Effects of water saturation on P-wave propagation in fractured coals: An experimental perspective.

Internal structure of coalbed methane (CBM) reservoirs can be evaluated through ultrasonic measurements. The compressional wave that propagates in a fractured coal reservoir may indicate the internal coal structure and fluid characteristics. The P-wave propagation was proposed to study the relations between petrophysical parameters (including water saturation, fractures, porosity and permeability) of coals and the P-wave velocity (V p ), using a KON-NM-4A ultrasonic velocity meter. In this study, the relations between V p s and water saturations were established: Type I is mainly controlled by capillary of developed seepage pores. The controlling factors on Type II and Type III are internal homogeneity of pores/fractures and developed micro-fractures, respectively. Micro-fractures density linearly correlates with the V p due to the fracture volume and dispersion of P-wave; and micro-fractures of types C and D have a priority in V p . For dry coals, no clear relation exists between porosity, permeability and the V p . However, as for water-saturated coals, the correlation coefficients of porosity, permeability and V p are slightly improved. The V p of saturated coals could be predicted with the equation of V p -saturated = 1.4952V p -dry–26.742 m/s. The relation between petrophysical parameters of coals and V p under various water saturations can be used to evaluate the internal structure in fractured coals. Therefore, these relations have significant implications for coalbed methane (CBM) exploration. [ABSTRACT FROM AUTHOR]