汶川地震前后四川中坝气田的气井井口压力异常变化

Abnormal gas well pressure of Zhongba gas field,Sichuan,before and after the Wenchuan earthquake

分析了龙门山断裂带附近的四川中坝气田气井井口压力数据,发现汶川地震前,多口气井出现井口压力异常升降的变化.开采层深3 100 m的7口雷三储气层的气井压力,在4月8日—5月5日井压同步出现了大于35%—130%的上升;开采层深2 400 m的24口须二储气层的气井中,多数井压在震前出现缓慢下降的异常变化,幅度最大为45%,少数在震前出现上升.特别是52和63号井在5月5日前后,分别出现了井压上升24%和15%的异常变化.发震前数天,两个储气层的井压基本上都恢复到正常背景值附近.震后各气井的压力变化不完全一致,但多数在突升7—8日后恢复.结合地震前兆的扩容理论和临震阶段油气运移的气体动力学过程,对两个不同层深储气层在临震阶段井口压力异常变化的可能物理机制进行了初步分析.

The gas well pressure data of Zhongba gas field in the vicinity of Longmenshan fault zone were analyzed in this paper and significant changes were found in most oil wells before the 12 May 2008 Wenchuan earthquake.The well pressures of 7 deep wells at Lei-3 gas storage layer with mining depth of 3100 m simultaneously rose with rates greater than 35%—130% from April 8 to May 5 and recovered from May 6 to the occurrence of earthquake.At the same time,the pressures of most of 24 gas wells at Xu-2 gas storage layer with mining depth of 2400 m decreased slowly with the maximum rate of 45%.The pressures of a few wells rose before the earthquake,especially No.52 and No.63 wells showed 24% and 15% well pressure increase around May 5.Several days before the earthquake,the well pressures at the two gas storage layers basically returned to normal state.After the earthquake,changes of well pressure varied,but most of them recovered after increasing for 7—8 days.The physical mechanism of abnormal change of wellhead pressures at the two gas storage layers with different mining depths before the earthquake is also discussed.Based on the dilatancy theory of earthquake precursors and gas dynamic process of hydrocarbon migration during the development of earthquake,this paper gave a possible explanation of the physical mechanism of abnormal well pressure changes.