ROLE OF IMMATURE FAULTS IN INJECTION-INDUCED SEISMICITY IN OIL/GAS RESERVOIRS—A CASE STUDY OF THE SICHUAN BASIN, CHINA

doi:10.3969/j.issn.0253-4967.2014.03.007

Abstract

Abstract:

In order to understand the role of underdeveloped immature small faults and conditions of the occurrence of medium-size earthquakes induced by fluid injection into deep reservoirs, we carry out an integrated study involving geological survey of small faults, seismological investigation of typical cases of injection-induced seismicity, and rock fracture test in laboratory. At first, we briefly summarize the general features of several important cases of injection-induced seismicity in gas/oil reservoirs in the Sichuan Basin, China. It is suggested that major induced earthquakes, especially those of a moderate size, result from the reactivation of pre-existing faults. We also present some pieces of field and petrologic evidence showing such small faults exist widely in reservoirs and are important channels for fluid migration. Then, we present experimental results with a focus on the formation and the after-slip of fault in typical sedimentary rocks under tri-axial compression. Finally, methodology for risk assessment and injection management is discussed based on insights gained from the integrated approaches.
Our results are helpful for understanding the question—why injection-induced seismicity is so significant in the Sichuan Basin. Major Pre-Triassic sedimentary rocks, including dolomite, shale, and dolomitic limestone are strong and demonstrating brittle fracturing behaviors. Such properties are necessary conditions for maintaining high level of reservoir stress and leading to seismic fracturing. Insights gained from this study may shed some lights to the general earthquake seismology and provide a better understanding of why damaging injection-induced earthquakes occur so that they can either be avoided or be mitigated. In general, existing of critically or sub-critically stressed faults of a dimension of a few kms is a necessary condition for M～5 level earthquakes. In addition, AE, or in other words, micro-seismicity monitoring is useful in risk assessment and injection management and should be fully utilized in injection applications.

Key words: injection-induced seismicity, fault reactivation, oil/gas reservoir, Sichuan Basin, rock fracture experiment, acoustic emission

摘要：

以四川盆地为例，通过分析沉积岩储藏系统中断层的发育特征和典型沉积岩断层形成前后岩石变形破坏过程的实验研究，探讨沉积岩层中小断层在油气田注水诱发地震中的作用及较大地震发生条件。首先，通过分析已有的有关四川盆地深部注水诱发地震活动情况的资料得到具有一定普遍性的认识：诱发地震的空间分布明显受已有断层或破裂的控制，有感地震尤其是可导致一定灾害的中强地震往往都是已有断层的重新活动所致。其次，结合石油地质资料及盆地边缘地区出露的一些未成熟小型断层的地质与构造特征观察分析，发现这些断层的存在具有普遍性而且是地下油气纵向运移的主要通道。进而，在实验室条件下对四川盆地几个典型沉积岩样本进行了三轴压缩破坏实验。采用多通道宽动态高速声发射观测技术与三维X射线CT成像技术分析断层形成过程及形成后的几何特征和摩擦系数的变化。最后，通过融合各方面研究结果分析注水诱发地震的发生机制和模型，探讨了较大地震发生条件，为提升注水诱发地震评估能力和建立安全注水操作管理指南提供理论基础。该结果有助于理解为何四川盆地油气田注水诱发地震活动如此剧烈。主要的前三叠系沉积岩（包括白云岩、页岩及白云质灰岩）均具有较高的破坏强度和脆性变形破坏特征，使四川盆地油气储藏系统有条件保持较高的应力水平和在一定条件下导致地震性断层破坏。一般，一条尺度达到几km的断层便足以产生M5级别的中强地震。这些条件使得该区具有发生破坏性注水诱发地震的可能。有必要在对注水孔周围的破裂及断层构造的空间分布、尺度大小及力学性质特征有基本了解的条件下，通过注水数值模拟和监测注水诱发地震的时空演化，建立适当的统计模型实现诱发地震的统计评估和预测，指导注水操作而达到减少灾害性地震发生的风险。

关键词: 注水诱发地震, 断层重新活动, 油气田, 四川盆地, 岩石破坏实验, 声发射

CLC Number:

P315.9

LEI Xing-lin, LI Xia-ying, LI Qi, MA Sheng-li, FU Bi-hong, CUI Yin-xiang. ROLE OF IMMATURE FAULTS IN INJECTION-INDUCED SEISMICITY IN OIL/GAS RESERVOIRS—A CASE STUDY OF THE SICHUAN BASIN, CHINA[J]. SEISMOLOGY AND GEOLOGY, 2014, 36(3): 625-643.

雷兴林, 李霞颖, 李琦, 马胜利, 付碧宏, 崔银祥. 沉积岩储藏系统小断层在油气田注水诱发地震中的作用——以四川盆地为例[J]. 地震地质, 2014, 36(3): 625-643.

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