In shale formations, a large amount of hydrocarbon fluid is stored inside the organic matters where the pore sizes are in the order of nanometer scales. Inside these nanopores, the interactions between the fluid molecules and porous walls play such an important role that can change the fluid properties of the stored hydrocarbons, causing adsorption. For unconventional gas condensate reservoirs, the adsorption of gas could be multi-layers adsorption, because of the small pore size and heavy hydrocarbon components in the kerogen source rocks. A Langmuir isotherm is not valid for modeling the adsorbed gas content and estimation of gas in place (GIP) for shale gas condensate reservoirs, because it is based on the assumption that the adsorption is a single molecule layer adsorption.
To accurately determine the film thickness and density of adsorbed gas, we introduced the Simplified Local-Density (SLD) theory coupled and combined it with modified Peng-Robinson Equation of State (EOS) to model the thermodynamic behavior of the adsorbed gas and free gas of multi-component hydrocarbons, considering the pore confinement of the source rocks. The proposed method is applied to compute the Gas-In-Place (GIP) at initial condition of shale gas condensate systems. A GIP model considering the effects of adsorption was developed based on results from the SLD model.
We first validated the model by comparing the results with available literature data. We also performed the sensitivity analysis and found that fluid density distributions in nanopores were temperature, pressure, pore size and fluid composition dependent. In general, the adsorbed amount increased by increasing pressure and decreased by increasing temperature. Heavier components tended to accumulate near the wall (adsorbed phase) while lighter component would like to stay in the center region of the pore (bulk phase). Then we performed a case study of real reservoir fluids in condensate window of Eagle Ford shale and calculate the adsorbed gas content and the total GIP using the introduced model. The preliminary computation results showed that the adsorbed gas could take more than 30% of gas in place in Eagle Ford shale.
By using the introduced method, the adsorbed gas content and the total GIP in unconventional reservoirs can be calculated with good accuracy under short computational time. This makes the model useful when implemented into reservoir simulators.