Response of CO2 Diffusion to Macro- and Microporosity in Carbonate Reservoirs

Abstract Carbonate reservoirs are known for complexity to oil recovery industry, one reason is the dual-porosity pore system. In macroporosity regions where pores are mostly effective for fluid flow and therefore viscous flow will play a role in oil extraction, while in microporosity regions contribution of pores to permeability is very limited and results in a large amount of residual oil entrapment. The purpose of this paper is to investigate the response characteristics of diffusion to porosity (include percent fraction and pore size ratio of macro- and microporosity) and permeability. Then with the help of permeability to evaluate the efficiency of CO2 diffusion for oil extraction from different porosity composition core plugs. In order to achieve this objective, we use integrated carbonate core samples by pore structure characterization to study the feasibility of diffusion macro- and micropore regions. First, SEM and MICP experiments were introduced to characterize the bimodal pore systems, these results were combined with permeability measurement values as basic parameters for evaluation. Then the response characteristics of CO2 diffusion to macro- and microporosity will be described by the performance of CO2 injection for oil recovery with dual porosity carbonates. With the result the first observation was the dual effect of macroporosity that it lowers the resistance for fluid flow and meanwhile reduces the exposure duration for CO2 to diffuse into the microporsoity regions. Furthermore, percent fraction, tortuosity and ratio of pore size between macro- and microporosity are critical parameters for CO2 diffusion. Also with different dual porosity component core plugs, various ultimate oil recoveries were acquired and all recovery curves were divided into two different parts that contributed by viscous flow and diffusion respectively. Through comparison the different diffusion performance in various dual porosity core plugs, response characteristics of CO2 diffusion to macro- and microporosity were finally concluded. Acquire the knowledge of CO2 diffusion response characteristics to macro- and microporosity makes oil extraction from microporosity regions which have limited contribution to permeability possible. It shows an optimistic prospect of oil recovery by diffusion mechanism which ever been neglected and provides one more option for EOR method design in low permeable reservoirs. In addition, suggestion will be presented to extract oil effectively using CO2 injection in case of high percent fraction of microporosity reservoirs.