A Critical Investigation of Optimization of Steam-Assisted Gravity Drainage SAGD in Naturally-Fractured Heavy Oil Carbonate Reservoirs Under Uncertainty
Authors: Temizel Cenk , Tuna Tayfun , Putra Dike 
Published: March 2016

Abstract
Advancing technology in thermal recovery and declining conventional oil resources are leading to the increased application of thermal heavy oil recovery processes. Heavy oil resources in carbonate rock are estimated at 1.6 trillion bbl, one-third of which is in the Middle East, making it a crucial part of the global energy supply. A thorough understanding of reservoir properties (including but not limited to natural fracture systems, fluid saturation, permeability and rock matrix) is fundamental to optimizing recoveries with existing uncertainties, where efficient optimization through simulation can lead to the application of optimum operational decision parameters combined with improved understanding of the significance of such uncertainty parameters.
Steam-assisted gravity drainage (SAGD) provides many advantages compared to conventional surface mining extraction techniques and alternate thermal recovery methods. These advantages include significantly greater per-well production rates, greater reservoir recoveries, reduced water treating costs, and dramatic reductions in steam-oil ratios (SORs). However, carbonate reservoirs exhibit varying properties in terms of porosity, permeability, and flow mechanisms. Many other engineering considerations exist for SAGD, including recovery rate, thermal efficiency, capability and economics of drilling horizontal well pairs, steam quality, steam injection rate, steam pressure, sand production, reservoir pressure maintenance, and water intrusion. In this study, such parameters are simulated and optimized by coupling a numerical reservoir simulator with commercial optimization software featuring exploratory, gradient, and direct methods. A dual permeability approach to modeling naturally fractured reservoirs is used.
The results are discussed and the significance of both uncertainty and decision parameters is outlined in addition to the added benefits of optimized solutions using various optimization techniques. The results and observations show significant improvement using the optimum combination of decision parameters, such as steam injection rate, pressure, and temperature, to improve recoveries with a good understanding of the significance of uncertainty parameters, including rock and formation properties, in addition to overburden conditions that can affect the SAGD process. Reservoir management requires effective risk management where a sound understanding of uncertainties is fundamental to success. This study outlines all of these parameters in a comparative way, which can be useful in future industry applications.
Modeling fractures in carbonates in heavy oil reservoirs is not an easy task because of the modeling and computational difficulties. It is important to have solid thorough examples, including building the model, setting up the optimization workflow of uncertainty, and outlining the results in a comparative manner to enhance understanding for more efficient reservoir management during SAGD processes.