Influence of Rigid Side Chains on the Structural Stability of High-Temperature Resistant Fluid Loss Additives for Oil Well Cements: An Experimental Study and Molecular Simulation

Abstract

A fluid loss additive (named AAMN) for cementing ultra-deep reservoirs at high temperature (210 °C) was prepared by a free radical copolymerization method using 2-acrylamido-2-methylpropane sulfonic acid (AMPS), acrylamide (AM), acrylic acid (AA), and N-vinyl-2-pyrrolidinone (NVP) as monomers. Filtration tests at 210 °C demonstrated that AAMN reduced the amount of fluid loss by 22% in fresh cement slurry, 23% in 18 wt% NaCl cement slurry, and 16% in 36 wt% NaCl cement slurry compared to AAM (without NVP) when both were added at 6% bwoc ("bwoc" denotes the addition by the weight of the cement). The pore structure was analyzed using the Brenner-Emme-Teller (BET) method and environmental scanning electron microscopy (ESEM), confirming that AAMN could be adsorbed onto the cement particles surfaces, filling the pores and blocking fluid loss channels. The degradation temperatures of the two copolymers were tested by thermogravimetric analysis (TGA-DTGA) and the molecular dynamics behavior of two Amorphous Cell (AC) models (the amorphous structures with randomly arranged Ca2+, Cl−, Na+ and H2O with the AAMN or AAM) built by Materials Studio 7.0 software, were compared: the introduction of five-element NVP side chains didn’t significantly improve the thermal stability of the AAMN relative to the AAM, but resulted in the AAMN molecular chain maintaining a stable geometry at a temperature of 483 K (210 °C). The AAMN also showed more adsorption groups (–SO3 − and –COO−) at the high temperatures, which allowed the AAMN to form a dense "network structure" with Ca2+ and H2O molecules, thus effectively blocking the channels for water molecule loss from the cement slurries.