The Barnett Shale in north Texas illustrates successful (and persistent!) commercialization of an unconventional reservoir. The extremely low permeability shale reservoir benefits from hydraulic fracturing treatments that establish long and wide fracture pathways, which results in connecting very large surface areas of the formation with an extremely complex fracture network.  However, gas production in such tight shale has been technically challenging. The primary challenge facing gas producers is rapid initial depletion rate of new wells,with the average first year decline  reported to be 64% for Barnett Shale. The gas recovery factor of Barnett shale is reported to be only 15-30% even in stimulated fracture-shale systems, and there are very limited investigations on the low gas recovery factors in shale plays.Gas recovery in this tight shale formation is probably diffusion-limited by tortuous pathways in shale matrix characterized by nano-sized pore spaces. In this work, we used mercury intrusion porosimetry (MIP) to measure pore-size distribution and derive permeability and tortuosity for Barnett shale samples from five different depths. MIP can intrude connected pores with pore-throat diameters down to 3 nm, and the pores in Barnett shale are predominantly in the nm size range (with a measured median pore diameter of 6.5 nm), and tortuosity is approximately 10,000. We also directly measured chemical diffusion in the saturated Barnett shale using a suite of tracers, followed by chemical mapping using laser ablation-ICP-MS. Tortuosity obtained from tracer diffusion profiles is consistent with those from mercury intrusion porosimetry. Gas production decline behavior from producing wells is analyzed from the topological aspects of shale pores, using the perspective of percolation theory; several possible explanations about observed decline behavior are offered.

Tortuous migration pathway and gas production behavior of Barnett shale

246th National Meeting (2013)

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