This study elaborates the evolving techniques implemented to address challenge of thin oil rim (5 to 10-ft) development in relatively thick reservoir-Z (90-ft thickness). The issues are related to the presence of gas cap, active bottom aquifer, and presence of high permeability streak at the top 4-6 ft. of reservoir layer. Thin dense layer (1-2 ft.) present inside the high-K streak and underneath the layer, reservoir properties drop significantly (ca. 2-permeability order). Log signature markedly influenced by the rock property contrast and unable to differentiate fluid types. The oil-water contact varies between wells, driven more by the rock-type contrast rather than structural/depth position.

Horizontal wells are implemented and deliver sustainable oil production for all reservoirs in the field except for the Reservoir-Z. Due to its complexity, no horizontal wells drilled in the early production scheme (EPS) delivered any oil from the respective reservoir. The wells were mis-placed at either gas cap and/or aquifer leg. The subsequent development implemented cased and perforated completion with 60deg. inclination along reservoir interval to overcome well placement challenge. These wells delivered sub-optimal result due to high-drawdown (limited entry of perforated interval) and suffered from early gas and water breakthrough.

Accordingly, well configuration is improved by having 85deg. inclination along reservoir section. It lengthens oil column penetration and facilitate longer perforation interval but inefficient due to the long-wasted interval inside the transition zone. Ultimately, in perspective of efficiency, an ambitious goal was set to drill horizontal wells along the peripheral oil rim. Materializing the goal practically left no room for error in well placement. Meanwhile, the field has cluster-based drilling, implying long step-out/ departure and some degree of wellbore survey uncertainty (1σ of trajectory uncertainty ca. 30 ft.).

A comprehensive program was prepared to tackle the challenges. This subsume:

  • Feasibility evaluation of deep azimuthal resistivity tool usage (forward model).

  • Pilot hole and relevant data acquisition (fluid analyzer/sampling).

  • Update of deep azimuthal resistivity forward model with the pilot hole result.

  • Geosteering and risk mitigation plan.

The pilot hole result met its very objective, i.e.: delineating the areal outline of GOC around the horizontal target location and provide the exact stratigraphic target for horizontal well placement. It is 2-3 ft TST target below thin dense act as baffle toward high-K streak layer. Below this stratigraphic target, water saturation (Sw) increases abruptly above 45%. The deep azimuthal and at-bit resistivity tool was used to geosteer the well and successfully delivered 2000 ft. section of dry oil without any crossing to the high-K layer.

After the failure of the early horizontal wells, it becomes dogma that placing horizontal section along oil column of reservoir-Z and at the same time avoiding the high-K streak as an impossible feat. The success proves otherwise and opens a wide door of alternative development scheme with huge cost-saving potential.

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