The Condition of Capillary Condensation and Its Effects on Adsorption Isotherms of Unconventional Gas Condensate Reservoirs

Abstract Traditionally, Langmuir isotherms are used as the adsorption isotherms of coal bed methane and shale gas. To model the adsorbed multi-components of hydrocarbons, the Langmuir isotherm for the single component is extended to the Langmuir isotherm for multi-components. A Langmuir isotherm is simple, but it is based on the assumption that the adsorption is a single molecule layer adsorption. Its adsorption mechanism is adsorption-site filling. For shale gas reservoirs, the adsorption of gas could be multi-layer adsorption, because of the small sizes and wettability of nanopores in the kerogen source rocks. Particularly for gas condensate reservoirs, such as the Marcellus shale gas reservoir, capillary condensate could exist, due to the confinement of kerogen pores, i.e., the adsorbed gas becomes condensate to fill the pores at the pressure below the dew-point pressure. For capillary condensation, therefore, a Langmuir isotherm is not valid. The conventional Kelvin equation can be used for computing the isotherm of capillary condensation. However, it is limited to the single component. We introduce a method, which is based on the generalized Kelvin equation, to compute the adsorption isotherms of capillary condensation for multi-component hydrocarbons. This new computation method enables us to investigate the condition of capillary condensation and relation between pore size and condensation line. Furthermore, we check the capillary condensation effects on the adsorption isotherm caused by fluid composition, pore-size distribution, temperature, and pressure. We predict the phase behavior for the Marcellus shale reservoir. The computation results show: When the temperature is less than pore critical temperature (lower than the cricondentherm temperature), capillary condensation can occur.For shale gas formations, if the hydrocarbon composition includes some heavy components, capillary condensation can occur, depending on pore-size distribution, composition, and pressure.Large standard deviation of pore size will smooth the adsorption isotherm for capillary condensation.As pore size increases, capillary condensation occurs at pressures close to the dew-point pressure.Capillary condensation can occur in the Marcellus shale formations.