Real Time Virtual Flow Metering, A Digital Enhancer for Gas Injection Monitoring and Optimization

Abstract

Abstract Reservoir gas re-injection is a reservoir management technique able to avoid flaring, boost oil production and preserve resources for future usage. Quite often, reinjected gas volume measurements are affected by poor reliability. This uncertainty affects also total gas injection flow rate, which has not the same accuracy as fiscal oil produced. On the other hand, a huge amount of real time data is available in the smart oil field framework and it can provide an insight to mitigate this issue. In this context, Real Time Virtual Flow Metering (RTVFM) is a digital tool able to significantly improve the accuracy of reinjected gas flow rates estimation both at well and field level. This work presents the successful design and application of RTVFM to an offshore oil field in West Africa with significant uncertainty on gas injection rates. RTVFMs are orchestrated with an in house-built workflow; more specifically two virtual meters are integrated in this application: one for the tubing model and another one for the choke model. The implementation foresees a preliminary calibration phase, where both virtual meters are matched against reliable measured data. Next, the workflow runs in real time and estimates gas injection flow rates on well basis according to pressure and temperature data from permanent gauges installed on flowline, tubing head and downhole. The integration of two virtual meter models enables also to withstand periods with sensor measurement errors or data unavailability, for example bottom hole pressure readings. In fact, permanent downhole gauges are often subject to temporary or permanent failures and their maintainability is difficult, especially for offshore wells. Three years of gas injection history were analyzed and simulated for three injector wells, highlighting significant discrepancy of measurement, both at well and field level, in comparison with previous back-allocation based on ultrasonic flow meters. By improving the total gas measure and the back allocation at well level, RTVFM greatly improved reservoir management. Indeed, RTVFM highlighted periods of under-injection where full injection potential was not achieved as well as periods of over-injection that could compromise future injectivity in the reservoir or trigger earlier gas breakthrough at producers. In conclusion, RTVFM is a digital enhancer for gas injection monitoring and optimization and in this specific application it helps the reservoir management in the correct allocation of available gas volumes. Moreover, it represents a cost-effective parallel solution even in presence of physical metering. Finally, the key takeaway of RTVFM is making the most out of already available data, often unused. A novel procedure to calibrate RTVFM was developed for gas injectors with intermittent bottom hole pressure readings, thus integrating data from well testing and wireline formation testing. Moreover, a new validation methodology based on well performance was created to check RTVFM simulation versus ultrasonic flow meters measures.