Unconventional shale reservoir evaluation and development are extremely challenging. One of the most dominating aspects is permeability, which is measured in the nano-darcy range. Although these wells are stimulated to enhance production, the presence or absence of natural fractures can have a large impact on the production results. In addition, the fracture variation across a reservoir can be substantial, leading to large production variations even in adjacent wells. Gaining insight about the natural fracture system, both intersecting and around the borehole, is crucial and can often help determine the economic success of a well and/or reservoir.
The standard means of fracture evaluation, such as borehole imaging, Stoneley permeability analysis, and azimuthal shear-wave anisotropy evaluation from cross-dipole, provide valuable information when evaluating fractures. These standard methods, however, can only investigate a limited area around the borehole—imaging looks at the borehole wall and the other borehole acoustic methods rely on refracted and guided modes that respond to an area as large as 2 to 4 ft out into the formation. The flexural wave from the dipole is one of the guided modes that generally reads the deepest and is used in the standard cross-dipole analysis. In addition to flexural mode, the dipole source creates shear body waves that radiate away from the borehole and into the formation. When these shear waves impinge on a fracture, their energy reflects back to the borehole, allowing the facture to be imaged. The reflection strength is a function of the shear-wave polarization and the nature of the fracture, with the strongest response occurring from the shear waves intersecting a fluid/gas-filled fracture and polarizing in the fracture’s strike direction.
Another important aspect is that these shear waves have azimuthal sensitivity, providing a means to determine the fracture direction. These features permit the evaluation of fractures over a much larger area around the well, often looking out in excess of 60 ft from the borehole and even detecting major fractures that do not intersect the well.
We will look at the application of this deep shear-wave imaging technology in several unconventional reservoirs across North America. Our review includes conventional methods and the deep shear-wave imaging analysis, showing its value in gaining important insight about the natural fracture system around the borehole.