Development of an integrated approach for modeling gas adsorption and coal swelling

Abstract

Accurate descriptions of gas adsorption, adsorbent swelling and permeability in coalbed reservoirs are essential for the optimal design of enhanced natural gas recovery and CO2 sequestration processes. In practice, reservoir simulators provide a convenient way for estimating natural gas recovery rate and CO2 injectivity for these processes. Such simulators require adsorption capacity and reservoir permeability estimates, which, in turn, require adsorption and swelling models for predicting permeability changes. As a result, a model capable of providing accurate description of adsorption behavior and adsorbent swelling is needed.

Knowledge of adsorption isotherms is essential for modeling adsorbent swelling. Further, to account carefully for gas-adsorption-induced swelling, swelling models are applied at reservoir conditions, where geothermal gradients and different types of geological formations such as coals and shales may exist. Thus, an integrated approach for modeling adsorption and swelling requires: [1] an adsorption model capable of describing the temperature-dependence of gas adsorption over the expected range of reservoir temperatures, [2] an adsorption model capable of describing adsorption behavior on a variety of coals and shales and [3] a theoretically consistent model for describing swelling. To meet these requirements, the Simplified Local-Density (SLD) model has been integrated with the Pan and Connell (PC) swelling model.

The primary objectives of this study are to: [1] modify the SLD model to describe temperature dependence of adsorption over significant temperature ranges, [2] extend the SLD model to describe adsorption behavior on shales and [3] integrate the SLD and PC models to describe the adsorption and swelling behavior of several coals and use the results obtained from the SLD-PC swelling model to calculate permeability changes and compare the results with available experimental data.

To facilitate the model development, a comprehensive database of experimental measurements was assembled for gas adsorption on activated carbons, coals and shales, as well as data for coal swelling and permeability changes. The SLD model was then modified by introducing a new temperature-dependence expression in the model. The model was tested with adsorption data on activated carbons, coals and shales. The results indicate that the SLD model can describe accurately the adsorption data on these carbonaceous adsorbents. In addition, the SLD model has better representations for gas adsorption over significant temperature ranges. Further, the SLD model was integrated with the PC model to describe coal swelling data and permeability changes. The results reveal that the integrated model is capable of describing accurately the coal swelling, thus providing useful input for permeability predictions.