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KRYSTINIK, LEE F., Krystinik Litho-Logic, Fort Worth, TX

ABSTRACT: Predicting Fractures 5 Km Down: Integrated Reservoir Characterization, UPR/DOE Rock Island 4-H Well

The UPR Rock Island 4-H horizontal well (RI-4H) was part of a five-year cooperative research partnership between Union Pacific Resources, the U. S. Department of Energy, and the Gas Research Institute, to find ways to economically produce gas from the fractured, over-pressured, very tight sandstone of the Frontier Formation in southwestern Wyoming. Prior to the drilling of the RI-4H well, no economic production had been established from this large basin-centered gas accumulation (estimated to exceed 100 tcf in place) because of the very low permeability of the reservoir rock at the target depth of 5 km.

Site selection required the integration of extensive regional outcrop, core, log, and production test analyses, combined with 3600 km2 of regional 3-D seismic data. The reservoir is massive to hummocky cross-stratified, very fine grained, nearshore-marine sandstone, with 10-12% porosity, 25 microdarcy permeability, and intense fracturing. Fractures in a vertical offset well and in the horizontal RI4-H indicate recurrent shear and dip-slip displacement with a dominant strike orientation ranging from 80 to 110.

A critical factor in the analysis was the identification of faulting, which controlled the deposition of the Frontier Formation as well as the later fracturing of the reservoir rock. These faults were active prior to, during deposition of the Frontier, and well after lithification of the Frontier Formation. Similar relationships are more subtly expressed in shallower Cretaceous horizons and cause localized preservation of reservoir sandstone and localization of "sweet spots" of maximum producibility in most of the major shallow fields in the area.

Repeated fault reactivation produced well-developed fracture sets. These fractures are open to partially filled with calcite, quartz, kaolinite, and bitumen. Fracture spacing is variable and is most intense near several small faults penetrated by the well. Open fractures that strike east-west, nearly perpendicular to the horizontal well bore, occur along the entire length of the well, as observed in core and microresistivity images. Dissolution of feldspar and lithic grains along microfractures significantly enhances permeability.

Results include (1) the cost for drilling deeper than 5 km reduced to 50% below the previous industry average; (2) over 400 fractures intersected in a 550 m horizontal leg (4980 m vertical depth); (3) the deepest horizontal tight-gas sandstone cores in the world; (4) the highest  gas-flow capacity ever encountered in the tight-gas province of the Frontier Formation (>14 MMCFD); and (5) initiation of ongoing drilling to exploit this resource. Three new wells are presently drilling to extend the limits of the play.

AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures