Rheology of Viscous Fracturing Fluids
Authors: Sinclair A. Richard 
Published: December 2007

A rheological study of a new viscous fluid system reveals that fluid mixtures containing highly viscous oils can be pumped at fracturing rates with tubular friction losses less than that obtained with water alone because of annular water ring lubrication and because of the slippage layer in the water-in-oil dispersion.

Introduction

Many different fluids, from water to very viscous oils, have been used to break down or fracture formations. The use of viscous or low-penetrating fluids was discussed in early patents on fracturing, but full appreciation of the advantages of very viscous Newtonian fluids is a recent development. These oils and asphalt residue are called Newtonian because they have a constant viscosity at any reasonable shear rate. By contrast, viscous water-based gels have a variable viscosity with shear rate and are called non-Newtonian fluids. Some of the advantages of viscous Newtonian fluids for fracturing are increased fracture width, good fluid-loss control, and better sand-carrying ability. The increased fracture width obtained with the viscous fluids is due to increased propagation pressure or friction loss in the fracture and by the inherently high viscosity of the fluids in the fracture. This increased fracture width can be ascertained either by allowing large sand (8–12 mesh) to be used routinely at high concentrations, or by using an impression packer to actually measure wellbore fracture widths. packer to actually measure wellbore fracture widths. The predicted width of the fractures varies approximately as the cube root of the viscosity. This is shown in Fig. 1, which indicates that a 50-cp fluid generates a fracture width of 0.2 in., whereas a 1,000-cp fluid generates a fracture width of almost 0.5 in. These results are calculated by using Kristianovich and Zheltov's solution, modified by Kiel, for a partially pressurized crack, and involving a flow rate of 5 bpm, pressurized crack, and involving a flow rate of 5 bpm, fracture length of 180 ft, and time of 30 minutes after fracture initiation. Both increased length and increased width are obtained as a result of the improved fluid-loss control secured with very viscous oils and with viscous oils containing fluid-loss additives. This makes fracturing applicable to many more formations. Reasonable fluid efficiencies can be obtained in formations with very high permeabilities. The settling rate of sand in viscous oils is low. High viscosity facilitates the transportation of 8–12 mesh sand through surface equipment, down the tubing, and into the fracture. Good conductivity contrast between the fracture proppant and the formation can be obtained in very permeable formations, including those with permeabilities greater than 1 darcy. As a result of these advantages, the use of viscous Newtonian fluids can result in productivity improvements considerably greater than can be obtained by conventional fracturing. Some drawbacks to the use of viscous Newtonian fracturing fluids have been the extremely high friction losses, and the high horsepower and high pumping rates required to initiate the fractures. Viscous fluids are difficult to pump if the formation is very deep or if fracturing is carried out through tubing. An example of the way normal fluid friction loss depends upon viscosity is shown in Fig. 2. The friction loss for a 10,000-ft well at 5 bpm can be calculated for fluid of any viscosity by multiplying by 100 the friction loss per 100 ft, shown in Fig. 2.

JPT

P. 711