关键词: 水力压裂, 诱发地震, 时空异质性, b值, 地震危险性分析

Abstract:

Induced earthquakes and the corresponding seismic risk are rising concerns for the smooth implementation of new industrial activities such as the exploitation of unconventional oil and gas resources and has attracted broad attention from both the public and academia. As a result, many associated scientific problems need to be further examined. The magnitude-frequency distribution(FMD)is fundamental to seismicity characterization, where systematic study of b values for induced earthquakes could reveal the regional accumulated stress, subsurface structural characteristics, as well as seismic risks of induced seismicity.
In this study, we systematically reviewed the values, spatial-temporal heterogeneity, physical mechanism, dominating factors, as well as the application status of b values on hydraulic fracturing induced earthquakes in the past ten years. Multiple case analysis shows that the b value varies over a wide range(0.6~2.9)and exhibits large spatiotemporal heterogeneity. Felt earthquakes are often preceded by decreased b values and often occur in the regions with relatively low b backgrounds. In addition, fault activation or felt induced earthquakes are often accompanied by b values less than 1.0, despite that b>1.0 are commonly observed in the process of fracture expansion. Thus, the b value is promising for estimation of the state of faults(i.e., maturity and criticality).
This study further assessed the factors that may affect b values, including objective factors such as in-situ stress field, fault geometry, fault maturity and focal depth, as well as subjective factors associated specific construction conditions, such as injection volume and injection rate. We then summarized multiple possible physical mechanisms, including the pore pressure, in-situ differential stress, maximum shear stress, and the non-uniformity of geological conditions. Although the discussed factors and physical mechanisms imply the multiple complexities associated with the b value that may challenge the effectiveness of its utilization, the b value remains a preliminary but effective evaluation for first-order estimation induced-earthquake hazard. For example, in the cases of deep hypocenter, high differential stress, high fault maturity, developed initial fracture network or bedding, or when the pore pressure, fault geometry and in-situ stress field meet the conditions of high probability fault slip tendency, the b value is often less than 1.0. In fact, due to the limited understanding of the seismic risk induced by hydraulic fracturing, b value still serves as a key parameter for the seismic risk analysis such as earthquake rate prediction and maximum magnitude prediction, as well as risk control technologies such as “Traffic Light System”(TLS), the b value has been widely used in hydraulic fracturing.
Finally, we discussed the misunderstandings and challenges of b-value estimations for hydraulic fracturing induced earthquakes. For instance, b values calculated from different methods are less comparable and the quality of seismic catalogues, especially the reliability of magnitude measurement, also impact the accurate estimation and physical interpretation of b value. In addition, the mutation point of when the fault is about to reach its critical stage cannot be accurately identified through the temporal evolution of the b value alone. Even in cases where the mutation point is identified, the shut-down of current industrial operation does not guarantee the prevention of a subsequent felt event. Such challenges limit the effective utilizations of b values toward mitigating the seismic hazard associated with hydraulic fracturing induced earthquakes.
After clarifying the consensus and controversial scientific issues, we speculate that the b value for induced earthquakes may serve as one preliminary criterion for the evaluation of reservoir reconstruction, the estimation of reservoir stress state and the mitigation of induced seismicity hazard. Our study summarized and evaluated the b-value characteristics for hydraulic fracturing induced earthquakes. The paper could serve as a scientific reference for the industrial, regulating and research communities that are interested in non-conventional energy exploration and/or seismic safety supervision.

Key words: hydraulic fracturing, induced earthquake, spatiotemporal heterogeneity, b value, seismic risk analysis