沁南-郑庄区块深部煤层气“临界深度”探讨

Discussion about “critical depth” of deep coalbed methane in Zhengzhuang area,Qinshui Basin

基于沁南—郑庄区块35层次煤层气井注入/压降及地应力实测数据,系统分析了郑庄区块地应力垂向变化规律,并在此基础上探讨了煤储层渗透性、含气性、气水产出垂向差异性演化,揭示了郑庄地区深部煤层气界限。郑庄区块地应力状态在垂向上发生转换:575 m以浅,σH>σv>σh,表现为大地动力场,现今地应力状态为压缩状态;575 m^675 m,水平主应力较浅部有所降低(σH≈σv>σh),表现为准静水压力场,现今地应力状态为过渡状态(由压缩状态过渡为拉张状态);675~825 m以深,σv>σH>σh,表现为大地静力场型,现今地应力状态为拉张状态;825 m以深,σH>σv>σh,现今地应力状态为压缩状态。煤储层试井渗透率随埋深的变化与地应力场状态的转变基本一致,其实质是地应力作用下煤体孔隙结构的变形与破坏;含气量与埋深之间存在一个"临界深度"范围(800~1 000 m),超过此埋深范围之后煤层含气量随埋深增大而趋于降低。整体来说,825m以深煤层气资源处于地应力转换状态和(或)含气量"临界深度"之下,其赋存和开发地质条件发生转换,气体采收率相对较低,属于深部煤层气范畴。该埋深(825 m)以下煤层气开发将面临"低渗透率、低含气量、高地应力"的挑战。

Based on the in-situ stress measurement data of 35 CBM wells in Zhengzhuang area, the characteristics of in-situ stress was analyzed, and the different evolutions of permeability and gas content vertically were also discussed. The results show that the in-situ stress state transformation vertically：①Within 575 m,the stresses σH〉σv〉σh shows the earth stress field ; ② From 575 m to 675 m, the stress relationship is σH≈σv〉σh, and the recent in-situ stress state is situated in transition zone;③ From 675 m to 825 m,the stress relationship is σv〉σH〉σh, and the recent in-situ stress state is located in extension zone ;④ When 825 m below the surface ,the stress relationship is σH〉σv〉σh ,which shows the dynamic field. The change of the well test permeability of the coal reservoir is almost the same with the change of stress state, the essence of which is that the in-situ stress controls the function of original permeability by ehanging the width and density of cracks. There is a ＂critical depth＂ range （800 - 1 000 m） between gas content and depth,when below this depth, the gas content shows a decrease tendency with the increase of depth. Resources of coal-bed methane below 825 m is both under the depth of in-situ stress transformation and the ＂critical depth＂ of gas content with poor gas recovery, its occurrence and development of geological conditions are converted, which belongs to the category of deep coalbed methane. In general, the development of CBM under 825 m faces the challenge of ＂high in-situ stress,low permeability, and low gas content＂.