High‐pressure diffusion measurements by Mach‐Zehnder interferometry

Abstract

The present work was initiated to study nitrogen diffusion in hydrocarbons at high temperature and pressure. Such data are needed for mathematical modeling of gas diffusion into the porous matrix of fractured oil reservoirs (da Silva and Belery, 1989). Few measurements of mutual diffusion coefficients of gadoil systems at high pressures and temperatures have been reported. Erkey and Akgerman (1989) used the Taylor dispersion technique with binary mixtures of alcanes and n-octane in the temperature range from 31 to 162°C at 1.72 MPa. This appears to be the most accurate measurements for hydrocarbon mixtures at elevated pressures; the estimated accuracy was reported to be f 1%. In addition to selected nitrogen/hydrocarbon mixtures, we have therefore included the methane/n-octane system to compare with the data of Erkey and Akgerman. We chose to use the method of optical interferometry because it is sensitive, nonintrusive, and rapid. A good overview of optical methods for diffusion measurements is presented by Tyrrell and Harris (1984). In the interferometric methods, the differences in phase and amplitude between two interfering light beams are related to the light intensity. A Mach-Zehnder interferometer is particularly attractive because it allows a visualization of the cell. A phase shift technique has been implemented to determine the phase difference between the interfering beams. The diffusion cell is constructed for pressures up to 45 MPa and temperatures up to 200°C. Application of MachZehnder Interferometry The Mach-Zehnder interferometer used in this work is shown in Figure 1. Light from a He-Ne laser (A) is attenuated by a polarization filter (B) and expanded by a collimator (C). The parallel beam is split by the semitransparent mirror (D), recombined at the similar mirror (E), and observed by a video camera (not shown). The separate beam paths contain either a prism (F) or a mirror (G). The diffusion cell (H) is described in detail in the subsequent section. The mirror (G) is mounted on a piezoelectric position transducer, which enables the use of the phase shift technique (see