Niobrara Development Program, Washington County, CO

Summary The gas producing section of the Niobrara formation is a low pressure, low permeability chalk. To evaluate the potential of this tight gas reserve, reservoir data were obtained in a systematic program. These data demonstrate that many Niobrara wells can be produced profitably when stimulated by hydraulic fracturing and that an economically optimal fracture treatment can be designed. The described techniques generally may be applied for development of a variety of tight reservoirs through hydraulic fracturing. Introduction Gas bearing chalk of the upper Cretaceous Niobrara formation is encountered at depths of 1,000 to 3,000 ft (300 to 900 m) in parts of Colorado, Kansas, and Nebraska. The general area of the Niobrara gas producing potential is shown in Fig. 1. This low pressure, low permeability reserve has been recognized for many years. However, commercial development has become feasible only recently as a result of higher gas prices and improved stimulation techniques. Cities Service Co.'s Niobrara development program began in Washington County, CO, early in 1979. This program was designed to evaluate the economic potential of this tight gas reserve and to determine an optimal exploitation scheme. On the basis of data obtained in a systematic well testing program, it was demonstrated that many Niobrara gas wells can be produced profitably with proper stimulation. Furthermore, the well testing data enabled the determination of an optimal stimulation treatment for a given well. This paper describes our Niobrara development efforts focusing on the Washington County activities. The geology, reservoir characteristics, and historical aspects of the Niobrara gas play are reviewed briefly. The specific procedures used in drilling, completion, stimulation, and testing are detailed and representative well data are presented. The techniques described here generally may be applied for the judicious development of a variety of tight gas reservoirs. Geology Structure Gas accumulations in the Niobrara formation generally are related to low relief structural features found along the eastern margins of the Denver basin. These structures are domal to oval and do not exhibit a strong regional lineation. Regional dips normally are less than 1 deg. and closures range from 50 to 200 ft (15 to 60 m). The structural features are modified frequently by normal faults, many of which are considered listric. These are believed to be early diagenetic features. The most recent Niobrara wildcats have been drilled on structural features defined by existing wells where deeper beds were the objective. We have relied to a large extent on data from wells drilled to the Cretaceous D and J sands and shale sequence for locating new prospects. Stratigraphy The upper Cretaceous Niobrara formation was deposited over much of the central and western U.S. in a major transgressive stage of marine deposition. Two members of the Niobrara are recognized in the Denver basin area: the lower Fort Hays limestone and the upper Smoky Hill chalk. Gas production appears to come exclusively from a relatively clean chalk zone at the top of the Smoky Hill chalk member. JPT P. 628^