Production allocation is required in hydrocarbon accounting to determine the hydrocarbon volume at the point of sale and for back allocation to the field, platform, well, and down to the individual reservoir levels. Production allocation is not only important for the purpose of reporting to the host government but also to understand the remaining hydrocarbon reserves which are crucial for reservoir management and input to the full field review studies.
For wells producing from commingle zones, the individual zonal contribution determination is important. The Production Logging Tool (PLT) is commonly used to measure each reservoir's contribution downhole. Latest technology advancement in directional drilling over time has allowed for more deviated and horizontal wells. Well deviation is one of the factors affecting fluid flow pattern in a borehole apart from the phase holdups and fluid properties (PVT). As production fluid flows upwards in a deviated well, the movement of the lighter phase to the high side of the well displaces the dominant heavier phase liquid, causing it to flow downwards. This borehole phenomenon is commonly known as Apparent Down Flow (ADF).
A standard PLT has a centralized spinner configuration and when run in wells experiencing ADF will likely cause the spinner to measure an incorrect fluid velocity. Depending on the degree of the holdup of the heavier phase, the spinner may show a reduced or even negative rotation if it is immersed in the heavier phase fluid. Conversely, the spinner may show faster rotation if it is located in the lighter phase fluid. The advanced PLT, with its array of mini spinners and holdup sensors, was developed in part, to measure the effects of ADF and was designed to cover the well's cross section area, giving a more accurate description of the flow behavior; thus better measurement and understanding of ADF phenomena.
It has been observed from many production logging surveys that were conducted using a standard PLT, where the spinner shows negative readings during the flowing condition, indicating fluid re-circulation (or fluid fallback). However, information from other sensors such as fluid density identifier and temperature tool does not support these findings (of fluid re-circulation), which results in inaccurate rate calculation to determine zonal contribution. To overcome this challenge, the advanced PLT can be used to measure the contribution for each zone more accurately as the effects of ADF can be further understood. The calculated production rates from the advanced PLT were found to be more representative despite the presence of ADF in the wells.
This paper discusses some case studies on the application of the advanced PLT in overcoming the challenges of quantifying zonal contribution in wells experiencing ADF.