G. Peter Matthews

Practical observation of deviation from Lucas-Washburn scaling in porous media

Abstract This work analyses the applicability of the Lucas–Washburn equation to experimental observations of imbibition into real network structures. The experimental pore structures used in this study are constructed from tablets of two finely ground calcium carbonates, with defined differences in particle size distribution. These are compressed under a range of different applied pressures to achieve a controlled series of porosities while maintaining the surface chemical, particulate and morphological pore characteristics constant. The porosities are determined by mercury intrusion porosimetry applying corrections for mercury compression and penetrometer expansion together with a correction for sample skeletal compression (Gane et al., J. Am. Chem. Soc., 35 (1996)). Imbibition studies are made by bringing each porous sample into contact with a supersource of liquid and the dynamic imbibition is recorded gravimetrically. Results follow a long timescale macroscopic absorption rate depending on the square root of time but show a failure to scale according to pore size in the Lucas–Washburn equation even though the constants of surface energy, contact angle and fluid viscosity have been maintained. Furthermore, values of average measured pore radius are shown to be finer than the Lucas–Washburn predicted equivalent hydraulic capillary radius. The predominance of a relevant pore size within a given pore size distribution forming a selective pathway filling based on inertial retardation of larger pores and short-term linear time wetting in finer pores is argued to account for the departure from simple pore size scaling.

An intermolecular pair potential energy function for methane

A numerical spherically averaged intermolecular pair potential energy function is presented which describes the interactions of methane molecules. The function incorporates accurate estimates of the long-range dispersion force coefficients, and closely reproduces shear viscosity and second virial coefficient data over a wide temperature range. Agreement with other experimental data which is generally satisfactory is also discussed. The potential is shown to be superior to a large number of analytical potentials previously proposed for this gas.

Elimination of the Schlieren effect in the determination of reactive phosphorus in estuarine waters by flow-injection analysis

Abstract Conventional flow-injection manifolds with sample injection for the determination of reactive phosphorus in estuarine waters are limited by the Schlieren or refractive index (RI) effect which can cause major errors in quantification. A simple flow-injection (FI) manifold which obviates this RI error in reactive phosphorus measurement is reported. It involves the injection of acidic molybdophosphate reagent into a carrier stream of sodium chloride solution of similar refractive index, which is then sequentially merged with a sample (the salinity of which may vary widely from sample to sample) and a reductant. Despite the occurrence of sizeable blank signals, reactive phosphorus has been successfully measured in samples with salinities ranging from 0 to 34%. using calibration standards prepared in deionised water, with a detection limit of 6 μg l −1 P.

The effects of correlated networks on mercury intrusion simulations and permeabilities of sandstone and other porous media

Abstract A void space network is presented for the simulation of mercury intrusion and the calculation of the absolute permeability of porous media. Mercury intrusion is simulated by the Laplace/Washburn equation within a percolation algorithm. A ‘Dinic’ operational research network analysis algorithm gives the maximal flow capacity of the unit cell. A combination of the Darcy and Poiseuille equations of flow is then used to derive the absolute gas permeability of the simulated structure. Mercury intrusion curves and permeabilities are calculated for networks with banding, clustering of small pores and throats, or clustering of large pores and throats. The modelling method is then applied to banded sandstone samples with edge corrections, and it is shown that the constraint of fitting to experimental mercury intrusion data suppresses the permeability differences induced by structural changes alone. The same networks can model mercury hysteresis, porosity, connectivity, pore/throat size correlation, tortuosity and other properties, and can be applied to any consolidated porous medium.

Modelling characteristic properties of sandstones

A computer model of a porous medium is described. Written in FORTRAN, it consists of a three-dimensional array of cubes and cylinders representing pores and throats, respectively. The pore-size distribution (p.s.d.) is calculated from the mercury porosimetry curve of Clashac outcrop sandstone, a relatively clay-free, well characterised rock. Surface area and porosity are calculated from the program and the original porosimetry curve is simulated as a check of the p.s.d. Fifty values of the tortuosity factor of the sandstone are then simulated via a weighted random walk through the model. The tortuosity is then defined as the median of the resulting distribution. The tortuosity is simulated for two different cases of an assumed potential field.

Modelling of the Void Space of Tablets Compacted Over a Range of Pressures

A previously developed computer model, named Pore‐Cor, has been used to simulate the changes in the void‐space dimensions which occur during the compaction of tablets over a range of pressures.

Modelling of simulated clay precipitation within reservoir sandstones

The purpose of this study was to investigate whether a previously developed computer model, named Pore-Cor, could simulate the subtle changes in void space dimensions which occur during the artificial deposition of small amounts of illite and kaolinite within Fontainebleau sandstone. Clay precipitation was carried out by placing a sandstone plug in a gold capsule, with an aluminosilicate gel, and treating the plug hydrothermally with potassium hydroxide solution. Using experimental conditions of 350°C and 1.0 kbar (100 MPa), illite, illite-smectite and kaolinite were precipitated in parts of the sandstone void space with morphologies similar to those of authigenic clay minerals in sandstone petroleum reservoirs. Mercury intrusion curves were then measured for the untreated and clay precipitated sandstones. The Pore-Cor package simulated these intrusion curves, and generated void space models of the correct porosity. By this means, subtle changes in void space dimensions and connectivity could be identified, which give rise to large changes in permeability.

Measurement and modelling of diffusion, porosity and other pore level characteristics of sandstones☆

Abstract The diffusion coefficients of methane, iso-butane and n-butane through dry and water-saturated sandstones and their dependence on relative molar mass are discussed. The diffusion of iso-butane is anomalously fast in dry sandstones and that of methane is anomalously low in water-saturated sandstones. A three-dimensional computer model of the pores and throats in the sandstone has been developed. Simulations have been made of the sandstones porosity, surface area, pore and throat size distribution, tortuosity, mercury porosimetry primary drainage curve, average pore connectivity and diffusion coefficient for mixed gaseous alkanes.

Isotope fractionation factors controlling isotopocule signatures of soil-emitted N₂O produced by denitrification processes of various rates.

RATIONALE This study aimed (i) to determine the isotopic fractionation factors associated with N2O production and reduction during soil denitrification and (ii) to help specify the factors controlling the magnitude of the isotope effects. For the first time the isotope effects of denitrification were determined in an experiment under oxic atmosphere and using a novel approach where N2O production and reduction occurred simultaneously. METHODS Soil incubations were performed under a He/O2 atmosphere and the denitrification product ratio [N2O/(N2 + N2O)] was determined by direct measurement of N2 and N2O fluxes. N2O isotopocules were analyzed by mass spectrometry to determine δ(18)O, δ(15)N and (15)N site preference within the linear N2O molecule (SP). An isotopic model was applied for the simultaneous determination of net isotope effects (η) of both N2O production and reduction, taking into account emissions from two distinct soil pools. RESULTS A clear relationship was observed between (15)N and (18)O isotope effects during N2O production and denitrification rates. For N2O reduction, diverse isotope effects were observed for the two distinct soil pools characterized by different product ratios. For moderate product ratios (from 0.1 to 1.0) the range of isotope effects given by previous studies was confirmed and refined, whereas for very low product ratios (below 0.1) the net isotope effects were much smaller. CONCLUSIONS The fractionation factors associated with denitrification, determined under oxic incubation, are similar to the factors previously determined under anoxic conditions, hence potentially applicable for field studies. However, it was shown that the η(18)O/η(15)N ratios, previously accepted as typical for N2O reduction processes (i.e., higher than 2), are not valid for all conditions.

Viscosity of nitrogen and certain gaseous mixtures at low temperatures

The viscosity of nitrogen has been measured at temperatures in the range 120–320 K using a capillary flow technique. These results, together with previous data, are used to define a new standard for low-temperature viscosity determinations. In addition, the viscosities of the binary gas mixtures He + Ar, Ar + Kr and CH4+ CF4 have been measured in this temperature range using the same apparatus. Agreement to within 1% is found with the mixture data of previous workers at room temperature, and there is good agreement for the pure component viscosities over the entire temperature range. The mixture viscosities are analysed in terms of the so-called interaction viscosities η12, which are related to unlike pair potential energy functions. The interaction viscosities for He + Ar are combined with previous high-temperature results and are used to obtain a potential energy function in the repulsive region.