Pabitra Sen

Molecular Composition and Dynamics of Oils from Diffusion Measurements

We discuss examples and methods for using NMR diffusion measurements to obtain information about molecular sizes, their distributions, and dynamics. Scaling relationships between chain lengths and diffusion constants are derived and tested on diffusion measurements of many samples, including crude oils that are high in saturates. The diffusion constants of asphaltenes are also measured as a function of asphaltene concentration, indicating the formation of asphaltene aggregates at a concentration of approximately 0.2 g/L, and the sizes of the individual asphaltene molecules and aggregates are obtained. The examples and methods discussed in this paper can become the basis for in situ characterization of crude oils. Crude oils are complex mixtures of molecules encompassing a broad range of shapes and sizes.1−3 They include molecules ranging from alkanes, which are chain-like and relatively simple, to asphaltenes, which are complex and may interact strongly with one another.4 The composition determines the properties of crude oils, such as their viscosity and phase behavior. These properties are very important in the production of the oils. For example, the heavy oil components may precipitate and clog the formations and wells, depending on how oils are being lifted to the surface. There are several reasons why it is also important to characterize the composition of the oil in situ. First, many properties of the fluid depend critically on temperature and pressure, so it can be advantageous to make the measurements downhole. In some cases, oil samples even undergo irreversible changes as they are extracted from the well and transferred to the laboratory for analysis. Second, the fluid composition in a reservoir can exhibit large heterogeneity, and strong compositional gradients have been reported.5 Because downhole measurements

The low-frequency dielectric response of charged oblate spheroidal particles immersed in an electrolyte

We study the low-frequency polarization response of a surface-charged oblate spheroidal particle immersed in an electrolyte solution. Because the charged spheroid attracts counterions which form the electric double layer around the particle, using usual boundary conditions at the interface between the particle and electrolyte can be quite complicated and challenging. Hence, we generalize Fixmans boundary conditions, originally derived for spherical particles, to the case of the charged oblate spheroid. Given two different counterion distributions in the thin electric double-layer limit, we obtain analytic expressions for the polarization coefficients to the first nontrivial order in frequency. We find that the polarization response normal to the symmetry axis depends on the total amount of charge carried by the oblate spheroid while that parallel to the symmetry axis is suppressed when there is less charge on the edge of the spheroid. We further study the overall dielectric response for a dilute suspension of charged spheroids. We find that the dielectric enhancement at low frequency, which is driven by the presence of a large ζ potential (surface charge), is suppressed by high ion concentrations in the electrolyte and depends on the size of the suspended particles. In addition, spheroids with higher aspect ratios will also lead to a stronger dielectric enhancement due to the combination of the electric double layer and textural effects. The characteristic frequency associated with the dielectric enhancement scales inversely with the square of the particle size, the major radius of the spheroid, and it has a weak dependence on the shape of spheroids.

A system for acoustical and optical analysis of encapsulated microbubbles at ultrahigh hydrostatic pressures

Acoustics are commonly used for borehole (i.e., oil well) imaging applications, under conditions where temperature and pressure reach extremes beyond that of conventional medical ultrasonics. Recently, there has been an interest in the application of encapsulated microbubbles as borehole contrast agents for acoustic assessment of fluid composition and flow. Although such microbubbles are widely studied under physiological conditions for medical imaging applications, to date there is a paucity of information on the behavior of encapsulated gas-filled microbubbles at high pressures. One major limitation is that there is a lack of experimental systems to assess both optical and acoustic data of micrometer-sized particles data at these extremes. In this paper, we present the design and application of a high-pressure cell designed for acoustical and optical studies of microbubbles at hydrostatic pressures up to 27.5 MPa (271 atm).