Monitoring of Rock Stress Redistribution in Geological CO2 Sequestration

E. Sun, E. Westman, B. Fahrman, Xu Ma
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Abstract

Shale gas has become an increasingly important clean energy, which has been explored worldwide in recent decades. Supercritical CO 2 acts as fracturing fluid for shale gas production. The safety monitoring is essential to prevent any kinds of leakage from the reservoir as the supercritical CO 2 physically stored hundred kilometres underground. Seismic tomography is an imaging technique that uses induced seismic waves to create three dimensional images of the subsurface. It is an effective monitoring method to evaluate the caprock integrity in the carbon dioxide sequestration storage (CCS). In this experimental research, a simulated uniaxial compressive load is applied on a granite sample to analyze the stress redistribution for long-term in-situ caprock integrity during CO 2 injection. The induced seismic waves are recorded and seismic events are traced based on the Geiger algorithm. The frequency of seismic events correlates with the caprock failure evolution. The acquired seismic data is divided into four regimes based on the frequency of seismic events and the failure process to examine the failure evolution. Furthermore, the travel time and distance is plotted to analyze the variation of velocity. Finally, the double difference tomography (TomoDD) algorithm using arrival time is adopted to recalculate the locations of seismic events and velocity structure in each regime. The results indicate that the passive seismic system can map the caprock stress distribution and allow for imaging of the caprock integrity. TomoDD exhibits sound improvements to relocate seismic events both in relative and absolute locations as well as to characterize the local velocity structure. The study further reveals that seismic monitoring along with TomoDD could evaluate the caprock failure accurately in the CCS.
地质CO2封存过程中岩石应力重分布的监测
页岩气已成为一种日益重要的清洁能源,近几十年来在世界范围内得到了广泛的开发。超临界二氧化碳作为页岩气生产的压裂液。由于超临界co2物理储存在地下数百公里处,因此安全监测是防止水库泄漏的必要条件。地震层析成像是一种成像技术,它使用诱发地震波来创建地下的三维图像。对二氧化碳封存(CCS)中盖层完整性进行评价是一种有效的监测方法。本实验研究在花岗岩试样上施加模拟单轴压缩载荷,分析co2注入过程中原位盖层完整性的应力分布。利用盖革算法记录了诱发地震波,并对地震事件进行了跟踪。地震事件的发生频率与盖层破坏演化有关。根据地震事件的发生频率和破坏过程,将采集到的地震资料划分为4个区,以考察破坏演化。此外,还绘制了行进时间和距离,以分析速度的变化。最后,采用基于到达时间的双差层析成像(TomoDD)算法,重新计算各区域的地震事件位置和速度结构。结果表明,被动地震系统可以绘制盖层应力分布图,实现盖层完整性成像。TomoDD在定位地震事件的相对位置和绝对位置以及描述当地速度结构方面都有长足的进步。研究进一步表明,结合TomoDD进行地震监测可以准确地评估CCS的盖层破坏情况。
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