Chromatographic separation and liquid drop-out in unconventional gas reservoirs

Abstract

A study is presented on the impact of heavier components on gas production in unconventional reservoirs. The competing effects of Knudsen and molecular diffusion are incorporated by using Maxwell-Stefan equations and the Dusty-Gas approach. The total flux of gas considers contribution of viscous flow, molecular and Knudsen diffusion, adsorption/desorption and surface diffusion. The model is used to perform numerical experiments demonstrating the effects of transport mechanisms, medium permeability and initial gas composition on production performance. We evaluate pressure decline and chromatographic separation effects in the presence of a wider range of components in flow through both adsorbing and poorly-adsorbing porous media.During gas production, the flow of different chemical species will result in chromatographic separation. Therefore, gas composition will change over time, causing shifts in the phase envelope. Depending on the degree of chromatographic separation, liquid drop-out may occur in the near-wellbore.In this study, we demonstrate that for poorly adsorbing media, the degree of chromatographic separation is small due to the balance between Knudsen and molecular diffusion fluxes. In this case, chromatographic separation is intensified with decreasing permeability, since molecular sieve mechanism becomes dominant with smaller pore sizes at lower pressures. However, when adsorption is present, species separation is more relevant, resulting in significant shifts in the phase envelope. Production behavior is affected by the higher affinity of hydrocarbon molecules with adsorption sites. As a result, condensate formation in the reservoir is likely to occur as production progresses.Many analytical/semi-analytical models available for gas transport in unconventional reservoirs are based on single component or binary mixtures. In this work, we demonstrate the effect of including heavier fractions in modelling gas transport when they are present, which is relevant in prediction of pressure decline, produced gas composition and natural gas liquids (NGLs) yields. Moreover, we demonstrate that gas composition at the wellbore can be significantly influenced by a balance between viscous and diffusion fluxes, and we highlight the relevant transport mechanisms to be considered when modelling multicomponent gas flow in tight reservoirs.