No abstract

Unconventional oil and gas reservoirs have broad exploration and development prospects. Fracture parameters and effectiveness evaluation are two of the key tasks for the evaluation of these types of reservoirs. Array acoustic logging can be used for fracture evaluation to compensate for the deficiencies of the image logging fracture evaluation method. Therefore, to develop acoustic logging evaluation methods as well as nondestructive testing methods for fractures, experiments were conducted to study the shear wave transmission in fractured media. Experiment data demonstrate a good correlation between the shear wave attenuation coefficient and fracture width, and the shear wave attenuation coefficients rise logarithmically with the increase in the fracture width for all models with different porosities and distinct dip angles of fractures. The shear wave attenuation coefficient changes relatively faster with the fracture width when the fracture width is within 250 μm. In addition, the shear wave attenuation is affected by the core porosity and fracture dip angle. When the fracture width is constant, the shear wave attenuation caused by the 0° fracture is relatively larger and is obviously greater than that of the fractures at other angles, which is consistent with the existing experimental results. The results of this study can be used to guide further research on amplitude compensation methods for sonic signal transmission in fractured media and fracture evaluation methods.

Taking into consideration the difficulties in predicting the properties of liquid production and evaluating the irreducible water saturation of low-porosity and ultra-low-permeability sandstone reservoirs, the relationships between the irreducible water saturation and logging reservoir evaluation parameters are explored based on a large number of core mercury injection experiment. When the permeability cumulative contribution value reaches 99.9%, the lower limit of pore throat radius is determined as 0.025 μm and the corresponding irreducible water saturation is more accurate. The experimental results of 77 samples in the research area showed that the irreducible water saturation has a good correlation with the median pore throat radius which has a good correlation with the porosity. The irreducible water saturation is consistent with the nuclear magnetic experiment results. Therefore, a new method to determine the irreducible water saturation based on logging data is established. The application results of this method in tight sandstone reservoirs in the Keshen area show that the fluid types identified by the relationship between the calculated irreducible water saturation and total water saturation are consistent with the actual test results, which confirms the accuracy and feasibility of this method. This method solves the problem of irreducible water saturation evaluation in low-porosity and ultra-low-permeability sandstone reservoirs. In the field or other situations where core testing is lacking, accurate irreducible water saturation can be obtained based on only porosity. It also lays a foundation for further improving the prediction accuracy of fluid properties, which has broad application prospects.