Unified PTA and PDA Approach Enhances Well and Reservoir Characterization

Abstract

Abstract As permanent downhole gauges have become more reliable, increasingly operators are installing them on injection/production wells. Each time the well is shut in, often solely for operational reasons, pressure transient analysis (PTA) can be performed on the resulting falloff/buildup transient lasting from a few hours to a few days. When the well is flowing, the permanent gauge provides a long term record of pressure behavior during injection/production. Usually the well rate is also determined on a continuous basis, daily or monthly, although often at a much lower data rate than the pressures recorded by the permanent gauge. Recorded pressure and flow rate data are typically used for production data analysis (PDA). Due to the difference in data collection between PTA and PDA, these analyses are performed independently, yielding multiple interpretations from a diverse group of people and software programs. At times the results may conflict, and creating one consistent well and reservoir characterization can be quite challenging and time consuming. A unified interpretation of both analyses would reduce analysis time and increase confidence in the results. In this study, relatively short-duration PTA data and long-term PDA data are combined to provide a more complete analysis than either PTA or PDA alone can provide. The unified analysis is displayed on a combined plot of pressure change and its derivative from a single transient flow period (preferably a falloff/buildup), and rate normalized pressure (RNP) and its derivative based on injection/production data. Data processing techniques follow a stepwise procedure to construct the combined plot. The result is a virtual drawdown response that can be diagnosed like pressure and pressure derivative and matched with an appropriate model for the well and its drainage area. The method is most effective when flow remains single phase or at least segregated in the reservoir. Apart from the unified PTA and PDA approach, this study also offers significant improvements in the RNP data processing that leads to a much more conclusive PDA. Synthetic examples illustrate the effectiveness of the technique for known models. Then two field examples show its practical application.