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Deep Shear Wave Imaging Helps Making Sidetrack Decisions in Fracture-Cave Carbonate Reservoir


 The development of a vuggy carbonate reservoir is very challenging because of low porosity and permeability in the matrix. Natural fractures and caves play a dominate role in this kind of reservoir. Production of these reservoirs highly depend on whither fracture-cave cubes are successfully targeted. Consequently, it is very important to determine the entire picture of the natural fracture-cave systems near to and away from the borehole.
 The traditional ways to evaluate fractures or caves in acoustic logging are Stoneley permeability, azimuthal shear-wave anisotropy analysis, along with resistivity image logging. However, the depth of investigation of all these methods is limited from less than one inch to a few feet from the borehole wall. Deep shear wave imaging is a new way to image the reflectors offside the borehole. This imaging technique uses the shear body wave that is reflected back to the borehole by the acoustic impendence differences caused by planar features (such as fractures and caves) in the formation. After suppression of the direct wave mode and migration of the reflected wave, it gives a picture of the fracture system around the borehole. In addition, it has a much deeper depth of investigation, which depends on the shear slowness and attenuation of the formation. Normally, in carbonate formations with shear slowness at 80 ft/us, deep shear wave imaging manages to see reflectors as far as 100 ft from the borehole. Compared to methods using monopole wave to imaging reflectors, deep shear-wave imaging has another important feature. Because it is using cross-dipole waveforms, which is azimuthally sensitive, therefore, the deep shear wave imaging can provide the azimuthal of the fractures—usually very important information for penetration and stimulation of the formation, also the possible locations of the reflectors are defined to help making sidetrack drilling decisions.
 This paper describes a case study of deep shear-wave imaging application in a vuggy carbonate reservoir in Tarim basin. Conventional pertrophsical interpretation does not show good signatures in the borehole. Deep shear-wave imaging analysis shows two major reflectors away from the borehole. Combining seismic attributes and inversion analysis, the caves offside the borehole are located, helping the operator to make sidetrack decision. The sidetrack is successfully drilled into the cave. The value of deep shear-wave imaging is to provide a further insight of the natural fracture and cave system around the borehole, which cannot be achieved by conventional petrophysical methods.