This paper will describe the details of experimental conditions and will present gelling systems for potential water shut-off and profile modification applications under hostile environmental conditions.
A range of copolymers of acrylamide with an experimental comonomer has been prepared to evaluate gelation of the polymers and thermal stability of the gels under hostile environmental conditions. The experimental monomer was obtained using commercially available chemicals by a simple synthetic route. Gels of high thermal stability have been revealed with polymer concentrations below 1 weight %. The polymer was dissolved with crosslinker in synthetic North Sea water (SNSW). The crosslinking systems include phenol with formaldehyde (HCHO) or hexamethylenetetramine (HMTA). The results were evaluated after aging for 30 days at 120 C. The thermal stability of the gels depends not only on chemical composition of the polymers and the crosslinker, but also polymer and crosslinker concentrations used for gelation. For one of the polymers, viscometric study of gel formation was carried out in addition to the studies of gel stability.
In this work, statistically designed experiments were used to determine which variables had the most significant effect on thermal stability of the gels so that the optimum gelling composition could be determined. These variables include polymer composition, polymer solution viscosity and concentration as well as the crosslinker concentration. A ranking of the gels was made according to an overall gel quality parameter based on gel strength and degree of syneresis. The results illustrate the advantage of using experimental design during testing and evaluation of synthetic polymers.
The amount of residual oil can be up to 40–70% in oil producing fields. To increase the production from off-shore fields, sea water is injected into the formations. The sea water follows the high permeability zones in the reservoir, and after some time, a water break-through may occur, and the result is a higher water to oil ratio. In such a case, it is desirable to plug high permeability zones in the formation by using water soluble polymers. Generally, a solution of the polymer along with an appropriate crosslinking system, is injected into the formation. With appropriate placement techniques the polymer solution can permeate into and form gel in the regions having the highest permeability. However, for near-wellbore applications on deep off-shore fields, the salinity and the temperature limit the use of many currently available water soluble polymers for enhanced oil recovery applications in these regions.
Polyacrylamides with phenol/HCHO as crosslinking system have been used successfully for plugging or permeability reduction, but the thermal stability of gels from polyacrylamide is very poor above 90 C in sea water due to extensive syneresis.