Viscous acid fracturing

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. 

Viscous acid fracturing has become the more common acid fracturing treatment. The basic viscous acid fracturing treatment design includes the following ... 

The basic method of viscous acid fracturing is sufficient for most applications. However, alternating-stage and alternating-acid techniques have been used very successfully. For the stimulation of a new well or a well in a field with no previous acidizing history, though, it is best to keep the treatment design as simple as possible. 

CFA has been used very successfully following viscous fingering treatments and conventional viscous acid fracturing treatments. Regular 15% HCl and HCl/organic add hlends are most commonly used as the CFA treatment fluid. 

Acid fracturing is a treatment in which the fracturing and etching fluids are acid. The fracture is created with a viscous acid system, for example, which also etches the walls of the fracture dining the injection process. These days, either term, fracture acidizing or acid fracturing, may be used to describe the process of creating an acid-etched fracture in a carbonate formation. 

Currently, the commercial systems available that will result in the deepest penetration of acid are emulsified acid and, perhaps, surfactant-gelled acid. Viscous acids may contain a fluid-loss additive, such as an oil-soluble resin or polymer, to reduce leak-off. Particulate diverters are not effective in fracture acidizing, but in matrix treatments, they can make a difference. 

Viscous acid stage. The purpose of the acid stage is to simultaneously propagate the fracture and differentially etch its walls. The acid stage is typically gelled, emulsified, or foamed add. Combinations of the three are possible. 

Overflush. The purpose of the overflush is to displace add from the wellbore and push the acid volume forward, thereby increasing the penetration distance. When viscous acid is used, a large overflush can effectively increase the etched fracture length. The overflush is a critical step in the treatment design. A high rate is beneficial. 

C-2. Generic acid fracturing treatment for a tight limestone (viscous fingering procedure closed-fracture acidizing [CFA] method).262... 

EPDM-ZnO-stearic acid systems could not be extruded even at 190°C. This is not unexpected since the material, in the absence of zinc stearate, shows no transition from the rubbery state to the viscous flow state (Fig. 1). In the presence of 10 phr of zinc stearate, the m-EPDM-ZnO-stearic acid system could be extruded but melt fracture occurred at a lower temperature (150°C) at all shear rates. At 160°C and 170°C, however, the extrudates showed melt fracture only at high shear conditions. At 20 phr loading of zinc stearate, melt fracture of the extrudate occurred at high shear conditions at 150°C, but at higher temperatures no melt fracture occurred and the extrusion was smooth under all shear conditions. At 30 and 40 phr loadings of zinc stearate, the extrudates were smooth under all shear conditions at all temperatures. 

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... 

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