• Energy Research

Abstract Accurate predictions and precise control of the allowable water content in CO2-rich fluids are required in large-scale pipeline operations. Especially during transient shut-in and re-start operations, the pressure decrease associated with cooling may cause the CO2-rich mixture to pass through its dew point, producing an aqueous liquid phase. The pH of this liquid aqueous phase will rapidly decrease as carbonic acid is formed, greatly accelerating the corrosion rate of the carbon steel pipeline. The phase behaviour of CO2-rich fluid mixtures is qualitatively different to that of hydrocarbons, and standard oil and gas property packages in process simulation software may be inadequate for predicting dew points and other key properties. An extensive literature survey reveals 34 data sets where water contents of CO2-rich fluids have been measured near conditions relevant to CO2 pipelines. Following consistency tests, 23 data sets were found to be of good quality and 11 data sets were found to be of poor quality. The good-quality data were compared with predictions from 6 equations of state. Overall, Multiflash’s RKS (Advanced) model was found to provide the best agreement with the aqueous dew point data of CO2-rich fluid phases. A case study is presented wherein it is demonstrated that the formation of a corrosive aqueous phase can be avoided during shut-in via introduction of a relatively small volume of ethanol.

Abstract Enhanced gas recovery (EGR) is a promising technology offering both CO2 sequestration and enhanced recovery of natural gas when CO2 is re-injected into producing natural gas reservoirs. Mixing of CO2 and CH4 is, however, potentially problematic and could lead to asset contamination. Essential for effective assessment of EGR are reservoir simulations that require accurate descriptions of dispersive mixing of the two supercritical fluids. Here we systematically measure this supercritical dispersion data in sandstone rock cores, accounting for erroneous gravitational and entry/exit contributions. Using the measured value of dispersivity (α) as the characteristic length scale for mixing, we are able to reconcile our dispersion data with literature values for unconsolidated media (packed beds). Furthermore, our measurements of supercritical dispersion in consolidated porous media have almost 50 times less scatter than the data available in the literature for packed beds. The consequential dispersion correlation as a function of medium Peclet number captures variations with temperature, pressure and dispersing species.

Abstract The simulation of carbon capture unit operations often involves predicting the vapor liquid equilibrium (VLE) for mixtures containing polar, non-polar and quadrupolar compounds. In this work, we investigate how well a simple cubic equation of state (EOS) can predict the results of new low temperature, high-pressure VLE measurements of the ternary methane + carbon dioxide + methanol system, which is important to the Rectisol process used for capturing CO 2 from natural gas. The ternary p , T , x measurements presented here are the first such data for this system reported in the open literature. First, predictions made with the Peng Robinson (PR) EOS as implemented in a commercial process simulator were compared to binary p , T , x data measured in this work and also taken from the literature. Significant deviations were found between the measured liquid mole fractions and those predicted with the EOS using the default binary interaction parameters (BIP): the relative standard errors were 39%, 77% and 17% for the methane + methanol, methane + carbon dioxide and methanol + carbon dioxide binaries, respectively. Regression of the PR EOS to the binary VLE data by adjusting the BIPs improved the liquid phase mole fraction predictions for the ternary mixture data by a factor of about 2.5 for methane and methanol. However, improvement by a factor of 4.4 in the carbon dioxide liquid mole fraction was achieved by describing the carbon dioxide + methanol binary with an asymmetric composition and temperature dependent mixing rule and tuning the BIPs therein to VLE data for this binary over a wide temperature range. This reduced the standard error in the liquid phase CO 2 mole fractions predicted for the ternary mixture using the optimized model by 79% relative to the default model.

Abstract The enhanced recovery of natural gas by the injection and sequestration of CO 2 is an attractive scenario for certain prospective field developments if the risks of gas contamination or early CO 2 breakthrough can be assessed reliably. Simulations of enhanced gas recovery (EGR) scenarios require accurate dispersion parameters at reservoir conditions to quantify the size of the miscible CO 2 –CH 4 displacement front; several experimental studies using core-flooding equipment aimed at measuring such parameters have been reported over the last decade. However, such measurements are particularly challenging and the data produced are generally afflicted in their repeatability by limited experimental control and in their accuracy by systematic errors such as gravitational and core-entrance/exit effects. We review here the existing experimental data pertaining to EGR by CO 2 sequestration and also report new measurements of longitudinal CO 2 –CH 4 dispersion coefficients at temperatures of 40–80 °C, pressures of 8–12 MPa and interstitial velocities of 0.05–1.13 mm s −1 [14.2–320 ft day −1 ] in 5–10 cm long sandstone cores with permeabilities of 12 and 460 mD. The core-floods were conducted in both a horizontal and vertical orientation, with significant gravitational effects observed for low velocity floods in horizontal cores with high permeabilities. We also analyzed the effects of tubing and core entrance/exit effects on the measurements and found that the latter resulted in apparent dispersion coefficients up to 63% larger than would be due to the core alone. Our results indicate that dispersivities for CO 2 –CH 4 at these supercritical conditions are less than 0.001 m, which indicates that excessive mixing will not occur in EGR scenarios in the absence of conformance effects such as heterogeneity coupled with injection well pattern. Inclusion of such conformance effects is essential for detailed reservoir simulation.