HYDROCARBON STIMULATION BY ENERGETIC CHEMISTRY
Disclosed are methods and compositions for stimulating a hydrocarbon formation by generating heat and/or pressure in the formation, in either a fracturing or matrix treatment. This invention utilizes reactive fluids which comprise energetic chemistry that reacts in the formation to create heat and/or pressure. The heat may reduce the viscosity and increase the mobility of heavy oil, and/or the pressure may initiate or extend fractures in the hydrocarbon bearing formation. The reactive fluid may be buffered to slow the reaction and include an encapsulated activator to accelerate the reaction after suitable delay or when the fluid is placed in a zone of interest. Reactive fluids may be sequentially used, wherein each reactive fluid is successively less energetic than the preceding reactive fluid.
1 . A method of stimulating a subterranean hydrocarbon formation penetrated by a wellbore, the formation having a proximal zone adjacent the wellbore, and a distal zone outside the proximal zone, the method comprising:
(a) injecting a first reactive fluid comprising exothermic reactants into the proximal zone;
(b) displacing the first reactive fluid into the distal zone with a second reactive fluid, which is less energetic than the first reactive fluid; and
(c) activating the second reactive fluid such that heat generated by the second reactive fluid activates the first reactive fluid.
2 . The method of claim 1 wherein the second reactive fluid is less energetic than the first reactive fluid as a result of a lower concentration or quantity of reactants, or the absence of ammonium nitrate, or both.
3 . The method of claim 1 wherein the first reactive fluid comprises an ammonium compound and a nitrite compound.
4 . The method of claim 3 wherein the first reactive fluid comprises ammonium nitrate in addition to the ammonium compound.
5 . The method of claim 1 comprising the further step of displacing the first and second reactive fluids with at least one additional reactive fluid which is less reactive than the second reactive fluid, and activating the at least one additional reactive fluid such that heat generated by the at least one additional reactive fluid activates the second reactive fluid.
6 . The method of claim 1 wherein the ammonium nitrate is present in the first reactive fluid in a concentration greater than about 30%.
7 . The method of claim 4 wherein the ammonium nitrate is present in the first reactive fluid in a concentration greater than about 40%.
8 . The method of claim 4 wherein the ammonium nitrate is present in the first reactive fluid in a concentration greater than about 50%.
9 . The method of claim 3 wherein the nitrite compound is sodium nitrite.
10 . The method of claim 3 wherein the ammonium compound is ammonium chloride.
11 . The method of claim 3 wherein the second reactive fluid is buffered to a neutral pH and comprises an encapsulated activator acid.
12 . A method of stimulating a subterranean hydrocarbon formation penetrated by a wellbore, the formation having a treatment zone, the method comprising:
(a) injecting a buffered reactive fluid comprising an ammonium compound, a nitrite compound, an encapsulated acid into the treatment zone;
(b) wherein the encapsulated acid is configured to release the acid once the reactive fluid has been placed into the treatment zone.
13 . The method of claim 3 wherein the reactive fluid is placed at the tip of a fracture network created by a conventional fracturing step, and followed by at least one stage of proppant laden fracturing fluid.
14 . The method of claim 12 wherein the encapsulated acid is broken and released by the closing force of the fracture network, or by dissolving over time or with increased temperature.
15 . The method of claim 12 wherein the reactive fluid comprises ammonium nitrate in a concentration greater than about 30%.
16 . The method of claim 15 wherein ammonium nitrate is present in the reactive fluid in a concentration greater than about 40%.
17 . The method of claim 16 wherein the ammonium nitrate is present in the reactive fluid in a concentration greater than about 50%.
18 . A stimulation reactive fluid comprising reactants which undergo exothermic and/or gas-generating reaction or reactions into the formation, a neutral pH buffer comprising an alkaline substance, and an encapsulated acid activator, wherein release of the activator increases the rate of the exothermic and/or gas generating reaction or reactions.
19 . The stimulation reactive fluid of claim 18 wherein the activator comprises an organic acid encapsulated in a polymer.
20 . The stimulation reactive fluid of claim 19 wherein the polymer comprises one of guar, chitosan, polyvinyl alcohol, carboxymethylcellulose or xanthan.