Fluids used in stimulation are designed to open fractures and transport proppant along the length of the fracture. As a consequence, the rheological behavior of that fluid is very carefully designed. Throughout the past years many systems have been developed to achieve a subtle balance of properties in which the fluid initially develops a specified viscosity in the fracture, followed by rapid fluid rheology breaking and fracture cleanup at the end of the operation. In addition, a good fluid loss control is of key importance for treatment efficiency as well as for the prevention of formation damage. Current grades of derivatized guars show improved cleanup versus native guars but are still not fully satisfactory. In addition, polymer free systems based on viscoelastic surfactants (VES) have raised considerable interest due to their ease of cleanup leaving no residues in the fractures. Nevertheless, these systems still need high active concentrations to develop rheology at elevated temperature and in hard water conditions.
Alternative systems based on synthetic polymers have shown increasing interest over the past years for Oil and Gas applications but their utilization as rheology modifiers in stimulation fluids are still limited. Most of their application is in polymer flooding (EOR) or friction reduction.
Associative polymers based on polyacrylamide derivatives can demonstrate dramatically enhanced rheological performance over standard hydrophilic polymers. Hydrophobic associations behave like crosslinking points providing improved Proppant suspension. These crosslinking points are physical associations and can easily be disrupted just like VES with dilution or in the presence of hydrocarbons or surfactants and leave little to no residues in a Proppant pack.
In order to meet challenging Oil and Gas rheological performance targets focusing on high temperature efficiency, associative polymers are here prepared using the Micellar polymerization process. This process consists of the aqueous polymerization of hydrophilic monomers in presence of micelles containing hydrophobic monomers and yields hydrophilic polymers bearing a small amount of hydrophobic groups with multiple hydrophobic monomers per group. The amounts of groups as well as the number of hydrophobes per group are can be tuned to meet rheology targets. In addition, coupling this process with a controlled radical polymerization technique affords associative polymers of significantly higher performance through the reduction of chain to chain compositional heterogeneities which are known to be a significant limitation of the Micellar polymerization process.
The present study demonstrates that primary rheological performance can be met up to very high temperatures using this unique process. In addition, fluid-loss control data as well as a preliminary assessment of cleanup are presented and discussed versus classical gelling systems.