Long-term Polymer Degradation in High pH Solutions and Polymer Effect on Alkali-oil Phases

Abstract

Summary OMV's 16 Tortonian Horizon (TH) reservoir is considered a suitable candidate for EOR via Alkali-Surfactant-Polymer A(S)P flooding. Polymer stability in A(S)P applications is a major concern mainly due to effects of temperature and brine salinity composition. Within this work, we evaluate the effect of temperature, dissolved-oxygen and presence of alkali on polymer stability in a long-term. Moreover, we also describe the polymer effect in alkali-oil mixtures and emulsion development. We performed experiments for a period ~100 days using real injection water. Data was gathered through steady-shear rheology, pH tracking, dissolved oxygen control, phase-behavior screening and selected single-phase flooding. The workflow included a fourfold approach: 1) Assess polymer baseline behaviour with different concentrations at reservoir temperature. 2) Define optimum polymer concentrations in presence of alkali (7500 ppm), aiming to reach viscosity values reported in previous micromodel/core flooding. 3) Long-term stability assessment to define effects of temperature (50–70°C), dissolved-oxygen and presence of alkali on viscosity and average molecular weight (MW). 4) Phase-behaviour experiments to draw micro/macro emulsion phase maps. Different polymers/vendors were assessed and three included in this work. During baseline assessment a Homopolymer post-hydrolysed (Acrylamide) of high-MW (P-A) depicted the highest viscosifying power, followed by a co-polymer (Acrylamide–Sodium acrylate) of medium-MW (P-B) and a Terpolymers (Acrylamide– Sodium, Acrylate–ATBS) of high-MW (P-C). As expected, presence of alkali reduced polymer viscosity in all polymers, with optimum polymer concentrations ~2000 ppm. Long-term stability tests showed that presence of alkali enhances polymer residual viscosity6 ( η/ηo, corrected by ηbrine ), with residual viscosity decreasing once temperature is increased. However, viscosities at a given time were higher for solutions in presence of alkali compared to those without, presumably owing to increasing hydrolysis. Solutions aged at 50°C depicted 20-30% higher residual viscosity when compared to the solutions aged at 70°C. Solutions degassed with argon/nitrogen enhances polymer long-term stability. Results also showed that degassed solutions had 25% higher residual viscosity than untreated solutions and matched with previous results performed under anaerobic environment (glove-box). Complementarily, phase-experiments depicted that presence of polymer reduces water solubility. Once alkali concentration was increased a 3-phase mainly described as macro-emulsion was observed, microemulsions where only described by eye-looking/light-through observations. Overall, this study provides a workflow to perform long-term polymer degradation screening in a partially degassed environment in the absence of a glovebox. With a major observation that the experiments need to be performed without oxygen, otherwise results are different and too pessimistic.