AVO analysis and modeling applied to fracture detection in coalbed methane reservoirs, Cedar Hill Field, San Juan Basin, New Mexico

Ramos, Antonio C. Buginga; Anderson, John E.; Benson, Robert D.

doi:10.1190/1.1931953

Cited by 4 publications

(3 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…For the model used, the magnitude of the reflection amplitude was found to decrease with increasing offset, with the rate of decrease being greatest for acquisition perpendicular to the fractures. Benson et al (1994) and Ramos et al (1994) used AVO in the Cedar Hill Field in the San Juan Basin, New Mexico, to identify coal zones with large Poisson's ratio contrast and hence high fracture density. Two fracture trends orientated NW and NE have been recognized in this area, which conform to a natural cleat system for the coals.…”

Section: Introductionmentioning

confidence: 99%

Azimuthal variation in AVO response for fractured gas sands

Sayers

Rickett

1997

Geophysical Prospecting

View full text Add to dashboard Cite

Natural fractures in reservoirs play an important role in determining fluid flow during production, and hence the density and orientation of fractures is of great interest. In the presence of aligned vertical fractures, the reflection amplitude at finite offset varies with azimuth. The effect of natural fractures on the azimuthal AVO response from a gas‐sandstone reservoir encased within shale is investigated. A simple expression for the difference in P‐wave reflection coefficient from the top of the reservoir parallel and perpendicular to the strike of the fractures is obtained in terms of the normal and tangential compliances, ZN and ZT, of the fractures. This expression is valid for small anisotropy and material contrasts and is compared with the results of numerical modelling. For a given value of ZT, the azimuthal variation in reflection coefficient at moderate offsets is found to increase with decreasing ZN/ZT. For gas‐filled open fractures ZN/ZT ≈ 1, but a lower ratio of ZN/ZT may result from the presence of cement or clay within the fractures, or from the presence of a fluid with non‐zero bulk modulus. For ZN/ZT = 1 and moderate offsets, the variation with offset of the reflection coefficient from the top of the fractured unit is dominated by the contrast in Poisson's ratio between the gas sand and the overlying shale, the effect of fractures only becoming noticeable as the critical angle for the unfractured sandstone is approached. However, for reflections from the base of the fractured unit, the variation in reflection amplitude with azimuth is much greater at conventional seismic offsets than for the reflection from the top. Azimuthal variations in the strength of the reflection from the top of the reservoir depend only on the variation in reflection coefficient, whereas the raypath is also a function of azimuth for reflections from the base of the fractured unit, leading to stronger, more visible, variations of AVO with azimuth. It follows that an azimuthal variation in AVO due to fractures in the overburden may be misinterpreted as due to the presence of aligned fractures in the reservoir.

show abstract

Section: Introductionmentioning

confidence: 99%

Azimuthal variation in AVO response for fractured gas sands

Sayers

Rickett

1997

Geophysical Prospecting

View full text Add to dashboard Cite

show abstract

“…Most studies using this technology, however, have involved nonfractured reservoirs and two-component seismic data. In this study (Ramos, 1993), 3-D AVO analysis is performed on a multicomponent 3-D seismic data set and is used to better characterize the fractured coalbed methane reservoir at Cedar Hill Field, San Juan Basin, New Mexico (Figure 1). In this field, methane production is associated with the presence of enhanced fracture permeability in the coalbeds of the Fruitland Formation.…”

Section: Introductionmentioning

confidence: 99%

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

Ramos

1997

GEOPHYSICS

Self Cite

View full text Add to dashboard Cite

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.

show abstract

“…More recently, to improve prospect evaluations in new areas such as deep offshore environments, AVO attributes are being used as an analysis tool for quantitative prospect ranking (e.g., Adamick et al, 1994, Cardamone et al, 1998 attributes are considered valuable for evaluating anomalous seismic amplitude responses on large 3D datasets (e.g., Barton and Gullette, 1996). Evolving applications of AVO attributes are the detection and characterization of fractured reservoirs (e.g., Lefeuvre, 1994;Rueger and Tsvankin, 1995;Ramos, 1996) and in lithology determination (e.g., Nada and Shrallow, 1994). Crossplotting AVO attributes helps in establishing trends against which anomalous amplitude behavior can be seen (e.g., Smith and Gidlow, 1987;Foster et al, 1997;Castagna et al, 1998;Sams, 1998;Ross, 2000).…”

Section: Introductionmentioning

confidence: 99%

AVO polarization and hodograms: AVO strength and polarization product

Mahob

Castagna

2003

GEOPHYSICS

View full text Add to dashboard Cite

An alternative approach to identifying amplitudevariation-with offset (AVO) anomalies is to consider the AVO polarization in the AVO intercept-AVO gradient (A-B) plane. This method does not require deviations or separations from a background trend exhibited in traditional crossplots such as intercept-gradient (A-B) or near trace-far trace (N-F). A benefit of the hodogram or polarization method is that the wavelet is taken into consideration. Crossplotted intercept and gradient are polarized along a "background trend" for nonanomalous events and at angles different from the "background trend" for anomalous events. This allows recognition of anomalous behavior otherwise buried in a background. Attributes resulting from this methodology include (1) the polarization angle, (2) the polarization angle difference, (3) the AVO strength, (4) the polarization product, and (5) the linear-correlation coefficient. These different attributes can then be used to enhance AVO interpretation. Synthetic modeling for a succession of gas and brine layers encased in shale units indicates that the method can potentially be an effective hydrocarbon indicator. Application of the method to a real seismic dataset shows polarization anomalies associated with hydrocarbons.

show abstract

AVO analysis and modeling applied to fracture detection in coalbed methane reservoirs, Cedar Hill Field, San Juan Basin, New Mexico

Cited by 4 publications

References 0 publications

Azimuthal variation in AVO response for fractured gas sands

Azimuthal variation in AVO response for fractured gas sands

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

AVO polarization and hodograms: AVO strength and polarization product

Contact Info

Product

Resources

About