Recent Advances in Viscoelastic Surfactants for Improved Production From Hydrocarbon Reservoirs

Abstract

Summary Viscoelastic surfactants (VES) are used in upstream oil and gas applications, particularly hydraulic fracturing and matrix acidizing. A description of surfactant types is introduced along with a description of how they assemble into micelles, what sizes and shapes of micelles can be formed under different conditions, and finally how specific structures can lead to bulk viscoelastic-solution properties. This theoretical discussion leads into a description of the specific VES systems that have been used over the last 20 years in improved oil recovery for upstream applications. VES-based fluids have been used most extensively for hydraulic fracturing. They are preferred over conventional polymer-based fracturing-fluid systems because they are essentially solids-free systems that have demonstrated less damage to the reservoir-rock formation. In fact, approximately 10% of the fracturing treatments use VES-based fluids. Important advancements in VESs have been made by introducing “pseudocrosslinking agents,” such as nanoparticles, to enhance the viscosity. Fracturing-fluid systems modeled after VES have also been improved recently by developing internal breakers to lower their viscosity to flow back the well. The flexibility of VES-based fluids has been demonstrated by their application as foamed fluids, as well as their incorporation with brine systems such as produced water. A second key area that has benefited from VES-based systems is matrix acidizing of carbonate-based reservoirs. The viscosity of these VES-based fluids is mostly controlled by pH; at low pH (low viscosity), the acid system flows easily and invades pore spaces in the formation. During acidizing, the acid is spent, and the pH and viscosity increase. Because the spent acid has higher viscosity, fresh acid is diverted to low-permeability, uncontacted zones and penetrates the rocks to form wormholes. A number of experimental studies and field applications to these effects have been performed and will be described in this study. In order for VES-based fluids to play a more-prominent role in the field, inherent limitations such as cost, applicable temperature range, and leakoff characteristics will need to continue to be addressed. If we can efficiently and economically overcome these issues, VES-based fluids offer the industry an excellent clean and nondamaging alternative to conventional polymer-based fluids.