Emulsified acid (30 vol% diesel and 70 vol% HCl acid) has been used in both matrix and acid fracturing treatments. Injection the acid in this form has several advantages including: retard the reaction of the acid with rock, reduce corrosion to well tubulars and minimize acid additives. However, using this acid to treat wells with asphaltene deposition will require removing asphaltene first using a suitable aromatic-based solvent, and then using a matrix acid treatment. This additional step will increase the cost and time needed to execute acid treatments.
To remove asphaltene deposition and enhance well productivity, hydrochloric acid was emulsified in xylene. Xylene was the external phase, and was used to dissolve asphaltenes. Then the acid, present as the dispersed phase dissolved the carbonate rock, thus enhancing well productivity. Extensive lab work was performed to ensure the stability of acid-in-xylene emulsion and measure its apparent viscosity. Acid concentration was 15 wt% HCl, the acid volume fraction was 0.7 and the balance was xylene. All tests were conducted at room temperature and 160oF.
The stability and apparent viscosity of emulsified acids were found to be a function of the type of hydrocarbon phase used to prepare emulsified acid. Emulsified acids prepared with xylene had a lower apparent viscosity and were stable for relatively shorter period of times. A matrix acid treatment was that based on xylene emulsified acid was applied in four wells without encountering operational problems. Unlike previous matrix acid treatments using regular acid, the four wells responded to the new treatment, without increasing water-cut, except in one well which was wet before the treatment.
Diesel is commonly used to prepare emulsified acids. It acts as a diffusion barrier between the acid and the rock (Crowe and Miller, 1974; Bergstrom and Miller, 1975, Hoefner and Fogler, 1985; Daccord et al., 1987; Peters and Saxon, 1989). Thus, the reaction rate of the acid with carbonate rocks becomes slower. This gives the acid the ability to penetrate deeper into the formation by creating wormholes (i.e., channels with high permeability), which enhance well performance (Williams and Nierode, 1972; Guidry et al., 1989; Navarrete et al. 1998a,b)
Acid-in-diesel emulsion has several advantages besides its slow reaction rate with the rock. It has a relatively high viscosity, which results in a better sweep efficiency that will improve acid distribution in heterogeneous reservoirs (Buijse and van Domelen, 1998). The live acid does not come in contact with well tubulars. Therefore, there is minimum corrosion to well tubulars. As a result, the concentration of iron in the live acid reaching the formation will be low, which will reduce the amount of iron control agents (Al-Anazi et al., 1998).
Several studies (de Rozieres et al., 1994; Conway et al., 1999; Kasza et al., 2006; Al-Mutairi et al., 2008a) examined the reaction of emulsified acid with carbonate rocks. The rotating disk apparatus was extensively used and the results of these studies indicated that the reaction of emulsified acid with carbonate rocks is slower than that of regular acids. Al-Mutairi et al. (2008a) indicated that the size of the acid droplets affected the reaction rate. And this should be considered as an important parameter in designing acid treatments. In a second study, Al-Mutairi et al. (2008b) examined the impact of the droplet size on the rheological properties of acid-in-diesel emulsions.
Propagation of emulsified acids in carbonate cores was examined by several research groups (Bazin and Abdulahad, 1999; Lynn and Nasr-El-Din, 2001; and Siddiqui et al., 2006). These studies indicated that the acid propagated through the core plugs in almost straight lines. The acid enhanced core permeability by a factor that depended on the acid injection rate.