Study On The Stability Of Vertical Shale Gas Wells Penetrating Longwall Mining Areas

As the exploitation of shale gas booms in recent years, large amount of shale gas wells have been drilled in the Appalachian coal field. These wells are very sensitive to strata movements caused by coal mining as the diameter of wellbore is very small(15~40 cm). Shale gas wells are prone to fail especially when the longwall mining method is used. Based on the engineering background of coordinated extraction of the Pittsburgh coal seam and the Marcellus shale gas within the southwest Pennsylvania, this paper systematically investigated the stability of shale gas wells through theoretical analysis, calculation and numerical simulation. The main research contents and conclusions are as follows:(1) The fundamental cause for all modes of gas well deformation is the strata movements induced by mining activities. With this influence, failure modes of gas wells consist of shear, tension, compression, and distortion resulting from combined effects of shear and tension or compression, among which shear is the main fail mode for vertical gas wells.(2) Weak interface between soft and hard layers is the key factor affecting the stability of vertical shale gas wells. The horizontal shear offset, vertical tension and compression occur on interfaces are quite sensitive to the variation of interfaces’ stiffnesses and internal friction angle. With increasing stiffnesses of interfaces, the horizontal shear offset and compression decreases logarithmically and the vertical tension linearly. As the internal friction angle rises, the shear offset shows a logarithmical downward trend, while the vertical tension and compression ascends in an approximate linear manner. While the abovementioned deformations of gas wells lag in response to the rock elastic and strength parameters.(3) Through the simulation of influence of surrounding rock geological characteristics on wells deformation, we find that horizontal shear offset, vertical tension and compression are positively correlated to the mining height and single layer thickness. Normally, the shear offset occur on one weak interface with a stiffer layer above and a soft layer below is generally larger. Large shear offset won’t occur on the interface between bed rock and its cover soil if the top layer of bed rock is very thin; and shear offsets on interfaces between the very soft thin interlayer and its neighboring layers are also unlikely to be very large. The gas well becomes increasingly prone to shear when piercing an inclined coal seam and this tendency grows with the dip angle. The longwall panels should be arranged to ensure the deep panel is first extracted and then is the shallow one; as for the inclined longwall panels, both advancing and retreating mining are applicable.(4) Through the geological evolution on vertical gas wells stability in the longwall mining areas within southwest Pennsylvania, we find that: no matter the gas well is drilled from the horizontal surface or the hillcrest, wells deformation is always more severe when extracting a shallow seam. As the seam depth deepens, wells deformation mitigates to some extent and then intensifies again. Wells deformation of all kinds can generally be strengthened by the rolling mountain to varying degrees and usually peaks in the vicinity of the seam, ground surface, and hillcrest. Horizontally, the gas well should better be located 7.5~10 m deviating from the center of a 30~70 m wide coal pillar and close to the later mined panel.(5) This research also reveals the dynamic evolution of gas wells deformations during the whole mining cycles of the twin panels through the 3D numerical simulation. Wells deformations accelerate or decrease as each panel approaches and passes by the wells. Usually, wells are more susceptible to fail when each panel, especially the second panel, advances past the well by approximately 50 m in the case of deep mining(300 m) and more than 100 m for shallow mining(100 m) with the accumulative deformation reaching the maximum.Marine shale gas exploitation in China has entered a preliminary stage of scale development, the coordinated exploitation of deep shale gas and shallow coal seam has not come out in China. However, the Qinshui basin, Ordos basin and Junggar basin are abundant not only in shale gas but also in coal resource. For vertical shale gas wells penetrating longwall mining areas, the stability control of wells may emerge in these regions in the foreseeable future. Besides, this work partly shows similarity with the study on the stability of coalbed methane vertical wells in stress relief areas. The research approach, contents and derived conclusions can provide some reference for that study. This work is also of some scientific and practical significance to the harmonic extraction of coal seams and deep shale gas.