沁水盆地南部中深部煤层气储层特征及开发技术对策

Moderately deep coalbed methane reservoirs in the southern Qinshui Basin:Characteristics and technical strategies for exploitation

为了实现沁水盆地南部中深部煤层气高效开发,以郑庄北-沁南西区块为研究对象,基于参数井取心分析测试、注入/压降测试、地应力循环测试结果和大量动静态数据,通过与浅部对比,阐述了中深部煤储层特征,分析了从浅部到中深部煤层直井压裂和水平井分段压裂两种开发技术的改进,进而提出了中深部煤层气主体开发技术。结果表明,郑庄北-沁南西区块3号煤平均埋深1200 m左右,为中深部煤层气储层。随着埋深增加,研究区含气量和吸附时间均先增加后降低,含气量和吸附时间峰值分别位于埋深1100~1200 m和800~1000 m;随着埋深增加,研究区地应力场类型发生了2次转换,埋深小于600 m时,为逆断层型地应力场类型,水力压裂易形成水平缝,利于造长缝;埋深大于1000 m时为走滑断层型地应力场类型,水力压裂易形成垂直缝,裂缝延伸较短;埋深为600~1000 m时,地应力场由逆断层型向走滑断层型转换阶段,水力压裂形成的裂缝系统较为复杂。与浅层相比,中深部储层含气量、解吸效率和应力场发生明显转变。随着埋深增加,无论是直井(定向井)还是水平井,均应采用更大的压裂规模才能获得较好的效果。对于直井,埋深大于800 m后,压裂液量达到1500 m^(3)以上、排量12~15 m^(3)/min以上、砂比10%~14%以上,单井日产气量可以达到1000 m^(3)以上;对于水平井,埋深大于800 m后,压裂段间距控制在70~90 m以下,单段液量、砂量分别达到2000、150 m^(3)以上,排量达到15 m^(3)/min以上开发效果较好,单井产量突破18000 m^(3)。随着埋深增加,水平井开发方式明显优于直井,以二开全通径水平井井型结构、优质层段识别技术和大规模、大排量缝网压裂为核心的水平井开发方式是适用于沁水盆地南部中深部煤层气高效开发的主体工艺技术。

This study investigated the northern Zhengzhuang-western Qinnan block for the purpose of achieving effective 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 moderately 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 moderately 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 average burial depth of around 1200 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 1100 m to 1200 m and from 800 m to 1000 m,respectively.The in situ stress field in the study area shifts twice as the burial depth increases.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 1000 m,where short vertical fractures tend to be generated through hydraulic fracturing.At burial depths from 600-1000 m,the in situ stress field transitions from the reverse 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 1000 m^(3) at burial depths exceeding 800 m under fracturing fluid volumes greater than 1500 m^(3),injection rates of fracturing fluids above 12-15 m^(3)/min,and proppant concentrations greater than 10%-14%.For horizontal wells,the single-well daily gas production can exceed 18000 m^(3) at burial depths greater than 800 m under fracturing intervals less than 70-90 m,single-stage fracturing fluid volumes above 2000 m^(3),single-stage proppant volumes above 150 m^(3),and injection rates of fracturing fluids greater than 15 m^(3)/min.Horizontal wells significantly 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.