Viscoelastic Surfactant Fracturing Fluids: Applications in Low Permeability Reservoirs

Abstract

Abstract The introduction of viscoelastic surfactant (VES) base fracturing fluids has changed the way industry views fracturing fluids and proppant transport during a fracture treatment.1 Elimination of polymers allows one to achieve highly conductive proppant packs with no polymer damage. Retained permeability and leakoff control are two of the most important requirements for fracturingfluids. Traditional and new generations of cross-linked gels provide good leakoff control, but they often adversely affect the retained permeability of the proppant pack. In addition, minimizing frac-height growth and increasing effective fracture length are a few other advantages of using VES fluids.2 In the majority of cases in low permeability formations, a long and conductive fracture is the ultimate aim of hydraulic fracturing. Borate or metal crosslinked guar fluids, because of their inherent high viscosity, typically result in height growth rather than increased fracture length. With VES fluids proppant transport is based on the elasticity and structure rather than the viscosity of fluid. Therefore VES fluids efficiently transport proppants at lower viscosities. At the same time, VES fluids let one achieve abetter fracture geometry, that with minimum fracture height and maximum fracture length. Pressure transient analysis and tracer studies have shown that this non-damaging low viscosity fluid can give longer effective frac-length even when using much less fluid and proppant volumes (Figure 1). Reduced friction pressure is another added advantage while using VES fluids. Hence VES fluid is the fluid of choice when fracturing is performed through coiledtubing.3,4 Simplicity and reliability of this two component system are the other features of this fluid that attract the industry globally.5 The use of VES technology is now extended to other oilfield applications, such as selective matrix diversion,6 filtercake removal,7and coiled tubing clean out. VES technology is also defining new engineering practices in hydraulic fracturing that cannot be accomplished with conventional fluid systems, such as fracturing through coiled tubing. Introduction: Hydraulic fracturing has long been considered an effective stimulation treatment for low permeability formations. In these treatments, the goal is to create a long, thin fracture that provides a large surface area. Fracture half-lengths can be of the order of 100 to 1000 ft and have widths in the order of tenth of an inch. The perception that a successful hydraulic fracturing treatment is the one that was pumped without problems, has changed in the industry. The true measure of a successful fracturing treatment is increased production or injectivity. The key objective is to improve fluid communication between reservoir and the wellbore. Polymer residues that stay in the fracture contribute significantly towards lower proppant pack permeability, thus leading to a loss in treatment effectiveness. Laboratory experiments have shown that unbroken residues from polymer-base fluids can indeed plug the pores of the proppant pack. Analysis of the flowback fluid obtained from wells treated with conventional and low guarfluids indicates that even in low permeability reservoirs, only 35 to 45% of the polymer that is pumped during the treatment flows back during the flowback period.8 The remaining polymer stays in the fracture and will adversely affect the well productivity. An ideal fracturing fluid should show minimal pressure drop in the pipeduring placement, should have adequate proppant carrying ability and should inactivate the transport mechanism after the fracture closes. This will allow the fluid to break and flowback without leaving any residue that minimizes conductivity.