Risk assessment of fault reactivation considering the heterogeneity of friction strength in the BZ34-2 Oilfield, Huanghekou Sag, Bohai Bay Basin, China

Abstract

The hazards of fault reactivation caused by fluid injection are a growing concern. However, traditional evaluation methods of fault stability are likely to underestimate the risk in fault segments with a high clay content. Therefore, an extended evaluation method of fault stability (ECPP) incorporating the heterogeneity in friction strength caused by variation in the clay content within the fault zone is established in this study. After characterizing the current stress field of the BZ34-2 Oilfield in the Huanghekou Sag, Bohai Bay Basin, the reactivation potential of faults is evaluated using both traditional and ECPP methods. Traditional evaluation of fault stability shows that all faults are stable in the present stress field. Faults oriented ENE have a relatively high risk. The maximum sustainable fluid pressure Δp is approximately 8.8–8.9 MPa and 9.3–9.9 MPa. When considering the heterogeneity in fault friction strength, the fault stability is clearly controlled by the clay content of the faults. The high-risk fault segments assessed using traditional methods are no longer obvious, which reflects the importance of incorporating friction strength heterogeneity in the process of fault evaluation. Moreover, the results also show that most fault segments are activated when the fault zone is dominated by montmorillonite, reflecting the strong influence of clay mineral types on fault stability. The factors influencing the heterogeneity of fault friction strength are very complicated in actual situations. Therefore, future work should focus on establishing a database through a large number of experiments and investigating the relationship between the friction coefficient and the main controlling factors.