- Copyright © 2002 Society of Exploration Geophysicists
In this article we will discuss some horizon-attribute-based methods that we have been exploring to help identify productive fracture swarms in tight-gas reservoirs. Our objectives are:
to demonstrate that horizon attributes derived from conventional P-wave 3-D seismic surveys may be used to image productive fracture-swarms in tight-gas reservoirs
to illustrate some data exploration techniques that we have been using
to advertise some free software that may be used for analyzing seismic horizons
We illustrate our work with three examples, one carbonate and two clastic, from the San Juan Basin area of New Mexico.
Low matrix-permeability gas reservoirs (“tight-gas reservoirs”) are an important resource in the United States; some estimates suggest that approximately one half of the nation's future natural gas supplies will be produced from these technologically challenging reservoirs. In both clastic and carbonate reservoirs with low matrix permeability, natural fractures play an important role in enhancing bulk permeability, thus enabling wells to produce at commercial rates. Fractures need to be relatively densely spaced, and have the correct orientation with respect to in-situ stresses in order to act as effective high-permeability conduits.
Several surface seismic methods (including azimuthal variations in velocity, amplitude variation with azimuth, and shear-wave studies) can help detect the presence and orientation of in-situ natural fractures. However, because of the sometimes marginal economics of tight-gas reservoirs, cost-sensitive operators are typically reluctant to apply these methods.
As part of a program investigating tight-gas reservoirs in the San Juan Basin region, we have been working with conventional P-wave 3-D seismic surveys (integrated with wireline log and production data) to develop means of using these data to image “fracture-swarm sweet spots” (Figure 1). We simplistically distinguish “fracture swarms,” densely spaced fractures associated with faults or flexures, from “regional fractures” that have more or less the same fracture density …