The Effect of CO2 Adsorption on Permeability Anisotropy in the Eagle Ford Shale
We conducted pulse-decay and steady-state flow experiments on vertically and horizontally oriented Eagle Ford Shale samples, using adsorbing gas (CO2) and non-adsorbing gas (Helium) as pore fluids. The permeability of these samples is highly anisotropic. He permeability of the Eagle Ford sample normal to bedding was ~10 nanodarcies. He permeability parallel to bedding was ~2 microdarcies. This large permeability anisotropy appears to reflect vertical flow through interconnected nanoscale pores along tortuous pathways whereas horizontal flow can also flow along microcracks and bedding planes.
We found that the adsorption of CO2 molecules reduced the permeability normal to bedding by about an order of magnitude. However, the permeability reduction parallel to bedding decreased by only ~10%. In both cases, the permeability reduction was reversible. That is, when the CO2 was desorbed, the He permeability returned to its initial value. Both pulse-decay and steady-state flow experiments were conducted at a temperature of 38.5°C (± 0.1°C), confining pressures up to 6,000 psi (41.4 MPa), and pore fluid pressures ranging from 377 to 2,000 psi (2.6 to 13.8 MPa).
As the measurements with both types of gases were performed at the same mean free path to achieve the same impact of slippage flow on permeability, the observed difference in permeability anisotropy is due to the adsorption of CO2. A thorough understanding of these nanoscale transport processes in various directions is critical for defining the interactions between shale rock and CO2. Hence, the design and implementation of various CO2 applications in shale formations, such as hydraulic fracturing with CO2 and CO2 injection for enhanced oil/gas recovery, can be further optimized and their economic impact can be improved.