Robert L. Braun

Viscosity, granular‐temperature, and stress calculations for shearing assemblies of inelastic, frictional disks

Employing nonequilibrium molecular‐dynamics methods the effects of two energy loss mechanisms on viscosity, stress, and granular‐temperature in assemblies of nearly rigid, inelastic frictional disks undergoing steady‐state shearing are calculated. Energy introduced into the system through forced shearing is dissipated by inelastic normal forces or through frictional sliding during collisions resulting in a natural steady‐state kinetic energy density (granular‐temperature) that depends on the density and shear rate of the assembly and on the friction and inelasticity properties of the disks. The calculations show that both the mean deviatoric particle velocity and the effective viscosity of a system of particles with fixed friction and restitution coefficients increase almost linearly with strain rate. Particles with a velocity‐dependent coefficient of restitution show a less rapid increase in both deviatoric velocity and viscosity as strain rate increases. Particles with highly dissipative interactions result in anisotropicpressure and velocity distributions in the assembly, particularly at low densities. At very high densities the pressure also becomes anisotropic due to high contact forces perpendicular to the shearing direction. The mean rotational velocity of the frictional disks is nearly equal to one‐half the shear rate. The calculated ratio of shear stress to normal stress varies significantly with density while the ratio of shear stress to total pressure shows much less variation. The inclusion of surface friction (and thus particle rotation) decreases shear stress at low density but increases shear stress under steady shearing at higher densities.

Development of a detailed model of petroleum formation, destruction, and expulsion from lacustrine and marine source rocks

Abstract A variety of laboratory experiments, including programmed micropyrolysis, isothermal fluidized-bed pyrolysis, oil evolution from a self-purging reactor, pyrolysis-mass spectrometry, and hydrous pyrolysis are analyzed to derive chemical kinetic expressions for pyrolysis of lacustrine and marine kerogens. These kinetic parameters are incorporated into an improved, detailed chemical-kinetic model which includes oil and gas generation from kerogen, oil degradation by coking and cracking, gas generation from residual kerogen, and hydrogen consumption reactions. Oil is described by eleven boiling-point fractions of two chemical types. The model includes equation-of-state calculations of vapor/liquid equilibria and PVT behavior. The model can simulate closed, open and leaky systems, and the open system can include an inert-gas purge. The porosity is calculated for both unconstrained conditions as well as conditions simulating natural compaction and fracturing during sedimentary burial. Model calculations are compared to results from a variety of laboratory experiments, including hydrous pyrolysis. Oil expulsion efficiencies and properties are also calculated for a variety of geological conditions. The relative amounts of water and hydrocarbon phase(s) expelled are governed by saturation-dependent relative permeabilities. Gas/oil ratios in the expelled petroleum are related to organic content and geological heating rate.

Oil-shale pyrolysis: Kinetics and mechanism of oil production

Abstract Literature data on the thermal decomposition of the organic material in Colorado oil shale are analysed. Inclusion of a thermal induction period in the data analysis results in a concise interpretation of the kinetics of oil production in terms of a simple mechanism involving two consecutive first-order reactions. The rate constants and activation energies for these two reactions are deduced.

Temperature and pressure dependence of n-hexadecane cracking

The rates and product compositions of hexadecane cracking are reported for temperatures ranging from 300 to 370°C and pressures of 150 to 600 bar. The overall apparent activation energy at an intermediate pressure of 300–350 bar is about 74 kcal/mol. This is higher than the overall 60 kcal/mol energy consistent with higher temperature measurements, even though the rates from our highest temperatures overlap with those from earlier reports. The product composition is consistent with a free radical mechanism in which alkene intermediates react with primary and secondary radicals to form branched and normal liquid products both smaller and larger than the starting material. Pressure has a retarding effect on the rate of reaction, but the dependence is not measured precisely enough to say more than that the survival of oil is likely to vary by a factor of two or so under typical geologic conditions. A simple kinetic model having five first-order reactions is presented that predicts the lumped kinetic species of C1, C2–C4, C5–C9, C10–C15, C16, and C16+. The gas is depleted in methane compared to most natural gas.

A Model of Hydrocarbon Generation from Type I Kerogen: Application to Uinta Basin, Utah

We have developed a computer model that can predict when and how much oil and gas are generated from a source rock during its burial and later uplift. Kinetic parameters for the oil and gas generation reactions are obtained from high-pressure pyrolysis experiments carried out over a wide range of heating rates and temperatures. In our kinetic model, which applies only to Green River shale, we use a single activation energy of 52.4 kcal/mole and different pre-exponential factors for different products of primary pyrolysis, which allows us to extrapolate laboratory-derived kinetics to geologic heating rates. This model is in contrast to the wide distributions of activation energies or artificially low apparent activation energies used in some models of petroleum formation. hen extrapolated to geologic heating rates on the order of 10°C/m.y., our kinetics show that the temperature of the maximum rate of oil generation (Tp) changes by about 15°C when the heating rate is changed by an order of magnitude. Changes in pressure have relatively minor effects on the kinetics of oil generation but are important for gas generation reactions. We used geophysical data from oil fields in the Uinta basin of Utah to develop a thermal history model of Green River Formation source rocks. This time-temperature history was used to predict the maturation level of the kerogen at a given depth and to predict changes in the compositional characteristics of the oil. The shape of calculated oil generation rate curves, as a function of depth in the basin, mimics the shape of the overpressure curves; this similarity suggests that oil-gas generation may be an important cause of overpressuring. Maturation levels and compositional characteristics of the oil predicted by our model agree very well with characteristics of the oil recovered from the basin.

PMOD: a flexible model of oil and gas generation, cracking, and expulsion

A new computer program, PMOD, is used to develop and test a wide variety of global models of varied complexity for the generation and destruction of hydrocarbons from petroleum source rocks. Chemical reaction models are constructed interactively by supplying the empirical formula of the reactants and products, desired reactions, and reaction constraints. PMOD calculates stoichiometric coefficients that conserve elemental balance. The chemical reactions are integrated into two compaction and bulk-flow expulsion models in order to predict the amounts and compositions of products expelled from the source rock under geological conditions. A variety of published and unpublished experiments are used to show when various model types are required and to derive the parameters for the various models. This exercise provides a good means to compare the strengths and weaknesses of the various models and provides guidance on the types of information needed to calibrate them. Chemical properties calculated by PMOD for all models include apparent Rock-Eval parameters and other common geochemical measurements.

Further comparison of methods for measuring kerogen pyrolysis rates and fitting kinetic parameters

Abstract We compare rates of product generation during pyrolysis of several petroleum source rocks and isolated kerogens by nonisothermal techniques, including Rock-Eval pyrolysis, condensed oil evolution, and pyrolysis MS/MS. We discuss problems related to temperature calibration in the Rock-Eval instrument, and confirm that standard Rock-Eval temperatures are in error by about 40°C. Calculations using Gaussian and discrete activation energy distributions and associated frequency factors derived from Rock-Eval data by nonlinear regression agree within a few °C with oil evolution at 2°C/min and pyrolysis-MS/MS at 4°C/min. This comparison also demonstrates that activation energy distributions are needed for oil alone as well as for gas and for total hydrocarbons from many source rocks. Discrepancies between Rock-Eval kinetics and oil evolution at 2°C/h and hydrous pyrolysis are related to mass transport limitations. Finally, we discuss the sensitivity of the extrapolation of reaction rates to geological conditions and how the extrapolation may be influenced by erroneous data or kinetic analysis.

A test of the parallel reaction model using kinetic measurements on hydrous pyrolysis residues

Open-system kinetics are measured on extracted residues from hydrous pyrolysis to test aspects of the parallel reaction model and the assumption of a single frequency factor for all energies. The absolute rates of the residues form a series supporting the concept that hydrous pyrolysis removes primarily the more reactive components at lower temperature. The parallel reaction model works well for the marine kerogen, except for the small amount of refractory material in the highest temperature residue that is not represented adequately in kinetics from the initial sample because of measurement limitations. The kinetic parameters derived from the unreacted coal do not work as well and predict too much remaining reactive material. The discrepancy may be caused by a weakness in the parallel reaction model, or due to the fact that the open-system kinetics are not intended to predict the generation and subsequent removal of extractable organic matter. That the parallel reaction model works less well for coal confirms earlier concerns about the complexity of oxygen-related reactions. The constant frequency factor approximation works fairly well for the kerogen but marginally well for the coal. The frequency factor shifts about ten-fold over the activation energy range in the kerogen and about 100-fold over the activation energy range in the coals.

Kinetics of Colorado oil shale pyrolysis in a fluidized-bed reactor

Abstract Previous hydrocarbon evolution data were reanalysed to determine improved rate expressions for oil generation from Colorado oil shale under rapid pyrolysis conditions. Contributions from low-molecular-weight gases were subtracted from flame-ionization detector data to obtain the rate of oil generation alone. Equally good fits to the data were obtained using two parallel first-order reactions or a single reaction with an effective reaction order of 1.51. The latter expression was easier to incorporate into global process models. The rate expressions were independent of shale source (Anvil Points or Tract C-a) and particle size (0.5–2.4 mm). The kinetic data were consistent with the previous conclusion that the small incremental oil yield possible for fluidized-bed pyrolysis requires a longer residence time than that estimated by kinetic expressions derived from slow-heating data.

Chemical Kinetic Model of Hydrocarbon Generation, Expulsion, and Destruction Applied to the Maracaibo Basin, Venezuela

This paper describes the development and application of a compositional chemical model of hydrocarbon generation, expulsion, and destruction for the Cretaceous La Luna Formation source rock of the Maracaibo basin, Venezuela. Applications include both laboratory and geological settings. Laboratory pyrolysis experiments were used to study bulk oil generation, expulsion, and associated changes in composition of the kerogen, extractable organic matter, and expelled and unexpelled hydrocarbons. The laboratory experiments were also used to determine kinetic parameters to quantitatively describe organic reactions, via a computer model that also includes simulation of pressure-driven primary expulsion, over widely varying conditions. We show that the chemical model accurately sim lates the experimental results. Thermal history models for wells in the Maracaibo basin were used to simulate hydrocarbon generation and pore pressure development in the La Luna Formation and expulsion into nearby Cretaceous reservoirs. Results of the modeling indicate that both compaction disequilibrium and organic maturation play important roles in the development of excess pore pressure in the La Luna Formation. The model simulation of the variation of indicators such as Rock-Eval parameters and extract and oil compositions shows generally good agreement with measurements from remaining kerogen, oils, and extracts recovered from the La Luna Formation and from nearby Cretaceous reservoirs.