Michael Ben Clennell

Molecular simulation studies of hydrocarbon and carbon dioxide adsorption on coal

Sorption isotherms of hydrocarbon and carbon dioxide (CO2) provide crucial information for designing processes to sequester CO2 and recover natural gas from unmineable coal beds. Methane (CH4), ethane (C2H6), and CO2 adsorption isotherms on dry coal and the temperature effect on their maximum sorption capacity have been studied by performing combined Monte Carlo (MC) and molecular dynamics (MD) simulations at temperatures of 308 and 370xa0K (35 and 97xa0°C) and at pressures up to 10xa0MPa. Simulation results demonstrate that absolute sorption (expressed as a mass basis) divided by bulk gas density has negligible temperature effect on CH4, C2H6, and CO2 sorption on dry coal when pressure is over 6xa0MPa. CO2 is more closely packed due to stronger interaction with coal and the stronger interaction between CO2 molecules compared, respectively, with the interactions between hydrocarbons and coal and between hydrocarbons. The results of this work suggest that the “a” constant (proportional to Tc2/Pc) in the Peng–Robinson equation of state is an important factor affecting the sorption behavior of hydrocarbons. CO2 injection pressures of lower than 8xa0MPa may be desirable for CH4 recovery and CO2 sequestration. This study provides a quantitative understanding of the effects of temperature on coal sorption capacity for CH4, C2H6, and CO2 from a microscopic perspective.

A Link between the Pressure Dependency of Elastic and Electrical Properties of Porous Rocks

We present a technique to invert for the stiff and compliant porosity from velocity measurements made as a function of differential pressure on saturated sandstones. A dual porosity concept is used for dry rock compressibility and a squirt model is employed for the pressure and frequency dependent elastic properties of the rocks when saturated. The total porosity obtained from inversion shows satisfactory agreement with experimental results. The electrical cementation factor was determined using the inverted porosity in combination with measured electrical conductivity. It was found that cementation factor increased exponentially with increasing differential pressure during isostatic loading. Elastic compressibility, electrical cementation factor and electrical conductivity of the saturated rocks correlate linearly with compliant porosity, and electrical cementation factor and electrical conductivity exhibit linear correlations with elastic compressibility of the saturated rocks under loading. The results show that the dual porosity concept is sufficient to explain the pressure dependency of elastic, electrical and joint elastic-electrical properties of saturated porous sandstones.

Overpressure prediction in shales

Accurate overpressure detection and prediction is essential to the scientific understanding of the history and safe engineering exploitation of a sedimentary basin. We predict the velocity of shales using the Clay-Plus-Silt (CPS) model from logging measured parameters, the modeled velocity is then used as normal velocity trend to detect and predict overpressure using Eaton equation. The results show that the CPS model predicts accurate velocity in normal-pressure shales, and the modeled shale velocity can be used to detect and predict overpressure more accurately than the established normal velocity trend in overpressured wells.

Elastic Properties of Lochaline Sandstone - Numerical Experiment vs. Measurements

P- and S- velocities of clean quartz Lochaline sandstone are numerically simulated using microtomographic images with resolution of 2 micron and compared with the velocities measured at ultrasonic frequencies. The numerically simulated velocities are in a good agreement with velocities measured at confining pressure of about 30-40 MPa which is high enough to close soft pores but do not cause noticeable deformation of equant pores yet. The obtained results shows that while effects of soft porosity on elastic properties of sandstones cannot be directly simulated from microtomograms, such simulation give reliable results when soft pores are closed.

An Estimation of Sonic Velocities in Shale Using Clay and Silt Fractions from the Elemental Capture Spectroscopy Log

Anisotropic differential effective medium approach is used to simulate elastic properties of shales from elastic properties and volume fractions of clay and silt constituents. Anisotropic elastic coefficients of the wet clay pack are assumed to be independent of mineralogy and to be linearly dependent on clay packing density (CPD), a fraction of clay in an individual wet clay pack. Simulated compressional and shear velocities normal to the bedding plane and are shown to be in a good agreement with measured sonic velocities. Further, elastic coefficients of shales, and , calculated from the log sonic velocities, calibrated porosity and clay fraction obtained from the mineralogy tool are used to invert for elastic constants of clays, C33 and C44. The obtained elastic coefficients of clays show lower scatter than the original elastic coefficients of shales. The noticeable increase of the clay elastic coefficients with the depth increase is shown to result from the positive trend of the CPD with depth. Being interpolated to the same CPD = 0.8, elastic coefficients of clays show no depth dependency. Our findings show that the CPD and silt fraction are the key parameters that can be used for successful modelling of elastic properties of shales.