|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||Natural Fractures in Sonora Canyon Sandstones, Sonora and Sawyer Fields, Sutton County, Texas|
|Authors||Marin, B.A., Clift, S.J., Hamlin, H.S., Laubach, S.E., Bureau of Economic Geology, U. of Texas|
Low Permeability Reservoirs Symposium, 26-28 April 1993, Denver, Colorado
|Copyright||Copyright 1993, Society of Petroleum Engineers, Inc.|
The Sonora Canyon is one of several "Canyon Sands" (Virgilian-Wolfcampian) intervals that exist in the Val Verde Basin of southwest Texas. This paper describes the presence and attributes of natural fractures in Sonora Canyon sandstones in Sutton County, Texas. Data obtained from three cored wells through a cooperative research program conducted by the Gas Research Institute and industry show that natural fractures are locally abundant. At least three distinct natural fracture classes coexist that have contrasting distributions, characteristic sizes, and/or mineral fills. The most abundant fracture class consists of clay- or clay- and carbonate- filled fractures existing only in siderite-cemented zones in sandstones. Owing to their clay content, these fractures locally may be barriers to fluid flow and their presence could cause reservoir heterogeneity and anisotropy. Calcite- and quartz-cemented fractures are less common fracture classes. These fractures are larger than those in siderite layers and are partly open as a result of propping by diagenetic minerals. Our results show that fractures in Sonora Canyon sandstones should be considered in completion and stimulation design. For example, high treatment pressures observed in some Canyon Sandstone stimulations may be due to natural fractures promoting propagation of multiple fracture strands near the wellbore. Inconsistencies between hydraulic fracture strike and maximum horizontal stress may be due to natural fractures guiding hydraulic fracture growth.
A KEY RESERVOIR ELEMENT
In rock having low matrix permeability, natural fractures can significantly affect reservoir properties. Open fractures can enhance and filled fractures can impede fluid flow, and fractures in many cases produce anisotropic and heterogeneous rock permeability. When hydraulic fracture treatments are performed to stimulate gas reservoirs, treatment design should take into account the presence of natural fractures because these fractures can cause high treatment pressures by promoting development of multiple fracture branches. Because they are commonly planes of low tensile strength, natural fractures can also cause treatment fractures to grow in a direction that differs from that predicted from analysis of principal stress directions.
Sonora Canyon sandstones, a gas play in western Sutton County in the Val Verde Basin of southwest Texas (Figure 1), have yielded almost 2 trillion cubic feet (Tcf) of gas. Several trillion cubic feet of recoverable gas are thought to remain. Canyon gas reservoirs lie at depths between about 3,000 and 8,000 ft. Clean, matrix-free reservoir sandstones generally have less than 5% effective porosity and 0.1 md permeability. Hamlin and others summarized recent work on the stratigraphy and diagenesis of Sonora Canyon reservoirs, and aspects of Sonora Canyon reservoir engineering and hydraulic fracture treatment are described in recent Gas Research Institute topical reports. Natural fractures in Sonora Canyon sandstones have not previously been described. In this study, we document the occurrence and attributes of 191 natural fractures in 842.5 ft of Sonora Canyon core from 3 wells. Our discovery of natural fractures in Canyon sandstones and our descriptions of fracture attributes are useful for improving exploration and development strategies and the design and modeling of hydraulic fracture treatments in the Canyon Sandstone.
In the northern Val Verde Basin, the Sonora Canyon is a wedge-shaped interval composed primarily of coalesced submarine fans forming a slope apron.
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