Abstract
Adsorbed gas and free gas both exist in shale reservoirs simultaneously due to the unique nanoscale pore structure, resulting in the complex flow mechanism of gas in the reservoir during the development process. The dynamic performance analysis of shale reservoirs has mostly been conducted by the numerical simulation and theoretical model, while the physical simulation method for relevant research is seen rarely in the literature. Thus, in this paper, an experiment system was designed to simulate the degraded development experiments of shale, coal, and tight sandstone to reveal the output law of gas in different occurrence states of shale reservoirs and clarify the pressure propagation rules of different reservoirs, and then, adsorption gas and free gas production laws were studied by theoretical models. Research indicated the following: (1) The gas occurrence state is the main factor that causes the difference of the pressure drop rate and gas production law of shale, coal, and tight sandstone. During the early stage of the development of shale gas, the free gas is mainly produced; the final contribution of free gas production can reach more than 90%. (2) The static desorption and dynamic experiments confirm that the critical desorption pressure of adsorbed gas is generally between 12 and 15 MPa. When the gas reservoir pressure is lower than the critical desorption pressure in shale and coal formation, desorption occurs. Due to the slow propagation of shale matrix pressure, desorption of adsorbed gas occurs mainly in the low-pressure region close to the fracture surface. (3) The material balance theory of closed gas reservoirs and the one-dimensional flow model of shale gas have subsequently validated the production performance law of adsorbed gas and free gas by the physical simulation. Therefore, in the practical development of shale gas reservoirs, it is recommended to shorten the matrix supply distance, reduce the pressure in the fracture, increase the effective pressure gradient, and enhance the potential utilization of adsorbed gas as soon as possible to increase the ultimate recovery. The findings of this study can help for a better understanding of the shale reservoir utilization law so as to provide a reference for production optimization and development plan formulation of the shale gas reservoirs.
Highlights
Shale is a potentially unconventional gas reservoir, both the source rock and the storage space for shale gas
The difference in the gas occurrence state leads to different gas production performances, in which the pressure drop rate is ranked as tight sandstone > shale > coal sample for the difference in the adsorbed gas ratio for each sample
(1) The difference in gas occurrence state is the main factor of the different gas production laws of the shale, coal seams, and tight sandstone
Summary
Shale is a potentially unconventional gas reservoir, both the source rock and the storage space for shale gas. The indoor shale gas research mainly focuses on the static evaluation of reservoir physical properties, such as shale gas porosity, permeability test and pore connectivity characterization [12,13,14], and adsorption isotherm curve test for adsorbed gas volume [15, 16]. It can be seen from the previous studies that there are relatively few studies on the shale gas dynamic production performance, mostly focusing on theoretical models and numerical simulations [17,18,19,20]. Combined with isothermal adsorption experiments, material balance equations, and onedimensional flow model, the utilization mechanism and development rules of free gas and adsorbed gas were analyzed, which can provide a reference for production optimization and development plan formulation of the shale gas reservoirs
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