Numerical Analysis on the Effect of Fractures on Elastic Properties of Tight Clastic Reservoirs
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摘要: 有效预测裂缝发育区是寻找致密气甜点区的关键,但目前海陆过渡相致密碎屑岩储层裂缝预测的效果较差。利用DEM理论模型获得岩石孔隙纵横比α及干岩石体积模量,探讨裂缝对致密碎屑岩地层岩石弹性的影响及不同尺度裂缝间的关联性。利用模型确定的α可以较好地识别出裂纹发育段,随着孔隙纵横比由1.0转变为0.01,地层岩石体积模量和剪切模量也逐渐降低,表明裂纹相比孔隙而言更容易引起岩石弹性发生改变。地层岩石由干岩石到饱和地层流体过程中,裂缝不发育地层的体积模量增加幅度为3.1%;对于裂缝发育地层段,孔隙度小于4.7%时其体积模量的增加幅度平均为7.0%,孔隙度大于4.7%时其体积模量增加幅度平均为23.0%,拐点处所对应岩石孔隙度可作为岩石内部裂纹发育程度的评价指标。对于裂缝不发育段地层,干岩石体积模量与剪切模量的比值和孔隙度具有较好的正相关性;对于裂缝发育段地层,干岩石体积模量与剪切模量的比值和孔隙度具有较好的负相关性。研究结果表明,利用岩石弹性性质可以定量表征碎屑岩地层裂缝的发育特征。Abstract: The effective prediction of fracture development areas is the key to pinpointing sweet spots in tight gas reservoirs, but currently, the fractures in tight clastic reservoirs of transitional facies are poorly predicted. In this paper, the pore aspect ratio and dry rock bulk modulus were calculated by using the DEM theory model. Later, the effect of fractures on elastic properties of tight clastic formations was analyzed and a correlation between fractures of different scales was proposed. Results indicated that crack development sections could be well identified by virtue of the pore aspect ratio α which was determined by the DEM theory model. By decreasing the pore aspect ratio from 1.0 to 0.01, both the rock bulk modulus and the shear modulus decreased gradually. Results showed that the decline of rock elastic properties was more influenced by cracks than by pores. During the conversion of dry rocks into fluid saturated rocks, the increase amplitude of rock bulk modulus was 3.1% in the strata with undeveloped fractures. In the strata with developed fractures, however, the increasing amplitude of rock bulk modulus was 7.0% when the rock porosity was lower than 4.7%, and otherwise it was 23.0%. The rock porosity at the inflection point could act as an indicator to measure the development degree of fractures in rocks. A positive correlation between dry rock bulk modulus and shear modulus in the strata with undeveloped fractures was detected, but there was a negative correlation in the strata with developed fractures. It was shown that development characteristics of fractures in clastic formations could be characterized quantitatively by using elastic properties of rocks.
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Keywords:
- DEM theory /
- fracture /
- tight clastic rock /
- elastic properties /
- bulk modulus /
- porosity
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