The main objective of this thesis is to develop a practical, geology- and rock physics-oriented approach to constructing anisotropic velocity model for unconventional reservoirs using downhole microseismic datasets. The working procedure of the approach starts by addressing the geological sources of anisotropy. A priori knowledge of anisotropy is obtained by integrating geological information and rock physics studies. The prior knowledge serves as constraint on the microseismic inversion. The anisotropic velocity model obtained by the approach can reflect the heterogeneity of anisotropic parameters and cover the anisotropic symmetries of most importance in seismic exploration and reservoir characterization. The optimal anisotropic velocity model not only minimizes the data misfit, but also is reasonable from the perspectives of geology and rock physics. The results derived from downhole microseismic dataset are comparable with laboratory experiments. This demonstrates that the downhole microseismic monitoring, as a quasi in-situ experiment, has a potential to contribute to a better understanding of subsurface anisotropy beyond the laboratory.
The approach developed in this thesis uses a layered velocity model. This approximation is adequate due to the limited spatial range of microseismic monitoring and the relatively flat sedimentary background of unconventional reservoirs. The transverse
isotropy caused by the bedding-parallel fabric is defined by Thomsen parameters in each layer. The lateral heterogeneities within each layer are dismissed, while the vertical gradients of transverse isotropic parameters are kept. The fracture-induced
anisotropy is only defined in a specific layer of high brittleness and is characterized by normal and tangential fracture compliance. The approach uses the arrival-time of seismic waves recorded by sensor arrays. An anisotropic ray-tracing algorithm is modified to calculate the synthesized travel-time. Parallel computing is employed to accelerate the ray-tracing program. The inherent singularity problems in the ray-tracing method are fixed by applying numerical strategies. Two nonlinear inversion methods are involved in this approach to determine different components of anisotropy velocity model. The multi-layer TI model is inverted by an iterative gradient-based optimization (the Gauss-Newton method). The fracture-induced anisotropy represented only by two parameters is obtained by a global search method. Besides, as a possible source of uncertainties in the velocity model inversion and event locations, the issues of computing triggering time (T0) are analyzed theoretically and illustrated with examples. The approach developed in this study is partially applied to a completed project of downhole microseismic monitoring in a coalbed methane reservoir to verify the capability of iterative gradient-based inversion for anisotropic velocity model and illustrate the T0 issue in the configuration of limited aperture. Then, the approach is fully applied to a downhole microseismic dataset from Horn River Basin in Canada to investigate the fabric anisotropy and fracture-induced anisotropy of shales.
The fabric anisotropy of shale is caused by the alignment and lamination of the low aspect-ratio, compliant particles, such as clay minerals and organic matter. The existence of quartz minerals can prevent and interrupt such alignment and lamination and consequently weaken the fabric anisotropy of shale. Laboratory measurements show a strong positive correlation between the degree of fabric anisotropy and the volume contents of clay minerals and kerogen. Thomsen parameters ε and γ of shale samples are well correlated with each other, but not with δ. By integrating the geological information and experimental studies, the fabric anisotropy of Horn River shales is initially estimated. The quartz-rich shale gas reservoir is expected to show much
weaker transverse isotropy than the overlying clay-rich shale. An iterative optimization using the gradient-based method is then implemented on this initial model. The results derived from the downhole microseismic dataset are consistent with the laboratory measurements. The optimized VTI model reduces the time misfit by about 65% compared to the originally provided VTI model. The event locations are also significantly improved.
The preferred-oriented fracture set is another important source of shale anisotropy. Mechanical analyses show that the fractures in Horn River shales mainly occur in the quartz-rich formation showing much higher brittleness. According to the core
analyses and fracture mechanism, the fracture planes are commonly perpendicular to the bedding plane and the dominant fracture set strikes to NE-SW direction which is parallel to the current maximum horizontal stress. The elastic behaviors of the
fracture are effectively described by the normal and tangential fracture compliance (i.e., ZN, ZT) regardless of any physical details of fracture. Theoretical modeling and experimental measurements show, the magnitudes of ZN and ZT increase with the
fracture dimension scale, and the ZN/ZT ratio is sensitive to fluid fills and has the value less than or slightly larger than 1. These facts are used as physical constraints in the grid search for the optimal fracture compliance. The magnitudes of ZN
and ZT define the searching range and the ZN/ZT ratio is used as a quality control. The optimal ZN and ZT have the same order of magnitude as other measurements in the crosshole and microseismic scale. The ZN/ZT ratio corresponds to the extreme cases
of dry or gas saturated fractures.
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