Implications Of Surface Fracture Patterns And Sand Continuity In Relation To Stimulation Of Tight Gas Sands, Eastern Uinta Basin, Utah
Authors: Knutson C.F. , Ward Don C. 
Published: December 2007

The Mesaverde and Lower Tertiary formations of western Colorado, eastern Utah, and southwestern Wyoming have been identified as a major potential source of natural gas. Several industry and joint industry-Energy Research and Development Administration (EREA) massive hydraulic fracturing (MHF) experiments have been carried out in these Mesaverde/ Lower Tertiary rocks with varying results.

In an attempt to provide some insight into ongoing MHF projects, an outcrop study of fracture and sandstone geometries in the Mesaverde and adjacent formations was carried out around the periphery of the eastern Uinta Basin, Utah, the site periphery of the eastern Uinta Basin, Utah, the site of several current and recent industry and joint industry-ERDA MHF experiments.

The features observed on outcrop fracture surfaces are those typical of tensile failures; therefore, the surface fracture phenomena may well correlate with tensile failure from hydraulic fracturing. Thus, the surface outcrop information may provide an informative model for MHF design and provide an informative model for MHF design and evaluation.

Fracture density is highest in the nonreservoir rocks, i.e., thinly bedded, brittle siltstones, silty shales, thinly bedded fine-grained sandstones, and coals; and the lowest densities are in the plastic mudstones and shales and the massive sandstone reservoir rocks.

The fluvial sandstones of the Neslen and Farrer intervals display the lowest continuity of the various facies of the Mesaverde, and the marine margin sandstones of the Castlegate and Sego intervals display the greatest continuity.

The effective reservoir volume of the discontinuous Neslen and Farrer facies is strongly dependent on the sandstone/shale fraction of the interval. The higher sandstone fractions generally represent interconnected sand lens systems and the lower fractions generally represent systems or isolated channels with little interconnection between lenses. This effect can be illustrated in the following table.

FARRER INTERVAL

Volume (100-ft net sand) Sandstone 2,000-ft 3,000-ft Fraction Drainage Radius Drainage Radius (percent) (acre ft) (acre ft)

10 4,500 7,500 20 5,400 9,500 30 6,700 13,000 100 27,500 62,000

The Farrer and Neslen intervals have typical sandstone lens thicknesses of from 10 to 30 ft, width-to-thickness ratios of about 20 to 1, length-to-width ratios of more than 10 to 1, and a widely scattered easterly lens orientation.

The fluvial massive "reservoir" sandstones generally are interbedded with the easily fractured brittle siltstones and thin, very fine-grained sandstones. The marine margin sandstones are frequently associated with coals. Thus, the reservoir rocks in all the Mesaverde and adjacent strata may be interbedded with layers more susceptible to fracturing than the reservoirs. Hence, the subsurface model developed from the outcrop study would suggest the following hydraulic fracturing strategy.The well completion design (and execution) should be such that the hydraulic fracture must be initiated in the reservoir rock, i.e., the fracture has no opportunity to start in the adjacent nonreservoir material such as brittle siltstones, shales, fine-grain sandstones or coals.