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ABSTRACT: This paper examines microseismic events recorded during hydraulic fracturing which outline faults in the carbonate layers above the target formation. The faults can be grouped into two systems: North-South system (system 1) and North-Northeast-South-Southwest system (system 2) exhibiting different temporal-magnitude patterns. System 1 showed typical patterns similar to a natural earthquake sequence where the main shock is followed by smaller aftershocks. Whereas system 2 showed increasing larger magnitudes with time along with smaller magnitudes varying with the pumping stages. Regional stress orientation suggests that fault system 1 has higher shear to normal stress ratio, which might be the main reason why those faults are capable of hosting large events without much drive from fluid injection. Geomechanical modeling was performed combining hydraulic fracture modeling with dual-porosity-dual-permeability reservoir simulation. Simulation results show that the events on fault system 2 are more likely to be fluid driven (induced), while events on fault system 1 are more likely to be natural events triggered by fluid injection (triggered). 1. INTRODUCTION Microseismic data has been used as a qualitative measure for assessing hydraulic fracture geometry during the stimulation process in unconventional plays (Maxwell et al. 2010; Grechka and Heigl 2017). It is an effective tool in outlining the stimulated reservoir volume (SRV) to estimate fracture height, length, complexity, and further, this information can be used to optimize well spacing and landing decisions (Tan et al. 2014a; Tan 2015; Warpinski 2009). Microseismic can also shed light to stress regime and pressure depletion level (Dohmen et al. 2014; Tan et al. 2019; Wang et al. 2019). Common source mechanisms observed for microseismic events include bedding-plane-slip/dip-slip (BSDS) and strike-slip. The mechanisms of BSDS events are still under debate: the disagreement centers around which plane actually slipped (Tan et al. 2014 b; Tan and Engelder 2016; Tan et al. 2021; Tan et al. 2022). The strike-slip events are relatively better understood, which are associated with either natural fracture swarms or faults being reactivated during the hydraulic fracturing process (Yang and Zoback 2014). |
