Study On Geological Theories Of Stress Relief Coalbed Methane Drainage From The Distant Protected Seam By Vertical Surface Wells And Their Application In Huainan Coal Mine Area

This study introduces a way to drainge stress relief coalbed methane(CBM) from the distant protected coal seam after the lower protective coal seam mined through surface vertical wells over the longwall panel which is drilled in advance of mining. The compression in the overlying protected coal seam is relieved and gas flow ability is greatly increased because of the relaxation of the overlying strata after the protective coal seam is mined, which make that the wells get high gas production. This study has proved that the stress relief drainage technology through surface vertical wells is scientific in theory through sheared coal reservoir characteristics experiments, physical and numerical simulation tests, moreover, dynamic variation of sheared coal reservoir permeability during the protective coal seam mining and after the protective coal seam mined is summarized to design the well deployment in the area where the high gas flow ability would maintain a long time. Some engineering problems resulted in surface wells stopping producing are analyed too. The position where the wellbore is easily broken is found and some solutions are provided to maintain the wellbore integrity. The design methods of wellbore structure are summarized through engineering practice, and some drilling and completing methods to strengthen the wellbore stability are gave to help the surface well get high drainage efficiency.The results also indicate that reservoir adsorption capacity and gas storage ability is correlated positively with the level of the coal structure destruction. Sheared coal has better gas flow ability than primary coal while the compression is relieved. All these have proved that sheared coal reservoir is convenient for stress relief CBM drainage by surface vertical wells.The permeability of the protected coal seam varied dynamically during the protective coal seam mining and after the protective layer mined. Well developed abscission layer and high permeability area in the protected coal seam locate in the central panel in plane behind the face during mining , it moves with face advancement and has a short life. However, after the lower protective coal seam mined, well developed abscission layer and high permeability area in protected coal seam shifts from the central panel to the zone aound the gob, it may be kept for a long time.The wellbore stability is closely relevant to casing strength, borehole diameters, geological conditions , mining factors of the lower protective coal seam. The wellbore structure of surface wells for stress reilief CBM is destroyed at the beginning of the mining influence because of the axial tension failure and shear failure resulted from overburden movement. Axial tension failure and shear failure to casing is initiated by vertical and horizontal movements of overburden. The extent of the casing failure is different in various position. The failure is serious while wells locate in the central panel in plane and casing is near mined coal seam in vertical direction. The difference of rock layer thickness and the lithology combination have great influence on wellbore stability too. The bigger mining height and greater face advance speed would strengthen all kinds of the above-mentioned failure.Mining fractures and high permeability area are located in zones where lines of compaction and the panel boundary line are determined after its effect has become relatively stable, in this area, the wellbore stability is better than central panel too. So it is the optimum well location for surface vertical wells to drain stress relief CBM.The technology parameters of wellbore structure and their applicalble geological conditions and mining factors are summarized from the engineering practice. And a new kind of wellbore structure is designed to strengthen the wellbore integritybased on the above research results.The drilling and completing technologies are discussed and some suggestions are provided to strengthen the wellbore stability during the lower protective coal seam mining.