Elucidating Kinetic, Adsorption and Partitioning Phenomena from a Single Well Tracer Method: Laboratory and Bench Scale Studies

This study is aimed at giving some insights on kinetics, adsorption and partitioning of ethyl acetate during a single well tracer test. Synthetic formation water, an specific crude oil and a silicate-dolomite rock were used during experiments performed in laboratory and bench scale systems. Independent sets of experiments were designed to calculate the partition coefficient of ethyl acetate between the formation water and the oil, to develop a kinetic model for the hydrolysis of ethyl acetate, and to derive isotherm and kinetic models for the adsorption of ethyl acetate on the rock. These tracer experiments were evaluated at a concentration range (100-300 mmol. L-1) similar to that supposed to be used in the single well tracer method. All parameters determined from these experiments were validated describing observations from stirred batch and column systems, in which kinetic, adsorption and partitioning phenomena occurred at the same time. Pseudo-heterogeneous models, accounting for three phases namely the formation water, the rock and the oil, were applied to elucidate the interaction of the different mechanisms involved in these set-ups. Main results are summarized as follows: (i) partition coefficients (KEA) were apparent varying from ca. 5-8 because of thermodynamic constraints; (ii) kinetic models for the hydrolysis of ethyl acetate were developed under acid and basic conditions since at neutral ones there were negligible conversions; (iii) the combined Langmuir-Freundlich isotherm and the Langmuir kinetics were the most suitable models describing equilibrium and adsorption rate observations, respectively; (iv) the studied rock adsorbed significant amounts of ethyl acetate, leading to a maximum adsorption capacity (qEAm) of ca. 7.0 mmol. g(-1) at studied operating conditions; (v) the adsorption kinetic model rather than the simplified isotherm model seems necessary to describe this phenomenon from the single well test evaluating ethyl acetate as the tracer; and (vi) partition, hydrolysis and adsorption parameters evaluated from independent experiments allowed us to describe observations from both stirred batch and column systems. These results disclose the importance of accounting for partition, hydrolysis and adsorption mechanisms in a single well method using ethyl acetate as the tracer.