Anisotropy Analysis in Shale Using Advanced Sonic Data — Bakken Case Study

Incorporating anisotropy in seismic data processing will improve accuracy in prestack depth migration, amplitude variation with offset (AVO) and hydraulic fracture monitoring, among which hydraulic fracing plays a critical role in production enhancement of tight reservoirs such as Bakken Formation. Considering shales as being vertically transverse isotropic (VTI), three Thomsen anisotropy parameters with five independent stiffness coefficients could be used to fully characterize them. So far, different methods are applied to measure these parameters directly such as walkaway VSP(vertical seismic profiling), multi-offset and multi-azimuthal VSP. Although these surveys can provide us reliable anisotropic models, various constrains including the cost could limit recording the VSPs. Recent innovations in the acquisition of broad band sonic waveforms and dispersion analysis have changed the industry’s perspective on cost of anisotropy analysis.

In this paper, advanced sonic data in two producing wells in Bakken Fm, each drilled through different geologic features has been acquired and analyzed. One is located on the crest of Nesson anticline, the major geologic structure in the area. The other is away from the anticline and none of the Nesson geologic feature is visible. First, a total of five stiffness moduli were estimated from the Stoneley, two flexural waves and formation density. Second, the Thomsen parameters in three members (upper, middle and lower) of Bakken Fm. were calculated and compared one another. It has been found that the Upper and Lower Bakken are highly VTI because of the clay platy particles, while the middle member is isotropic or slightly anisotropic. The well on the top of the anticline shows a higher degree of anisotropy in the middle member; and we interpreted this as the presence of fractures due to the folding. Finally, data inversion of Stoneley and two flexural waves was performed to create slowness radial profile around the borehole and in the far field to elaborate the idea of existence of intrinsic fractures and to discriminate them from the drilling induced ones. The well on the crest of the anticline shows higher amount of variations in the Stoneley wave slowness. Stoneley wave is proved to be sensitive to the mobility and could be an indicator of higher permeability.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California