Field Application of Emulsified Acid-Based System to Stimulate Deep, Sour Gas Reservoirs in Saudi Arabia
Authors: Nasr-El-Din H. A. , Solares J. R. , Al-Mutairi S. H. , Mahoney M. D. 
Published: December 2007

Description of Paper
This paper describes optimization of an emulsified acid system to stimulate deep, sour gas reservoirs. High temperatures encountered in deep wells have a tendency to destabilize emulsified acid and, as a result, this acid may loose its retardation effect. In addition, some of the commonly used acid additives were found to adversely affect the stability of the emulsified acid. Therefore, extensive experimental studies were performed to evaluate the influence of temperature on emulsion stability and retardation effect. In addition, the effects of various acid additives on emulsion stability were examined in detail.

Results, Observations, Conclusions
An acid-in-diesel emulsified acid was evaluated to stimulate deep, sour reservoirs in Saudi Arabia. The acid (28 wt% HCl) to diesel volume ratio is 70 to 30. Experimental results indicate that the emulsified acid is stable for more than two days at ambient conditions and more than four hours at 250°F. The retardation factor of the emulsified acid was found to be greater than ten times that of the conventional acid systems. Coreflood tests using tight carbonate plugs (dolomite cores) indicated that the emulsified acid could be injected into tight cores (permeability less than 10 md) without encountering any injectivity problems. The acid created deep wormholes, which significantly increased the permeability of the treated cores.

Application
A non-associated gas is being produced from deep carbonate reservoirs in Saudi Arabia. The lithology of the formation is mainly dolomite with some calcite and streaks of anhydrite. The non-associated gas is sour with hydrogen sulfide content that varies from 0 to 10 mol%. The average reservoir temperature is 275 ☐F. Based on extensive lab work, emulsified acid was used to stimulate eleven vertical, cased wells in these reservoirs. The acid treatments were comprised from three distinct acid formulations. Each acid formulation was specifically designed to provide incremental conductivity over a given fracture half length. The pumping sequence included a stable emulsified acid (extended region, 150–250 ft), a leak-off control acid or in-situ gelled acid (intermediate region, 50–150 ft) and a conventional acid system to ensure that the conductivity for the near wellbore region was maximized. Viscous pads were used alternately between stages to aid in cooling the formation and enhancing fracture propagation. Initial results following these treatments indicated substantial increases in gas production and flowing wellhead pressures. Gas production of some wells has increased by a factor greater than 25.

Technical Contributions
1. Emulsified acid can be effectively used to stimulate deep sour, gas wells, 2. The stability criteria developed in this study enhances the performance of emulsified acids, especially for high temperature wells, 3. Emulsified acids require minimum amounts of additives and maintain the integrity of well tubulars, and 4. Field results indicated conclusively that emulsified acid is a cost effective option to stimulate deep wells. They also suggest that this acid can be used to stimulate wells completed with special types of tubing.