ADVANCEMENTS IN CARBON-OXYGEN SURVEILLANCE OF THE DEEPWATER GULF OF MEXICO MARS WATERFLOOD

摘要

The primary purpose of the Mars (Mississippi Canyon 807) waterflood in the deepwater of the Gulf of Mexico (GOM) is to increase recovery efficiency in three main reservoirs. A robust surveillance logging program has been conducted in the field since 1996. Measurements have included strain, flow profile, reservoir layer pressure, casing inspection, and multi-component fluid saturation evaluations. The fluid saturation surveillance results derived from log measurements have historically been based on original porosity estimates. More recent analysis techniques have included reductions in porosity due to compaction to better understand fluid distributions and eliminate anomalous saturation indications. The newly developed modeling can observe and more accurately identify injected seawater at the observation wells. This fluid identification is accomplished through recently developed interpretation methods using pulsed neutron sensors including inelastic carbon-oxygen (C/O mode) and capture sigma ( mode). The method is demonstrated with several Mars examples including discussion of interpretations. Surveillance of the Mars field is critical to optimizing recovery. The cased hole logging data are integrated with other subsurface data, including 4D seismic, additional openhole logs from new wells, production data, and injection data. This dataset allows the team to properly operate existing wells, identify additional opportunities, and plan future activities. Carbon-oxygen and sigma surveillance has provided an understanding of the fluid changes within the reservoirs under the influence of compaction, water injection and aquifer movement. Through experiences in several reservoirs, anomalous tri-fluid (oil, native formation water and injected waterflood seawater) saturation estimates were identified. The proper evaluation of multiple fluids is dependent on the porosity of the rock matrix system. After application of measured strain and porosity reduction modeling, estimates obtained were more representative than what conventional analysis would derive. While log measurements investigate the near wellbore, reservoir modeling can predict a compaction profile spanning from most dramatic in the near wellbore region to lesser impact further in the reservoir. Techniques are demonstrated that help identify fluid and porosity changes in the volume observed by the pulsed neutron tool. Selected Mars well log examples are described in detail to highlight the results of compaction and fluid saturation with time. This discussion is focused on best practices learned during the 14 year surveillance program. Included are highlights on how recent log responses have encouraged the development of more advanced methods of interpretation. In some cases the interpretation became problematic since log responses did not indicate a unique solution. The new techniques presented have provided more accurate multi-fluid volumetric evaluation and assisted in quantification of compaction estimates in wells lacking strain surveillance. Potential added benefits from this recent method include fluid saturation evaluation in complex lithology reservoirs with enhanced recovery. The new method better defines fluid types and allows a more independent determination of porosity reduction. The use of improved methods will hopefully increase interest in reservoir surveillance and allow waterfloods to be operated more effectively.