基于微震特性的相对震级技术研究及应用

随着非常规气藏的开采开发, 微地震监测成为压裂效果评估的关键技术.四川盆地非常规油气藏开采开发处于早期, 井网密度极低导致在压裂井附近难以找到匹配深井作为观测井, 而地面、浅井等替代观测方式面临无法有效探测微地震信号的风险.微地震事件能量弱和辐射的方向性使得观测方位预判及有效监测距离的评估成为微地震监测成败的关键因素.本文提出一种基于压裂微地震能量辐射模式和地层传播特征的相对震级计算技术, 模拟微地震事件能量辐射模式及在地层传播过程中的动力学特征, 达到评估微地震相对震级与检波器方位、地层传播距离的非线性关系的目的.通过理论分析和实际微地震监测资料验证, 该方法能有效地解决微地震监测最佳观测方位的优选和有效传播距离的评估问题.

Research and application of the relative magnitude technique based on microseism

With the development of the unconventional exploration, the microseismic monitoring (MSM) has been the key technique for assessment of the fracturing effect. In the Sichuan basin, exploitation of unconventional reservoirs remains in the early stage, where the very sparse wells lead to difficulty to find a suitable deep well nearby the fracturing well as the monitoring borehole. While other alternative wells such as those on the ground or shallow ones cannot detect microseismo event effectively. Because of the weak energy and strong orientation of microseismic radiation, pre-determination of observational azimuths and estimation of effective monitoring distances are critical for MSM. The objective of this work is to study how to deploy the observational system of MSM based on microseismic mechanism and propagation features. Methods: This paper proposes a calculation method for relative magnitude based on the microseimic event radiation pattern and propagation characters in strata, and simulates the kinetic parameters to assess the non-linear relationship among the relative magnitude, sensor azimuth and the propagation distance. This method takes the source mechanism, spherical spreading, attenuation and refraction into consideration to reveal the energy attenuation in different directions and locations of the MSM signal.#br#With the detailed theoretical analysis of different effects of the source mechanism, spherical spreading, attenuation and refraction and the relative magnitude relationships of different source mechanisms, this method is able to reveal the energy attenuation of MSM signal at varied locations of the layer. The relative magnitude has applied in two different MSM projects. One is the downhole MSM observation geometry that the simulated relative magnitude accorded with the actual recording data both in shallow and deep observational wells. The other is the surface MSM observation geometry with 17 receiving arrays, and the simulated relative magnitude keeps consistent with the actual recording data. The theoretical and actual data show this technique can effectively resolve the optimum observation azimuth and distance of the MSM.#br#Because of different geology and rock-physics properties in the early stage of the unconventional exploitation, the statistics-based detection distances of MSM must have big differences in different areas. The relative magnitude technique based on the microseismic characteristics can provide the qualitative and quantitative guidance for MSM observation geometry with the consideration of the fractured source mechanism and property of the layer, and also provides the optimum scheme for the receiving azimuth and distance for the surface, deep well and shallow well observation design. The source mechanism and spherical spreading play a dominant role in energy attenuation at the nearer location to the source, the former affects the relative magnitude direction and the latter affects the energy gradient in the radial direction. At the farther distance to the source, the attenuation and refraction have bigger effects on the energy. Therefore, the farther distance to the source, the smaller relative magnitude decrease gradient, which means the wider optimum space for surface and shallow well MSM.