A rapid optimization methodology of produced gas reinjection EOR in carbonate reservoirs

Abstract

The Carboniferous carbonate reservoir of the Pre-Caspian basin has abundant oil and gas resources. The typical features of low porosity and low permeability and reservoir oil with high concentrations of H2S and CO2 and heavily dissolved gas-oil ratio allow produced gas reinjection as an effective and economic way to enhance oil recovery (EOR). However, the optimal design of injection/production parameters for gas injection processes is the premise for development scheme and technical decision. Compared to the conventional method of one-parameter-at-a-time approach with lack of interactions between various factors and tremendous challenge of main controlling factors identified, in this work we present a rapid optimization methodology to identify the main factors from numerous candidate parameters and obtain optimum levels of key factors. Our chosen methodology is a combination of experimental design with response surface method (RSM). Firstly, the two-level Plackett-Burman experimental design was used to screen and rank ten operational variables for the gas reinjection process. The top five significant parameters with the sum of their influencing weight greater than 94.3 % are bottomhole pressure for producers, reinjection ratio, injection period, injection gas timing and completion interval. Then the three-level Box-Behnken experimental design was employed to perform optimization exploration with the RSM. Regression models of the oil recovery factor (RF) were constructed by using these five key factors, which fits excellently with numerical simulation results. Finally, the optimal levels of controlling factors were determined to maximize oil recovery. This optimization procedure is very practical for reservoir engineers to guide a rapid evaluation of gas injection processes.