Moderately deep coalbed methane reservoirs in the southern Qinshui Basin:Characteristics and technical strategies for exploitation
This study investigated the northern Zhengzhuang-western Qinnan block for the purpose of achieving effect-ive exploitation of moderately deep coalbed methane(CBM)reservoirs in the southern Qinshui Basin.Based on results from the analyses and tests of parametric wells,including core analysis and tests,injection/falloff tests,and in situ stress cyclic tests,as well as a large amount of dynamic and static data,this study expounded on the characteristics of moder-ately deep CBM reservoirs in the study area by comparison with shallow counterparts.Then,it explored the technical improvements in fracturing through vertical wells and staged fracturing through horizontal wells for shallow to moder-ately deep coal seams.Accordingly,this study proposed the primary technology for exploiting moderately deep CBM reservoirs.The results indicate that the No.3 coal seam in the northern Zhengzhuang-western Qinnan block has an aver-age burial depth of around 1 200 m,suggesting moderately deep CBM reservoirs.With an increase in the burial depth,both the gas content and adsorption time increase at first and then decrease,peaking at depths from 1 100 m to 1 200 m and from 800 m to 1 000 m,respectively.The in situ stress field in the study area shifts twice as the burial depth in-creases.Specifically,the study area exhibits a reverse fault type of in situ stress field at burial depths less than 600 m,where long horizontal fractures are prone to form through hydraulic fracturing.In contrast,the study area displays a strike-slip fault type of in situ stress field at burial depths exceeding 1 000 m,where short vertical fractures tend to be generated through hydraulic fracturing.At burial depths from 600-1 000 m,the in situ stress field transitions from the re-verse fault type to the strike-slip fault type,with an intricate fracture system tending to form via hydraulic fracturing.Compared to shallow counterparts,moderately deep CBM reservoirs in the study area manifest significantly different gas content,desorption efficiency,and stress field.As a result,to achieve higher fracturing performance,a larger fracturing scale is required for both vertical(directional)and horizontal wells as the burial depth increases.For vertical wells,the single-well daily gas production can exceed 1 000 m3 at burial depths exceeding 800 m under fracturing fluid volumes greater than 1 500 m3,injection rates of fracturing fluids above 12-15 m3/min,and proppant concentrations greater than 10%-14%.For horizontal wells,the single-well daily gas production can exceed 18 000 m3 at burial depths greater than 800 m under fracturing intervals less than 70-90 m,single-stage fracturing fluid volumes above 2 000 m3,single-stage proppant volumes above 150 m3,and injection rates of fracturing fluids greater than 15 m3/min.Horizontal wells signi-ficantly outperform vertical wells at large burial depths.Horizontal wells with a two-spud-in structure and full bore sleeve each,combined with the technique for identifying high-quality CBM intervals and fracture-network fracturing with high fracturing fluid injection rates,serve as the main technology for the efficient exploitation of moderately deep CBM reservoirs in the southern Qinshui Basin.
southern Qinshui Basinnorthern Zhengzhuang-western Qinnan blockmoderately deep coalbed methanereservoir characteristicslarge-scale fracturinghorizontal wellcoalbed methane development
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