Nirmala, Ika Rima (2026) Evaluasi Penurunan Cadangan Tersisa (Remaining Reserves) Akibat Invasi air pada Reservoir Batu Pasir (sandstone) Menggunakan Data produksi. Other thesis, Universitas Islam Riau.

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Abstract

Field X is a mature oil field in the Central Sumatra Basin that has been producing since 1984 and faces serious challenges from water influx from an active aquifer. By the end of 2023, cumulative water production reached 46.53 MMSTB, far exceeding cumulative oil production of 8.84 MMSTB, with water cut increasing from 60% to more than 85%. This study aims to evaluate the decline in remaining oil reserves due to water invasion using the Material Balance Analysis method with IPM-MBAL software and the Carter-Tracy model for water influx calculation. To accommodate uncertainties in aquifer parameters that cannot be directly measured, three scenarios were developed: Low (pessimistic), Mid (base case), and High (optimistic) with variations in aquifer permeability parameters, outer/inner radius ratio, aquifer thickness, and encroachment angle. History matching results showed high accuracy with reservoir pressure error less than 10% and cumulative oil production error less than 5%, confirming that the main drive mechanism is natural water drive. Analysis of three scenarios yielded highly variable recovery factors: Low Scenario of 11.66% with final reservoir pressure of 95.33 psi (very strong aquifer), Mid Scenario of 36.74% with pressure of 786.03 psi (moderate aquifer), and High Scenario of 61.85% with pressure of 833.29 psi (weak aquifer). Sensitivity analysis results indicate that aquifer permeability and outer/inner radius ratio are the most influential parameters on water influx magnitude and recovery factor. Water invasion became significant from 1995 when reservoir pressure dropped to 60-70% of initial pressure, causing increased residual oil saturation (Sor) due to uneven sweep in the heterogeneous sandstone reservoir. The wide range of recovery factors (11.66%-61.85%) provides important uncertainty information for strategic decision-making in field development. Keywords: Material Balance Analysis, Water Influx, Recovery Factor, CarterTracy Model, Remaining Reserves

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