Abstract

Determining the most influential parameters affecting the reservoir flow responses is a vital step in the integrated reservoir studies for evaluation and analyses. More specifically, removing the non-influencing parameters leads to reach the optimal process design. The conventional procedure to determine the most sensitive parameters combines regression analysis with analysis of variance. However, that approach produces one reduced regression model after eliminating the non-influential parameters (deterministic approach). In this paper, Bayesian Model Averaging (BMA) was applied to stochastically identify the geological parameters that control the immiscible CO2-assisted gravity drainage process performance in a multilayer heterogeneous sandstone oil reservoir in South Rumaila oil field. After achieving acceptable history matching within approximately 57 year of production, the CO2-assisted gravity drainage injection was evaluated in 10 year future prediction. In that process, vertical wells were placed at the top of the reservoir for CO2 injection to formulate a gas cap that make oil drains down towards the bottom of reservoir. Above the oil-water contact, a series of horizontal production wells were installed to produce oil. The main geological parameters that controls the immiscible CO2 flooding are horizontal permeability, anisotropy ratio (Kv/Kh), and porosity that were investigated for their impact by layers or multilayer. Many low-discrepancy designed simulations were created by the Latin Hypercube Design and evaluated by the reservoir flow simulation to disregard the no-impact parameters using the linear BMA approach. Among multi- candidate models, BMA select the best model of optimal subset variables based on the highest posterior probability. BMA led to quantify accurate impact of each geological control on the CO2 EOR process. Based on the concept of the CO2-assisted gravity drainage process, it was concluded that horizontal permeability has a higher impact of the reservoir flow response than vertical permeability (Kv/Kh) with no effect of porosity.

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