DIFFUSION-LIMITED FRACTURE-MATRIX INTERACTION AND GAS RECOVERY IN BARNETT SHALE
HU, Qinhong1, GAO, Zhiye1, EWING, Robert P.2 and GAO, Xubo3, (1)Department of Earth and Environmental Sciences, University of Texas at Arlington, 500 Yates Street, Arlington, TX 76019, (2)2101 Agronomy Hall, Iowa State University, Ames, IA 50011, (3)College of Environmental Sctudies, China University of Geosciences, 388 Lumo Road, Wuhan, 430074, China, maxhu@uta.edu
As the birthplace of successful commercialization of an unconventional reservoir, the Barnett shale currently has some 16,000 producing wells.The reservoir produces at commercially viable levels only with hydraulic fracturing that establish long and wide fracture pathways, which connect large surface areas of the formation through a complex fracture network. However, the main challenge facing hydrocarbon producers is the rapid initial depletion rate of new wells, with first year declines averaging around 64% for Barnett shale. Total gas recovery from the Barnett shale was reported to be only 8-15% in 2002, and 12-30% in 2012, even with stimulation, and there are few investigations into root causes of this low gas recovery.
Gas recovery in the tight shale formation is probably diffusion-limited by tortuous pathways in shale matrix characterized by nano-sized pore spaces. This work discusses various approaches to investigating pore structure (both geometry and topology) of the Barnett shale. For example, we 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 Barnett 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.
