This paper describes the chemical theory behind the unexpected features of an environmentally acceptable temporary fluid-loss control pill. This liquid gel concentrate pill meets EPA and OSPARCOM standards for use in on- and offshore operations, and has been used for a variety of applications throughout the world.
Economic reasons and environmental regulations compelled one oil-service company to find a replacement for an existing field product. Initially, research was aimed toward the development of a pill that had the following characteristics.
It blocks fluid flow into and out of the wellbore.
It is mixable in brines 8.35 to 19 lb/gal.
It can be used effectively at temperatures as high as 257°F/125°C.
Researchers concentrated their efforts on optimizing the synthesis, manufacture, and field-mixing procedures pertaining to a replacement crosslinkable hydroxyethyl cellulose fluid (RXHEC). RXHEC degrades by uncrosslinking and unzipping the backbone, which simplifies the disposal of returns. It is capable of breaking with weak acids, which allows the use of external breakers in acid-sensitive wells. The first field application took place in May 1993. Since then, some evidence has revealed additional features that were not conceived in the initial design.
Dispersion occurs with no "fish-eyes."
Crosslinking occurs with most metals and non-toxic metals.
The fluid can be pumped fully crosslinked.
Reversible shear degradation of crosslinked RXHEC fluids allows good fluid-loss control.
Tubulars slip through the RXHEC pill with little difficulty, making a remedial pill unnecessary.
The RXHEC pill remains in place during tripping and retrieving operations.
It is indefinitely stable to 275°F/135°C.
Laboratory data and field data are presented. Chemical explanations for the unexpected features are discussed in detail.