Lessons Learned From Using Viscoelastic Surfactants in Well Stimulation
- Hisham A. Nasr-El-Din (Saudi Aramco) | Mathew M. Samuel (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- February 2007
- Document Type
- Journal Paper
- 112 - 120
- 2007. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.2.3 Fluid Loss Control, 1.10 Drilling Equipment, 3.3.1 Production Logging, 5.2 Reservoir Fluid Dynamics, 5.8.7 Carbonate Reservoir, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 1.8 Formation Damage, 1.6 Drilling Operations, 3.2.4 Acidising, 3 Production and Well Operations, 4.3.4 Scale
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Viscoelastic surfactant systems are used in the industry for several applications. Initially, the application was focused on low-friction and solids-suspension (fracturing and CT-cleanout) characteristics of the fluid. In the last 4 years, the application of viscoelastic surfactants was extended to acid-based systems for carbonate stimulation. These surfactants have the ability to significantly increase the apparent viscosity and elastic properties of the treating fluids. This is because of the ability of surfactant monomers to associate and form rod-shaped micellar structures under certain conditions.
Viscoelastic surfactant-based acid systems have been used in Saudi Arabian fields in matrix acid stimulation, and in leakoff control acids during acid-fracturing treatments. These surfactants were used to provide diversion during acidizing of vertical, long horizontal, and multilateral wells. They were used in sour environments where hydrogen sulfide levels reached nearly 10 mol%. They were also utilized in gas wells to reduce acid leakoff, and create deep fractures in dolomitic carbonate reservoirs (250 to 275°F). In addition, they were successfully employed to stimulate seawater injectors and disposal wells where the bottomhole temperature was in the range of 100 to 150°F.
More than 250 wells (oil, gas, water injectors, and disposal wells) were treated with viscoelastic surfactant-based acid systems. The acid was placed either by bullheading, by using coiled tubing with or without a tractor. In some cases, these treatments included stages of emulsified or regular acids. All these wells responded positively to the treatment. There were no operational problems encountered during pumping these acids even when low-permeability reservoirs were treated. Because these acid systems do not contain polymers, there was no need to flow back water injectors. The spent acid in oil and gas wells was lifted from the treated wells in a very short period of time. Finally, wells treated with surfactant-based acid systems showed sustained performance for longer times than those treated with other acid systems.
Matrix acidizing and fracturing treatments have been used to enhance the performance of oil, gas, and water wells for several decades. Water-soluble and acid-soluble polymers have been used in these treatments to increase the viscosity of the treatment fluids and hence enhance diversion during matrix acidizing treatments. High-viscosity fluids are needed during acid-fracturing treatments to reduce leakoff rate during acid injection into the fracture.
Various chemicals were developed to enhance acid diversion by increasing the viscosity of the injected acid. Depending on the viscosifiying agent, these systems can be divided into two main categories: polymer-based and surfactant-based.
Acid-soluble polymers have been used to increase the viscosity of HCl, and to improve its performance (Pabley et al. 1982; Crowe et al. 1989). As the viscosity of the acid increases, the rate of acid spending decreases and, as a result, deeper acid penetration can be achieved (Deysarkar et al. 1984).
The addition of uncross-linked polymers to HCl improved acid penetration; however, acid placement did not significantly improve (Yeager and Shuchart 1997). Crosslinked acids were introduced in the mid 70's as was cited by Metcalf et al. (2000). These acids have much higher viscosity than regular acids or acids containing uncrosslinked polymers. Two types of crosslinked acids are available. The first type consists of a polymer, a crosslinker, and other acid additives (Saxon et al. 2000). The acid in this case is crosslinked on the surface and reaches the formation already crosslinked. The second type of crosslinked acid consists of a polymer, a crosslinker, a buffer, a breaker, and other acid additives (e.g., corrosion inhibitors and surfactants). The acid in this case reaches the formation uncrosslinked, and the crosslinking reaction occurs in the formation (Taylor and Nasr-El-Din 2003).
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