Production Data Analysis Aids Fracture Treatment Design/Cherokee Group in Western Oklahoma
Authors: Shelley Robert F. , Stacy Alan 
Published: December 2007

The quality and quantity of information available in the public domain is growing rapidly. New commercial sources of digital data are available. Companies are generating in-house databases to track and improve operating and service performance. The software and hardware required to obtain and manipulate large blocks of information has improved, making data analysis feasible on a routine basis. Access to large blocks of data in a convenient, usable form has created an opportunity for operators and service company engineers to work together, integrating data from multiple sources. By performing overall analyses of this integrated data, operators and engineers can develop an effective exploitation strategy.

This paper details the benchmarking study of about 560 wells completed in the Cherokee Group of western Oklahoma. The study attempts to provide enough information about the area that service company engineers can determine optimal treatment designs. The Cherokee Group was chosen as the subject of this benchmarking study because of the large number of producing wells in the area and the variety of completion and stimulation methods used. The study examines wells with initial production dates from Jan. 1, 1988 to Jan. 1, 1996. Completion and stimulation techniques are compared to production results from a statistically large number of wells. Specific areas of interest included defining reasonable expectations and success in well projects and analyzing completion and stimulation procedures. Study results provide relevant comparisons that can help optimize completions and stimulations in the Cherokee Group. The paper also summarizes the methodology used for generating the report.

Cherokee Group Background

The Cherokee Group is a prolific, low-permeability natural-gas and gas-condensate reservoir deposited in the middle-Pennsylvanian (Des Moinesian) period. The reservoir is located in the deep Anadarko Basin of west-central Oklahoma. Most producing activity occurs in Beckham, Custer, Roger Mills, and Washita counties (Fig. 1). This four-county area has produced more than 1.24 Tcf since the area was first developed in the early 1970s.

The Cherokee Group is divided into four informal subsurface formations consisting of deltaic/marine sandstones and limestone. The lowest formation consists of the sandstone Redfork formation, followed by a layer of Pink Limestone, the sandstone Skinner formation, and the uppermost layer of Verdigris Limestone. The Prue formation overlays the Verdigris Limestone and is considered by many operators to be part of the upper Skinner formation, although it is not officially part of the Cherokee Group.

The Redfork, Skinner, and Prue are composed of very fine to fine-grained sand deposited in a deltaic complex. The sandstone deposits are found at the transition between continental and marine environments and continue into the deeper marine basin.

The more productive Redfork formation is divided into lower, middle, and upper members. The formation consists of successions of shale and siltstone that are up to 700 ft thick, as well as thinly interbedded sandstones and shales. The channels of fine, sandy shale consist of feldspathic sandstone with interstitial clay materials. Horizontal permeability ranges from 0. 1 to 5 md with occasional streaks of high-permeability reservoir sandstones. Core samples indicate the presence of quartz, feldspar, and volcanic rock fragments. The Prue/Skinner is divided into upper and lower members. The Prue formation overlays the Skinner in areas where the Verdigris Limestone is shaly.

Because of differences in depositional environments, the reservoir characteristics vary from area to area in the Cherokee Group. The depth of the Cherokee ranges from about 9,900 ft at the top to more than 14,300 ft at the bottom. These extreme depths limit coring efforts, which limit detailed depositional analysis. Engineers generally rely exclusively on well logs and mud-log data to determine sand-bed thickness and porosityl permeability data.

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