Preparation of thermo-responsive polymer and its application for plugging in hydrate-bearing sediments

Abstract

Hydrate phase equilibrium can be disrupted by the drop in pressure and increase in temperature due to the drilling fluid invasion into hydrate-bearing sediments (HBS) during drilling operations. In this study, in order to alleviate hydrate dissociation in the drilling process, thermo-responsive polymers as reversible plugging nanoparticles were employed in drilling fluids. First, by introduction of butyl acrylate monomers into N-isopropylacrylamide with different molar ratios, a series of thermo-responsive polymers with different lower critical solution temperatures (LCSTs) were prepared to adapt the formation temperature conditions, and then their plugging performance and the reversibility in hydrate-bearing sediments were studied. Results showed that the hydrate decomposition only occurred in a limited zone within 2 cm during the injection of polymer solution, and the hydrate saturation decreases only by 4.4% at 10 min at a position 2 cm from the inlet end. A dense filter cake formed by the polymers can effectively prevent external fluid from entering hydrate sediments for a long time under the condition of 3 MPa differential pressure, and thus reduce the decomposition of hydrates in porous media. Moreover, the differential pressure dramatically decreased from 3.2 MPa to 0.8 MPa when the temperature declined to below the LCST, indicating that the particles blocking the pore throat can be gradually re-dissolved and expelled from the pores. Consequently, the reservoir permeability can be recovered due to the reversibility of thermo-responsive polymer, allowing fluid flows in the exploitation process. Thermos-responsive polymers can be smart plugging agents to block and unblock the well walls in the whole drilling and production stages in hydrate-bearing sediments with intelligence.