The effect of orthorhombic anisotropy and its implication for oil recovery and reservoir exploitation

Abstract

Results of an experimental study of shear‐ and compressional‐wave propagation in an orthorhombically anisotropic medium are presented. The experiments were performed on a physical model consisting of two sets of fractures. The first set consisted of orientated rubber inclusions simulating weak material‐filled cracks. The second set consisted of a system of closely spaced parallel fractures simulated by thin plates of epoxy resin, superimposed orthogonally on the first set. Three cases of fracture orientations within the model were identified and studied. Case 1 is analogous to a jointed fracture reservoir with one vertical set of fluid‐filled cracks or fractures and one non‐filled horizontal set. This case is referred to as JFV. Case 2 is analogous to a double fracture reservoir with one horizontal set of fluid‐filled fractures or cracks and one non‐filled vertical set. This is referred to as DFH. Case 3 is analogous to a double fracture reservoir with two vertical sets of fractures or cracks, with only one fluid‐filled. Case 3 is referred to as DFV. A pulse transmission method was performed on all three modelled cases along the three principal axes. A directional variation in the compressional‐ and shear‐wave velocities, as well as distinct shear‐wave splitting, was observed. The elastic constants for each case were determined and differences between them were noted and compared with the controlled results of both layered (transverse isotropy, TI) and vertically fractured (azimuthally anisotropic models, VF) media. The differences in elastic moduli and velocities indicate the potential of recognizing the different fracture orientations and suggest an approach to designing a method of drilling to further enhance oil recovery and reservoir exploitation.

Estimating fracture orientation from elastic‐wave propagation: An ultrasonic experimental approach

“…The seismograms recorded at M‐1 and M‐3 show a visible shear‐wave anisotropy of 3.2 μ s between the fast (S1) and slow (S2) components. Because of previous theoretical [ Crampin , 1985; Thomsen , 1986] and experimental work [ Assad et al , 1992, 1996; Assad , 2005; Tillotson et al , 2012] on shear‐wave anisotropy, we can infer from the record at M‐1 that the crack distribution of this region shows an orientation of 0°, i.e., it is parallel to the X axis. The seismograms recorded at M‐4 and M‐5 still show a minor anisotropy.…”

Effective orthorhombic anisotropic models for wavefield extrapolation

“…Transversely isotropic media (TI) have gradually become a useful assumption of the Earth, representing in one form (tilted) the first order nature of azimuthal anisotropy influence of the Earth [20]. However, a more realistic representation of the Earth's subsurface that may include the natural thin horizontal layering and parallel vertical cracks may be given by orthorhombic anisotropic models, which assume three mutually orthogonal planes of mirror symmetry [3,4,5,6].…”

“…Orthorhombic symmetries are considered to be very important models to describe wave propagation in fractured reservoirs with specific geometries [3,42,43,5]. Although, the implementation of this symmetry may conduct to better descriptions in such media, it may also lead to more complicated and computationally expensive processes.…”

“…As a matter of fact, most important geological settings for hydrocarbon exploration (e.g., reservoir formations) are created by large tectonic and irregular stresses that generate systems of vertical fractures in a vertical or tilted thin-layering background (see Figure 1.1(b)). As shown in Figure 1.1(a), the type of systems described by this geological representation can be approximately modeled by orthorhombic symmetries [3,4,5,6]. …”

An optimized 2D/3D Finite-difference Seismic Wave Propagator Using Rotated Staggered Grid for Complex Elastic Anisotropic Structures

“…Nowadays, the utilization of Thomsen's parameter is common in industry and this approximation has led better quality seismic imaging (Tsvankin et al, 2010). However, as computational capabilities progress there is a growing realization (Assad, 2005;Cheadle et al, 1991;Malehmir & Schmitt, 2017;Schoenberg & Sayers, 1995;Tsvankin, 1997) that Thomsen's approximations are not sufficient to describe more complicated situations of tilted and fractured formations necessitating that elastic wave solvers capable of handling more general, lower symmetry cases of anisotropy.…”