Abstract
During the hydraulic fracturing procedure in shale-gas exploitation, the poroelastic properties of shale formation can be altered significantly. However, it is difficult to evaluate these variations using microseismic field data. In this study, we conduct a hydro-fracturing experiment using Longmaxi shale, which is a major formation for shale-gas production in China, to simulate the water injection and rock fracturing procedure. The variation of the velocity and attenuation for primary/secondary (P/S) ultrasonic waves was investigated throughout the entire experimental procedure. The results show that the attenuation is more sensitive to sample rupture than the velocity. However, P-wave attenuation loses sensitivity to the water injection after the fractures are saturated with water. In that case, it is preferable to use S-wave attenuation to identify the opening/closing of the fractures. Based on the experimental results, we can conclude that the variation of the attenuation must be considered during microseismic data processing and interpretation.
Highlights
Unconventional oil and gas reservoirs usually exhibit extremely low porosity and permeability.Hydraulic fracturing is the primary exploitation technique used to improve gas/oil extraction efficiency [1]
When more and more fluid is injected into sediment, water transport occurs in the rock matrix and differently-scaled pore spaces can alter the physicochemical properties of a shale reservoir significantly, resulting in a noticeable variation of velocity and the attenuation value [30,31,32]
The hydro-fracturing operation changed the physical properties of the shale sample significantly
Summary
Unconventional oil and gas reservoirs usually exhibit extremely low porosity and permeability. Most methods of microseismic data processing or inversion do not consider the dynamic variations of petrophysical parameters, i.e., the primary/secondary (P/S)-wave velocity, attenuation, and anisotropy [7,8]. To determine the physical characteristics of sediment rock, many researchers have implemented petrophysical experiments to compensate for the deficiencies of seismic exploration techniques [13,14,15,16]. When more and more fluid is injected into sediment, water transport occurs in the rock matrix and differently-scaled pore spaces can alter the physicochemical properties of a shale reservoir significantly, resulting in a noticeable variation of velocity and the attenuation value [30,31,32]. The sensitivities of velocity and attenuation changes to the hydraulic fracturing were compared
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