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

摘要

The primary purpose of the Mars (Mississippi Canyon807) waterflood in the deepwater of the Gulf of Mexico(GOM) is to increase recovery efficiency in three mainreservoirs. A robust surveillance logging program hasbeen conducted in the field since 1996. Measurementshave included strain, flow profile, reservoir layerpressure, casing inspection, and multi-component fluidsaturation evaluations. The fluid saturation surveillanceresults derived from log measurements have historicallybeen based on original porosity estimates. More recentanalysis techniques have included reductions inporosity due to compaction to better understand fluiddistributions and eliminate anomalous saturationindications. The newly developed modeling canobserve and more accurately identify injected seawaterat the observation wells. This fluid identification isaccomplished through recently developed interpretationmethods using pulsed neutron sensors includinginelastic carbon-oxygen (C/O mode) and capture sigma(∑ mode). The method is demonstrated with severalMars examples including discussion of interpretations.Surveillance of the Mars field is critical to optimizingrecovery. The cased hole logging data are integratedwith other subsurface data, including 4D seismic,additional openhole logs from new wells, productiondata, and injection data. This dataset allows the team toproperly operate existing wells, identify additionalopportunities, and plan future activities. Carbon-oxygenand sigma surveillance has provided an understandingof the fluid changes within the reservoirs under theinfluence of compaction, water injection and aquifermovement. Through experiences in several reservoirs,anomalous tri-fluid (oil, native formation water andinjected waterflood seawater) saturation estimates wereidentified. The proper evaluation of multiple fluids isdependent on the porosity of the rock matrix system.After application of measured strain and porosityreduction modeling, estimates obtained were morerepresentative than what conventional analysis wouldderive. While log measurements investigate the nearwellbore, reservoir modeling can predict a compactionprofile spanning from most dramatic in the nearwellbore region to lesser impact further in the reservoir.Techniques are demonstrated that help identify fluidand porosity changes in the volume observed by thepulsed neutron tool.Selected Mars well log examples are described in detailto highlight the results of compaction and fluidsaturation with time. This discussion is focused on bestpractices learned during the 14 year surveillanceprogram. Included are highlights on how recent logresponses have encouraged the development of moreadvanced methods of interpretation. In some cases theinterpretation became problematic since log responsesdid not indicate a unique solution. The new techniquespresented have provided more accurate multi-fluidvolumetric evaluation and assisted in quantification ofcompaction estimates in wells lacking strainsurveillance. Potential added benefits from this recentmethod include fluid saturation evaluation in complexlithology reservoirs with enhanced recovery. The newmethod better defines fluid types and allows a moreindependent determination of porosity reduction. Theuse of improved methods will hopefully increaseinterest in reservoir surveillance and allow waterfloodsto be operated more effectively.