This paper presents how cased-hole formation testing technology solves the mystery of fluids distribution in a reservoir in a South China Sea well with very challenging operational conditions. Two wireline cased-hole formation testers were deployed together on more than 7000 m of drilpipe, which would be one of the world's longest such operations in a highly deviated well. The significant oil column that was identified results in a completely new picture of the field.The L#4 through L#8 formations of the Wenchang Group in A-fieldwere originally believed to be gas pay. But the big question whether this belief was correct was suddenly raised when oil was identified in a deeper zone right beneath L#8 by a mini-drillstem test (DST) conducted in an appraisal and development well looking for gas. Because the L#4-L#8 formations were already cased, a cased hole formation tester was identified as the best approach to characterize the vertical fluid distribution across the column and acquire pressure measurements and PVT samples from each zone. But the operational conditions are extremely tough: more than 7000 m of pipe-assisted operation required in a highly deviated well, zero tool failure tolerance due to the time-consuming and costly long pipe-conveyance operation offshore, and thermal expansion of this long drillpipe in a nonhomogeneously distributed high-temperature profile along the wellbore could potentially jeopardize the formation tester operation.Careful planning, smooth operation, and real-time technical support ensured flawless execution of the cased-hole formation testing. Oil instead of gas was identified from three sands to make a big oil column that exceeded everyone's expectations. Well testing showed #,#00 bbl/day of oil production with a restricted choke. More significantly, this breakthrough discovery has opened a brand new exploration and development area and has a significant impact on field development planning. This paper gives a detailed description of how this great success was achieved and a summary of technical findings from petrophysics and reservoir engineering points of view. Distinguishing features of the oil properties have been observed, which has triggered a deeper investigation of the regional petroleum system, PVT behaviors, and production and development optimization. The preliminary results of these investigations are also discussed.
The Huizhou 6S and 3S oil fields in the Pearl River Basin, Offshore South China Sea are mature fields which have produced 40% to 60% of their original oil in place since 1991. Currently the field production is rapidly declining and water production is increasing. However, through reservoir surveillance data, geologic and reservoir modeling, significant recoverable oil was identified in shaly sandstone reservoirs and attic structural locations of clean sandstone reservoirs. As a result, a comprehensive portfolio of prospects has been built for a robust development program. Horizontal wells were utilized to improve oil recovery in shaly sands and to reduce water coning in thin remaining oil columns. Horizontal drilling best practices were applied during well planning and drilling executions, such as optimum well designs, specific LWD/MWD tool selections, low fluid loss drilling fluids, real-time geosteering data monitoring and the cleaning of the pay zone during completions were applied to maximize reserve recovery and successfully reverse the field's production decline.
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