黔西煤层气成藏特性空间分异及其对开发的启示

Spatial distribution and variation of coalbed methane reservoir characteristics and its significance for CBM development in Western Guizhou

黔西地区煤层气资源丰富,但赋存条件复杂,开发难度较大。基于该区煤层气井注入-压降试井数据,重新厘定了本区煤储层渗透率划分方案,分析了地应力及其控制下渗透率、压力系统、气藏类型的空间分异特征,探讨了地应力的控藏效应及其对煤层气开发的启示：区域上,六盘水煤田地应力显著高于织纳煤田,煤储层渗透率为中低渗—低渗水平,异常超压发育,具有＂应力封闭＂特征,而织纳煤田煤储层渗透率为中渗—中低渗水平,以欠压状态为主,表现为＂压力驱动＂特点,可实现压降自由传递;垂向上,300 m和600 m为黔西地区地应力的转换深度,也是控制煤层气成藏特征的临界埋深线,300 m以浅以水平应力为主导,300-600 m水平应力与垂直应力共同作用,异常高压发育,600 m以深以垂直应力占主导。600 m以浅有利于统一流体压力系统的发育,以深有利于多层叠置流体压力系统的形成,并使气藏资源类型由＂压力驱动型＂向＂应力封闭型＂转变;基于成藏要素的台阶式跃变特征和多层次模糊综合评价,推测300-600 m为本区煤层气勘探开发的黄金带,织纳煤田高产井产层普遍位于该深度范围。

Western Guizhou has the most abundant coalbed methane resource in South China with complicated CBM occurrence conditions and development disadvantage. In this study,based on the test data of CBM wells,the scheme for dividing the coal reservoir permeability in the research area is determined. The spatial distribution and variation characteristics of in-situ stress,together with permeability,pressure system and gas reservoir types under its control are analyzed. The reservoir control effect of in-situ stress and its significance for CBM development is discussed. Regionally,Liupanshui coalfield has significantly higher in-situ stress than Zhina coalfield,which leads to a medium-low and low permeability and abnormal over-pressure reservoir,and is characterized by ＂in-situ stress sealing＂. Zhina coalfield develops a medium and medium-low permeability and under-pressure coal reservoir,and is characterized by ＂pressuredriven＂,the pressure drawdown can spread freely. Vertically,the critical conversion depth of in-situ stress is defined as300 m and 600 m,which are also the critical depth controlling CBM accumulation characteristics. In different depthranges of above 300 m,300-600 m and below 600 m,it is represented by horizontal stress dominant,horizontal stress and vertical stress jointly dominant,vertical stress dominant,respectively. Overpressure reservoirs develop within the depths of 300-600 m. Unified pressure system tends to develop above 600 m whereas multi-layer superposed pressure systems tend to develop below 600 m,and the reservoir energy type is transformed from ＂pressure-driven＂to ＂in-situ stress sealing＂. Based on the stepped change characteristics of reservoir parameters in vertical and multi-level fuzzy synthetic evaluation,the depths of 300- 600 m is speculated as the golden zones for CBM exploration and development,the producing layers of high production well in Zhina coalfield are all within this depth range.