In the South China Sea, an operator initiated a horizontal well campaign to develop a laminated edge-water sand reservoir, including multiple irregular and thin (0.5 to 3 m) sand layers and shaly/calcareous interbeds. The development success greatly depends on optimal well placement along irregular and discontinuous sweet sand layers to minimize bypassed oil. However, the high stratigraphic heterogeneity and dip uncertainties complicate this laminated reservoir profile to affect well placement efficiency.

In the first development stage, conventional service proved to be inefficient for mapping the accurate reservoir profile for optimal well placement. Accordingly, an upgraded deep directional electromagnetic (EM) resistivity logging-while-drilling (LWD) tool was introduced in the second stage to provide a high-definition (HD) multi-boundary mapping service. During prejob planning, a feasibility study with this HD multi-boundary mapping service was carried out which revealed its capability in this multilayer environment, and this result gave the operator the confidence of utilizing this HD service as fit-for-purpose solution to place the horizontal wells optimally and enhance reservoir understanding with uncertainty management. This HD service could detect high-resolution multiple key boundaries (4 to 5) and map the profiles of multiple layers (>3) simultaneously from the distance up to 20 ft in this given case, highlighting its capability of detecting other discontinuous sand layers separated by interbeds while the tool remains inside one sand layer. With the result, the subsurface resistivity profile, including multiple layers, could be clearly identified in real time for efficient, proactive geosteering along sweet sand layers with less bypassed oil.

In the first well using the HD service, after the trajectory entered the shaly sand layer below a tight interbed, the HD resistivity inversion simultaneously revealed up to four boundaries and five layers vertically, identifying a 1m conductive shaly interbed and a thick resistive layer below. Then the decision of cutting down softly to enter this resistive layer was proved right because this resistive layer was found oil bearing. The remaining 412-m section was placed successfully within the sweet layer. The HD mapping service remotely revealed the lateral reservoir thickness variation and its vertical property heterogeneity which were validated by intentionally approaching the mapped sweet layer top and bottom. The initial production (2,300 BOPD) exceeded the prognosis by 15%.

The HD multi-boundary mapping service had demonstrated its capability to significantly reduce uncertainty for optimal well placement, consequently, to improve oil recovery. The successful case validated that this laminated reservoir can be profitably and efficiently developed with the use of this fit-for-purpose solution. Following the first well, another successful case was drilled by implementing the same service, and a six-well campaign was initiated to use this HD service to squeeze field potential.

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