EOS Modeling, The First Step in Detailed EOR Potential Evaluation - A Case Study
- Pashupati Sah (Calsep A/S) | Karen Schou Pedersen (Calsep A/S) | Jawad Azeem (Calsep Dubai)
- Document ID
- Society of Petroleum Engineers
- SPE Enhanced Oil Recovery Conference, 19-21 July, Kuala Lumpur, Malaysia
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 5.2.2 Fluid Modeling, Equations of State, 5.2 Reservoir Fluid Dynamics, 5.4.2 Gas Injection Methods, 5.5 Reservoir Simulation, 4.6 Natural Gas, 5.2.1 Phase Behavior and PVT Measurements
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The paper presents an Equation-of-State (EOS) modeling work carried out for a Middle East reservoir fluid for which gas injection was considered for increasing ultimate recovery. The aim of the work was to develop an EOS model that would accurately reproduce the phase behavior in a reservoir on injection of either a hydrocarbon gas (mix of gas condensate and associated rich gas) or a CO2 rich gas. A single EOS model was developed, which provided a good match of data for both injection gases. This EOS model enables compositional reservoir simulation studies to be carried out comparing and contrasting the recovery from the field with each of the two injection gases.
Extensive PVT data was available and to be matched by a 9-component 'lumped' EOS model. Available data included classical PVT data as well as gas injection (EOR) data including solubility swelling, equilibrium contact and slim tube tests. A major challenge was to develop a model which, in addition to classical PVT data, which can easily be regressed to, also matched slim tube minimum miscibility pressures (MMPs). A multi-component tie-line method was used considering combined vaporizing/condensing drives, and the tie-line MMP was afterwards verified using a cell-to-cell simulator.
Depth gradient simulations indicated that the transition from liquid-like to vapor-like properties in the reservoir did not take place through a sharp gas-oil contact (GOC), but happened continuously in a 'transition zone'. An EOS model neglecting such 'transition zones' or simulating a sharp gas-oil contact may lead to severe misinterpretations in reservoir simulations. A segregation model based on irreversible thermodynamics was used to investigate the influence of an observed vertical temperature gradient on the compositional variation with depth.
The oil industry shows a high level of confidence in miscible gas injection as a mean of increasing ultimate recovery from an oil field. Still the theoretical aspects of the fluid phase behavior in a field with gas injection are items of discussion. However much of the theoretical work is based on model systems and not easily related to reservoir fluids. This case study is for a project where the amalgamation of laboratory experiments and numerical PVT simulations provided a robust EOS model, which, using two different injection gas compositions, could be used to represent the miscible gas injection process in a compositional reservoir simulation model. The extensive amount of experimental data provided a unique opportunity to enlighten the theory behind and test the validity of tie-line simulation methods for determining the combined vaporizing/condensing MMP of a reservoir fluid composition.
Reservoir fluids are usually classified as oils, gas condensates or gas mixtures and different correlations and simulation models may be applied dependent on fluid type. Distinction by fluid type is, however, inappropriate for oil mixtures undergoing miscible gas injection. Miscibility develops through a critical composition, i.e. a gas and a liquid in equilibrium become identical. An EOS model to be used for an oil mixture undergoing gas injection should therefore ideally match the critical composition at miscibility. As miscibility develops after numerous (several thousands) contacts between gas and oil it is, in reality, impossible to measure the critical composition at equilibrium. What can be measured is an approximate critical composition (shift from bubble to dew point) in a swelling experiment. Though there is only one contact between gas and oil in a swelling experiment the experiment still provides useful information about the gas-oil mixing ratio needed to develop a critical composition. To measure the MMP slim tube experiments are required. MMP is the lowest pressure at which a critical composition can develop as a result of multiple contacts. Though the slim tube experiment does not provide any info about the critical composition, it does tell what pressure is needed to develop a critical composition.
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