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Qazvini Firouz A.,Computer Modelling Group Ltd. | Torabi F.,University of Regina
Fuel | Year: 2014

Previous studies showed that using non-condensable gases in huff-and-puff is successful in producing light oil components, but heavier components cannot be produced during the puff cycle. This study aims to validate the feasibility of solvent based huff-and-puff method with respect to enhancing heavy oil recovery for a wide range of operating conditions. Thus, an experimental approach was applied and a physical model with 1.774 μm2 absolute permeability and 24% porosity Berea core mounted in a high pressure stainless steel core holder was designed. For all tests, the core was saturated with a sample of Saskatchewan heavy oil with viscosity of 952 mPa s at 28 C. Over eight sets of huff-and-puff experiments at different operating pressures and a constant temperature of 28 C were performed, utilizing the pure carbon dioxide (CO2) and pure methane. A soaking time period of 24 h and a cut-off pressure of 276 kPa were considered for all cycles. In addition, each set of cyclic injection process were continued until production dropped below 1% of the original oil in place. Hence, 66 huff-and-puff cycles were performed. Considering the performance of all tests, at higher operating pressure the produced oil was lighter and the recovery factors were higher due to higher diffusion coefficient and solubility factor of solvent. Ultimate oil recovery of 71% was obtained by injecting pure CO2 at near supercritical condition of 7239 kPa and 28 C, while that of pure methane at the highest operating pressures of 6895 kPa was 50%. The governing mechanisms contributed to the production were recognized to be solution gas drive, viscosity reduction, extraction of lighter components, formation of foamy oil and to a lesser degree diffusion process. The produced oil viscosity was reduced to 62 mPa s (at 28 C) by injecting CO2 at 7239 kPa. In addition, most of the produced oil occurred during the first five cycles for each set of huff-and-puff test. The highest incremental recovery for all CO2 tests occurred at the 2nd, while that of methane happened at the 3rd cycle. Also, the solvent diffusion process was identified qualitatively by pressure depletion around the core during each experiment. © 2013 Elsevier Ltd. All rights reserved. Source

Uddin M.,Alberta Innovates Technology Futures | Coombe D.,Computer Modelling Group Ltd.
Journal of Physical Chemistry A | Year: 2014

Molecular dynamics simulations of gas hydrate dissociation comparing the behavior of CH4 and CO2 hydrates are presented. These simulations were based on a structurally correct theoretical gas hydrate crystal, coexisting with water. The MD system was first initialized and stabilized via a thorough energy minimization, constant volume-temperature ensemble and constant volume-energy ensemble simulations before proceeding to constant pressure-temperature simulations for targeted dissociation pressure and temperature responses. Gas bubble evolution mechanisms are demonstrated as well as key investigative properties such as system volume, density, energy, mean square displacements of the guest molecules, radial distribution functions, H2O order parameter, and statistics of hydrogen bonds. These simulations have established the essential similarities between CH4 and CO2 hydrate dissociation. The limiting behaviors at lower temperature (no dissociation) and higher temperature (complete melting and formation of a gas bubble) have been illustrated for both hydrates. Due to the shift in the known hydrate stability curves between guest molecules caused by the choice of water model as noted by other authors, the intermediate behavior (e.g., 260 K) showed distinct differences however. Also, because of the more hydrogen-bonding capability of CO2 in water, as reflected in its molecular parameters, higher solubility of dissociated CO2 in water was observed with a consequence of a smaller size of gas bubble formation. Additionally, a novel method for analyzing hydrate dissociation based on H-bond breakage has been proposed and used to quantify the dissociation behaviors of both CH4 and CO2 hydrates. Activation energies E a values from our MD studies were obtained and evaluated against several other published laboratory and MD values. Intrinsic rate constants were estimated and upscaled. A kinetic reaction model consistent with macroscale fitted kinetic models has been proposed to indicate the macroscopic consequences of this analysis. © 2014 American Chemical Society. Source

Uddin M.,Alberta Innovates Technology Futures | Wright F.,Geological Survey of Canada | Coombe D.,Computer Modelling Group Ltd.
Journal of Canadian Petroleum Technology | Year: 2011

Gas hydrates are a potentially vast untapped source of natural gas. Recent numerical and field studies suggest the Mallik gas-hydrate field in Canada's Mackenzie Delta may represent a technically producible and potentially economically viable reservoir of natural gas. Our initial reservoir simulations using a kinetic reaction approach indicate that gas evolution and transport within porous geologic reservoirs have a significant effect on fluid production characteristics, while field and laboratory data suggest that significant amounts of evolved gas can be trapped for some time within the reservoir, depending on the field operation. In this work, we invoke modelling concepts extensively employed in quantifying gas ex-solution from viscous oils to further assess the kinetic behaviour of gas-hydrate ex-solution through depressurization. Here, the gas bubbles can be categorized into three groups with explicit transport behaviour: small bubbles (water phase), large bubbles (immobile) and connected bubbles (or free gas). These concepts allow the development of a new set of kinetic reactions for hydrate dissociation: one representing the (possibly delayed) conversion of hydrate into water and dispersed gas bubble phases, and one representing the evolution from dispersed bubbles to connected bubbles. These reactions can effectively capture the nonequilibrium fluidflow behaviour observed in field production tests. For modelling of the transport phenomenon, we assumed two explicit mobility formulations: (1) trapped bubbles (no mobility) and a flowing water phase and (2) large connected gas bubbles and flowing water (with relative mobility). Relative mobility can be estimated by using traditional gridblock-relative permeability curves. We then develop a simple mechanistic gas bubble trapping tool as a function of the capillary number, which can easily be incorporated into our numerical simulator. This entrapment of the nonwetting gas-phase results in higher values of critical gas saturation. Two case studies based on alternative representations of a Mallik-like gas-hydrate reservoir demonstrate that significant errors can result in reservoir modelling if these fluid transport phenomena are not adequately represented in numerical simulations. Aspects of the model developed here have been applied to history matching and prediction of natural gas recovery from a clastic, sand-dominated reservoir at the Mallik site. Source

Bourgault G.,Computer Modelling Group Ltd.
Mathematical Geosciences | Year: 2012

The likelihood of Gaussian realizations, as generated by the Cholesky simulation method, is analyzed in terms of Mahalanobis distances and fluctuations in the variogram reproduction. For random sampling, the probability to observe a Gaussian realization vector can be expressed as a function of its Mahalanobis distance, and the maximum likelihood depends only on the vector size. The Mahalanobis distances are themselves distributed as a Chi-square distribution and they can be used to describe the likelihood of Gaussian realizations. Their expected value and variance are only determined by the size of the vector of independent random normal scores used to generate the realizations. When the vector size is small, the distribution of Mahalanobis distances is highly skewed and most realizations are close to the vector mean in agreement with the multi-Gaussian density model. As the vector size increases, the realizations sample a region increasingly far out on the tail of the multi-Gaussian distribution, due to the large increase in the size of the uncertainty space largely compensating for the low probability density. For a large vector size, realizations close to the vector mean are not observed anymore. Instead, Gaussian vectors with Mahalanobis distance in the neighborhood of the expected Mahalanobis distance have the maximum probability to be observed. The distribution of Mahalanobis distances becomes Gaussian shaped and the bulk of realizations appear more equiprobable. However, the ratio of their probabilities indicates that they still remain far from being equiprobable. On the other hand, it is observed that equiprobable realizations still display important fluctuations in their variogram reproduction. The variance level that is expected in the variogram reproduction, as well as the variance of the variogram fluctuations, is dependent on the Mahalanobis distance. Realizations with smaller Mahalanobis distances are, on average, smoother than realizations with larger Mahalanobis distances. Poor ergodic conditions tend to generate higher proportions of flatter variograms relative to the variogram model. Only equiprobable realizations with a Mahalanobis distance equal to the expected Mahalanobis distance have an expected variogram matching the variogram model. For large vector sizes, Cholesky simulated Gaussian vectors cannot be used to explore uncertainty in the neighborhood of the vector mean. Instead uncertainty is explored around the n-dimensional elliptical envelop corresponding to the expected Mahalanobis distance. © 2012 International Association for Mathematical Geosciences. Source

Feng H.-Y.,University of British Columbia | Endrias D.H.,TS Technology Canada Inc. | Taher M.A.,Boeing Company | Song H.,Computer Modelling Group Ltd.
CAD Computer Aided Design | Year: 2013

This paper presents a novel combinatorial search algorithm for determining minimum circumscribed (MC) circles and spheres from discrete data points. The presented algorithm is able to efficiently identify the essential subset of the input data points to construct the MC circle/sphere for the entire data set. The common issue of computational explosion for a large data set due to a greatly increased number of combinatorial searches is thus of no concern. The main feature of this work is the derivation of an innovative geometric property, named the Integrated Property (IP), for a unique three-point subset in 2D or a unique four-point subset in 3D. The significance is that the MC circle/sphere for the identified IP point subset is in fact the MC circle/sphere for the entire data set. As the unique IP point subset can always be found, the presented algorithm is guaranteed to yield exact MC circle/sphere solutions. A related data exchange scheme is formulated to efficiently identify the unique IP point subset from the input point set. The expected computational complexity of the search algorithm is quantified to be O(nlogn) through a large number of computational test cases. © 2012 Elsevier Ltd. All rights reserved. Source

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