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In order to truly describe the fracture network of the enhanced geothermal system (EGS), the tree-like fractal bifurcation theory is used to reflect the heterogeneity of the fracture network. The temperature-seepage coupling model is set up, the influences of bifurcation fracture networks and well layouts on thermal recovery performance are analyzed. The results show that the increase of bifurcation level, angle and length ratio all have positive effect on the thermal recovery performance. A higher bifurcation level produces a greater number of secondary fractures, a higher bifurcation angle and length ratio leads to a relatively spread fracture distribution, thereby improving thermal recovery performance. A higher aperture ratio is prone to premature thermal breakthrough, which limits the thermal recovery process. Under the conditions and scopes of our study, the optimal bifurcation level is 5, angle is 120°, length ratio is 0.55 and aperture ratio 0.4. This study provides some good guidance of efficient fracturing design for better thermal recovery performance.

Abstract A novel multilateral well for coalbed methane extraction was proposed in the study. There is a main wellbore at the longitudinal center of coal seam and four lateral wells at the horizontal center in the multilateral-well system. Compared with traditional drainage holes, multilateral-well system has a better performance on coalbed methane development. A hydraulic-mechanical coupling model of multilateral well was established, the pressure and permeability ratio distribution of the gas extraction process were analyzed comprehensively. The sensitivity analysis of lateral number, length distribution and intersection angle of multilateral-well system were studied. The results indicate that there is a minimum gas pressure distribution around the multilateral well and the overall permeability of coal seams increases with production time and the permeability around the multilateral well is larger than the area away from the multilateral well which induced by the gas desorb and matrix shrink. The quantity of lateral wells has a positive effect on cumulative production. When the total length of lateral wells is equal, the uniformity and symmetry of lateral length distribution are two key factors on the gas extraction performance. The minimum intersection angle has a positive effect on cumulative production. This study provides a better alternative for traditional drainage hole to obtain greater coalbed methane performance.

Abstract The shale gas reservoir is regarded as a dual medium consisting of fracture (hydraulic fracture and discrete natural fracture network) and rock matrix, the seepage process in the fracture and rock matrix is fully considered and a mathematical model of seepage flow in accordance with Darcy's law was established. The results show the influence order of hydraulic fracture geometry on the cumulative production. Compared with the hydraulic fracture aperture of 10–4 m, when the aperture is 10–5 m and 10–6 m, the cumulative production is reduced by 88.0% and 99.7%, respectively. Compared with the hydraulic fracture length is 100 m, when the length is 200 m and 300 m, the cumulative production is increased by 38.2% and 62.4%, respectively. The increase in the natural fracture aperture increases the fracture permeability, which make it more conducive to gas flow into the fracture, thereby increasing the cumulative production. The increase in the number of natural fractures makes the connectivity of the shale reservoir becomes better and the cumulative production increases more.