Geomechanics and elastic anisotropy of the Bakken Formation, Williston Basin

Many of the earth’s rocks exhibit anisotropic characteristics. Anisotropy is particularly common in many sedimentary rocks, such as shales. Anisotropy is defined as the spatial alignment of mineral grains, layers, fractures and stresses which causes elastic wave velocity and other elastic properties to vary with direction. There are two types of anisotropy: intrinsic and stress-induced. Intrinsic anisotropy is caused by beddings, microstructures or aligned fractures formed during deposition. Stress-induced anisotropy is caused by strain associated with external stresses. Intrinsic anisotropy originates in the absence of external stresses, while stress-induced anisotropy results from tectonic and overburden stresses. The style of earth material alignment causes two simplified, but convenient models of anisotropy: vertically transverse isotropy (VTI), like shale, and horizontally transverse isotropy (HTI), like vertically fractured medium. These models have been used to describe how physical properties of rock vary in a medium. Identifying the anisotropy in a formation is important in reservoir characterization seismic data processing and oil-field development. Deep shales are the most abundant yet least characterized sedimentary rocks in the Williston Basin of North Dakota. They are significant sources of hydrocarbon unconventional resources in this basin. This dissertation aims to fulfill an investigation of anisotropy in this rock type in several different facets through exploiting of field data. I seek to generate key information for better interplay of field in-situ stress and the existing natural fracture systems for the purpose of drilling, well completion, perforating, hydraulic fracturing and defining reservoir properties. In this study advanced sonic logging data has been processed and interpreted to calculate three independent shear moduli. These parameters then will be used to estimate Thomsen (1986) anisotropy parameters, elastic stiffness coefficients and principal stresses of deep shales in the Williston Basin. The parameters then will be used to generate shear radial profiles and slowness-frequency plots analyze formation anisotropy type and origin as well as reservoir quality. The next step will be to evaluate direction and magnitude of the minimum and maximum anisotropic principal horizontal stresses as the governing element in geomechanical modeling. I will analyze wellbore stability and predict wellbore behavior under stress alteration caused by drilling. Elastic anisotropy of the formation will be included in the 3 D numerical models. In addition the effects of local geological features on the mode of anisotropy both in the far-field and around the borehole to get an in-depth insight of the fractures will be studied. Finally, by generating stress polygons for the reservoir, before and after production and pressure decline, I will try to study how reservoir depletion may cause future geological natural hazards such as faulting and induced seismic events in the region.