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Foam fracturing with a base fluid of delayed crosslinked gel has expanded the use of foams in fracture stimulation and greatly improved job success rates. Foamed delayed crosslinked gels allow the placement of much higher proppant concentrations in the fracture than foamed linear gels. Also, the fracture fluid efficiency is greatly enhanced all owing better control of downhole fracturing parameters (e. g. leakoff) critical for successful stimulation.
Laboratory results indicate that the foaming of a delayed crosslinking gel increases apparent viscosity, improves proppant transport capabilities, and extends fluid loss properties beyond those of foamed linear gels.
More than 200 treatments have been completed in various formations in California, Texas, and Oklahoma. A review of local formation characteristics and treatment conditions are discussed regarding the application and successful use of a crosslinked versus conventional linear gel foam.
Noteworthy is that crosslinking the gelled phase of a foam system requires limited modification of current foam fracturing procedures or equipment.
Virtually every aspect of foams fracturing has been addressed in numerous technical papers and articles through the years. Topics have ranged from the foaming and application of various liquids (water, acid, hydrocarbon, and alcohol) to the theoretical and practical analysis of their rheology and leakoff. This paper focuses, on the properties of a foamed delayed crosslinked gel, which are critical properties of a foamed delayed crosslinked gel, which are critical in fracture. stimulation design, and the application of these improved properties in the field. properties in the field. Attempts to utilize crosslinked gels in foam systems were made soon after their introduction to the industry with little success. This indicated that Surface crosslinked borate and titanate gels were either too viscous and unyielding for uniform foam generation, or more susceptible to shear conditions than subsequent delayed crosslinked products. Early exposure of a crosslinked gel to a shear environment (pump, surface lines, pipe, perforations) has been shown to reduce the apparent viscosity ultimately perforations) has been shown to reduce the apparent viscosity ultimately achieved in the fracture. Thus the advent of delayed crosslinked systems, whether the or temperature activated, has permitted foam generation of linear gels at the surface while reaping the benefits of a foamed fully crosslinked gel in the fracture.
Historically, foam fracture treatments have been limited to maximum downhole proppant concentrations of 4 to 5 pounds per gallon (ppg). This has been due to pumping equipment limitations, with respect to proppant handling, and the inherent downhole properties of linear gels, foamed or non-foamed.
When employing a foamed linear gel, the gas (nitrogen or carbon dioxide) to liquid ratio must typically be equal to or greater than 60%. This is necessary to ensure an apparent viscosity greater than the linear gel alone. This can result in high proppant to liquid ratios of as much as 20 ppg at the surface in order to achieve proppant to foam ratios of 5 ppg downhole. proppant to foam ratios of 5 ppg downhole. P. 235
