Laboratory-based mathematical modelling of graded proppant injection in CBM reservoirs

The graded proppant injection into a connected cleat system of coal bed methane (CBM) reservoir allows deeper particle penetration and straining in the remote open cleats yielding higher well productivity index. The analytical model for axisymmetric flow has been derived for exponential stress–permeability relationship and accounting for permeability variation outside the stimulated zone. Laboratory proppant injections into coal cores have been performed for different proppant sizes and water salinities. It is shown that the proppant suspension based on low salinity water prevents the particle–particle and particle–coal attraction with the consequent core inlet plugging and external cake formation. However, low salinity of the injected water may cause mobilisation, migration and straining of the natural reservoir fines resulting in high formation damage. The interval where salinity is low enough for the rock inlet not to be plugged by the injected proppant, and is high enough for large formation damage due to fine migration not to occur, is proposed for the cores under investigation. The analytical model is tuned from the laboratory data and used for well index prediction. Ignoring the model matching by the laboratory results causes the overestimation of the incremental productivity index, as achieved by the graded proppant injection into coal beds below the fracturing pressure.