Unlocking the Potential of Remaining Hydrocarbons Using Time Lapse Pulsed Neutron Capture Logs in Complex Completions - A Case Study from UK Central North Sea

摘要

Reservoir surveillance using pulsed neutron capture (PNC) sigma data has shown its importance historically (Madhavan et al. 2015; Odom et al. 1994; Gournay and Maute 1982). This paper proposes to discuss the planning, acquisition and analysis of the PNC sigma data in complex well completions with multiple casing strings and hole sizes. A case study from the Gannet A field located in the Central North Sea of the United Kingdom has been presented to demonstrate the importance of having the correct logging environment and its subsequent effect on the PNC sigma processing and interpretation. The consequence of not making such detailed checks may result in mis-interpretation of the sigma data leading to incorrect conclusions about fluid dynamics, and risky decisions in water shut-off (WSO) opportunities and additional perforations. The integration of PNC sigma data with other surveillance techniques such as production logs and 4D seismic has also been discussed to understand the reservoir dynamics for Well Reservoir and Facility Management (WRFM) planning purposes. The Gannet A field comprises an Eocene age deep marine amalgamated turbidite sand reservoir with combined dip/stratigraphic closure. The field initially had a gas cap and a thin oil rim. In the early 1990's, eleven horizontal producer wells were drilled to produce solely from the oil rim. Two additional wells were used to re-inject a portion of the produced solution gas into the gas cap. Gas injection ceased in 2001 due to unexpected strong aquifer ingress from the North-East. The oil rim and aquifer moved upwards into the former gas cap in the North resulting in a tilted contact from the North to the South and watering out of the producers progressively over time. The PNC sigma data has played an important role in accessing the migrated oil rim and understanding reservoir movement. In 2015, PNC sigma data helped to add perforations in three wells in the Gannet A field adding approximately 5,000 bbl/d of incremental oil. This paper focuses on the three key areas in PNC sigma interpretation which are often overlooked. Firstly, quantifying the effect of the type of annuli fill, hole size, and multiple casing/tubing strings on computed saturations; secondly, the importance of using consistent environmental parameters for time lapse comparison of the PNC sigma data; and thirdly, the added uncertainties in computed saturations due to the presence of three fluid phases; i.e. migration of oil rim into the original gas cap and its subsequent sweep with water.