Proxy-Based Metamodeling Optimization of Gas-Assisted-Gravity-Drainage Process
Authors: Wilson Adam 
Published: September 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 185701, “Proxy-Based Metamodeling Optimization of the Gas-Assisted-Gravity-Drainage Process in Heterogeneous Sandstone Reservoirs,” by Watheq J. Al-Mudhafar and Dandina N. Rao, SPE, Louisiana State University, prepared for the 2017 SPE Western Regional Meeting, Bakersfield, California, USA, 23–27 April. The paper has not been peer reviewed.
Unlike continuous gas injection and water-alternating-gas injection, gas-assisted gravity drainage (GAGD) takes advantage of the natural segregation of reservoir fluids to provide gravity-stable oil displacement. The feasibility of carbon dioxide (CO2) GAGD was investigated for immiscible injection through equation-of-state compositional reservoir simulation with design of experiments (DOE) and proxy modeling to obtain the optimal future-performance scenario. After history matching, Latin-hypercube sampling (LHS) was used as a low-discrepancy and more-uniform DOE approach to create hundreds of simulation runs to construct a proxy-based optimization approach.
Introduction
Many enhanced-oil-recovery studies have been conducted for CO2-flooding optimization in real oil fields; however, to the best of the authors’ knowledge, no study has been made for GAGD implementation and optimization in a real oil field. To implement the optimization process, a full compositional reservoir simulation was constructed to evaluate the reservoir performance through CO2-GAGD flooding for 10 years of future reservoir prediction. Then, proxy-model optimization was conducted through manipulating the operational decision factors that influence the CO2 flooding through GAGD by means of DOE. More specifically, DOE and proxy modeling were combined to create a simplified alternative (metamodel) to the compositional reservoir simulation to optimize the operational decision factors affecting GAGD. Four proxy models were adopted and validated as metamodels for the compositional reservoir simulator: polynomial proxy model, multivariate additive regression splines (MARS), fuzzy logic/genetic algorithm, and generalized boosted modeling (GBM).
GAGD
The GAGD concept involves placing horizontal producers at the bottom of a pay zone. Then, gas is injected in a gravity-stable mode, either immiscible or miscible, through vertical wells from the top of the formation. Because of gravity segregation resulting from the distinct fluid densities at reservoir conditions, the injected gas accumulates at the top of the pay zone to form a gas cap, providing gravity-stable oil displacement that drains down toward horizontal producers and leading to better sweep efficiency and higher oil recovery. Fig. 1 illustrates the basic concepts of GAGD.
GAGD Simulation
The main-pay reservoir in the South Rumaila oil field was selected for a full, detailed compositional reservoir simulation to enhance oil recovery through GAGD. The main pay has only three lithology types—sand, shaly sand, and shale—with distinct areal permeability distributions. A high-resolution geostatistical reservoir model was reconstructed for lithofacies and petrophysical properties considering multiple-point geostatistics.