Horizon Attributes and Fracture-Swarm Sweet Spots in Low-Permeability Gas Reservoirs

Abstract

Abstract Horizon attributes, i.e., attributes that numerically describe geometric characteristics of interpreted horizons from conventional (i.e., p-wave) 3-D seismic volumes, hold considerable potential for identifying fracture-swarm sweet spots in low permeability reservoirs. Typically, these attributes (e.g., dip, azimuth, and curvature) are used to define subtle faults that are near the limit of seismic detectability. These subtle structures can play important roles in compartmentalizing conventional reservoirs. However, in low permeability gas reservoirs where fracture permeability is critical, these same attributes can be used to define high-permeability fracture swarms. We illustrate this point with three case studies, two clastic the other carbonate, from the San Juan Basin area of northwestern New Mexico. The Paradox Formation is a Pennsylvanian age low permeability carbonate reservoir. At Ute Dome Field, production characteristics indicate that fractures are the main control on gas production from these carbonates. Comparison of horizon attributes from this level with production data shows that these attributes are defining high-permeability fracture swarms associated with faults. The Mesaverde Group consists of Cretaceous age clastics and is a tight reservoir in the Blanco Field. Again, horizon attributes (including curvature attributes) can be used to define fault-related fracture swarms that will produce at higher rates than surrounding areas. The Dakota Sandstone is another Cretaceous tight gas sandstone. A map of horizon dip in one area shows a trend that is associated with anomalous production from two wells. However, these two wells cannot be described as "sweet spot" wells because their production is not anomalously high. These observations indicate that development drilling plans for low permeability reservoirs should take into account geologic heterogeneity that can be associated with fracture swarms. Undrilled fracture swarms should be targeted to produce high-rate wells. On the other hand, offset wells should specifically avoid drilling into previously tapped fracture swarms to avoid drainage interference. Other factors that need to be considered are: a) the orientation of the fractures with respect to in-situ stress directions, and b) lithologic (i.e., stratigraphic) control on fracture density.