Abstract
Shale gas mainly stores in shale matrix, and gas production in shale matrix is very important during exploration. In order to clarify gas production and transport mechanism in shale matrix, an experimental modeling of gas production in shale matrix was designed and conducted with Longmaxi shale samples collected from South of Sichuan. The experimental results show that gas production decline curve displays a “L” pattern which indicates initial production is high and declines rapidly, while late production is low and declines moderately; meanwhile, pressure propagation in shale matrix is quite slow due to ultralow permeability. Based on the results, a mathematical model was derived to describe gas production in shale matrix. The comparison between numerical solution of mathematical model and experimental results shows that the mathematical model can well describe gas transport in shale matrix. In addition, factors affecting gas production were investigated on the basis of the mathematical model. Adsorbed gas can replenish gas pressure in pores by desorption and delay pressure propagation, and gas production decreases very quickly when there is no adsorbed gas. Other parameters (diffusion coefficient, permeability and porosity) also need to be considered in shale gas development.
Highlights
Hydraulic fracturing technology is widely applied in shale reservoirs to significantly increase gas production
Due to extremely low permeability of shale matrix, shale gas production is strongly relied on gas supply of shale matrix
Javadpour found that gas flow in shale nanopores can be modeled with a diffusive transport regime and negligible viscous effects (Javadpour et al, 2007); in 2009, presenting another formulation for gas flow in the nanopores of mudrocks based on Knudsen diffusion and slip flow, he drew a conclusion that the ratio of apparent permeability to Darcy permeability increases sharply as pore sizes reduce to smaller than 100 nm
Summary
Hydraulic fracturing technology is widely applied in shale reservoirs to significantly increase gas production. Javadpour found that gas flow in shale nanopores can be modeled with a diffusive transport regime and negligible viscous effects (Javadpour et al, 2007); in 2009, presenting another formulation for gas flow in the nanopores of mudrocks based on Knudsen diffusion and slip flow, he drew a conclusion that the ratio of apparent permeability to Darcy permeability increases sharply as pore sizes reduce to smaller than 100 nm. Swami studied the contribution of Knudsen diffusion, gas slippage, gas desorption and gas diffusion from kerogen to total shale gas production by a proposed pore scale gas flow model (Swami et al, 2012). Previous studies mainly focused on theoretical calculation in a micro scale Those previous investigations did not obtain the experimental data to answer questions concerning gas production in shale matrix or pressure propagation in shale matrix. We conducted an experiment to model gas production in shale matrix and studied gas transport mechanism in shale matrix in a macro scale
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