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.

Shutting the aging well for good towards the end of field life requires a lot of operational and technical considerations as well as high cost to ensure project viability and safe operation. Procedures to evaluate competent cap rock layers as natural formation barriers, well plug and abandonment (P&A) design, assurance on well barrier integrity through cement bond logging and best practices in well abandonment will be highlighted through three case studies in this paper. Data integration starting from the early exploration or development drilling phase, during production and towards the abandonment phase play key roles in the project and will be the ultimate pinnacles in determining cap rock intervals, cement integrity, tubulars condition, potential reservoir communication and plug setting depth in the well. Adding an extensive behind casing acoustic noise, pulsed neutron and cement bond logging campaign and full integrated formation evaluation in the picture will further enhance the feasibility of abandonment project with additional discovery of shallow gas potential to prolong the field economics. Detail analysis of petrophysical logs, mud log gas reading, well pressure monitoring, shallow gas hazard site survey and acoustic-noise log deliverables have revealed the presence of shallow gas as deep as 300-400 m below the sea bed and few more intervals at deeper or shallower depth. Incorporating all available data to date had resulted in extensive volume of gas-in-place being evaluated, thus driving additional value through potential workover or shallow gas appraisal project prior to abandonment. In addition, operational cost, time and safety will be further optimized through cap rock restoration method and cement recipe upgrading. In a nutshell, packaging shallow gas potential and possible work-over or appraisal campaign for quick field monetization prior to abandonment will prove to justify the project economics. Changing the game from conventional gas producer to shallow hanging fruits opportunity is worth to be exploited in this case.