Optimization of ThermalProcesses in Heavy Oil Recovery

Abstract

The past few years has been marked by high oil prices, apparent inability of traditional oil supplies to meet world energy demands, and a move towards evaluating untraditional (higher-cost) energy resources such as heavy oil, tar, and renewable energy from solar, wind, and nuclear sources. The petroleum industry has made major investments towards developing heavy oil resources, mainly located in Canada and Venezuela.

The achievements of the thesis provide the industry with technology and methods to optimize heavy oil resources. The main objective of the present work is divided in two categories: (a) Modeling the thermodynamics of bitumen (heavy oils) and solvent mixtures, and (b) Optimization of thermal process.

First part describes a systematic approach to model the phase behavior and viscosity of Athabasca bitumen and light solvent mixtures for a wide range of temperatures. The description of heptanes and heavier components (C7+) in reservoir fluids can be important for equation of state (EOS) predictions of phase and volumetric behavior. A procedure for C7+ characterization of heavy oil based on crude assay data which are typically measured for refining and marketing applications is first described.

Then a cubic equation of state (EOS) is developed to model the volumetric and phase behavior of bitumen and solvent mixtures. PVT data of pure solventbitumen mixture are analyzed and used to update the model parameters. Moreover, new technique for modeling viscosity of the bitumen and solvent mixtures is developed for a wide range of temperatures. The application of this approach for viscosity modeling is briefly described.

The final EOS/viscosity model is successfully used to predict the measured PVT and viscosity data for the mixture of the Athabasca bitumen and synthetic combustion gas solvents.

Finally, the developed EOS/viscosity model is used to calculate the required properties for the fluid model used by thermal simulators.

The second section of the thesis provides new approach for modeling steam assisted gravity drainage (SAGD) process. It helps to increase the speed of the computation in an optimization problem. In this approach SAGD is modeled using an isothermal black-oil (BO) reservoir simulator. The oil viscosity reduction caused by heating in the actual SAGD process is emulated by a tuned saturated pseudo-oil viscosity relation where solution gas-oil ratio is used as a “proxy for temperature”. In the black-oil formulation: (1) fully saturated oil viscosity at reservoir pressure equals the oil viscosity that would be attained at steam-chamber temperature in the actual SAGD process; (2) initial dead-oil viscosity with zero solution gas-oil ratio represents initial oil viscosity at reservoir temperature; and (3) black-oil gas properties represent steam at steam temperature.

Upon careful analysis of the SAGD process, one finds that oil flows only along a narrow zone along the outer edge of the steam chamber. The temperature gradient within this narrow zone is perpendicular to the oil flow direction and is practically impossible to model with any precision because of the large temperature variation and dynamic steam chamber shape over time. The black-oil model solubility gradient also varies, analogous to temperature in a thermal model, from zero to fully-saturated with an associated drop in oil viscosity from initial oil viscosity to the viscosity at steam temperature.

The proxy model saturated pseudo-oil viscosity relation used is found by history matching a full-physics thermal model performance prediction of oil rate, BHFP, and cumulative oil for a 2D homogeneous model. It is found a singleconstant viscosity equation that yields a good match to thermal SAGD performance. The tuned pseudo-oil viscosity relation honors the measured initial reservoir and fully-heated oil viscosities. Its dependence on gas-oil ratio is not physical, but reflects the use of gas-oil-ratio as a transform variable for temperature, capturing the strong spatial variation of temperature and oil viscosity within the localized steam-oil boundary region where oil has been mobilized.

The pseudo-oil viscosity relation appears to be applicable for a wide range of reservoir heterogeneity, injection and production rates, and well placement. Consequently, it should be possible to use the black-oil proxy model for SAGD optimization.

Moreover, the section describes the detailed mechanism of the SAGD process for homogenous and heterogeneous viscosity along the formation using the developed fluid model from first phase of the work.

Also in this section, detail analysis of solvent additives with steam in a SAGD process is studied. The mechanism is briefly described through series of numerical simulations and developed analytical approach in this work. It is also shown that under which condition the solvent-based SAGD is not efficient.

Finally, Integrated-optimization of solvent-based SAGD process is developed. The optimization of reservoir, surface process and pipeline models are conducted to maximize the net-present-value defined as objective function. The work also provides recommendations for future research and development in this area.