Gas-bearing shales are characterized by pore systems that are present in the organic (kerogen) matrix, inorganic matrix, natural fractures, and induced fractures. Each pore system has different physical properties. The organic matrix essentially has gas-wet nanopores with adsorptive properties. The inorganic matrix is an ultra-low permeability matrix that is most likely water-wet but could have more complex wettability. Two alternative but complementary approaches, namely a lumped tank model and a continuum model, for describing gas transport in shale are developed and their capabilities are demonstrated by various applications. Our continuum approach describes gas transport in a manner analogous to the methods employed by commercial simulators. The model is discretized and petrophysical and fluid parameters incorporated into the model that may then be calibrated to obtain a history match. Often, this process leads to non-uniqueness in the match because the number of observations is likely to be smaller than the number of parameters to be resolved. Our lumped tank approach treats the porosity systems within shales as tanks interacting with each other such that relevant physical phenomena, such as adsorption, are described by rate equations. Calibration of this model to production data enables the quantification of grouped parameters and not necessarily the individual storage- and transport-related parameters, thereby reducing the severity of the non-uniqueness of the history matching process. Several case studies conducted exclusively delineate the advantages and disadvantages of both approaches in shale-gas production analysis and simulation. Comparisons between the tank model and the continuum approach for different case studies show good matches within an acceptable measure of tolerance. Reservoir parameters determined from the tank approach are compared with the values used in the continuum approach. The new models are shown to accommodate the inherent complexities of transport processes occurring in shale-gas reservoirs.

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