Recent Advances in the Fluid Mechanics and Rheology of Fracturing Fluids

Abstract

Abstract Numerous advances have been made in rheology of fracturing fluids over the past decade. This paper describes the state-of-the-art fracturing fluid technology. In particular it gives a comprehensive review of the current literature on fracturing fluid rheology. It describes various fluids available today for the fracturing process and provides a review of the presently available methods and procedures to characterize the simple as well as more complex fluids. Further, our intention is to review and describe current laboratory methods for prediction of friction pressure under simulated field conditions within the wellbore. This includes uncrosslinked and crosslinked fluids with and without proppant. proppant. This paper also describes the use of pipe and rotational viscometers to simulate laminar flow, of these non-Newtonian and time dependent fluids within the fracture. Application of data collected during these various laboratory simulations to predict wellbore hydraulics and fracturing fluid rheology is predict wellbore hydraulics and fracturing fluid rheology is described. Finally, the paper summarizes findings, addresses future needs, and presents recommendations. Introduction The hydraulic fracturing process has been used commercially as a means of stimulating oil and gas production for over 40 years, in the application of this process, a viscous non-Newtonian fluid containing a high proppant concentration is injected down the wellbore at high rate and pressure to create and extend a fracture system in the formation of interest. Fracturing fluids are generally in turbulent flow in the wellbore and perforations and in laminar flow in the fracture system. Characterization of the hydraulic and rheological properties of these fluids in both laminar and turbulent flow regimes is important for the successful application of this process. Predicting turbulent flow friction loss in the wellbore and perforations is important when designing and executing a fracture treatment. In the design phase, friction loss estimates are used to predict the expected surface treating pressure and the hydraulic horsepower necessary to achieve the desired injection rate. During the execution of a treatment, friction, loss estimates are used to provide an estimate of bottomhole treating pressure for use in various real-time pressure analysis procedures. This latter use is particularly helpful in the absence procedures. This latter use is particularly helpful in the absence of downhole gauges or "live" static annular or tubular fluid columns. Predicting the laminar flow behavior of these fluids in the fracture is critical to the design of fracture geometry and the prediction of proppant transport. When the fluid rheology is prediction of proppant transport. When the fluid rheology is coupled with an appropriate fracture design model, it is possible to predict fracture width, length, and height as a function of time and/or volume pumped. When the fluid rheology is coupled with an appropriate proppant transport equation, it is also possible to predict the location of proppant within the fracture as a function predict the location of proppant within the fracture as a function of time and at the end of pumping. To some extent, fluid rheology also has an influence on fluid loss and fracture conductivity. Many of the non-Newtonian fluids being used today are considerably more complex than the oil and water soluble polymer and micellar solutions initially used for hydraulic polymer and micellar solutions initially used for hydraulic fracturing fluids. Although many of these latter fluids are still being used today, they are being replaced by more rheologically complex foams and metal-ion crosslinked polymer fluids. Crosslinked fluids are formulated continuously by adding a crosslinking chemical to the polymer solution just before its injection downhole. Because crosslinking then occurs in the wellbore and/or fracture, the resultant fluid rheology is a complex function of temperature-shear history, fluid pH, crosslinker concentration, and mixing efficiency. Despite the fact that crosslinked fluids are difficult to characterize rheologically, the industry has found that they offer advantages in terms of improved proppant transport, viscosity, and stability at elevated temperatures. P. 621