Polymer Degradation in high pH solutions and the Effect of Polymers on the Phase Behaviour of Alkali – Oil.

With a continuous increase in the worldwide demand for the crude oil and considering the fact that conventional oil and gas resources are depleting, the Enhanced Oil Recovery processes (EOR) play a vital role to boost the production from mature reservoirs. Chemical Enhanced Oil Recovery (cEOR) is one of the most implemented methods to produce the remaining oil in-situ. Alkali-Polymer flooding is a cEOR technique in which the alkali should react chemically with the oil to produce in-situ surfactants/soaps. Polymers solutions are expected to provide fluid mobility control and improve the sweep efficiency. Hence, it is crucial that the viscosity of a polymer solution does not decrease throughout its propagation along the reservoir. The 16 Tortonian Horizon (TH) of the Matzen field has been screened for the application of different EOR and is considered a suitable candidate for Alkali-Polymer flooding due to its reactive oil properties (high acidity). This work presents a set of criteria/workflow for defining polymer degradation in high pH solutions using fluid samples from the 16 TH reservoir. Based on this workflow, the procedures and the effect of those solutions on the alkali-oil phases are described. This combination allows performing long-term polymer degradation screening in a partially degassed environment without using a glovebox. Long-term thermal stability was assessed for three commercial partially hydrolyzed polyacrylamide (HPAM). The workflow can be divided into four different steps. First, an assessment of polymer viscosifying power behaviour -with different polymer concentrations- at reservoir temperature. Second, the definition of optimum polymer concentrations in presence of alkali (7500 ppm) for each of the three studied polymers. This approach aimed to reach viscosity values reported in previous micromodel/core flooding experiments. Third, Long-term stability assessment, including periodic steady-shear rheological, pH and dissolved oxygen measurements, to define effects of temperature (50-70°C), dissolved-oxygen and presence of alkali on viscosity and average molecular weight (MW). Finally, Phase-behaviour experiments with and without the presence of polymer, in order to define the effect of polymers in the phase behavior of alkali-oil mixtures. The overall findings and observations presented in this work showed that the presence of alkali enhances the residual viscosity values for polymer solutions roughly 20% after 20 days. Moreover, the apparatus used for the degassed set can be used as an economic alternative to perform long-term stability assessments. Flopaam 3630 S and Flopaam 5205 VHM showed residual viscosities values ranged between 82% to 99% and 90% to 91% respectively for the degassed set. In addition, solutions aged at 50°C showed a half-life time 4- to 10- times bigger in comparison with solutions aged at 60- and 70°C. For the phase experiments, polymer presence affects the phase behavior of alkali-oil solutions. It reduces the amount of water that can partition into the emulsion phase. Finally, no major differences in phase behavior were observed for the three studied polymers regardless of the temperature.