Evolution of Completion and Fracture Stimulation Practices in the Jonah Field, Sublette County, WY
Authors: Finch R.W. , Skees J.L. , Aud W.W. , Johnson J.L. , Esphahanian C. , Hansen J.T. 
Published: December 2007

This case history discusses the evolution of the completion strategies and fracture stimulation techniques that have turned a large marginal gas resource in the Jonah field, WY into a commercially viable field. Completion practices have evolved from single zone, high-quality nitrogen foam fracture stimulations to multiple zone completions utilizing crosslinked borate fluid systems. Using these new techniques, total field production has increased approximately 180%. A normalized comparison of productivity (q/ P) per foot of gross clean sand indicates the new completion and stimulation techniques yield shallower decline rates with initial production equal to or higher than the earlier foam treatments.

Introduction

The Jonah field, located in Sublette County, southwest Wyoming, has the potential to be a significant gas resource with field reserve estimates ranging up to a trillion cubic feet. Over-pressured gas is found in the tight sands of the Lance formations' fluvial sand-shale sequence that range from 1,500 to 3,000 feet in gross thickness. The primary challenge presented in the Jonah field is effectively and economically fracture stimulating (i.e., contacting) as much pay section as possible within a low gas price environment.

Initially, a reservoir/stimulation description was developed to establish a baseline and to begin developing an optimal economic completion and stimulation approach. Using this approach, an integrated strategy was created and implemented to 1) complete more pay intervals with pseudo-point source perforated, multiple-staged treatments; 2) modify the borate fluid chemistry to address complicated fracture geometry and improve far-field proppant placement; 3) reduce multiple fracture initiation; and 4) eliminate screen-out problems.

Initial results using these approaches in 5 wells indicated significant improvement in field economics. Both the average production and the normalized productivity (q/ P) per foot of gross clean sand are higher than the previous foam completions. The decline rates at three months are shallower suggesting better lateral and vertical proppant placement. Previously uncompleted reserves in uphole Lance pay sections were proven economic. In two of the wells reserve recovery from these shallow intervals was accelerated by commingling all the lance pay intervals in the initial completion phase. Completion costs per pound of proppant placed were reduced 35% compared to an offset foam treatment.

Background

The Jonah field is operated by McMurry Oil Co. and Snyder Oil Co. Figure 1 shows the location of the field and the regional geologic setting. Figure 2 is the production history of the field.

Regional Geologic Setting. The Jonah field is located on the west flank of the northern Green River Basin between the Moxa Arch and the Wind River Mountains (Figure 3). Production is from the fluvial sandstone reservoirs of the Upper Cretaceous Lance Formation. The Lance Formation comprises more than 2,500 feet of heterolithic strata, including fluvial channel sandstones and siltstones, floodplain shales, and minor coals that were deposited in a broad alluvial plain. Sandstone comprises 40-50% of these sediments, which were derived from uplift of the Thrust Belt to the west and the Wind River Mountains to the northeast. These sandstones were deposited in channels 10–20 feet deep and 150–4,000 feet wide, though some amalgamated sandstone intervals are greater than 100 feet thick and over a mile wide.

Fluvial architecture varies in the Lance Formation from isolated meandering river deposits to stacked, amalgamated braided river deposits. These intervals form roughly correlatable units ranging from 100 to 700 feet in gross thickness. Typically 50 to 150 feet of reservoir quality rock is found within a 300 foot interval with three to four intervals per well. The field terminology assigned to these correlatable intervals are (shallow to deep): Upper Lance, Middle Lance, Jonah, Yellow Point, and Wardell.

Sandstones are fine to medium grained and are composed of detrital chert and quartz with minor feldspar, authigenic clay, quartz cement, and calcite cement.

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