Arthur Cheng

A rock physics model for tight gas sand

Tight gas reservoirs are often defined as gas-bearing sandstones or carbonates having in-situ permeabilities to gas less than 0.1 mD (Holditch, 2006; Smith et al., 2009). Tight gas reservoir rocks can be at different in-situ physical conditions: deep or shallow; over- or underpressured; high temperature or low temperature; and under different stress states. The reservoir-forming rock can have different textures such as shaley and silty unconsolidated sandstones or clean-cemented sandstones. These different rocks produce gas at low rates. Tight reservoir rocks can be blanket or lenticular, homogeneous or heterogeneous, and can contain a single layer or multiple layers, be fractured or unfractured, and mainly produce dry natural gas.

Time-lapse VSP data processing for monitoring CO2 injection

As a part of the effort of the Southwest Regional Part-nership on Carbon Sequestration supported by the U.S. Department of Energy and managed by the National Energy Technology Laboratory, two sets of time-lapse VSPs were acquired and processed in oil fields undergoing CO2 injection. One set of VSPs was acquired at the Aneth oil field in Utah, the other set at the Scurry Area Canyon Reef Operators Committee (SACROC) Field in West Texas.

Velocity anisotropy and heterogeneity around a borehole

The knowledge of the in situ stress state around a borehole is of primary importance when estimating the stability of the borehole, or designing an optimal drilling trajectory in order to minimize the circumferential stress around the borehole. In this paper, we present an approach for calculating the stress field surrounding a borehole in rock with nonlinear anisotropic elastic properties and for predicting how that stress field changes with applied tectonic stress. Using these results we then resolve the complex velocity field in the region near the borehole.

Evaluation of dispersion estimation methods for borehole acoustic data

Acoustic/Elastic wave energy propagating in a boreholegeometry consists of several different wave types. The energy is split into free (refracted P wave), guided (pseudoRayleigh), or interface waves (Stoneley, flexural). The wave type is also dependent on the source pattern such as monopole, dipole, etc. The same velocity in a formation may be expressed through different types of waves such as refracted P-waves, or a leaky-P wave. Acoustic/elastic energy introduced into a borehole is partitioned between all these different types of waves, and the partition of energy is dependent on frequency and formation properties. Furthermore, most of the wave types and often the most energetic wave types are dispersive. Regular semblance processing (Sheriff, 1999) is sensitive to dispersive waves since semblance is simply a coherency measure and a dispersive wave will change shape between receivers, and does not propagate with a single well-defined velocity. Furthermore, formations with stress induced anisotropy generates waves with crossovers in dispersion depending on direction of measurements.

SEG Advanced Modeling (SEAM) today and tomorrow

This presentation focuses on what we felt are the three most important foundational concepts of the SEG Advanced Modeling Consortium (SEAM). In the our view, these three issues form the basis not only of SEAM, but to a considerable degree the geophysical road ahead. The first is on the emerging techniques for generating models of the Earth’s subsurface with realistic geologic styles. The second discusses the actual algorithms for simulation of seismic acquisition over such models. The third is directed toward the kind of existing and emerging mathematical, physical, and computational technologies that enable extraction of the kind of information required to improve the entire exploration industry’s success rate. We argue that solution of the first leads naturally to application of the second, followed by successful verification of the validity of the techniques of the third. Finally, we suggest reasons for why the technological infrastructure that should emerge from a successful integration of the three basic topics can have a significant impact on more successful exploration with substantially reduced risk.

A method to extract fast and slow shear wave velocities in an anisotropic formation

Summary Current methods for the estimation of fast and slow shear wave velocities, depend on the assumption that the two waves have identical propagation properties. Furthermore, they do not easily lend themselves to standard error estimates of the determined velocities and strike angle. We present a method, which decouples the estimation of fast and slow shear wave, so that the two shear waves can have different propagation properties. The method is strictly based on modeling/inversion, which in turn makes it possible to use standard error estimation techniques. The decoupling furthermore immediately produces an error estimate for the determination of strike angle. We show the method for synthetic anisotropic data and two field data examples. The field data is from an anisotropic and an isotropic formation

Formation stress determination from borehole acoustic logging: A theoretical foundation

Summary This paper presents a theory for modeling the interaction of borehole acoustic waves with the stress in and around the borehole. The theory predicts that the two principal stresses perpendicular to the borehole produce a splitting in the crossdipole-measured shear wave data. The stresses also produce an even greater splitting in the monopole-shear wave data. Thus, by combining the two measurements, one can detect the stressinduced shear-wave anisotropy and estimate both the orientation of, and the difference between, the two principal stresses. The theory provides a foundation for determining formation stress from borehole acoustic monopole and crossdipole measurements.

Dependency of flow and transport properties on aperture distributions and compression states: Dependency of flow and transport properties

Fluid conductivity and elastic properties in fractures depend on the aperture geometry – in particular, the roughness of fracture surfaces. In this study, we have characterized the surface roughness with a log-normal distribution and investigated the transport and flow behaviour of the fractures with varying roughness characteristics. Numerical flow and transport simulations have been performed on a single two-dimensional fracture surface, whose aperture geometry changes with different variances and correlation lengths in each realization. We have found that conventional measurement of hydraulic conductivity alone is insufficient to determine these two parameters. Transient transport measurements, such as the particle breakthrough time, provide additional constraints to the aperture distribution. Nonetheless, a unique solution to the fracture aperture distribution is still under-determined with both hydraulic conductivity and transport measurements. From numerical simulations at different compression states, we have found that the flow and transport measurements exhibit different rates of changes with respect to changes in compression. Therefore, the fracture aperture distribution could be further constrained by considering the flow and transport properties under various compression states.