| Popis: |
Abstract Borate crosslinked guar fracturing fluids have recently been re-engineered to allow use at higher temperatures in both fresh water and seawater. A key part of this work involves control of the crosslink delay time of the fluid. This report presents an overview of the technology available to control delay time with an emphasis on the types of processes used in current commercial systems. Control of delay time requires control of either the pH or the availability of borate ions or both. To date, control of pH can be effective in freshwater systems. Control of borate is effective in both fresh water and seawater. This may be accomplished by using low-solubility borate species or completing the borate with a variety of organic species. Introduction Crosslinked-guar or hydroxypropyl-guar fracturing fluids have a long history of use in the industry. Borate, titanate and zirconate crosslinkers have been the most popular. Initially borates were limited to fluid temperatures less than 200F in fresh water and titanates and zirconates were preferred for higher temperature applications. Used properly, the titanates and zirconates still provide good, reliable performance up to fluid temperatures of approximately 350F in fresh water and seawater. Borate-based fluids differ from the zirconates and titanates because the borate-guar bond is rapidly reversible. Therefore, fluid performance is much less sensitive to the shear history of the fluid and fluid viscosity is more predictable. Disadvantages for the borates included the need for high pH (9-10 is preferred) and high friction pressures because of the lack of adequate delay mechanisms. Both of these disadvantages have been addressed over the past few years. For example, buffer systems have been developed to minimize the highest pH seen by the fluid and resin coated proppants (RCPs) have been modified to be more compatible with borate fluids. The effectiveness, or lack thereof, of the newer RCPs has been discussed. This paper discusses recent approaches to controlling the delay time of borate fluids. In an oversimplification, the crosslinking process can be represented by Eq. 1. Delay of the crosslinking process can be controlled by controlling the pH or the availability of boron species. (1) The process illustrated by Eq. 1 is shifted to the left with increasing temperature. This can be reversed by increasing the concentration of B(OH)4- which is believed to be the active crosslinking species. P. 517^ |
