Analytical Investigation of Horizontal Well Performance and Drainage Architecture Optimisation in Thin Oil Columns of OML 77, Offshore Bayelsa State, South South Nigeria

Thin oil column reservoirs with strong aquifer support and overlying gas caps present a constrained production envelope where horizontal well placement and completion architecture critically determine economic well life and recovery efficiency. This study analytically investigates horizontal well performance and optimises drainage architecture for thin oil column reservoirs in OML 77, offshore Bayelsa State. Static reservoir models for three thin oil column sands were constructed using available seismic and well log data, with oil column thicknesses ranging from 6 to 18 metres, and were used as the basis for horizontal well performance modelling. Critical rate calculations using the Chaperon and Joshi models were applied to determine maximum stable production rates below coning onset as a function of horizontal well length, standoff from water-oil contact, and reservoir permeability anisotropy. Eclipse 100 reservoir simulation was used to forecast cumulative recovery under horizontal well scenarios at lengths of 600, 900, and 1,200 metres, and under conventional vertical well development for comparison. Simulation results indicate that 900-metre horizontal wells placed at 60 percent of oil column height above the water-oil contact provide the optimal balance between critical rate avoidance and reservoir contact, delivering 31 percent cumulative oil recovery at water breakthrough compared to 18 percent for vertical wells in the same column. Sensitivity to horizontal permeability anisotropy and oil column thickness is quantified. Keywords: horizontal well, thin oil column, drainage optimisation, OML 77, Bayelsa State.