A Comparative Study of Borate-Crosslinked Gel Rheology Using Laboratory and Field Scale Fracturing Simulations
Authors: Goel Naval , Willingham Joe D. , Shah Subhash N. , Lord David L. 
Published: December 2007

Abstract
Accurate rheological characterization of hydraulic fracturing fluids in a laboratory is extremely important prior to their use in the field. A borate-crosslinked Guar gel rheology study was performed to compare and evaluate laboratory measurements with a field-scale characterization of the gel. Field-scale fracturing operation was simulated by slot measurements at the Fracturing Fluid Characterization Facility (FFCF) of the University of Oklahoma and laboratory-scale simulations were obtained from viscometer measurements at three service company test facilities. These companies volunteered to participate in a joint effort to understand borate-crosslinked gel behavior. Both the FFCF and laboratories used identical chemicals, water, and fluid formulation procedure for the present study. The results show that slot yields reproducible borate-crosslinked Guar gel rheology data under various conditions. The comparison of results show that the slot and laboratory measurements yield different viscosities. Moreover, the laboratory viscosities show disagreement among themselves. The results suggest that the laboratory measurements must consider shear preconditioning in their rheological characterization. Furthermore, a standardized laboratory borate-crosslinked gel preparation and evaluation procedure can provide reproducible data from laboratory measurements.

Introduction
The hydraulic fracturing technique is widely used in the Petroleum Industry to stimulate production from a reservoir. This technique enhances the production of oil and gas and improves the economics of the formation. The reservoir is hydraulically fractured with a specially formulated fluid system. Several case histories have shown that an effective fluid system improves fracture treatment results. The ideal characteristics of a fracturing fluid are detailed in Recommended Practices prepared by the API subcommittee on Fracturing Fluid Rheology. The most important property of the fluid is its viscosity. The fluid viscosity must be sufficient to produce a wide fracture, eliminate premature proppant screenout and carry proppant deep into the formation. About 75% of fracture treatments performed today are with borate-crosslinked fluids. Borate-crosslinked fluids exhibit a non-Newtonian rheological behavior. Furthermore, their viscous properties are influenced by steady shear during flow in surface equipment, wellbore and fracture. These characteristics of borate-crosslinked fluids necessitate a study of their rheology in the laboratory. The laboratory testing of these fluids is extremely important prior to their use in an actual fracturing treatment. API recommends a standard testing procedure for measuring the viscous properties of crosslinked water-based fracturing fluids. Some researchers have suggested dynamic oscillatory measurements as a useful tool to measure the rheological properties of crosslinked fluids. The main objective of these measurements is to provide reproducible rheological data for borate-crosslinked fluids. However, the crosslinked fluid may exhibit ideal fluid properties under laboratory conditions, under actual field conditions its behavior may be completely different. Hence the borate-crosslinked fluid rheology must be determined under conditions that closely resemble actual field and downhole conditions.
The FFCF was established to study fluid behavior under representative surface and downhole conditions. Fluids are prepared and handled at the facility using field-scale mixing and pumping equipment. The fluids are pumped through coiled tubing lengths of up to 5000 ft, and through a formation simulator/heat exchanger to provide shear and thermal preconditioning that are representative of field conditions. P. 503^