Optimization of an in-situ polymerized and crosslinked hydrogel formulation for lost circulation control

Abstract

Lost circulation is a common problem during drilling and completion, and it contributes much to the nonproductive time of the operation. The Brazilian pre-salt is composed of highly heterogeneous carbonates and is highly susceptible to circulation loss. In this work, a removable, solid-free polymer gel formulation based on in-situ polymerization and gelation was proposed and optimized using design of experiments. The proposed gel contains acrylic acid (monomer), carboxymethylcellulose (thickener), ammonium persulfate (initiator), chromium (III) salt (crosslinking agent) and 240,000 NaCl brine as a solvent. The concentrations of these components, and the solution pH, were the variables considered in this study. The plateau modulus, maximum elastic modulus, gel stability, and gelation times were obtained and analyzed. The gelation time was obtained from oscillatory rheology time curves fitted by the Hill 5 model and from low-frequency nuclear magnetic resonance. Most of the studied gels presented elastic moduli from 1000 to 7000 Pa, close to and higher than the moduli of other gels for lost circulation. The gelation times ranged from 5 to 30 min, which could be improved by changing the initiator, protecting or inhibiting it. A pH of 9 led to faster gelation times, syneresis, and lower elastic moduli; therefore, pH values from 5 to 7 are ideal. Carboxymethylcellulose and the chromium salt increased slightly the gelation time, the elastic moduli, and the stability. Acrylic acid concentration controlled the overall elastic moduli, and higher concentrations led to shorter gelation times. Ammonium persulfate was able to both initiate the polymerization and also internally break the gel, depending on the concentration. Molecular explanations for these effects were provided. This proposed system is a promising candidate for lost circulation control for operations in the Brazilian pre-salt. Future steps involve studying the plugging and removable properties of the gels with optimized composition in high pressure and temperature settings. • A promising gel formulation for lost circulation control was proposed and optimized. • The pre-gel solution has low viscosity, but very high elasticity after gelation. • The gelation mechanism involves in-situ crosslinking and polymerization. • Rheological properties and gelation time can be modeled with R 2 ≥ 0.95. • Rheological properties match or surpass those from the literature.