Foamed acid fracturing

Foamed Acid Fracturing. The goal of add fracturing is to create a fracture within the reservoir and to etch the sides of the fradure face. Etching of the fracture face with add dissolves a portion of the fradure face and allows a conductive path to be created after the fradure has closed. The further the live add can reach into the fracture, the longer the conductive channel will increase well productivity. 

Cocamidopropyl betaine and cocamidopropyl hydroxysultaine, discussed later, are also used in petroleum production. Their relatively high foaming nature, electrolyte tolerance and hydrolytic stability make them useful for foam acidizing and foam fracturing fluids. 

Desirable foams Foam drilling fluid Foam fracturing fluid Foam acidizing fluid Blocking and diverting foams Gas-mobility control foams... 

Foamed acid fluids were developed to control accelerated leak-off without the addition of fluid loss additives. Foams are clean fluids and therefore less damaging. Because foams are not wall-building fluids, leak-off control is not affected by fracture face erosion because of the presence of acid. 

Foam fracturing and stimulation fluids Foam acidizing fluid Gas well unloading foam... 

The employment of foamed acid and acid-external emulsions (oil as the dispersed phase and gelled acid as the continuous phase) are other methods used to control the loss of acid solutions. The disadvantage of using oils is that a large concentration of oil is required to increase the viscosity of the emulsion formulation, which reduces the acid concentration and, therefore, the amount of acid available for fracture etching. Foaming the acid also reduces the amount of acid available for etching since less acid is present per unit volume injected. 

Gas/Liquid Systems Producing oilwell and well-head foams Oil flotation process froth Distillation and fractionation tower foams Fuel oil and jet fuel tank (truck) foams Foam drilling fluid Foam fracturing fluid Foam acidizing fluid Blocking and diverting foams Gas-mobility control foams... 

Viscous acid fracturing uses viscous acid systems such as gelled, emulsified, and foamed acid, or chemically retarded adds, to both create the fracture and differentially etch the fracture face. Treatments with viscous acid are applicable in heterogeneous carbonates such as dolomites or impure limestones. 

Foamed acid can be useful in increasing effective fracture length, as well as in improving contact in longer treatment intervals. Foamed acid is essentially a gas-in-add emulsion stabilized with a foaming agent. The amount of gas in the foam on a volume basis is called the quality for example, a foam composed of 70% gas and 30% liquid is a 70-quality foam. The gas phase is usually nitrogen, but CO can also be used. Most foamed acids are 60-75 quality. 

Foamed acids can be effective in improving contact with longer treatment intervals. As in fracture acidizing, most foams are 60-75 quality. The lightness of foam makes it an effective stimulation fluid for damaged gas wells. As with emulsions, the pumping of foam at high rates is not always possible. 

Emulsions. Emulsion fluids and foams came into routine use in competition with crosslinked fluids during 1970-80. Simple, barely stable emulsions had been used early in fracturing. These were mainly emulsified acids that "broke" when the acid spent on the formation surfaces. In the late 1960 s Kiel became a proponent of very high viscosity oil fluids as a method to place exceptional (at the time) amounts of proppant(337,338). To avoid the frictional resistance typical of gelled oils he advanced the concept of preparing a very viscous oil-external emulsion with one part fresh water, 0.1% sodium tallate surfactant, and two parts oil. The viscous emulsion had to be pumped simultaneously with a water stream to minimize frictional pressure. This process was clumsy and still... 

A foam, aqueous or non-aqueous, that is injected into a petroleum reservoir to improve the productivity of oil- or gas-producing wells. Some mechanisms of action for foam stimulation fluids include fracturing, acidizing to increase permeability, and diversion of flow. 

Hydroxysultaines find use in personal care products, where they function as secondary surfactants to enhance the properties of anionic-based formulations, in much the same way as betaines. They are also among the best lime soap dispersants known, so they are used effectively in natural soap based products where they make the use of hard water practical. They are also used in petroleum production chemicals were they serve as foaming agents for acid and foam fracturing procedures. 

Hydraulic fracturing and acidizing are the most common of the four stimulation methods. Foamed fluids are used in both hydraulic fracturing and acidizing, and thus will be the focus of this chapter. 

Successful fradure acidizing treatments do not depend solely on good fluid-loss properties (30). Adequate fracture flow capadty must be established by the add system used. The quantity of rock removed and the pattern in which it is removed from the fracture face are important. Fracture flow capadty is dependent on the nature of the rock and the characteristics of the add, such as add type, volume, and concentration. Other factors that lead to increased fracture flow conductivity include foam quality and pumping rate. As long as the foam is stable, foamed add of any quality increases the fracture flow conductivity when compared with non-foamed adds. As well, increases in total treatment pumping rates achieve better fracture conductivities because of greater acidized fradure lengths. 

Foamed Matrix Acidizing. Matrix addizing is a stimulation treatment used to remove damage near the wellbore without deating a fracture. The process involves the injection of a reactive fluid into the porous medium at a pressure below the fracturing pressure. The fluid dissolves some of the porous medium and consequently increases its permeability. 

Some fracturing fluids may include nitrogen and carbon dioxide to help foaming. Oil-based fluids find use in hydrocarbon-bearing formations susceptible to water damage, but they are expensive and difficult to use. Acid-based fluids use hydrochloric acid to dissolve the mineral matrix of carbonate formations (limestone and dolomite) and thus improve porosity the reaction produces inert calcium chloride salt and carbon dioxide gas. 

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