Applications of cross-dipole acoustic logging for formation fracture and anisotropic identifications

Abstract

Cross-dipole borehole acoustic logging has been used for identifying vertical or deviated fractures, evaluating formation stresses, and monitoring hydraulic fracturing effects. It also can be used for predicting horizontal transverse isotropy of a formation that apparently generates shear wave separations. This characteristic is used for identifying formation anisotropy distributions near the boreholes, so that formation stress and fracture in a real formation can be evaluated. However, in practice, because of complicated logging environments, such as formation borehole condition variations, and some times there existing abnormal formations, it has a limited applications. Therefore, a comprehensive procedure should be used for detailed data interpretation, with the help of the other logging data. In this work, firstly, a 3-dimensional staggered finite-difference method is used to simulate borehole acoustic wave propagation excited by a cross-dipole source in a pre-stressed formation. The wave equations of motion are based on nonlinear acoustoelastic theory. The effects of formation pre-stress on borehole flexural modes are discussed. Then, based on the numerical modelling results, the cross-dipole acoustic logging data in 400 wells from the Daqing oilfield in China are used for fracture identifications in deep formations with complex lithology. Also, the metamorphic formations outside of the main area in Daqing oilfield are studied. Hydraulic fracturing efficiency, formation stress distributions in a damaged casing area are investigated together with other well logging. Finally, the results are validated indirectly with other local formation stress detections, deformed casing analysis, and water injection fracturing data.