成像测井评价致密碎屑岩储层的裂缝与含气性

Fracture and Gas-bearing Evaluation of Tight Clastic Reservoir with Image Logging

川西地区须家河组为特低孔渗的裂缝性致密碎屑岩储层,储层岩性多变,裂缝和溶蚀孔发育不均,储层各向异性极强,常规测井方法难以准确评价其含气性。利用成像测井图像显示的斜缝与高角度裂缝是可以识别有效裂缝;利用微电阻率图像与方位电阻率图像的对比可以评价裂缝的有效性,声波能量衰减与低频斯通利波反射图像也可以评价裂缝有效性。当裂缝走向与现今最大水平主应力方向一致或夹角很小时,裂缝能最大程度地发挥其渗流通道作用;反之,裂缝走向与现今最大水平主应力方向垂直或大角度斜交时,裂缝的渗透作用大大降低,削弱了裂缝的有效性。利用偶极横波测井提供的纵横波速度比可以大致判断致密碎屑岩的含气性,纵横波速度比与纵波时差进行交会可更有效指示致密碎屑岩气层。

The Xujiahe formation in west Sichuan is a fractured tight clastic reservoir with very low porosity and permeability, characterized by inconstant lithology, heterogeneously developed fracture and solution pore and very strong anisotropy. It is difficult to evaluate its gas-bearing correctly with conventional logging methods. However, the effective fracture can be identified by inclined fracture and high angle fracture displaying on features of imaging loggings; and the fracture effectiveness can be evaluated by comparison between images of full-bore micro scanner imager FMI and azimuthal resistivity imager ARI, as well as by the energy attenuation of sonic wave and reflection image of low frequency Stoneley wave. When the trend of fracture intersects the direction of maximum horizontal main stress at small angle or parallel, the fracture can furthest exert its seepage channel function; or contrarily, when the intersecting is at big or right angle, the effectiveness of fracture will decrease since its osmose function decreases a lot. The gas bearing capability of tight clastic rock can be estimated by velocity ratio of compressional wave to shear wave derived from dipole sonic imager DSI, while tight clastic reservoir can be indicated more ef- fectively by a crossplot of the velocity ratio and slowness of compressional wave.