SPE 163651 Beyond Dual-Porosity Modeling for the Simulation of Complex Flow Mechanisms in Shale Reservoirs

The state of the art of modeling fluid flow in shale reservoirs is dominated by dual porosity models which divide the reservoirs into matrix blocks that significantly contribute to fluid storage and fracture networks which principally control flow capacity. However, recent extensive microscopic studies reveal that there exist massive microand nanopore systems in shale matrices. Because of this, the actual flow mechanisms in shale reservoirs are considerably more complex than can be simulated by the conventional dual porosity models and Darcy’s Law. Therefore, a model capturing multiple pore scales and flow can provide a better understanding of complex flow mechanisms occurring in these reservoirs. Through the use of a unique simulator this paper presents a micro-scale multiple-porosity model for fluid flow in shale reservoirs by capturing the dynamics occurring in three separate porosity systems: organic matter (mainly kerogen), inorganic matter, natural fractures. Inorganic and organic portions of shale matrix are treated as sub-blocks with different attributes, such as wettability and pore structures. In the organic matter or kerogen, gas desorption and diffusion are the dominant physics. Since the flow regimes are sensitive to pore size, the effects of nanopores and vugs in kerogen are incorporated into the simulator. The separate inorganic sub-blocks mainly contribute to the ability to better model dynamic water behavior. The multiple porosity model is built upon a unique tool for simulating general multiple porosity systems in which several porosity systems may be tied to each other through arbitrary transfer functions and connectivities. This new model allows us to better understand complex flow mechanisms and in turn is extended into the reservoir scale considering hydraulic fractures through upscaling techniques. Sensitivity studies on the contributions of the different flow mechanisms and kerogen properties give some insight as to their importance. Results also include a comparison of the conventional dual porosity treatment and show that significant differences in fluid distributions and dynamics are obtained with the improved multiple porosity simulation. Finally a case for reservoir-scale model covering organic matter, inorganic matter, natural fractures and hydraulic fractures is presented and will allow operators to better predict ultimate recovery from shale reservoirs. Introduction The development of unconventional resource plays in North America has achieved great success towards satisfy the growing energy demand. The organic shale formations which provide the basis of unconventional oil and gas production continue as an enigma as far as understanding production characteristics are concerned. Because of this many investigators have been inspired to establish suitable models to characterize fluid flow in shale encountering great challenges along the way. Shale is referred to as extraordinarily fine-grained sediments commonly showing fissility (Javadpour, 2009). Loucks et al. (2012) systematically classified nanometer to micrometer sized pores in the shale matrix into interparticle pores and intraparticle pores associated with mineral particles and organic-matter pores within kerogen. The organic matter has different physical properties from common rock constituents and could significantly affect gas storage and flow in shale. Curtis et al. (2010) found that mostly kerogen is scattered in inorganic minerals, and pores within it are basically round in cross-section interestingly with numerous small pores residing on the interior walls of larger pores. Due to forming during the process of hydrocarbon generation, the pore networks in the organic matter in shale are mainly considered to be oilor gas-wet (Wang et al., 2009; Odusina et al., 2011). Experiments on Barnett Shale demonstrated that the both adsorbed gas and free gas stored in the shale matrix are linearly increased with TOC content (Jarvie, 2004), and Javadpour (2009) also theoretically proposed that beside free gas storage in shale, gas could also be adsorbed on the surface of kerogen and dissolved within it. Hill et al. (2000)