Borate crosslinked fracturing fluids have been successfully utilized in fracturing operations for several years. These fluids provide excellent rheological, fluid loss, and fracture conductivity properties over fluid temperatures up to 225 °F [107 °C]. The mechanism of borate crosslinking is an equilibrium process which can produce very high fluid viscosities under conditions of low shear. While this property is very desirable once the fluids enters the fracture, it can cause high frictional pressure to be developed in the tubular under certain flow conditions. A crosslink activator system has been developed which allows the crosslinking rate of these fluids to be reliably and easily controlled. Using this system, crosslinking rates can be controlled to give low friction pressure in the wellbore and yet maximum viscosity in the fracture for improved proppant transport.
Field case histories demonstrate the successful use of the delayed borate crosslinked fluids and include results of an injection procedure used to determine friction pressure at different pump rates. The friction pressure data also profiles the changes that occur as the fluids is crosslinking at different stages in the tubing. Results indicate that different crosslinking rates can be recognized by their "friction pressure crosslinking" and that the crosslinking rate can be easily altered during a treatment to reduce total treating pressure. This technique with its resulting friction pressure profile has proven to be an extremely useful tool for field optimization or "crosslink times". Using this technique, delayed borate crosslinked fluids can be custom designed for each individual well configuration.
Data from laboratory development, full scale friction tests, and fracturing treatments are included which illustrate the properties of the fluid and the ability to couple fluid performance with treatment design.
Field usage of borate crosslinked fluids has been limited in some tubing/pump rate situations due to the large frictional pressure developed in the wellbore prior to entry into the fracture. A reliable method to delay the crosslinking of these fluids would have the advantages of providing:
low friction pressure in the wellbore,
increased fluid efficiency, and
maximum viscosity in the fracture for improved proppant transport capabilities.
A slurred delayed borate crosslink activator system has been developed that will predictably crosslink borate fluids in the 0.5 to 4 min range. The crosslinking rate is varied by changing the concentration of activator in the fluid. This is accomplished in field operations by changing the metering rate of the activator. As the concentration of crosslink activator slurry is increased the crosslinking rate increases.
Crosslinking rate can be custom designed to conform to a specified "crosslink time"or individual well configuration.
Laboratory development of the delayed borate crosslinked fluids focused on developing an activator system which could reliably control the crosslinking rate in tubular and also produce excellent rheological properties under the temperature and shear conditions found in the fracture.
Full scale field tests were performed to determine the friction pressure of the delayed borate crosslinked fluid at different pump rates and to compare changes In friction pressure that occurred in the tubing to "crosslink time"determined by sampling fluid al the missile.
