基于微震监测的页岩气开采区b值特征研究

Study on b-value Characteristics of Shale Gas Exploration Field Based on Microseismic Monitoring

四川盆地南部的长宁页岩气开发区附近地震频发,近年来已发生近10次ML>4.0的中型地震和万余次ML1.0~3.0的小微地震,灾害风险持续增高。由于国家地震台网的固定台站较为分散,难以捕捉到1级以下微震事件的精确信息,通过近场微震监测数据来分析页岩气开发区的地震风险演化趋势,已经成为势在必行的科学问题。本文基于专门布设的13个近场流动台站和国家地震台网固定台站2017年2月至2018年6月监测拾取到的1万余个地震事件,采用最大似然估计法计算了双差定位后地震目录的b值,详细探讨了长宁页岩气开发区的b值演化特征。基于更低的最小完整震级,估计出区域总体b值为0.98±0.02,略高于前人研究结果。拟合直线的双线性特征和错位分布指示出b值演化可能存在明显的时空差异特征。通过分析验证了这一结果,并且发现5个地震事件数量快速增加的时间段恰好伴随着b值的强振荡特征和相关空间距离SCL值的稳定低值分布,认为可能与附近的页岩气压裂开采存在关联。分析认为4个较大震级事件发生之前出现的b值下降应归因于震前的应力累积过程,类似于一般的大型天然地震事件。

In recent years,the Changning Shale gas exploration in the Southern Sichuan Basin has experienced frequently small earthquakes,as well as a number of medium-sized earthquakes with M L>4.0.Since the risk of disaster continues to increase,it has become an urgent scientific problem to analyze the evolution trend and development law of seismic risk in shale gas development zone by means of microseismic monitoring.From 13 portable near-field temporary seismic stations and national fixed network,more than 10,000 seismic events were picked up from February 2017 to June 2018.The maximum likelihood estimation method is used to analyze the seismic catalogue after double difference precise positioning and the evolution characteristics of b value in Changning shale gas development zone are put forward.The overall b value is estimated to be 0.98±0.02,slightly higher than previous results.The bilinear characteristics and differential distribution of the fitted line indicate that there exist obvious spatiotemporal differences in the evolution of b value,which has been verified with subsequent analysis.We found that the 5 periods of rapid increase of seismic events just corresponded to the oscillation characteristics of b value and the stable low value distribution of SCL value of relevant spatial distance,which was suspected to be associated with the nearby shale gas fracturing exploitation.The decrease of b value before the occurrence of four large magnitude events corresponds to the stress accumulation process before the earthquake.