Abstract
Shale gas is rapidly growing as a source of natural gas in China. Compared with the conventional gas reservoir, the shale gas reservoir is characterized by low porosity, low permeability, and adsorbed gas, making the flow mechanism of shale gas reservoir more complex. In this study, we investigated six factors influencing the gas flow: the Darcy flow, the slippage effect, the Knudsen diffusion effect, the desorption of gas on pore walls, the diffusion effect of gas in organic matter, and the matrix deformation effect. We simplified gas flow in the development process to only include gas flow in the capillaries and then considered the six influence factors. This study establishes a shale gas pseudo two-dimensional unsteady capillary seepage mathematical model based on the continuity equation, using the implicit difference method to solve the mathematical model. Certain capillary parameters were added to the calculation, and the study analyzed the effect of the different factors on both the pressure distribution and the cumulative gas production. Results show that the Knudsen diffusion effect and the desorption of gas from pore walls have lower impact on the pressure than the others factors. The diffusion effect of gas in organic matter, the slippage effect, and the matrix deformation effect have a stronger impact on the pressure. The gas in organic matter continuously diffuses into the capillary with the increasing of the production time, and the pressure drop becomes slow because of the gas diffusion.
Highlights
In shale-gas systems, nanometer- to micrometer-size pores, along with natural fractures, form the flow-path network that allows flow of gas from the mud rock to induced fractures during production (Loucks et al, 2012)
1) The capillary model is composed of a matrix pore system and a large number of organic inclusions; 2) Flow in the reservoir is isothermal, free gas is the real gas; 3) The diffusion of gas from organic inclusions flows radially, the diffusion of gas from the capillaries flows linearly; 4) In the initial state, a certain amount of free gas stays in the capillaries, and most of the gas stays on the inner surface of the matrix and in the adsorbed state in organic matter; 5) The flow in the capillaries reflects a multi-flow mechanism, without regard to gravity or the pressure effects; 6) Radial flow reflects a process of diffusion, and this gas diffusion process is in a non-equilibrium pseudosteady state, according to Fick’s first law
The Langmuir equation can be used to describe the phenomenon of the CoalBed Methane (CBM) adsorbed on the coal seam surface, and is used in the study of shale gas reservoirs because shale gas is similar to CBM
Summary
In shale-gas systems, nanometer- to micrometer-size pores, along with natural fractures, form the flow-path network that allows flow of gas from the mud rock to induced fractures during production (Loucks et al, 2012). The contributions of slip flow and Knudsen diffusion increase the apparent permeability of the reservoir while gas production takes place. The effects of both mechanisms explain the higher-than-expected gas production rates commonly observed in these formations (Shabro et al, 2011). Darcy’s law is modified by introducing apparent permeability to obtain the constitutive equation for gas seepage (Yao et al, 2012). These models are not based on shale’s nanometer pore structure, which can cause some deviations in the description of its percolation mechanism, leading to incorrect reservoir evaluations and prediction. We suggest some parameter values for the capillary model and analyze the impact of different factors influencing the pressure distribution
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
