Surfactant and polymer retention in chemical enhanced oil recovery processes
De la Cruz Vivanco, Carlos Alberto
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"Chemical Enhanced Oil Recovery by alkali, surfactant and/or polymer flooding could be o good option to produce remaining oil from brown fields. One parameter that affects performance and economics of a CEOR process is how much of the injected chemicals is adsorbed or retained by the porous medium. There may be significant interactions between transported molecules and the porous medium which cause the chemical to be retained and lead to the formation of a bank of injection fluid wholly or partially denuded of chemical. Clearly, this can lead to a reduction in the efficiency of the chemical flood. Therefore, the level of chemical retention can be considered as one of the key factors in determining the economic viability of a chemical flood. Thus, it is of great importance to establish the correct retention levels for a given proposed field chemical EOR process. The conditions under which such laboratory measurements should be made are extremely important so that relevant figures for retention are available for the simulation assessment of the chemical flood. In order to obtain relevant data for surfactant and polymer retention, reliable analytical methods must be available to determine their concentration. This is necessary to calculate retention by material balance in laboratory experiments or in the field. In this work, a bibliographic search was done to find analytical methods for surfactant and polymer determination. Several methods were found with good potential. Careful analysis of each methodology, equipment availability at BASF and previous experience, led to the conclusion that determination by Total Organic Carbon (TOC) and Total Nitrogen (TN) content was the most convenient option. It has the advantage that one single piece of equipment can analyze both TOC and TN and an autosampler can be used for automatic measurement. This makes it very convenient for retention determination experiments and coreflood tests in which many samples need to be analyzed. Therefore, this method was successfully implemented for determination of two surfactants and two polymers and calibration curves were obtained in the range from 0 to 200 ppm. Two calculation approaches were proposed for concentration determination, based on single- and two-variable linear regression. It was found that error is approximately 50% lower by using two-variable regression. Once a reliable analytical method was implemented, experiments were carried out to determine surfactant and polymer retention onto Bentheimer sand in static no-flow conditions and in dynamic conditions in a sandpack, at 23 °C. One sulfate surfactant and one HPAM polymer from BASF were studied in synthetic sea water brine, with 3.5% TDS. Adsorption isotherms obtained in static conditions showed that the surfactant was adsorbed in two layers, with maximum adsorption of 760 μg/g. For the polymer, a maximum adsorption of 426 μg/g was observed and possibly a two-layer adsorption behavior but further experiments are needed to confirm this. When both chemicals were mixed, competitive adsorption occurred and adsorption values decreased. Moreover, polymer prevented the adsorption of the second surfactant layer, reducing its adsorption by 80%. Dynamic retention experiments were carried out with a 0.2-mL/min flowrate. Maximum amounts of 206 and 132 μg/g were retained at 1500 ppm. This is only approximately 30% of the amount adsorbed in static conditions. After water flooding, only 16.4% of previously adsorbed surfactant was desorbed, reaching a residual retention of 172 μg/g. Regarding polymer, 31.7% was desorbed by water flooding, with a residual retention of 90 μg/g. It can be said that retention is largely irreversible."