鄂尔多斯盆地柳林地区煤储层地应力场特征及其对裂隙的控制作用

Characteristic of in-situ stress field in Liulin area,Ordos Basin and its control on coal fractures

为系统研究柳林地区煤储层地应力场展布及其对裂隙的控制作用,分析不同埋深煤层与地应力的关系,采用水力致裂法获取了地应力资料。通过统计分析,建立了主采煤层地应力与煤层埋藏深度之间的相关关系模型,分析了露头节理、煤层割理与现今地应力方向的耦合关系。研究表明：柳林地区最大水平主应力σmax为7.33~30.83 MPa,平均19.41 MPa,最小水平主应力σmin为6.50~24.00 MPa,平均13.12 MPa,主应力随埋深增大而线性增高。在埋深400~700 m,垂直应力σv≈σmax≈σmin,地层呈现准静水压力场特点;在700~850 m,σmax〉σv〉σmin,为大地动力场型,水平方向主应力占主导地位;在850~1 100 m,σv〉σmax〉σmin,为大地静力场型,仍未进入明显压缩带。水力压裂地应力检测表明现今主应力方向与煤层裂隙方向相近,以NNE向为主,地应力方向在割理形成后未发生明显改变。

To get a systematic analysis of in-situ stress filed in Liulin area and find out the relationship of in-situ stress and coal seams of different burial depth was built,hydraulic fracturing method was adopted to obtain the stress data. Through statistical analysis, the correlation and their models between in-situ stress in main ruinable coal seams and their burial depth was built, furthermore, the direction of outcrop joints, coal seam cleats and the present in-situ stress was demonstrated. The results show that the maximum horizontal principal stress in Liulin is 7.33-30.83 MPa, the av- erage is 19.41 MPa ;the minimum horizontal principal stress is 6.50-24.00 MPa,the average is 13.12 MPa. It is also noticed that the principal stresses all increase following a linear relationship as coal seam burial depth increases. The coal reservoir with a burial depth sallower than 700 m, in-situ stress state principally shows σv ≈ σmax ≈ σmin, recent in- situ stress state is water pressure field of quasi-static zone; the in-situ stress state of coal reservoirs with 700-850 m shows σmax 〉 σv 〉 σmin, the coal seams was under a state of extrusion from horizontal directions;the coal reservoirs with 850-1 100 re,the in-situ stress shows σv 〉 σmax 〉 σmin ,though still not under the state of compression. The detect of bydraulic fracturing shows that the direction of present in-situ stress is close to the direction of coal seam fractures,both mostly in NNE, and the direction of in-situ stress are relatively stable after the cleats were formed, part areas of stress concentrated are favorable area for coalbed methane accumulation.