Development and Field Application of a New, Highly Stable Emulsified Acid

Abstract

Abstract Acid-in-diesel emulsified acid has been used in the oilfield for many years. It is known that the emulsified acid should be stable at ambient conditions for a long period of time. It should be stable also at downhole conditions for a period of time long enough to pump the acid without encountering operational problems. Application of emulsified acid at higher temperatures requires 20 to 30 gpt of a cationic emulsifier. A thorough laboratory study was conducted to select an emulsifier that can be used at lower loadings, and meet the stability criteria that are needed to stimulate oil and gas wells. Laboratory work included measuring stability, apparent viscosity, and droplet size distribution as a function of emulsifier type and concentration, temperature, and additive type and concentration. A new emulsified acid was developed and used to acid fracture over ten wells in a deep gas reservoir in Saudi Arabia. The formation is predominantly limestone and dolomite with streaks of anhydrite. Acid fracturing practices consist of pumping 28 wt% emulsified acid (acid to diesel volume ratio = 70:30), a pad and in-situ gelled acid to create long conductive fractures. The new emulsified acid used a significantly lower amount (4 to 6 gpt) of the emulsifier, albeit it produced a stable emulsion over a wide range of temperatures (from 75 to 275?F). The droplet size of emulsions produced from the new emulsifier was much smaller and, as a result, the apparent viscosity of the acid-in-diesel emulsion was higher. Field data showed greater reduction in the time needed to prepare the new emulsion in the field. The performance of wells stimulated with the new emulsifier was significantly better than those stimulated with the old emulsifier. Introduction Hydrochloric acid is commonly used in matrix and acid fracturing treatments in carbonate reservoirs. However, application of HCl acids in deep wells is a concern because this acid is very corrosive and its reaction rate with carbonate minerals is fast at high temperatures. One way to address these issues is to emulsify the acid in a hydrocarbon phase, e.g., diesel (Crowe and Miller, 1974; Bergstrom and Miller, 1975). Diesel acts as a diffusion barrier between the acid and the rock (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 enhances well performance (Williams and Nierode, 1972; Guidry et al., 1989; Navarrete et al. 1998a,b). Depending on the droplet size of the dispersed phase, the emulsified acid can be categorized as micro (Hoefner and Fogler, 1985) or macro-emulsion (Al-Anazi et al., 1998). The latter has larger droplet sizes and uses smaller amounts of emulsifier. This study however will focus on macro-emulsions that were extensively used in the field (Al-Anazi et al., 1998; Mohamed et al., 1999; Nasr-El-Din et al., 2000; Kasza et al., 2006). 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). Also, 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 (Al-Anazi et al., 1998). The presence of iron in the acid is a major concern because it will precipitate once the pH reaches around 2 (Taylor et al., 1999). Iron(III) can adversely interact with corrosion inhibitors, and cationic surfactants at high acid concentrations (Al-Nakhli et al., 2008). It also enhances corrosion of well tubulars and aggravates the formation of acid-oil sludges (Nasr-El-Din, 2000).