Response characteristics of acoustic reflection imaging logging of fractures and formation interfaces outside horizontal wells in tight reservoirs
Dense small-sized and high-angle fractures are commonly developed in unconventional reservoirs,such as tight sandstone and shale reservoirs.These fractures play a positive role in increasing the permeability of the reservoir,but also play a negative role in causing horizontal casing deformation and inter-well frac-hit during oil and gas production,hydraulic fracturing,CO2 injection,etc.Therefore,it is of great significance to investigate the response characteristics of horizontal well acoustic reflection imaging logging for these fractures,and further identify these fractures using reflection imaging methods.This paper establishes a three-dimensional numerical model of fractures outside the horizontal well of a tight reservoir,and the forward modeling is carried out.The results indicate that when the angle between the fracture and the borehole is small,the received waveform in the borehole is mainly the fracture reflected wave.When the angle exceeds a certain critical value,the received waveform is mainly the scattered waves from the two endpoints of the fracture.And the scattered waves split into two wave packets.The concept of generalized scattered SH wave is proposed,which is a transverse wave with source polarization direction parallel to the fracture strike.It represents the direction where the scattered wave has the strongest energy.The presence of fractures and strong acoustic impedance formation interfaces outside horizontal wells is a more common situation.The simulation results indicate that,the main energy of the fracture scattered wave is concentrated on the generalized scattered SH wave.As the differences between the formation interface and the strike of fractures increase,the generalized scattered SH wave corresponding to the fractures are easier to identify.Finally,the accuracy of the forward simulation was verified through a reflection imaging example of a horizontal well.This study has important theoretical significance for identifying and evaluating small-scale fractures outside horizontal wells in unconventional reservoirs.
Horizontal wellAcoustic reflection imaging loggingFractureFormation interfaceScattered wavesGeneralized scattered SH waves
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