Acid Fracturing Technique for Carbonate Reservoirs Using Nitric Acid Powder
Authors: Liu X. , Zhao G. , Zhao L. , Liu P. 
Published: March 2010

Abstract
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 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 the 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.
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, leak-off, 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 a high-temperature carbonate reservoir. Therefore, it is an innovative and promising technique for well stimulation in carbonate reservoirs.

Introduction
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 screen-out 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 Tinker(1) 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. After the designed fracture length is created, the injection is continued at certain rates to maintain the equilibrium with the fluid leak-off rate from the created fracture faces. This technique was proven to be effective in stimulating the relatively cool dolomite. Closed acid fracturing techniques(2) were designed to obtain high conductivity, especially for the formations with extra high fracture close stress.