裂缝性致密砂岩储层声波测井数值模拟

Numerical Simulation of Sonic Logging in Fractured Tight Sandstone Reservoirs

摘要: 目前，用来评价致密砂岩储层有效性的测井方法主要有常规测井、成像测井及声波测井3种。由于裂缝性致密砂岩储层的裂缝宽度一般小于100 μm，常规测井和成像测井对微裂缝难以进行有效识别，而声波测井可通过斯通利波衰减系数等参数对微裂缝进行定性甚至定量识别。通过数值模拟计算，对含有裂缝的致密储层井孔声场进行研究，掌握裂缝对井内声场的影响规律，对于声波测井数据处理及解释具有重要意义。应用三维交错网格应力-速度有限差分方法，模拟了含有倾斜薄裂缝的孔隙介质地层中点声源所激发的井孔声场问题。针对单条水平裂缝，确定了不同裂缝宽度(20~1000 µm)对斯通利波识别微裂缝的影响，即裂缝宽度越大，波形振幅越小；裂缝宽度较小时(裂缝宽度小于100 μm)，波形幅度对裂缝宽度的变化非常敏感，递减非常快；裂缝宽度越大时，斯通利波衰减系数越大。此外，还计算了孔隙介质地层不同孔隙度条件下斯通利波、横波、纵波幅度及衰减系数与裂缝宽度的关系，即孔隙度越大，斯通利波、横波、纵波幅度越小，衰减系数越大。

Abstract: The logging methods to evaluate the effectiveness of the tight sandstone reservoir mainly include conventional logging, imaging logging and acoustic logging. It was difficult to effectively identify the microfractures by conventional logging and imaging logging, because the width of fractures in fractured tight sandstone reservoirs was less than 100 μm, but the acoustic logging makes an qua-litative identification and quantitative evaluation to microfractures through the stoneley wave at-tenuation coefficient and other parameters. It will be of great importance to master the influencing rule of fracture of borehole acoustic field for acoustic logging data processing and interpretation by numerical simulation, researching the borehole acoustic field on tight reservoir with the fractures. The method of three-dimensional staggered grid stress and speed finite difference was applied to numerically simulate the problems of borehole acoustic field, which was aroused by point source in layer of elastic media and pore media containing inclined thin cracks. The microfractures in fractured tight sandstone reservoirs were effectively identified and evaluated by using the parameters of energy amplitude of Stoneley wave and etc. In consideration of single horizontal crack, the influence of crack width (20 - 1000 µm) on the Stoneley wave used to identify microfractures was determined, the wider the fracture was, the smaller the amplitude of Stoneley wave was. When the width of fractures was narrower (crack width was less than 100 μm), the wave-form amplitude diminished rapidly. The wider the fracture was, the bigger the Stoneley wave atten-uation would be. Nevertheless, the relationship between its waveform amplitude, attenuation and fractural width was also obtained under the condition of different porosities of porous medium in formation, that was, the bigger the porosity was, the smaller the amplitude of Stoneley wave, P-wave and S-wave was, the larger the attenuation coefficient of Stoneley wave, P-wave and S-wave was.