Numerical study of in situ acoustic emission monitoring for small-scale hydraulic fracturing

Reservoir monitoring is necessary to achieve safe hydrocarbon extraction. It requires monitoring small acoustic emission (AE) events, assisting in determining the exact location, extension and direction of potential damage as early as possible. However, microcracks cannot be detected by the microseismic monitoring networks due to the limitations of frequency range and sensitivity. In contrast, the in situ AE monitoring system extends the detection range to higher frequencies and can detect very small deformation processes with high resolution and sensitivity. It provides detailed insights into ongoing deformation processes. However, the receivers of an in situ AE monitoring system need to be very close to hydraulic fracturing experiments due to the fast decay of high-frequency signals. In this work, by constructing four in situ AE monitoring models, the imaging effects of the interferometric imaging method for three different kHz-level frequency sources at different distances and orientations relative to the monitoring well are investigated. The numerical results show that: the higher the source frequency, the higher the imaging resolution; when the vertical orientation coverage of the source by the monitoring system is incomplete, the closer the source to the borehole axis, the worse the imaging resolution and location accuracy; when the vertical orientation coverage of the source by the monitoring system is complete, the imaging resolution and location accuracy are both improved, especially when the vertical azimuthal coverage angle is large. The integrity of the orientation coverage of the source by the monitoring system plays a critical role in improving source location accuracy.