The length of the etched fracture is rather limited utilizing traditional acid fracturing techniques, especially in a high-temperature carbonate reservoir. Although the propped fractures may have a deeper penetration, they have such drawbacks such as low fracture conductivity, unintended proppant bridging and subsequent proppant flow back.
This paper presents the development of a new acid fracturing technique, Nitric Acid Powder (NAP) acid fracturing, to improve the acid penetration and fracture conductivity. The NAP acid-fracturing technique has been applied in several oil fields in China. It has been shown that NAP acid-fracturing technique has the advantages of both hydraulic fracturing and acid fracturing such as long effective penetration, high fracture conductivity, low cost and easy field operation. The general process of this technique includes the following steps. First, the nitric acid is made into a solid powder. Second, the non-reactive fluid (oil-based fracturing fluid, diesel oil, etc.) is applied to create the fractures and carry the NAP into the fractures. Third, hydrochloric acid is injected at a low rate, and the NAP decomposes into nitric acid to etch the fracture surfaces together with hydrochloric acid.
We have developed a comprehensive mathematical model for the NAP acid-fracturing technique to facilitate the optimization of the field treatment design. The model presented considers fracture growth, acid transport and reaction, leakoff, etched width of the fracture and so on. The study has shown that the NAP acid-fracturing technique could reach a very high stimulation ratio even in the high-temperature carbonate reservoir. Therefore it is an innovative and promising technique for well stimulation in carbonate reservoirs.
Carbonate formations generally have a low permeability and can be highly fissured. Long fractures in acid-fracturing treatments are essential to maximize production. Acid must react with the walls of the fracture to form a channel that remains open after the treatment. Flow channels can be formed as a result of an uneven reaction with the rock surface or preferential reaction with minerals heterogeneously distributed in the formation. If the formation temperature is very high, the reaction rate will be fast. If this occurs, the acid treatment will tend to remain in the near wellbore vicinity, resulting in short penetration.
Acid-fracturing techniques are the primary preference in carbonate formations. Operationally, acid-fracturing is less complicated because no propping agents are used, which eliminates the risk of a screenout and subsequent problems of proppant flowback and cleanout from the wellbore. Generally, acid-etched fractures have high conductivity although they are quite limited in penetration, whereas the propped fractures have limited conductivity with deeper fracture penetration. The techniques to overcome the limitations of conductivity and penetration for the carbonate formation have been studied continuously to enhance the acid fracturing technology
Equilibrium acid fracturing was developed by S. J. Tinker1 to maximize the contact time of acid with the fracture face to get a high fracture conductivity in cool dolomite formations, which react slowly with acid. Maximum acid contact time is essential to create highly conductive etched channels on the fracture faces.
