Gelled acid

Abstract

Reducing agents or reducing agent precursors are provided for breaking ferric ion crosslinks in polymers in gelled acids used for diversion in matrix acidizing and used for leakoff control in acid fracturing. Previous reducing agents were very toxic to aquatic species or so active that they could be used only at low temperatures. The new reducing agents and reducing agent precursors are less reactive, less toxic, and leave less residue behind to impede fluid flow after the gel is broken after the treatment. Suitable compounds are sources of one or two hydrazines or sources of hydroxylamine. Such compounds are carbohydrazides, semicarbohydrazides, ketoximes, and aldoximes.

Description

BACKGROUND OF INVENTION [0001] The invention relates to stimulation of production in hydrocarbon wells. More specifically it relates to gelled acids for diversion and leakoff control in matrix acidizing and acid fracturing. Most specifically it relates to reducing agents and reducing agent precursors that are less toxic to aquatic species and that leave less residue than prior reducing agents upon destruction of the crosslinking agent after treatment. [0002] Acids are often used to treat subterranean formations to stimulate the formations to increase the production of fluids, such as hydrocarbons. When the acid is injected above the fracture pressure of the formation being treated, the treatment is called acid fracturing or fracture acidizing. The object is to create a large fracture that serves as an improved flowpath through the rock form ation. After such fractures are created, when pumping of the fracture fluid is stopped and the injection pressure drops, the fracture tends to c...

AI Technical Summary

Benefits of technology

[0007] One embodiment of the Invention is an aqueous acid composition containing an aqueous acid; a polymeric gelling agent that crosslinks in the presence of ferric ions at a pH of about 2 or greater; a soluble ferric salt in an amount sufficient to crosslink the polymeric gelling agent at a pH of about 2 or greater, but which does not crosslink the polymeric gelling agent at a pH below about 2; and an effective amount of a source of a reducing agent, which is hydroxylamine or a hydrazine. The source of the hydrazine or hydrazines is either a carbohydrazide having the formula

Problems solved by technology

The acid differentially etches the faces of the fracture, creating or exaggerating asperities, so that, when the fracture closes, the opposing faces no longer match up.

A problem with this technique is that as the acid is injected it tends to react with the most reactive rock and / or the rock with which it first comes into contact.

Thus, much of the acid is used up near the wellbore and is not available for etching of the fracture faces farther from the wellbore.

These highly conductive microchannels are called “wormholes” and are very deleterious because subsequently-injected fracturing fluid tends to leak off into the wormholes rather than lengthening the desired fracture.

In this case, the live acid penetration is commonly limited to within a few centimeters of the wellbore.

However, just as wormholing prevents the growth of large fractures, wormholing prevents the treatment of long horizontal or vertical regions of a formation.

However, hydrazines and their salts are suspected carcinogens, and hydroxylamine salts are very reactive and can be used only at low temperatures.

Furthermore, after these reducing agents have broken the crosslinked polymer gel, there is some residue left behind that can reduce the fluid conductivity of treated portions of the formation.