3-D AVO analysis and modeling applied to fracture detection in coalbed methane reservoirs

Abstract

Over the years, amplitude variation with‐offset (AVO) analysis has been used successfully to predict reservoir properties and fluid contents, in some cases allowing the spatial location of gas‐water and gas‐oil contacts. In this paper, we show that a 3-D AVO technique also can be used to characterize fractured reservoirs, allowing spatial location of crack density variations. The Cedar Hill Field in the San Juan Basin, New Mexico, produces methane from the fractured coalbeds of the Fruitland Formation. The presence of fracturing is critical to methane production because of the absence of matrix permeability in the coals. To help characterize this coalbed reservoir, a 3-D, multicomponent seismic survey was acquired in this field. In this study, prestack P‐wave amplitude data from the multicomponent data set are used to delineate zones of large Poisson's ratio contrasts (or high crack densities) in the coalbed methane reservoir, while source‐receiver azimuth sorting is used to detect preferential directions of azimuthal anisotropy caused by the fracturing system of coal. Two modeling techniques (using ray tracing and reflectivity methods) predict the effects of fractured coal‐seam zones on angle‐dependent P‐wave reflectivity. Synthetic common‐midpoint (CMP) gathers are generated for a horizontally layered earth model that uses elastic parameters derived from sonic and density log measurements. Fracture density variations in coalbeds are simulated by anisotropic modeling. The large acoustic impedance contrasts associated with the sandstone‐coal interfaces dominate the P‐wave reflectivity response. They far outweigh the effects of contrasts in anisotropic parameters for the computed models. Seismic AVO analysis of nine macrobins obtained from the 3-D volume confirms model predictions. Areas with large AVO intercepts indicate low‐velocity coals, possibly related to zones of stress relief. Areas with large AVO gradients identify coal zones of large Poisson's ratio contrasts and therefore high fracture densities in the coalbed methane reservoir. The 3-D AVO product and Poisson's variation maps combine these responses, producing a picture of the reservoir that includes its degree of fracturing and its possible stress condition. Source‐receiver azimuth sorting is used to detect preferential directions of azimuthal anisotropy caused by the fracturing system of coal.

Azimuthal offset‐dependent attributes (AVO and FVO) applied to fracture detection

“…To check the validity of the straight-line fit and the oscillation in the amplitudes and frequency data, the mean absolute deviation from the fit lines is one criteria for measuring reliability. Studies from Ramos and Davis (1997) show that undulations in the normalized RMS amplitude data are caused by converted waves and coherent noise. Additionally, the CDPs with [9][10][11] very limited offset ranges have a small degree of reliability.…”

“…The thickness of member '0' does not vary greatly over the research CDP range (60 CDPs, 1.06km), averaging 30 m (12 ms in terms of travel time). Studies from Ramos and Davis (1997) show that tuning has an important effect on the magnitude of AVO gradients and that varying the thickness of a reservoir can lead to variations of AVO gradients when the maximum thickness is smaller than half a wavelength. To investigate the distortion of tuning on azimuthal variations of AVO and FVO, we use the same background model parameters as in previous studies (Shen and Toksoz, 1998) and decrease the thickness of the fractured reservoir to within A14.…”

“…If the fracture layer has a small thickness, the application of P-wave stacking velocity is limited because the azimuthal travel time depends on both the fracture parameters and the thickness of the fractured reservoir. Unlike stacking velocity analysis methods, azimuthal AVO studies, relying on the reflection amplitudes, have been used to detect fractures (Perez and Gibson, 1996;Ramos and Davis, 1997;Mallick et aI., 1998). Generally, amplitudes are related to reflection coefficients and thus to elastic contrasts across the reflection boundary. The predication of fracture orientation based on azimuthal variations is still ambiguous when a priori knowledge of some variables, such as layer thickness and spatial heterogeneity caused by fluid content and lithology, are not available.…”

Azimuthal offset‐dependent attributes applied to fracture detection in a carbonate reservoir

“…If a fractured reservoir has a small thickness, the application of P-wave stacking velocity is limited because the azimuthal traveltime depends on both the fracture parameters and the reservoir thickness. In contrast to stacking velocity analysis, azimuthal AVO analysis, relying on the reflection amplitudes, has been used to detect fractures in this case (Perez and Gibson, 1996;Ramos and Davis, 1997;Mallick et al, 1998). Generally, amplitudes are related to reflection coefficients and thus to elastic contrasts across the reflection boundary.…”

Simultaneous inversion for model geometry and elastic parameters

“…In this paper, I show that by properly taking care of the structural effect, one can improve the estimation of the elastic parameters of subsurface targets in the traditional analysis of amplitude variation with offset (AVO). AVO analysis has been used extensively for lithology and fluid prediction in many regions (e.g., Ostrander, 1984;Hilterman, 1990;Hampson, 1991;Castagna et al, 1993;Spratt et al, 1993;Mallick, 1995;Buland et al, 1996;Simmons and Backus, 1996;Ramos and Davis, 1997).…”