The Unexpected Rheological Behavior of Borate-Crosslinked Gels
- Kevin W. England (Schlumberger) | Mike D. Parris
- Document ID
- Society of Petroleum Engineers
- SPE Hydraulic Fracturing Technology Conference, 24-26 January, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 2 Well Completion, 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2.1 Phase Behavior and PVT Measurements, 4.3.1 Hydrates, 2.1.7 Deepwater Completions Design, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.2.2 Perforating
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Fracturing fluid testing methodology and equipment has evolved, as have industry-accepted standards for testing. These standards have undergone a number of revisions over the years and were published first as a recommended practice under the American Petroleum Institute (API RP-39) and subsequently under the International Standards Organization as ISO-13503-1.
These standards define the rotor/bob geometry (for a concentric-cylinder viscometer), shear-rate, temperature control and other parameters selected to determine the fluid performance under in situ conditions. Little focus was given to the pressure applied to the fluid, other than to ensure that it would not boil under test conditions. This lack of emphasis on pressure is completely understandable as condensed matter (solids and liquids) typically have a limited response to the pressures experienced during hydraulic fracturing operations.
It has been discovered that pumping pressures encountered during hydraulic fracturing treatments can materially reduce the viscosity of a borate-crosslinked fluid. The loss in viscosity, depending on fluid formulation, temperature, and pressure can range from negligible to complete resulting in a fluid with the viscosity of an uncrosslinked polymer solution (linear gel).
This phenomenon has been studied using high-pressure rheometry and high-pressure nuclear magnetic resonance (NMR). This paper presents the results of recent testing of a variety of aqueous-based fracturing fluid systems under a range of pressures typical of hydraulic fracturing treatments. Detailed results for various borate-crosslinked fluids commonly commercially available will be presented as well as non-typical systems.
Borate-crosslinked polymeric fluids have been used for hydraulic fracturing treatments for more than forty years. Boratecrosslinked fluids are fairly simplistic systems to handle operationally and are relatively economical compared to some alternative fluid systems. These fluids can be formulated to crosslink and achieve an increase in viscosity instantaneously or the crosslinking can be delayed as required for various circumstances and conditions. Another advantageous behavior of borate-crosslinked systems is the ability for crosslink rehealing after being exposed to regions of high shear-rate. This is due to the type of crosslink bonding to the polymer chains for these fluid systems. A properly formulated fluid will quickly regain its viscosity upon the reduction in shear-rate, for example when exiting the tubulars and entering the hydraulic fracture. However it has been pointed out recently1 that for some formulations of borate-crosslinked fluids, there is an element of time to the rehealing behavior which can have an impact on overall treatment success. As a result of these properties and the continued improvement of borate-crosslinked fluid formulations they are by far the most widely used aqueous-based crosslinked fluid system in use today.
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