New Acoustic Log Data Interpretation for Gas-bearing Shaly Sands

摘要

Acoustic wave propagation in a sandstone formation depends on porosity, mineralogy, rock matrix elastic moduli, and fluid saturation. The wave velocities are directly linked to pore space compressibility, which forms the basis of many acoustic interpretation methods for identifying gas-bearing sands, e.g., Castagna’s cross-plot method and Murphy et al.’s modulus decomposition method, etc. The latter method detects the presence of gas by comparing acoustic log data with synthetic data computed for full saturated water, oil and water conditions. A basic assumption of this method is that the ratio of the bulk and shear moduli under gas-saturation is invariant for sandstones. This assumption limits the application of this method only to the clean sand scenario. For shaly sands with significant clay content, this moduli ratio is not a constant value, and the interpretation based on the constant ratio assumption may become invalid. We developed a new acoustic interpretation method for gas indication in the shaly sandstone formation. This method determines the variation of the dry frame bulk-to-shear moduli ratio by calibrating it in a water-bearing zone. This calibration establishes the correlation between the moduli ratio and shale content. The established correlation is then applied throughout the zone of interest to calculate pore fluid bulk modulus and gas saturation. The calculation result enables a direct interpretation of gas-bearing sands. To apply this method, acoustic compressional and shear velocity, gamma ray, and density log data are required. In case there is no water-bearing zone for the calibration, default ratio values can be used. This method has been successfully tested in several wells. Two examples are used to demonstrate the method and its application. In both examples, the new results are compared with those from Murphy et al.’s method to demonstrate the marked improvement of the new method for shaly sands. Neutron and density results and resistivity log data further validate the new results.