滇东老厂矿区煤层气储层地应力特征研究

Study on in-situ stress characteristics of coalbed methane reservoir in Laochang Mining Area,eastern Yunnan

地应力特征是煤层气勘探开发,尤其是压裂设计的基础研究内容。为了探明滇东老厂矿区地应力分布特征,基于测井资料、岩石力学试验及水力压裂曲线,利用带附加构造应力项的Anderson模型,计算了滇东老厂矿区煤层气储层地应力特征参数,分析了地应力分布规律及对储层渗透率的影响。结果表明:滇东老厂矿区煤储层地应力整体为10~30 MPa,属中高应力区。最大水平主应力σH,最小水平主应力σh和垂直主应力σV随埋深增加而增加,并且垂向上应力场随埋深增加逐渐由水平主应力主导变为垂直主应力主导,800 m为应力转换点,800 m以浅以σH>σV>σh为主,表明有利于走滑断层应力场活动,煤层处于压缩状态;800 m以深以正断层应力场(σV>σH>σh)为主,垂直主应力σV为主控应力,煤层处于拉张应力状态。侧压系数整体上位于中国和Hoek-Brown内外包络线之内,且随埋深增加逐渐由大于1为主,减小到应力转换带深度下的普遍小于1。地应力通过埋深对渗透率起控制作用,煤层埋深在800 m内,孔隙在挤压应力作用下逐渐闭合,渗透率随埋深增加呈负指数减小,800 m以深垂直主应力为渗透率主控因素,储层处于拉张应力状态,且天然裂缝近乎垂直发育,天然裂缝在垂直主应力作用下开启,渗透率随埋深增加而增加,渗透率变化点与应力场变化点基本一致。

The characteristics of in-situ stress is critical for coalbed methane exploration and development,especially for fracturing design.The paper is devoted to explore the distribution characteristics of in-situ stress in Laochang,eastern Yunnan.Based on log data,rock mechanics test and hydraulic fracturing curve,using Anderson model with added stress term,the in-situ stress of coalbed methane reservoir in Laochang area was calculated.The distributions of in-situ stress and its influence on reservoir permeability were also analyzed.The results show that the in-situ stress of coal reservoir in Laochang Mining Area is between 10 and 30 MPa,which is defined as high stress zone.The maximum horizontal principal stress(σH),the minimum horizontal principal stress(σh),and the vertical principal stress(σV)increase with the depth.As the increase of depth,the dominate stress field gradually turns from horizontal principal stress into vertical principal stress,with transition point at 800 m.When depth is less than 800 m,the horizontal stress is largest,which indicates that the strike-slip faults(σH>σV>σh)are active and coal seam is in compression state.When depth is higher than 800 m,the vertical stress is the largest,where is mainly the normal faults(σH>σV>σh),the vertical stress is the dominant stress and the coal seam is in the tension state.The results also show that the lateral pressure coefficient is within the inner and outer envelope of China and Hoek-brown.With the increase of depth,the lateral pressure gradually decreases from above 1 to below 1 at the depth of stress transition zone.Permeability at different depth is also determined by in-situ stress.When depth of coal seam is less than 800 m,the pore closes gradually under the action of compressive stress and the permeability decreases exponentially with the increase of depth.When depth is larger than 800 m,the vertical principal stress is the main factor to determine permeability.Reservoir is in tension stress state and nature fractures develop vertically.As nature fractures open under the effect of vertical principal stress,permeability increases with depth.The variation of permeability at different depth is basically the same as that of stress field.