt of a polyprotic acid.
Said acid (AA) may suitably be any water soluble Brønsted acid.
Said acid (AA) may be a mineral acid, an inorganic acid or an organic acid.
Acid (AA) may be selected from: HCl, HF, formic acid, lactic acid, phosphoric acid, sulfamic acid, ammonium bisulfate, acetic acid, oxalic acid, nitric acid, carbonic acid, nitrous acid, sulphuric acid, citric acid, propionic acid, lactic acid, trihaloacetic acids and acid precursors that provide the acid under wellbore conditions such as anhydrides, salts of trihaloacetic acid for example ammonium trichloroacetate, or esters like triacetin, methyl acetate, or dimers, oligomers or polymers of hydroxy acids such as lactic acid. Acid (AA) may be selected from: HCl, sulfamic acid and ammonium bisulfate or mixtures thereof. Acid (AA) preferably includes one or more nitrogen atoms.
Said acid (AA) is preferably arranged to react, for example with other materials contacted with the formation to produce a gas, wherein suitably the gas produced includes nitrogen atoms originating in the acid (AA). Thus, the method is preferably a method of treating a subterranean formation to generate gas within the formation. The gas may be arranged to fracture the formation in a region adjacent to, or within an area where said gas is produced.
Reference herein to a gas is intended to cover products which are gaseous at standard temperature and pressure (STP) (0° C. and 1 atm).
Preferably, said gas produced by reaction of said acid (AA) as aforesaid is a nitrogen atom containing gas (e.g. N2).
Thus, by use of acid (AA) as aforesaid, the acid can be treated to produce gas which can supplement gas produced by reaction of said ammonium sulfamate and said oxidizing agent, for example nitrite.
Preferably, said acid (AA) includes a nitrogen-atom.
Preferably, said acid (AA) includes a moiety




for example a moiety




or a moiety




Said acid (AA) may include a NH2 moiety such as found in sulfamic acid or in ammonium bisulfate (e.g. wherein the NH2 moiety is part of a NH4+ ion).
Said acid (AA) is preferably selected from sulfamic acid and ammonium bisulfate. A mixture of acids, for example the aforesaid acids may be selected.
A ratio (B) defined as the number of moles of ammonium sulfamate divided by the total number of moles of acid (contacted with the formation and/or reacted with ammonium sulfamate and oxidizing agent in the formation) may be greater than 0 and may be 10 or less. Ratio (B) may be in the range 0.1 to 2.5, suitably in the range 0.2 to 1.5, preferably in the range 0.4 to 1.1.
The total number of moles of acid may comprise the sum of the number of moles of acid (AA) and any other acid contacted with the formation and/or reacted with sulfamate and oxidizing agent in the formation.
In a preferred embodiment, acid used in the treatment may itself donate atoms (other than hydrogen atoms) to a gas generated in the method. A ratio (C) defined as the number of moles of ammonium sulfamate divided by the sum of the number of moles of one or more acids which are arranged to react, for example with other materials contacted with the formation, to donate atoms (other than hydrogen atoms) to a gas (e.g. nitrogen) produced in the method may be greater than 0 and may be 10 or less. Ratio (C) may be in the range 0.1 to 2.5, suitably in the range 0.2 to 1.5, preferably in the range 0.4 to 1.1.
In order to donate atoms (other than hydrogen atoms) to a gas produced, the acid may include a nitrogen atom. A ratio (D) defined as the number of moles of ammonium sulfamate divided by the sum of the number of moles of one or more acids (contacted with the formation and/or reacted with sulfamate and nitrite in the formation) which include a nitrogen atom, for example a NH2 moiety as described, may be greater than 0 and may be 10 or less. Ratio (D) may be in the range 0.1 to 2.5, suitably in the range 0.2 to 1.5, preferably in the range 0.4 to 1.1.
Preferred acids which donate atoms to a gas as described may be sulfamic acid and ammonium bisulfate. A ratio (E) defined as the number of moles of ammonium sulfamate divided by the sum of the number of moles of sulfamic acid and ammonium bisulfate (contacted with the formation and/or reacted with sulfamate and nitrite in the formation) may be greater than 0 and may be 10 or less. Ratio (E) may be in the range 0.1 to 2.5, suitably in the range 0.2 to 1.5, preferably in the range 0.4 to 1.1.
A ratio (H) defined as the number of moles of oxidizing agent divided by the total number of moles of acid (contacted with the formation and/or reacted with said ammonium sulfamate and oxidizing agent in the formation) may be in the range 0.5-10, preferably 0.6-5.0, more preferably 0.75-4.0 and most preferably from 0.9-3.5.
A ratio (I) defined as the number of moles of oxidizing agent divided by the sum of the number of moles of one or more acids which are arranged to react, for example with other materials contacted with the formation, to produce a gas (e.g. nitrogen) as described may be in the range 0.5-10, preferably 0.6-5, more preferably 0.75-4.0 and, especially, 0.9-3.5.
Thus, preferably, the acid does not simply catalyse another reaction, but rather is directly involved in gas generation by donating atoms other than hydrogen (e.g. by donation of nitrogen atoms) to the gas produced.
Said ammonium sulfamate may be provided as a slurry, an emulsion or a solution. Said ammonium sulfamate may be provided in water and the method may comprise selecting an aqueous solution of ammonium sulfamate. The solution may be of any suitable concentration up to a saturated solution. Said ammonium sulfamate may or may not be encapsulated, for example with an encapsulant arranged to delay reaction with the nitrite and/or acid (AA) on contact therewith. Said ammonium sulfamate is preferably not encapsulated
Said oxidizing agent, for example said nitrite may be provided as a slurry, an emulsion or a solution. Said nitrite may be provided in water and the method may comprise selecting an aqueous solution of said nitrite. The solution may be of any suitable concentration up to a saturated solution. Said nitrite may or may not be encapsulated, for example with an encapsulant arranged to delay reaction with the sulfamate and/or acid (AA) on contact therewith. Said nitrite is preferably not encapsulated.
When the method includes use of acid (AA) as described, acid (AA) may be provided in water for example as a solution or slurry in water. Said acid (AA) may or may not be encapsulated. Said acid (AA) is preferably not encapsulated, for example with an encapsulant arranged to delay reaction with the ammonium sulfamate.
In addition to the production of gas as described, said method may also produce heat to facilitate treatment of the formation.
Said method of treating said subterranean formation may be used in any subterranean formation that may benefit from the gas or heat rapidly generated by the reaction, for example to facilitate hydrocarbon production. The method may comprise treatment of said subterranean formation in a drilling operation, a stimulation operation, a hydraulic stimulation operation, a sand control operation, a completion operation, a scale inhibiting operation, a water-blocking operation, a clay stabilizer operation, a foam fracturing operation, a frac-packing operation, a gravel packing operation, a wellbore strengthening operation, a sag control operation, an acidising operation, an alkaline treatment operation, deposit removing operation, a ‘Huff and Puff’ operation, in a process for inhibiting ‘frac hits’, a wellbore damage removal operation, clean-up of a perforation, reduction of the hydrostatic pressure of the well, free stuck coiled tubing and/or pipe, a reservoir re-pressurisation operation, a depletion control operation, for far-field hydraulic fracture diversion, to reduce proppant settling, to reduce sand settling, an operation for increasing fracture complexity, or a fracturing operation.
Said method of treating a formation may be a ‘Huff and Puff’ operation.
‘Huff and Puff’ is a process that re-pressurises the near well area of the reservoir and reducing the viscosity of the oil in the surrounding formation. The reduction in oil viscosity can be achieved by pressurising the reservoir with a gas or fluid, comprising carbon dioxide which dissolves into the oil and reduces its viscosity. The pressurisation of the reservoir may be achieved by using any of the gas-generating reactions according to the invention. A typical ‘Huff and Puff’ operation would comprise a first step (i) of placing the gas generating chemicals within the wellbore and reacting them until the desired pressure is reached and a second ‘shut-in’ step (ii) wherein the well is sealed. Said shut-in step may be a full day or overnight. Once the well is opened production can resume.
Said method of treating a subterranean formation may be a process for inhibiting ‘frac hits’.
A ‘frac hit’ occurs when wells have been drilled in close proximity and fractures formed in the more recently drilled well grow into and through the production area of the older well and in some cases cause damages to the older well. Fractures preferentially propagate through the weaknesses within the formation and so increasing the pressure in and about the old well can divert and/or deflect the new fractures away from the older wells. The pressurisation of the older well can be achieved by contacting the ammonium sulfamate, oxidising agent, especially said nitrite, and acid (AA) within the older wellbore. This may be carried out as a one off treatment or the ammonium sulfamate, oxidising agent, especially said nitrite, and acid (AA), may be continuously injected to maintain a desired pressure.
Said method may comprise treatment of said subterranean formation, for example to fracture the formation or increase the complexity of a fracture network and/or stimulate hydrocarbon, for example oil and/or gas, production. By stimulate hydrocarbon production we mean, providing a method that improves the flow of hydrocarbons from the formation into the production well. More preferably, said method comprises treatment of said subterranean formation to fracture the formation or increase the complexity of a fracture network to facilitate hydrocarbon, for example oil and/or gas, production. For example, said method may extend an existing fracture, create new fractures or create microfractures extending out from a hydraulic fracture.
Preferably, said method is used in: a stimulation operation, a hydraulic stimulation operation, a ‘Huff and Puff’ operation, in a process for inhibiting ‘frac hits’, a wellbore damage removal operation, clean-up of a perforation, reduction of the hydrostatic pressure of the well, freeing stuck coiled tubing and/or pipe, a re-pressurisation operation, a depletion control operation, for far-field hydraulic fracture diversion, to reduce proppant settling, to reduce sand settling, an operation for increasing fracture complexity, or a fracturing operation.
Said method of treating a formation may comprise a wellbore damage removal operation.
Said method of treating a formation may be to free stuck coiled tubing and/or pipe.
Said method of treating a formation may comprise cleaning equipment, for example drilling equipment such as coil tubing underground. Gas produced may be arranged to clean equipment by the gas pressure blowing off oil and/or other solid/liquid contaminants from the equipment.
Said method of treating a formation may comprise a reservoir re-pressurisation operation.
Said method of treating a formation may comprise far-field hydraulic fracture diversion.
Said method of treating a formation may comprise reducing proppant settling.
Said method of treating a formation may comprise a stimulation operation.
In one embodiment, ammonium sulfamate described in (a) and an oxidizing agent selected from a perchlorate and a nitrite as described in (b) may be injected into different wellbores such that the reactants diffuse through the formation until they contact each other and react with each other within the formation. In this case, preferably, the wellbores are adjacent to each other. The pressure and concentrations of the reactants may suitably be selected to control where within the formation the reaction substantially occurs. This method of placing the reactants downhole may be used for a reservoir re-pressurisation operation, a depletion control operation, a damage removal operation or an operation for the far-field diversion of hydraulic fractures.
The subterranean formation may comprise a source rock comprising hydrocarbons (e.g., oil or natural gas) and may include shale, sandstone, or limestone or mixtures thereof. Said subterranean formation may be subsea.
Said method of said first aspect is preferably a method of treating said formation to stimulate the formation, for example to facilitate production of hydrocarbons, for example oil or gas from the formation. The method may comprise treating the formation to create or enhance a fracture in the formation. The method preferably comprises treatment of a formation which has already been fractured, wherein the method is arranged to enhance an existing fracture network and/or stimulate further hydrocarbon production from an existing formation.
The method may include introducing proppant and/or microproppant into the formation to enter fractures formed in the method. Proppant and/or microproppant may be included in a formulation introduced to the formation after the formation has been treated with said ammonium sulfamate, oxidizing agent and optional other reagents as described.
The method may also include introducing the proppant and/or microproppant in one or more of the formulations used in said method, so as to prop any fractures or microfractures formed as a result of the method.
Said method may comprise introducing said ammonium sulfamate, for example in aqueous solution, into the formation. Said ammonium sulfamate may be directed towards a region of said formation it is desired to treat, for example fracture and/or stimulate. Said method may involve introducing said ammonium sulfamate via an injection well. Coil-tubing (or the like) may be used to direct the ammonium sulfamate towards said region.
Said method may comprise introducing said oxidizing agent, preferably said nitrite, for example in aqueous solution, into the formation. Said oxidizing agent, preferably said nitrite, may be directed towards a region of said formation it is desired to treat, for example fracture and/or stimulate. Said method may involve introducing said oxidizing agent, preferably said nitrite, into the subterranean formation, for example via a suitable well, such as an injection well. Said method may involve introducing said oxidizing agent, preferably said nitrite, into the production well. Coil-tubing (or the like) may be used to direct the oxidizing agent, preferably said nitrite, towards said region.
Said method may comprise introducing said acid (AA), for example in aqueous solution, into the formation. Said acid (AA) may be directed towards a region of said formation it is desired to treat, for example fracture and/or stimulate. Said method may involve introducing said acid (AA), via an injection well. Coil-tubing (or the like) may be used to direct the acid (AA) towards said region.
In some embodiments of the method, said ammonium sulfamate and said oxidizing agent, preferably said nitrite, are preferably not contacted with one another above ground. They are preferably contacted underground, during passage towards, or preferably adjacent to or within the region of said formation it is desired to treat.
In other embodiments, it may be desirable to contact ammonium sulfamate and oxidizing agent, preferably nitrite, above ground and suitably taking a step to eliminate or minimise reaction between the reagents. Such a step may comprise controlling the pH of a formulation of ammonium sulfamate and oxidizing agent, preferably nitrite. Such control may involve inclusion of an alkali in the formulation of ammonium sulfamate and oxidizing agent, preferably nitrite, to increase and maintain the pH within a range where the ammonium sulfamate and oxidizing agent, preferably nitrite do not react. For example, the pH may be greater than 7, for example at least 8.
In the method, acid (AA) is preferably not contacted with said ammonium sulfamate and oxidizing agent, preferably said nitrite, above ground. It is preferably contacted with said ammonium sulfamate and/or nitrite underground, preferably within and/or adjacent to the region of said formation it is desired to treat.
In the method, for example in fracturing of a formation by production of gas within the formation, the sum of the wt % of a formulation (F1) (e.g. an aqueous formulation) comprising said ammonium sulfamate, a formulation (F2) (e.g. an aqueous formulation) comprising said oxidizing agent, preferably said nitrite, a formulation (F3) (e.g. an aqueous formulation) comprising said acid (AA) introduced into the formation is at least 80 wt %, preferably at least 90 wt %, more preferably at least 98 wt % of the total weight of materials introduced into the formation as part of the fracturing of the formation by production of gas within the formation, as described. For the avoidance of doubt, the aforementioned sum of the wt % is not intended to include a formulation (eg an inert spacer) which may be introduced into the formation (and may contact formulation (F1), (F2) and/or (F3)) but which does not include an active ingredient which is involved in production of gas in the formation as described herein.
In another embodiment of the method, for example in fracturing of a formation by production of gas within the formation, the sum of the wt % of a formulation (F3) (e.g. an aqueous formulation) comprising said acid (AA) and a formulation (F4) (e.g. an aqueous formulation) comprising said ammonium sulfamate, said oxidizing agent, preferable said nitrite and an alkali, introduced into the formation is at least 80 wt %, preferably at least 90 wt %, more preferably at least 98 wt % of the total weight of materials introduced into the formation as part of the method of treating of the formation by production of gas and/or heat within the formation, as described in the first aspect. For the avoidance of doubt, the aforementioned sum of the wt % is not intended to include a formulation (eg an inert spacer) which may be introduced into the formation (and may contact formulation (F3) and/or (F4)) but which does not include an active ingredient which is involved in production of gas in the formation as described herein.
The sum of the wt % of ammonium sulfamate and water in formulation (F1), when introduced into the formation, is suitably at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt %.
The sum of the wt % of oxidizing agent, preferably said nitrite, and water in formulation (F2), when introduced into the formation, is suitably at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt %.
The sum of the wt % of acid (AA) and water in formulation (F3), when introduced into the formation, is suitably at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt %.
The sum of the wt % of ammonium sulfamate, oxidising agent, preferably said nitrite, alkali and water in formulation (F4), when introduced into the formation, is suitably at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt %.
In another embodiment of the method, for example in fracturing of a formation by production of gas within the formation, a formulation (F5) may be provided, wherein said formulation is aqueous and comprises said ammonium sulfamate and said acid (AA). In the method, for example in fracturing of a formation by production of gas within the formation, the sum of the wt % of formulation (F5) and a formulation (F2) (e.g. an aqueous formulation) comprising said oxidizing agent, preferably said nitrite, is at least 80 wt %, preferably at least 90 wt %, more preferably at least 98 wt % of the total weight of materials introduced into the formation as part of the fracturing of the formation by production of gas within the formation, as described. For the avoidance of doubt, the aforementioned sum of the wt % is not intended to include a formulation (eg an inert spacer) which may be introduced into the formation (and may contact formulation (F5) and/or (F2)) but which does not include an active ingredient which is involved in production of gas in the formation as described herein.
Any of formulations (F1), (F2), (F3), (F4) and (F5) may comprise additional components commonly used in the treatment of subterranean formations for example: biocides, breakers, co-solvents, corrosion inhibitors, cross-linking agents, fluid loss control additives, friction reducers, iron control agents, oxygen scavengers, pH adjusting agents, proppants, microproppants, salts, scale inhibitors, surfactants, sulfide scavengers, viscosifying agents, clay stabilisers and the like.
Co-solvents may be used in any of formulations (F1), (F2), (F3), (F4) and (F5) to improve the solubility of the reagents in water and/or the thermodynamic stability of the solution. The co-solvents are preferably polar solvents for example: alcohols, glycols, amides, esters, ketones, sulfoxides etc. Suitably, the co-solvents are methanol or formamide or mixtures thereof. Specific examples may be selected from methanol and/or formamide.
Any suitable method may be used to place reagents into a well and/or deliver to a desired position in a formation. The well may be a horizontal or vertical well. However, preferred methods keep selected reagents isolated from each other until they reach the desired location within the formation.
Coiled tubing may be used to place reagents downhole. In this case, the end of the tube is placed where gas generation is required. One solution is pumped through the tubing and another solution along the casing. For example, Formulation (F3) may be pumped through the coil and formulation (F4) may be pumped along the casing.
Coiled tubing may be especially useful to place the reagents downhole in: a fracturing operation, a perforation clean-up operation, a wellbore damage removal operation, an operation to reduce the hydrostatic pressure of a well, or to free stuck coiled tubing and/or pipe.
Spacers may be used to keep the reagents and/or compositions separate until they reach a desired position in the formation. In this technique, a fluid, preferably an inert fluid, would used to separate the two formulations of reactive components. Typically with this technique, 5-10 bbl of the inert fluid may be used. Examples of inert fluids suitable for this technique include, but are not limited to, pure water and oil.
In one embodiment, the formulations (F1), (F2) and (F3) are introduced, in any order, with an inert spacer separating each of the formulations. Formulation (F3) may be used as a spacer to separate formulations (F1) and (F2),
In another embodiment formulations (F3) and (F4) may be introduced into the formation with an inert spacer separating the two formulations.
Spacers may be used to place the formulations downhole in the following operations: reservoir re-operation, a depletion control operation, a damage removal operation, for far-field hydraulic fracture diversion, a fracturing operation, to reduce sand or proppant settling.
The formulations may be provided as part of an emulsion, for example water-in-oil emulsions or double emulsions, for example water-in-oil-in-water. In a double emulsion, the inner water phase may be a formulation e.g. (F3) and the outer water phase may be a different formulation e.g. (F4).
In preferred embodiments described herein, the number of moles of gas generated per mole of reactants may be increased compared to prior art proposals.
The sum of the total weight in grams (g) of ammonium sulfamate, oxidizing agent, preferably said nitrite, and acid(s) (including or consisting of acid (AA)) introduced into the formation is herein referred to as SUM-W. The sum of the total volume of gas (e.g. CO2 and/or N2) in cm3 generated by reaction of ammonium sulfamate, oxidizing agent, preferably said nitrite, and said acid(s) is herein referred to as SUM-V. Preferably, in the method, the Reaction Efficiency is defined as the volume of gases produced divided by the weight of reactants (i.e. SUM-V divided by SUM-W). The Reaction Efficiency is suitably at least 100 cm3/g, for example at least 160 cm3/g or at least 180 cm3/g, especially it is at least 190 cm3/g. It may be less than 300 cm3/g.
The Reaction Efficiency as described may suitably be calculated based on weights of the specified reagents selected and gas generated by reaction thereof in a reaction carried out under controlled conditions at the surface, based on amounts of reagents which are to be introduced into the formation, since measurements within the formation itself are not practical. Values referred to are suitably measured at STP, unless otherwise stated.
To minimise the quantity of one or more of the formulations leaking off into the formation and to maximise the fracturing effect, it is desirable that the gas is rapidly generated after the components have been contacted with each other. The gas generation may substantially be complete within 10 minutes of all the components being contacted with each other. Preferably, the gas generation is substantially complete within 5 minutes of the components being contacted with each other.
The quantities of formulations introduced into the formation as part of the method may be suitably selected dependent on the features of the formation, for example the confining pressure, and the pressure required to achieve the desired effect of said method of treating said formation. Thus, it is anticipated that any quantity of the formulations may be used. However, preferably at least 1 bbl may be used, for example 10 to 500 bbl, or from 100 to 350 bbl, preferably from 150 to 250 bbl.
The rate at which one of more of the fluids is injected may suitably be adjusted according to the method of treating the formation and the method of delivering the components. For example, it may be injected at a rate sufficient to build up a pressure such as that it fractures the formation.
In some methods of treating a subterranean formation, it may be preferable to generate pulses of higher and lower pressures within the formation. This effect may be achieved by repeatedly reacting a gas producing formulation within the formation. Either mechanical, chemical or combinations of mechanical and chemical methods may be used to control the manner in which the formulations are contacted with the formation to produce a series of pressure pulses. Said pulses of pressure may be created in treating a subterranean formation in a method comprising:
(i) introducing a first gas producing formulation into the formation so the formulation produces a gas in the formation;
(ii) reducing the rate of gas production within the formation, so the pressure produced in this step is lower than in step (i) and may be 0;
(iii) introducing a second gas producing formulation into the formation, which formulation may be the same or different to the first gas producing formulation, thereby to produce a pressure higher than in step (ii); and, optionally,
(iv) reducing the rate of introduction of said second gas producing formulation into the formation.
Steps (ii) and (iii) may be suitably repeated to produce further pressure pulses as required.
Steps (i) through to (iv) may be carried out continuously, intermittently or a mixture of continuously and intermittently.
In step (ii), the reduction of rate of gas production in the formation may be achieved mechanically, for example by reducing or stopping the amount of one or more gas generating reagents being introduced into the formation.
Step (ii) may be achieved using chemical means. For example, in one embodiment, step (ii) may be achieved by pumping an inert fluid e.g. a spacer in between the pumping of gas producing formulations. In another embodiment, step (ii) may be achieved by pumping an inert fluid concurrently with the first gas producing formulation, so as to reduce the concentration of the gas producing formulation and the rate at which the gas is produced. Then, step (iii) may comprise stopping the pumping of the inert fluid.
In some embodiments the gas generating reagents used in the gas producing formulation used in step (i) may be non-stoichiometric. In this case step (ii) may occur when one of the reagents (herein reagent (P)) is consumed so gas generation stops, leaving an excess of the remaining reagents (herein reagents (Q)). Step (iii) may then comprise injecting a formulation comprising an excess of reagent (P). Steps (i) to (iii) may be repeated with the injected formulations being alternated. For example the method may comprise contacting the formation with 20 bbl of a solution of ammonium compound and acid and 10 bbl of a solution of sodium nitrite in step (i). Step (ii) occurs when the 10 bbl of sodium nitrite is consumed. Step (iii) may comprise injecting 10 bbl or more of sodium nitrite to produce a second pressure pulse. If the method is to be repeated, step (iii) may use a large excess of sodium nitrite.
According to a second aspect of the invention, there is provided a mixture for treating a subterranean formation, the mixture comprising:
(a) ammonium sulfamate; and
(b) an oxidizing agent selected from a perchlorate or a nitrite or combinations thereof;
(c) an acid (e.g. acid (AA) referred to herein).
The mixture may be produced below ground, for example within a subterranean formation.
The mixture may include more than one acid, for example including acid (AA).
The sulfamate, oxidizing agent, acid, including acid (AA), may be as described in the first aspect.
According to a third aspect of the invention, there is provided a collocation adjacent a subterranean formation and/or adjacent an injection well of a subterranean formation, wherein said collocation comprises (P), (Q) or (R) as described below:
(P) a formulation comprising ammonium sulfamate (e.g. formulation (F1) of the first aspect), which is preferably provided in a receptacle (e.g. a receptacle (A));
a formulation comprising an oxidizing agent (e.g. formulation (F2) of the first aspect), which is preferably provided in a receptacle (e.g. a receptacle (B)); and, optionally (but preferably)
a formulation comprising acid (AA) (e.g. formulation (F3) of the first aspect), which is preferably provided in a receptacle (e.g. a receptacle (C));
(Q) a formulation comprising ammonium sulfamate and an oxidising agent, preferably a nitrite which is preferably provided in a receptacle; and, optionally (but preferably)
a formulation comprising acid (AA) which is preferably provided in a receptacle;
(R) a formulation (F5), wherein said formulation is aqueous and comprises ammonium sulfamate and acid (AA), wherein said formulation is preferably provided in a receptacle; and a formulation (F2) (e.g. an aqueous formulation) comprising oxidizing agent, preferably a nitrite which is preferably provided in a receptacle.
The collocation suitably includes pipework for delivering the formulations into the subterranean formation. Receptacle (A) may communicate with a pipe (which may comprise coil tubing) arranged to deliver formulation (F1) into the formation. Receptacle (B) may communicate with a pipe (which may comprise coil tubing) arranged to deliver formulation (F2) into the formation. Receptacle (C) may communicate with a pipe (which may comprise coil tubing) arranged to deliver formulation (F3) into the formation.
According to a fourth aspect, there is provided the use of the following for gas generation in a subterranean formation:
(a) ammonium sulfamate;
(b) an oxidizing agent selected from a perchlorate or a nitrite or combinations thereof; and, optionally,
(c) an acid (AA).
The number of moles of gas generated per mole of reactants may be increased compared to prior art proposals.
The use may be as described in the first aspect.
Any feature of any aspect of any invention or embodiment described herein may be combined with any aspect of any other invention or embodiment described herein mutatis mutandis.


WORKING EXAMPLES
A subterranean formation may be treated with reagents which are arranged to react to produce a gas and/or heat within the formation. This may stimulate the formation by improving a fracture network within the formation, for example by creating new fractures, extending existing fractures, opening up naturally-occurring fractures or creating microfractures. The examples which follow describe reagents which may be used in a treatment.
Example 1—General Procedure for Undertaking Reactions
An ammonium compound and a nitrite or perchlorate-containing compound were added to a round-bottom flask and dissolved in the minimum quantity of water. Suitable apparatus to measure gas released was arranged in position and the solution heated with stirring to 75° C. Once the solution had reached 75° C., a selected amount of acid also heated to 75° C. was injected into the reaction vessel. The quantity of gas generated was recorded.
In Examples 2 to 4, use of ammonium sulfamate as the ammonium compound was compared with use of other ammonium compounds, namely ammonium chloride and ammonium bicarbonate.
Examples 2 and 3—Comparison Between Ammonium Sulfamate and Ammonium Chloride Using Sulfamic Acid
In order to compare use of ammonium sulfamate with ammonium chloride, ammonium sulfamate was reacted with sulfamic acid (Example 2) and the gas volume determined. For comparison purposes, the same reaction and assessment was undertaken wherein the ammonium sulfamate was replaced with ammonium chloride (Example 3). In each case, 30 mmol of sodium nitrite was used as oxidizing agent.
Results are provided in the table below from which it will be noted that significantly more gas is generated when ammonium sulfamate is used compared to use of the other ammonium compounds.
















Gas generated on





reaction with acid/


Example No.
Ammonium Salt
Acid
cm3







2
ammonium sulfamate
sulfamic
1320


3
ammonium chloride
sulfamic
 960


(comparative)









Examples 4 to 6—Comparison Between Ammonium Sulfamate Other Ammonium Compounds Using Hydrochloric Acid
The procedure described for Examples 2 and 3 was followed except hydrochloric acid was used instead of sulfamic acid. Results are provided in the table below.
















Gas generated on





reaction with acid/


Example No.
Ammonium Salt
Acid
cm3







4
ammonium sulfamate
hydrochloric
920


5
ammonium chloride
hydrochloric
330


(comparative)





6 
ammonium bicarbonate
hydrochloric
560


(comparative)









Again, it should be noted that significantly more gas was generated when ammonium sulfamate was used compared to use of other ammonium compounds.
It is found that the solubility of sulfamic acid is relatively low. To address this, mixtures of sulfamic acid and ammonium bisulfate (which is extremely soluble) were used with the ammonium sulfamate as described below for Examples 7 to 18.
Example 7
2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodium nitrite (20 mmol) was added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 0.83 mL of a 12 M aqueous solution of hydrochloric acid (10 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 8
2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodium nitrite (20 mmol) was added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 4.0 mL of an aqueous solution containing sulfamic acid (7.5 mmol) and ammonium bisulfate (2.5 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 9
2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodium nitrite (20 mmol) was added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 3.0 mL of an aqueous solution containing sulfamic acid (5 mmol) and ammonium bisulfate (5 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 10
2.9 mL of an aqueous solution of ammonium sulfamate (5 mmol) and sodium nitrite (20 mmol) was added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 1.95 mL of an aqueous solution containing sulfamic acid (2.5 mmol) and ammonium bisulfate (7.5 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 11
2.2 mL of an aqueous solution containing ammonium sulfamate (7 mmol), sulfamic acid (1 mmol) and ammonium bisulfate (9 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 12
2.5 mL of an aqueous solution containing ammonium sulfamate (5 mmol), sulfamic acid (2.5 mmol) and ammonium bisulfate (7.5 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.5 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 13
2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol), sulfamic acid (1.5 mmol) and ammonium bisulfate (8.5 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 14
2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol), sulfamic acid (1.75 mmol) and ammonium bisulfate (7 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 15
2.2 mL of an aqueous solution containing ammonium sulfamate (5.5 mmol), sulfamic acid (2.63 mmol) and ammonium bisulfate (4.88 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 16
2.2 mL of an aqueous solution containing ammonium sulfamate (6 mmol), sulfamic acid (2.24 mmol) and ammonium bisulfate (4.76 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 17
2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol), sulfamic acid (2.50 mmol) and ammonium bisulfate (4.25 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Example 18
2.2 mL of an aqueous solution containing ammonium sulfamate (6.25 mmol), sulfamic acid (2.50 mmol) and ammonium bisulfate (3.75 mmol) were added to a round-bottom flask. Suitable apparatus to measure gas release was arranged in position and the solution heated to 75° C. Once the solution reached 75° C., 2.2 mL of an aqueous solution containing sodium nitrite (20 mmol), heated to the same temperature, was injected into the reaction vessel. The quantity of gas generated was recorded.
Results for Examples 7 to 18 are provided in the table below.


















mmol
mmol

mmol
Gas
Total
Efficiency/


Example
NH4NH2SO3
NaNO2
Acid
acid
generated/cm3
mass/g
cm3 per g






















7
5
20
Hydrochloric
10.0
460
2.95
156


8
5
20
Sulfamic/ammonium bisulfate (75:25)
10.0
740
2.97
249


9
5
20
Sulfamic/ammonium bisulfate (50:50)
10.0
700
3.01
232


10
5
20
Sulfamic/ammonium bisulfate (25:75)
10.0
660
3.06
216


11
7
20
Sulfamic/ammonium bisulfate (10:90)
10.0
630
3.31
190


12
5
20
Sulfamic/ammonium bisulfate (25:75)
10.0
660
3.06
216


13
6.25
20
Sulfamic/ammonium bisulfate (15:85)
10.0
640
3.22
199


14
6.25
20
Sulfamic/ammonium bisulfate (20:80)
8.8
660
3.07
215


15
5.5
20
Sulfamic/ammonium bisulfate (35:65)
7.5
660
2.82
234


16
6
20
Sulfamic/ammonium bisulfate (32:68)
7.0
665
2.83
235


17
6.25
20
Sulfamic/ammonium bisulfate (37:63)
6.8
670
2.83
237


18
6.25
20
Sulfamic/ammonium bisulfate (40:60)
6.3
650
2.77
235









Thus, ammonium sulfamate may advantageously be used to generate relatively large quantities of gas and, suitably, without generation of any carbon dioxide (or any carbon-containing gas).
The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

## Claims

1. A method of treating a subterranean formation, the method comprising contacting the formation with the following:
(a) ammonium sulfamate; and
(b) an oxidizing agent comprising a nitrite; and
(c) an acid (AA);
wherein a ratio (B) defined as the number of moles of ammonium sulfamate divided by the total number of moles of acid contacted with the formation and/or reacted with said ammonium sulfamate and oxidizing agent in the formation is greater than 0 and is 10 or less;
wherein a ratio (A) defined as the number of moles of ammonium sulfamate divided by the number of moles of said oxidizing agent contacted with the formation is from 0.05 to 2.0;
wherein said ammonium sulfamate and said nitrite are contacted with one another underground;
wherein the method comprises treating the formation to create or enhance a fracture in the formation; and
wherein in the method the sum of the wt % of a formulation (F1) comprising said ammonium sulfamate, a formulation (F2) comprising said oxidizing agent and a formulation (F3) comprising the acid (AA) contacted with the formation is at least 98 wt %, of the total weight of materials introduced into the formation.

2. The method according to claim 1, wherein said oxidizing agent is sodium nitrite and wherein said ammonium sulfamate and said oxidizing agent are contacted so they react and nitrogen is generated in the formation.

3. The method according to claim 1, wherein the ratio (A) defined as the number of moles of ammonium sulfamate divided by the number of moles of nitrite contacted with the formation and/or reacted in the formation is in the range 0.2 to 0.6.

4. The method according to claim 1, wherein the method comprises contacting the formation with said ammonium sulfamate, said oxidizing agent and the acid (AA), wherein said acid (AA) is arranged to react to produce a gas, wherein the gas produced includes carbon and/or nitrogen atoms originating in the acid (AA).

5. The method according to claim 4, wherein said acid (AA) is treated in the method to produce gas which supplements gas produced by reaction of said ammonium sulfamate and said oxidizing agent.

6. The method according to claim 5, wherein said acid (AA) includes a moiety,



and wherein said acid (AA) includes a nitrogen-atom.

7. The method according to claim 5, wherein said acid (AA) is selected from sulfamic acid and ammonium bisulfate; and mixtures of sulfamic acid and ammonium bisulfate.

8. The method according to claim 7, wherein the ratio (B) is in the range 0.4 to 1.1; and/or
wherein a ratio (C) defined as the number of moles of ammonium sulfamate divided by a sum of a number of moles of one or more acids which are arranged to react with other materials contacted with the formation to produce a gas is in the range 0.4 to 1.1; and/or
wherein a ratio (E) defined as the number of moles of ammonium sulfamate divided by a sum of the number of moles of sulfamic acid and ammonium bisulfate contacted with the formation is in the range 0.4 to 1.1.

9. The method according to claim 1, wherein a ratio (D) defined as the number of moles of ammonium sulfamate divided by the sum of the number of moles of one or more acids which include a nitrogen atom contacted with the formation and/or reacted with ammonium sulfamate and/or said oxidizing agent in the formation is in the range 0.4 to 1.1; and/or
wherein a ratio (H) defined as the number of moles of oxidizing agent divided by the total number of moles of acid contacted with the formation and/or reacted with said ammonium sulfamate is in the range 0.9 to 3.5; and/or
wherein a ratio (I) defined as the number of moles of oxidizing agent divided by the sum of the number of moles of one or more acids which are arranged to react to produce a gas are in the range 0.9 to 3.5.

10. The method according to claim 1, wherein said ammonium sulfamate is provided as a slurry, an emulsion or a solution; wherein said oxidizing agent is provided in water; and wherein the method includes contacting the formation with an aqueous solution or slurry of the acid (AA).

11. The method according to claim 1, wherein the method comprises contacting the formation with a formulation (F5) which is aqueous and comprises said ammonium sulfamate and the acid (AA) which is selected from sulfamic acid and ammonium bisulfate; and mixtures of sulfamic acid and ammonium bisulfate.

12. The method according to claim 1, wherein:
the sum of the total weight in grams (g) of ammonium sulfamate, oxidizing agent and acid(s) introduced into the formation is herein referred to as SUM-W;
the sum of the total volume in cm3 of gas generated by reaction of ammonium sulfamate, oxidizing agent and said acid(s) is herein referred to as SUM-V;
wherein, in the method, the Reaction Efficiency is defined as SUM-V divided by SUM-W;
wherein the Reaction Efficiency is at least 100 cm3/g and is less than 300 cm3/g.

13. The method according to claim 1, further comprising producing pulses within the formation by controlling contact and/or amounts of the ammonium sulfamate, the oxidizing agent and/or the acid (AA).

14. A method of treating a subterranean formation, the method comprising contacting the formation with the following:
(a) ammonium sulfamate; and
(b) an oxidizing agent comprising a nitrite; and
(c) an acid (AA);
wherein a ratio (B) defined as the number of moles of ammonium sulfamate divided by the total number of moles of acid contacted with the formation and/or reacted with said ammonium sulfamate and oxidizing agent in the formation is greater than 0 and is 10 or less;
wherein a ratio (A) defined as the number of moles of ammonium sulfamate divided by the number of moles of said oxidizing agent contacted with the formation is from 0.05 to 2.0;
wherein said ammonium sulfamate and said nitrite are not contacted with one another above ground;
wherein the method comprises treating the formation to create or enhance a fracture in the formation; and
wherein in the method the sum of the wt % of a formulation (F1) comprising said ammonium sulfamate, a formulation (F2) comprising said oxidizing agent and a formulation (F3) comprising the acid (AA) contacted with the formation is at least 98 wt %, of the total weight of materials introduced into the formation.

15. The method according to claim 14, wherein said oxidizing agent is sodium nitrite and said acid (AA) is selected from sulfamic acid and ammonium bisulfate; and mixtures of sulfamic acid and ammonium bisulfate.

16. The method according to claim 15, wherein said ratio (A) is in the range 0.2 to 0.6; and said ratio (B) is in the range 0.4 to 1.1.

17. A method of treating a subterranean formation, the method comprising contacting the formation with the following:
(a) ammonium sulfamate; and
(b) an oxidizing agent comprising a nitrite; and
(c) an acid (AA);
wherein a ratio (B) defined as the number of moles of ammonium sulfamate divided by the total number of moles of acid contacted with the formation and/or reacted with said ammonium sulfamate and oxidizing agent in the formation is greater than 0 and is 10 or less;
wherein a ratio (A) defined as the number of moles of ammonium sulfamate divided by the number of moles of said oxidizing agent contacted with the formation is from 0.05 to 2.0;
wherein said ammonium sulfamate and said nitrite are contacted with one another underground, and wherein:
the sum of the total weight in grams (g) of ammonium sulfamate, oxidizing agent and acid(s) introduced into the formation is herein referred to as SUM-W;
the sum of the total volume in cm3 of gas generated by reaction of ammonium sulfamate, oxidizing agent and said acid(s) is herein referred to as SUM-V;
wherein, in the method, the Reaction Efficiency is defined as SUM-V divided by SUM-W;
wherein the Reaction Efficiency is at least 100 cm3/g and is less than 300 cm3/g.

18. The method according to claim 17, wherein the method comprises treating the formation to create or enhance a fracture in the formation.

19. The method according to claim 17, wherein said oxidizing agent is sodium nitrite and wherein said ammonium sulfamate and said oxidizing agent are contacted so they react and nitrogen is generated in the formation.

20. The method according to claim 17, wherein the ratio (A) defined as the number of moles of ammonium sulfamate divided by the number of moles of nitrite contacted with the formation and/or reacted in the formation is in the range 0.2 to 0.6.

21. The method according to claim 17, wherein the method comprises contacting the formation with said ammonium sulfamate, said oxidizing agent and the acid (AA), wherein said acid (AA) is arranged to react to produce a gas, wherein the gas produced includes carbon and/or nitrogen atoms originating in the acid (AA).

22. The method according to claim 21, wherein said acid (AA) is treated in the method to produce gas which supplements gas produced by reaction of said ammonium sulfamate and said oxidizing agent.

23. The method according to claim 21, wherein said acid (AA) includes a moiety,



and wherein said acid (AA) includes a nitrogen-atom.

24. The method according to claim 21, wherein said acid (AA) is selected from sulfamic acid and ammonium bisulfate; and mixtures of sulfamic acid and ammonium bisulfate.

25. The method according to claim 24, wherein the ratio (B) is in the range 0.4 to 1.1; and/or
wherein a ratio (C) defined as the number of moles of ammonium sulfamate divided by a sum of a number of moles of one or more acids which are arranged to react with other materials contacted with the formation to produce a gas is in the range 0.4 to 1.1; and/or
wherein a ratio (E) defined as the number of moles of ammonium sulfamate divided by a sum of the number of moles of sulfamic acid and ammonium bisulfate contacted with the formation is in the range 0.4 to 1.1.

26. The method according to claim 17, wherein a ratio (D) defined as the number of moles of ammonium sulfamate divided by the sum of the number of moles of one or more acids which include a nitrogen atom contacted with the formation and/or reacted with ammonium sulfamate and/or said oxidizing agent in the formation is in the range 0.4 to 1.1; and/or
wherein a ratio (H) defined as the number of moles of oxidizing agent divided by the total number of moles of acid contacted with the formation and/or reacted with said ammonium sulfamate is in the range 0.9 to 3.5; and/or
wherein a ratio (I) defined as the number of moles of oxidizing agent divided by the sum of the number of moles of one or more acids which are arranged to react to produce a gas are in the range 0.9 to 3.5.