微地震震源机制与页岩储层衰减系数（Q值）的联合反演

Hydraulic fracturing plays a very important role during the exploration of unconventional resources. The only way to quantitatively estimate the effect of hydraulic fracturing is the microseismic monitoring technology. Fracture directionality, scale and stress state of hydraulic fracturing areas are extremely important for the exploration of unconventional oil and gas resources. These information can be obtained from the study of microseismic focal mechanisms. When a microseismic event happens, its seismic wave will propagate in the shale layer. Due to the strong attenuation of shale reservoir, the signal’s amplitude and frequency will change. Previous studies on the microseismic focal mechanism inversion generally failed to consider the shale reservoir’s attenuation effect. Existing attenuation studies about the shale layer typically use only spectral information to make rough estimates of the attenuation factor..This study intends to conduct a joint inversion study on the microseismic focal mechanism and the shale layer’s attenuation factor. When synthesizing theoretical waveforms, we add attenuation items. When calculating microseismic focal mechanism, we invert the shale layer’s attenuation factor as well. Through this study, we can quantitative analysis the shale’s attenuation effect on microseismic focal mechanism inversion, improve the resolution of focal mechanism solutions and understand the changes of layer’s attenuation due to hydraulic fracturing activities. These results can provide guidance and quantitative basis for the shale gas exploitation and the oil reservoir modeling.

水压致裂是非常规油气开采过程中的关键环节，微地震监测技术则是对水力压裂效果定量评估的重要手段。研究微震震源机制，获得压裂区域的破裂方向、尺度和应力状态等信息，有助于有效压裂体积的计算和油藏的开发建模。微震发生后，地震波会在页岩层内传播，由于页岩衰减较强，信号的振幅、频率会发生改变。目前开展的微震震源机制反演研究，普遍并未考虑地层的衰减特性，已有的页岩层衰减研究，通常仅使用记录的频谱信息对地层衰减系数进行粗略估计。. 本研究拟开展微震震源机制与页岩衰减系数的联合反演研究，在合成理论波形记录时，加入地层衰减项，反演时，联合反演震源机制和地层衰减系数。通过此研究，定量分析页岩衰减效应对震源机制反演的影响，提高震源机制解的精度，并了解水力压裂活动对地层衰减特性的变化情况，从而更好地为页岩气开采和工区开发建模提供指导建议和定量依据。

非常规油气开发的关键技术是水力压裂，水力压裂会使岩石破裂，从而产生微地震。通过对微地震震源机制的研究，可揭示微地震的产生机理和地下储层的应力变化规律，有效地优化水力压裂和储层改造设计以提高采收率。（1）运用“剪切+张裂”广义位错模型来描述微地震发震机制，并优化完善了其表征和正反演方法。我们还发现广义位错模型的反演有两组解，其关系为斜交，并推导给出了两组解的转换关系。数值测试和实际资料处理结果表明，广义位错模型反演稳定，比剪切位错和矩张量模型更适合描述微地震震源机制。（2）建立了各向异性介质的震源机制反演方法，并研究发现各向异性会导致部分震相振幅翻倍、相位反转，且会显著影响倾滑断层和非剪切组分的反演。（3）通过数值测试，发现衰减会显著影响微地震震级和断层面解的反演，还研究了鄂尔多斯地区致密油开采水力压裂中的储层衰减，发现中段的低衰减异常与此处页岩成分增加对应，左右侧衰减强弱变化则与水力压裂有关，提供了使用衰减进行水力压裂储层解释和评价的成功案例。