Moderately deep coalbed methane reservoirs in the southern Qinshui Basin: Characteristics and technical strategies for exploitation
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摘要:
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1 200 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1 100~1 200 m和800~1 000 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1 000 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600~1 000 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1 500 m3以上、排量12~15 m3/min以上、砂比10%~14%以上，单井日产气量可以达到1 000 m3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2 000、150 m3以上，排量达到15 m3/min以上开发效果较好，单井产量突破18 000 m3。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。
Abstract: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 m3 at burial depths exceeding 800 m under fracturing fluid volumes greater than 1500 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 18000 m3 at burial depths greater than 800 m under fracturing intervals less than 70-90 m, single-stage fracturing fluid volumes above 2000 m3, single-stage proppant volumes above 150 m3, and injection rates of fracturing fluids greater than 15 m3/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.
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图 1 郑庄北−沁南西区块位置及地层剖面
Fig. 1 Location and stratigraphic section of the northern Zhengzhuang-western Qinnan block
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图 2 郑庄北−沁南西区块埋深分布
Fig. 2 Contour map showing the burial depth of the northern Zhengzhuang-western Qinnan block
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图 3 郑庄北−沁南西区块含气量分布
Fig. 3 Contour map showing the gas content of the northern Zhengzhuang-western Qinnan block
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图 4 郑庄北−沁南西区含气量随埋深变化情况
Fig. 4 Burial depth-varying gas content in the northern Zhengzhuang-western Qinnan block
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图 5 郑庄北−沁南西区块吸附时间随埋深变化
Fig. 5 Burial depth-varying adsorption time in the northern Zhengzhuang-western Qinnan block
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图 6 郑庄北−沁南西区块不同埋深条件下典型解吸速率
Fig. 6 Typical desorption rates under different burial depth ranges in the northern Zhengzhuang-western Qinnan block
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图 7 郑庄−沁南西区块不同埋深条件下应力状态
Fig. 7 Burial depth-varying stress state of the Zhengzhuang-western Qinnan block
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图 8 郑庄区块直井稳产气量与埋深关系
Fig. 8 Relationship between stable gas production and burial depth of vertical wells in the Zhengzhuang block
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图 9 充填预堵+大规模压裂+远端支撑重复压裂井生产参数曲线
Fig. 9 Production parametic curves of wells subject to refracturing using the technology of pre-plugging, large-scale fracturing, and remote proppant emplacement
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图 10 二开全通径L型水平井井型结构
Fig. 10 Two-spud-in structure and full bore sleeve of a L-shaped horizontal well
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图 11 不同煤质、煤体结构煤的自然伽马值对比
Fig. 11 Comparison of natural gamma-ray values of coals with different quality and structures
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图 13 郑庄北压裂段数与平均单井累产气关系
Fig. 13 Relationship between fracturing stage number and average single-well cumulative gas production in the northern Zhengzhuang block
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图 14 郑庄北大排量大液量压裂试验井生产曲线
Fig. 14 Production curves of wells subjected to fracturing tests with high injection rates and high volume of fracturing fluids in the northern Zhengzhuang block
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图 15 水力压裂规模对裂缝关键参数影响
Fig. 15 Effects of hydraulic fracturing scale on the critical parameters of stimulated fractures
表 1 郑庄−沁南西区块不同埋深直井压裂关键参数与稳产气量关系
Table 1 Relationship between critical fracturing parameters and stable gas production of vertical wells at different burial depths in the Zhengzhuang-western Qinnan block
类型 埋深/m 液量/m3 砂量/m3 排量/(m3·min−1) 砂比/% 平均单井稳产气量/(m3·d−1) 郑庄西南部一次压裂 400~600 500~700 30~50 4~6 8~10 2000 郑庄北部一次压裂 700~1 100 600~800 40~60 4~6 8~10 300 沁南西一次压裂 900~1 400 1 000~1 500 60~100 6 8~10 1000 郑庄北部二次压裂 700~1 100 1 300~2 000 100~150 10~14 12~15 1380 
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表 2 不同分段压裂工艺优缺点对比
Table 2 Pros and cons of different staged fracturing techniques
工艺名称 关键参数 优点 缺点 射孔段长度/m 最高施工排量/(m3·min−1) 砂比/% 普通油管底封拖动 0.6 8.0 0~11 工艺简单、成本低 排量受限、施工效率低 连续油管底封拖动 带压作业、施工效率高 排量受限 桥射联作分段压裂 1.0~3.0 22.0 0~20 任意方向定向射孔；排量调节范围大；
可实现多段多簇压裂枪串遇卡
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表 3 郑庄−沁南西区块不同埋深水平井分段压裂关键参数与稳产气量关系
Table 3 Relationships between critical staged fracturing parameters and stable gas production of horizontal wells at different burial depths in the Zhengzhuang-western Qinnan block
类型 埋深/m 阶段 段间距/m 单段液量/m3 单段砂量/m3 排量/(m3·min−1) 砂比/% 平均单井稳产气量/(m3·d−1) 郑庄西南部 400~600 早期 100~130 450~600 30~50 4~6 8~10 8 000 目前 80~100 450~600 30~50 6 8~10 15 000 郑庄北部 700~1 100 早期 80~100 450~600 30~50 6 8~10 8000 目前 70~90 2000 150 15 8~10 18 000 沁南西 900~1 400 目前 70~90 1000~1500 60~100 6 8~10 8 000
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