f Reservoir Surveillance by Production and Pulsed Neutron Logging in the West Qurna I Field, Southern Iraq

The West Qurna-1 field is comprised of a series of stacked carbonate and clastic reservoirs with highly variable reservoir properties. The main producing reservoir is the Cretaceous Mishrif carbonate, characterized by reservoir heterogeneity ranging from high permeability grainstones to low permeability microporosity-dominated facies. This reservoir quality heterogeneity produces high contrast flow units which pose a challenge to reservoir management and require extensive reservoir surveillance. Reservoir surveillance has many different aspects, including monitoring zonal flow contributions, zonal injection performance (waterflood), pressure monitoring, saturation changes, and oil-water contact (OWC) movement. Production logs (PLTs) are primarily run in wells completed in the carbonate Mishrif and the clastic Zubair reservoirs. Key objectives are (1) to measure well production rates in light of scarce fieldwide surface rate metering, (2) to determine zonal flow contributions and waterflood injection performance from acidized commingled perforated intervals, (3) to determine static/flowing cross flow patterns, (4) to measure flowing and static pressures for Psat maintenance, and (5) to obtain indications of cross-flow from behind pipe. An additional objective is the recent use of PLT toolstrings in the Mishrif for single rate production build up (PBU) analysis for permeability-thickness (kh), skin, and drainage area reservoir heterogeneity for wells with single and multiple perforated intervals. Where surface rates are available via test separators or Multi Phase Flow Meters, PLT results from the same time frame are integrated with the surface well performance data (ie. flowing upstream pressures and choke settings) to set optimal surface flow conditions within established reservoir surveillance guidelines (ie. guide flowing upstream pressures to ensure down-hole flowing pressure is above oil Psat). PLT results are used to guide current perforation and zonal shut-off strategies and will guide future water injection strategy. For saturation and OWC monitoring, Pulsed Neutron Capture (PNC) saturation logs are predominantly run in sigma capture mode. Mishrif saturation logs have been run since 2010 to obtain base logs for future time-lapse monitoring for (1) flank OWC movement, (2) saturation changes offsetting injection wells, and (3) monitoring water encroachment along recognized high kh zones. PNCs are also run on an as-needed basis for drillwells without complete open-hole logs and as diagnostic logs on high-water cut wells that have stopped flowing. Challenges for the PNC program include recognizing true saturation changes in heavily acidized completed intervals, and establishing an efficient repeat logging pace for time-lapse.