Abstract
After volume fracturing, shale reservoirs can be divided into nonlinear seepage areas controlled by micro- or nanoporous media and Darcy seepage areas controlled by complex fracture networks. In this paper, firstly, on the basis of calculating complex fracture network permeability in a stimulated zone, the steady-state productivity model is established by comprehensively considering the multi-scale flowing states, shale gas desorption and diffusion after shale fracturing coupling flows in matrix and stimulated region. Then, according to the principle of material balance, a transient productivity calculation model is established with the succession pseudo-steady state (SPSS) method, which considers the unstable propagation of pressure waves, and the factors affecting the transient productivity of fractured wells in shale gas areas are analyzed. The numerical model simulation results verify the reliability of the transient productivity model. The results show that: (1) the productivity prediction model based on the SPSS method provides a theoretical basis for the transient productivity calculation of shale fractured horizontal well, and it has the characteristics of simple solution process, fast computation speed and good agreement with numerical simulation results; (2) the pressure wave propagates from the bottom of the well to the outer boundary of the volume fracturing zone, and then propagates from the outer boundary of the fracturing zone to the reservoir boundary; (3) with the increase of fracturing zone radius, the initial average aperture of fractures, maximum fracture length, the productivity of shale gas increases, and the increase rate gradually decreases. When the fracturing zone radius is 150 m, the daily output is approximately twice as much as that of 75 m. If the initial average aperture of fractures is 50 μm, the daily output is about half of that when the initial average aperture is 100 μm. When the maximum fracture length increases from 50 m to 100 m, the daily output only increases about by 25%. (4) When the Langmuir volume is relatively large, the daily outputs of different Langmuir volumes are almost identical, and the effect of Langmuir volume on the desorption output can almost be ignored.
Highlights
Shale gas reservoirs have the characteristics of tiny pores and throats, extremely low permeability, abundant natural fractures and diverse gas storage modes [1,2,3,4,5,6,7,8,9]
There are few studies on the transient productivity of fractured wells in shale gas reservoirs, and the characteristics of multi-scale flow and non-linear flow are neglected in the majority of productivity models [21,22,23]
It can be seen that in the shale gas production process, the reservoir pressure gradually decreases from near the wellbore to the reservoir boundary. It reflects that the pressure wave propagates from the bottom of the well to the outer boundary of the volume fracturing zone, and propagates from the outer boundary of the fracturing zone to the reservoir boundary
Summary
Shale gas reservoirs have the characteristics of tiny pores and throats, extremely low permeability, abundant natural fractures and diverse gas storage modes [1,2,3,4,5,6,7,8,9]. There are few studies on the transient productivity of fractured wells in shale gas reservoirs, and the characteristics of multi-scale flow and non-linear flow are neglected in the majority of productivity models [21,22,23]. On the basis of the tri-line flow model, Swami et al [29] established a dual-porosity seepage model of shale gas reservoir considering adsorption and desorption in matrix This model still assumed that shale gas is linear seepage and the characteristic of multi-scale flow is neglected. Based on the previous studies, Deng [12] corrected the B-K model with different slip coefficients, and established a multi-scale flow model for shale gas reservoir considering diffusion, slippage, desorption and adsorption, while the assumption of homogeneous medium does not match with volume fracturing and not take into account the unsteady seepage of shale reservoir. Well the productivity horizontal well productivity prediction of influencing factors are carriedfactors out. are carried out
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