Diffra West is a medium size oil field in Muglad basin of Sudan. Oil is accumulated in multiple sandstone reservoirs of AD and BE formations. The field has been developed through conventional vertical wells and electrical submersible pumps (ESP's) are in use to lift the oil. Initial well productivity was very good. Currently the field is facing the challenge of high water cut which is masking the oil production and will result in lower recovery factor than envisaged in field development plan (FDP). Arresting the production decline by controlling the excess water production is crucial for achieving the targeted recovery factor. Well intervention programmes have given mixed results in view of inadequate diagnosis of source of water based on swab tests. This paper discusses the use of Production Logging Tool (PLT) for identification of water source and implemention of corrective action in order to revive and improve the oil production of an up-dip well in the Diffra West field. The decision to use the tool was guided by prudent reservoir monitoring and analysis after misleading swabbing test that would have led to permanent isolation of the main producing formation in this well.

The field was producing oil with water cut of about 60% when it was decided to investigate the source of the water. Performance of updip wells was also affected due to high water cut. D5 which is an up-dip well was selected for this study due to its better position and expected higher gain after workover job (WOJ) compared to other wells. A swabbing test was used as an initial attempt to identify the water source which indicated that the lower formation, zone-6, was producing 100% water. Obvious decision is to isolate it. However, a through performance analysis of the wells in the field revealed that downdip wells were producing oil from this formation. Most of these downdip wells have commingled production from zone#6 and other zones. Swab test of zone-6 confirmed oil. Two recently drilled wells completed in zone-6 are also producing significant amount of oil.

Therefore, the swabbing result of zone-6 in well D5 appears unreliable and inconclusive. Sometimes these inconclusive results are observed due to limited number of swabs. The ambiguity of the swab test together with the good performance of other wells completed in zone-6 warranted the resolution of this issue using PLT which is not so easy in wells completed with ESP. PLT results confirmed that zone-6 is producing significant volume of oil. This is quite consistent with the overall field assessment. Further, the PLT results indicate that most of the oil produced by this well is actually from this formation and other sub-layers are contributing only 25 to 35 percent. Thus a prolific contributing layer has not been prematurely isolated in this well due to improper analysis and this translates into reaping around 500 BOPD for the company. This is the first time of running PLT in ESP well in this field and the encouraging result will be useful in the analysis of remaining wells of the field.

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