FIG. 1 CORE FLOODS
Freiberg University of Technology
Wintershall Holding AG
China University of Geosciences
Recent studies investigated how pore structures influenced
enhanced oil recovery (EOR) in China’s Changqing field,
where CO2 pilots are under way despite recovery efficiency
methods having not yet been optimized.
Carbonate reservoirs are complex, largely because of
dual-porosity pore systems. Macroporosity regions feature
pores enabling effective fluid flow and oil production. Microporosity regions provide limited permeability, resulting in oil entrapment.
Tests on core samples from the US
states of Indiana and Wisconsin found
similar porosity and permeability to
Changqing. The samples showed dual-porosity, classified into micro and macro
types with a 1 µm pore-size serving as
Dominant gas flow changed from Darcy flow to diffusion
when pore diameters changed from macro to micro. CO2
flows through the macropores and produces oil by Darcy
flow. Micropores yield oil mainly by diffusion in lower-per-meability samples. Researchers evaluated the efficiency of
CO2 diffusion for oil extraction from different porosity core
PetroChina Co. Ltd. operates Changqing field in the Ordos basin. Baker Hughes Inc. has characterized Changqing as having low permeability, low pressure, and low-yield
sands (OGJ, June 14, 2010, p. 10).
The complex heterogeneous pore-structure inside carbonates is closely linked with permeability and fluid movement.
Heterogeneity can contribute to low recovery.
Literature shows earlier pore-dimension studies used
mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), scanning electron micrograph
(SEM), gas adsorption, computed tomography, and digital-image analysis. 1-2
Carbonate reservoirs demonstrate
pore diversity, which contributes to low
porosity-permeability correlation. Higher
porosity typically means higher permeability, but the relation between porosity
and permeability in carbonate reservoirs
is complex. 3
Carbonate reservoir fluid flow research
continues to emphasize microporosity’s
Study assesses CO2 EOR potential by pore structure in China’s Changqing field