Modifying the Hall Plot for Analysis of Immiscible Gas Injection Wells
- O. A. Talabi (Schlumberger)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference Asia, 22-25 March, Kuala Lumpur, Malaysia
- Publication Date
- Document Type
- Conference Paper
- 2016. Offshore Technology Conference
- 5.5 Reservoir Simulation, 5 Reservoir Desciption & Dynamics, 5.4 Enhanced Recovery, 5.4.2 Gas Injection Methods, 5.4 Enhanced Recovery
- Hall Plot, Gas, Monitoring, Analytic, Injection
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The Hall plot is a proven, useful tool for evaluating performance of injection wells. It was originally developed for single-phase, steady-state, radial flow of liquids. It continues to be used to analyse water-injection wells and has also been modified and applied to the injection of polymer micellar solutions for enhanced oil recovery (EOR). However, published studies into its applicability for gas injection scenarios are very limited. This paper discusses the applicability of, and modifications to, the Hall plot for evaluating injection of gas into a reservoir under specific conditions—particularly immiscible gas injection into a gas cap.
Three commonly used expressions of the Darcy equation applied to gas wells exist—the low pressure approximation, the high pressure approximation and the method of real-gas pseudopressures. For this study, a methodology similar to that which was originally used to derive the Hall plot for liquids is applied to these expressions in order to derive modified ‘Hall’ expressions for gas wells, making appropriate assumptions. The validity of the resulting derived expressions was then verified using results from a commercial reservoir simulator. Published correlations were used to evaluate pressure-dependent properties for the plot, where required.
Based on the analysis, it was found that at low pressures, using the original expression of the Hall plot introduces a nonlinearity that could be misinterpreted as a change in injectivity. The modified expression for the Hall plot, based on the low-pressure approximation and published correlations, removes this artificial nonlinearity and straightens the plot, greatly improving its diagnostic value. It was also seen that at high-pressures, the attempts to re-derive a Hall plot expression for gas flow lead to an expression of the same form as the original Hall plot. Therefore the original Hall plot can be used without modification as it produces a straight line with no curvature. The method of pseudopressures gives the most comprehensive solution over the entire pressure range as expected, although it is more difficult to evaluate due to the need for numerical integration.
The results from this work indicate that modified versions of the Hall plot can be used qualitatively by engineers to identify injectivity problems during injection of gas into a reservoir under specific conditions—immiscible gas injection into a gas cap—as long as the appropriate approximations and correlations are used.
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