Multi-finger caliper tools are widely used for inspection of casing inner wall condition, particularly for determining wear, corrosion, deformation and scaling. The maximum pipe wall penetration, and hence metal loss, inferred from this method is greatly dependent on the casing inner diameter value (provided by casing manufacturer) used as a reference. Evidence of overestimated casing wear, including new completions, has raised the question around the validity of such interpretations. Most of the casing is manufactured to API 5CT (ISO 11960) standard. The manufacturer‘s specification has an associated tolerance on casing outer diameter and thickness that is then used in the multi-finger caliper logs interpretation. The tolerance for casing with outer diameter (OD) greater or equal than 4.5″, is between -0.5% and +1% of OD. The pipe wall thickness tolerance ranges between manufacturer's nominal thickness and 12.5% metal loss. The calculation of casing internal diameter (ID) tolerance is a difficult task as this is controlled by the casing mass tolerance which in practice is not easy to measure. To overcome the challenge, we implemented a new approach of calculating the minimum and maximum casing inner diameter and then using the ID tolerance as a reference for multi-finger caliper logs interpretation. This approach has been tested with field data, and shows good correlation to actual casing inner diameter measurements performed at surface and is used as a baseline for new casing strings. This paper will describe in detail the approach taken and compare the difference between interpretation outcomes with and without using the casing inner diameter tolerance. The benefits of calculating casing inner diameter tolerance and applying it to multi-finger caliper log interpretation results will be backed up with evidence from field log data acquired in ACG.

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The late stage development of ACG field poses new challenges that can be addressed through completion technologies associated with new or improved methods of surveillance using Multidetector Pulsed Neutron (MDPN) instrumentation. In this paper we will describe the techniques developed and deployed to address specific well and reservoir challenges. The topics will include: –Evaluate the quality of the initial fill and subsequent changes to various types of proppant with stand-alone and time lapse logging for Open Hole Gravel Pack completions–Identification of scale depositing behind the screens–Monitoring the gas cap movement and sweep efficiency in difficult environments (large casing in holes enlarged by reamer, heavy mud in borehole,) The nuclear modelling has a key role in the early planning stage, enabling the selection of appropriate instruments and acquisition modes. This will reduce the interpretation uncertainty, and minimise the rig time and production deferment. Coupling MDPN measurements with innovative non-radioactive tracer doped gravel, enable a more efficient and accurate reservoir and well surveillance in Open Hole Gravel Pack Completions. The interpretational techniques combine the measured data with specific models of nuclear attributes benchmarked in yard tests that replicate some of ACG completions. Close quality control of the data during acquisition and proper communication with logging vendor is the key in to proper answers. Using multiple nuclear attributes leads towards a more unique solution in such complex environments. The approach taken along with the results from seven wells will be presented and discussed in detail.

Recent developments in pulsed neutron instrumentation open up new opportunities for petrophysical surveillance. Understanding the relation between a nuclear attribute and a recovery mechanism is a key element in selecting the appropriate running mode for a specific instrument. Developing a nuclear modelling capability allows us to screen various nuclear attributes for a given environment taking into account, specific rock, fluid and completion conditions to investigate the sensitivity of attributes to different recovery scenarios. There are a limited number of experimental tests pits limiting our ability to replicate the complexity of nuclear attribute responses to the variety of fluid displacements and well completions. Imperfections with cement and gravel pack further complicate the nuclear attribute response and should be taken into account during the nuclear model process. Benchmarking the nuclear models against controlled environments (man-made defects) provide some confidence in utilising nuclear forward modelling to design the logging program ahead of the job. This allows us to optimise rig time and maximise the data acquisition during well work opportunities. This method can be used in feasibility studies and petrophysical interpretation utilizing nuclear tools in different areas of petrophysical surveillance. Nuclear modelling can help to engineer gravel pack (specify the amount of Gadolinium enrichment), define the nuclear attributes that can be further used for gravel pack evaluation, and can also help in selecting the right tool from suppliers for monitoring multicomponent fluid saturations. In this paper we present some case studies to illustrate the application of nuclear modelling in the selection of attributes and tools for completion and reservoir surveillance. The results from experimental tests provide us with confidence in the quality of results from nuclear modelling. A robust model requires detailed information from core, fluids and completion. While all multidetector pulsed neutron instruments utilize the same physical principles and record similar basic measurements, there are differences due to tool geometry, electronics and interpretation algorithms. Screening nuclear attributes and selecting the appropriate tool for the job has a significant impact on rig time and acquiring quality data. Improving the quality of the acquired data will have a direct impact on the quality of the calculated results which will in turn lead to more effective well work and reservoir management.