H.J. Schenk

An investigation of the in-reservoir conversion of oil to gas: compositional and kinetic findings from closed-system programmed-temperature pyrolysis

Abstract The thermal alteration of reservoired petroleum upon burial was simulated by closed-system non-isothermal pyrolysis. Samples of a medium gravity oil from the Norwegian North Sea Central Graben were pyrolysed in microscale sealed glass or quartz vessels at heating rates of 0.1, 0.7 and 5.0 K · min−1 up to temperatures varying from 300 to 650°C. The composition of the oil and gas in each vessel was determined by a single-step on-line gas chromatographic analysis. With increasing rate of heating the onset of all oil degradation reactions was shifted to higher temperatures. The same successive compositional changes occurred in each case: increase in the total yield of GC-detectable compounds, significant gas (C1C4), generation accompanied by a decrease in yield of heavy components, aromatisation and attainment of maximum gas yield and finally a cracking of the C2+ gas components. The stable final mixture consisted of methane, aromatic hydrocarbons, pyrobitumen and possibly hydrogen. Kinetic modelling of the oil to gas conversion resulted in a narrow gas potential vs activation energy distribution between 66 and 70 kcal/mol assigning 35% of the total gas potential (460 mg per g of oil) to an energy of 66 kcal/mol and 29% to 67 kcal/mol (pre-exponential factor 1.1 × 1016 s−1). By extrapolation to natural maturation conditions the onset of gas generation is predicted to occur between 160 and 190°C for geological heating rates between 0.53 and 5.3 K·Ma−1. Predictions from the model are in accordance with the observed preservation of liquid hydrocarbons in a deep, hot (165°C) petroleum reservoir from the Saga 2/4–14 well, Norwegian Continental Shelf.

The conversion of oil into gas in petroleum reservoirs. Part 1: Comparative kinetic investigation of gas generation from crude oils of lacustrine, marine and fluviodeltaic origin by programmed-temperature closed-system pyrolysis

The thermal alteration of reservoired petroleum upon burial was simulated comparatively by closed-system programmed-temperature pyrolysis of produced crude oils of lacustrine, fluviodeltaic, marine clastic and marine carbonate origin using the microscale sealed vessel (MSSV) technique. Bulk kinetics of oil-to-gas cracking and accompanying compositional changes were studied at heating rates of 0.1, 0.7 and 5.0 K/min. The oil type related variations of experimental cracking temperatures were small compared to those related to heating rate, but the high-temperature shift of gas evolution curves with increasing rate of heating turned out to be more pronounced for the marine than for the non-marine oils. Accordingly the kinetic frequency factors were derived to be higher for gas generation from the lacustrine and fluviodeltaic oils (A ≈ 4·1019 min−1) than from the marine oils (A ≈ 2·1018 min−1) and the gas potential vs. activation energy distributions were calculated to be centered around 71–72 kcal/mol for the former and around 67 kcal/mol for the latter. These kinetic parameters and compositional observations give some evidence that gas generation is accompanied by the formation of aromatic compounds in the case of the marine oils whereas alkene intermediates seem to be involved in the case of the non-marine high was oils. Under geological heating conditions (e.g. 5 K/My), the onset of gas generation and peak gas generation are extrapolated to occur at about 180°C and 225°C for the high wax oils. The marine oils turn out to be slightly less stable with peak gas generation at 215°C and the onset of decomposition reactions predicted at about 170°C. In the absence of reservoir bitumen and minerals severe oil-to-gas cracking is very unlikely to take place at temperatures less than 160°C, whatever the crude oil type or the geological heating rate.

Determination of gross kinetic parameters for petroleum formation from Jurassic source rocks of different maturity levels by means of laboratory experiments

Abstract Solvent-extracted core samples of Lower Toarcian carbonate-rich shales (“Posidonia Shale”, Lias ϵ) of the Hils syncline area, Lower Saxony Basin, northern Germany, covering a wide maturity range (vitrinite reflectance, Rr = 0.48, 0.88 and 1.45%) and a Kimmeridge mudstone (Rr = 0.45%) are pyrolyzed at three different heating rates (0.1, 0.7 and 5.0 K/min). By application of non-isothermal first-order reaction kinetics to the resulting hydrocarbon evolution curves activation energy distributions between 46 and 70 kcal/mol and pre-exponential Arrhenius factors are obtained by an iteration method. Hypothetical hydrocarbon generation curves are calculated for the Lias ϵ assuming three geological heating rates (0.5, 5 and 50 K/106a). Peak hydrocarbon generation temperatures range from 135 to 165°C.

Kinetics of petroleum generation and cracking by programmed-temperature closed-system pyrolysis of Toarcian Shales

Abstract Primary kerogen-to-petroleum and secondary oil-to-gas conversion processes in marine source rocks have been studied contemporaneously by programmed-temperature closed-system (MSSV) pyrolysis of Toarcian Shale concentrates at heating rates of 0.1, 0.7 and 5.0K min −1 in the temperature range of 300–610°C. All pyrolysates were analysed by single-step on-line gas chromatography. The cumulative evolution profiles of liquid and gaseous compounds were deconvoluted into generation curves for oil (C 6+ ), primary gas and secondary gas using complementary open-system experiments and simple stoichiometric relationships. The subsequent kinetic analysis resulted in potential versus activation energy distributions which turned out to be comparatively broad for oil and primary gas and rather narrow for secondary gas, indicating that the formerer are generated from more inhomogeneous precursor materials than the latter. The dominant activation energies were found to increase from 52 (217.9) (oil) to 53 (222) (primary gas) and 55 (230.5) kcal mol −1 (kJ mol −1 ) (secondary gas); the best-fit frequency factors were calculated around 1015 min −1 . By extrapolation to a geological heating rate of 5.3K my −1 (10 −11 K min −1 ) the onset of oil generation is predicted to occur at 90°C, the maximum oil formation rate at 140°C and the onset (peak generation) of primary and secondary gas at 110°C (165°C) and 150°C (180°C), respectively.

Using natural maturation series to evaluate the utility of parallel reaction kinetics models: an investigation of Toarcian shales and Carboniferous coals, Germany

Abstract Open-system pyrolysis is routinely performed on immature samples in order to determine the kinetic parameters of petroleum generation at both bulk and molecular levels. This study tested such predictions for Type II and Type III organic matter by making calibrations with both artificial and natural maturity sequences of Toarcian shales (Posidonia shale; 0.48–1.44% Rr) and Carboniferous vitrains (0.74–2.81% Rr). Both natural series showed little or no compositional variability attributable to kerogen type. Artificially matured samples were prepared by non-isothermal heating (0.7 K/min) of the least mature samples up to end temperatures between 375 and 470°C under either closed- or open-system conditions. Measured generation rate versus temperature curves were analyzed assuming a distributed system of activation energies and a single frequency factor in each case. The resulting kinetic parameters were then used to assess bulk petroleum formation rates for geological heating conditions. In the case of all artificially matured samples, measured and predicted bulk petroleum formation rate vs. temperature curves for each maturation stage remain within the original envelope defined by the least mature sample, despite an upward shift of Tmax temperatures. This confirms that the reactions taking place during both the pyrolysis measurements and simulated maturation processes are the same, involving mainly homolytic cracking. A similar pattern of measured and predicted rate curves is reproduced by the natural maturation sequence of the Toarcian shales in the maturity range of 0.53 to 1.44% Rr suggesting that petroleum generation within these natural systems also results from cracking reactions, and therefore that petroleum generation over geological time can be reliably extrapolated from open-system pyrolysis of the appropriate immature sample. By contrast, significant deviations are observed for the natural coal series, with measured and predicted rate curves extending beyond the immature envelopes. In accordance with the pronounced increase of frequency factors and of protonated aromatic carbon concentrations, this behavior is attributed to solid state aromatization reactions which compete with product generation during natural coalification, but which are not reproducible to the same extent by experimental heating. It is concluded that petroleum generation from vitrinitic coals over geological time cannot be reliably extrapolated from open-system pyrolysis of low rank samples.

Kinetics of petroleum generation by programmed-temperature closed-versus open-system pyrolysis

Bulk petroleum generation by programmed-temperature pyrolysis of immature (Rr = 0.48%) Posidonia (Toarcian) Shale samples at heating rates of 0.1, 0.7, and 5.0 K/min has been studied comparatively under open- and closed-system conditions, using the microscale sealed vessel (MSSV) technique in the latter case. The comparison of formation rates required a differentiation (vs. temperature) of closed-system cumulative product evolution profiles. The kinetic analysis assuming twenty-five first order parallel reactions with activation energies regularly spaced between 46 and 70 kcal/mol and a single preexponential factory yielded the same value of A = 1.08 · 1016 min−1 and very similar petroleum potential vs. activation energy distributions centered around 54 kcal/mol in both cases. In particular, both approaches turned out to be in excellent agreement with respect to predicting temperature ranges of oil and gas formation under geological heating conditions. This is in contrast to the case of petroleum yield assessment which appears to be more system-dependent.

Kinetics of Petroleum Formation and Cracking

One of the most fundamental problems in basin modeling as related to petroleum exploration is assessing the temporal and spatial limits of petroleum generation in sedimentary basins. It is well known that petroleum is generated from macromolecular sedimentary organic matter as it thermally degrades upon burial. The multitude of chemical reactions involved are unknown in detail (Philippi 1965; Welte 1965) but are recognized to be quasi-irreversible Suck and Karweil 1955; Hanbaba and Juntgen 1969; Tissot 1969). The organic components of subsiding sedimentary rocks are generally far away from thermodynamic equilibrium (Dayhoff et al. 1967; Tackach et al.1987). Consequently, the formation of oil and gas in nature is controlled by chemical reaction kinetics, in particular by non-isothermal kinetics because temperature changes as a function of time under geological conditions (Tissot and Espitalie 1975).

Assessing the overlap of primary and secondary reactions by closed- versus open-system pyrolysis of marine kerogens

Abstract The evolution of various petroleum fractions during programmed-temperature pyrolysis of immature samples from the Toarcian Shale (Germany) and the Duvernay Formation (Canada) at a heating rate of 5.0 K min −1 has been studied comparatively under closed- and open-system conditions using the micro-scale sealed vessel (MSSV) technique in the former and a modified MSSV technique in the latter case. Whereas open-system heating gives raise to continuous primary product generation, closed-system experiments are marked by the overlap of primary generation and secondary cracking reactions; the apparent degree of overlap, however, turns out to be different for different compound classes. Primary generated C 15+ (total C 15+ fraction) compounds, in particular, are subjected to severe secondary cracking from the beginning. However, at temperatures lower than 460°C for both samples, secondary cracking essentially transforms C 15+ compounds into C 6–14 (total C 6–14 fraction) compounds. Because these processes take place only within the C 6+ fraction the sum of closed-system C 6+ compounds is close to the quantities of C 6+ compounds generated in the open-system. At temperatures higher than 460°C secondary oil-to-gas cracking becomes obvious from the decrease of C 6+ concentrations. Because the open-system generation of C 6+ compounds has come to an end at this temperature, the overlap of primary and secondary reactions only affects the overall composition of the C 6+ fraction in the closed-system but not the timing of secondary gas formation.

Heating rate dependency of petroleum-forming reactions: implications for compositional kinetic predictions

Abstract The generation of bulk petroleum, liquid and gaseous hydrocarbons from the Duvernay Formation was simulated by heating immature kerogens in a closed system (MSSV pyrolysis) at four different heating rates (0.013, 0.1, 0.7 and 5.0 K/min). Using the established parallel reaction kinetic model, temperature and compositional predictions were tested to be suitable for geological conditions by comparing the laboratory results with natural changes in source bitumens and reservoir oil maturity sequences from the Duvernay Formation. In the case of bulk liquid and gaseous hydrocarbons, the above kinetic calculations can be considered valid because their maximum yields are independent of laboratory heating rates. In contrast, the contents of paraffins, aromatics and sulfur compounds show a pronounced heating rate dependence. Extrapolated to geological heating rates, the compositional predictions are consistent with the bulk composition of natural products in the Duvernay-petroleum system showing an increase of paraffinicity and hydrogen content. In contrast to that, the “hump” decreases with decreasing heating rate, a trend which is confirmed by the low amounts of unresolved compounds in natural high maturity products. Because of these heating-rate dependent compositional changes, geological predictions of natural molecular composition by the commonly used kinetic models are not suitable.

Predicting Crude Oil Stability in Deep Hot Reservoirs

The stability of crude oil in deep hot reservoirs has been analysed using laboratory heating experiments and field studies in a three phase research project.