Abstract

When it was first drilled and completed in 1976 and again in the early 1980s, southwest Wyoming's Jonah field failed to produce substantial returns and was considered an economic failure. As a result, the field remained stagnant until the early 1990s, when McMurry Oil Company again examined the feasibility of producing the field. During this attempt, seven wells were drilled and completed, but returns were only marginal. In 1995, the production company established a working relationship with Flack Petroleum, Stim-Tech, Inc., and Halliburton Energy Services, Inc. The purpose of this relationship was to analyze all activities and practices in the Jonah field as a means of improving the ultimate recoveries of gas reserves with less expensive, more efficient drilling and completion techniques. This paper details the results of this program, which has dramatically improved the economics of this plan and turned the Jonah field into one of the most prolific and profitable exploration/development successes in Wyoming.

Introduction

Formations. The surface formation in the Jonah field is the Wasatch. The formations/zones currently being drilled below the Wasatch are the Fort Union and the Lance.

Fort Union Formation. Above the Lance, the Paleocene Fort Union formation consists of (from top to bottom) a massive zone, a coal zone, and a shaly zone. The Fort Union Sands, which are located directly above the Lance Overpressure, will be productive in areas of the Jonah field where the sealing shale barrier has been disturbed by faults. This conclusion is supported by significant gas shows that occurred when these zones were drilled on several wells in the Jonah field. Because of the differences in formation quality and average reservoir pressures, the Fort Union sands will be completed with a separate drilling program.

Lance Formation. The Lance consists of the Lance Derbis Wedge, Lance Overpressure Middle Lance, Jonah Sands, Yellow Point Sands and the Wardell Sands. The proven commercial gas-producing zones appear at the Lance Overpressure and continue through the Wardell. The Lance begins as shallow as 7,800 ft and continues to nearly 12,000 ft within various locations of the Jonah field.

The Lance formation is an approximately 3,200-ft thick continental deposit composed of fluvial sandstones, backswamp shales, coaly shales, and thick braided stream sandstone complexes. Individual sandstones are 5 to 80 ft thick and have an areal extent of a few acres to as much as a section. The entire Lance formation is overpressured. Directly above the Lance Overpressure is a shale barrier that has, in most cases, stopped the upward migration of natural gas. Once this shale is penetrated, the formation pressures change from 0.42 psi/ft to 0.56 psi/ft. As depth increases, the gradient increases to 0.63 psi/ft.

If permeability is great enough, all the sandstones below the top of the Lance formation are gas-bearing and productive. Commercial production has been established in sands with permeabilities ranging from 0.0074 md to 0.072 md. Table 1 (Page 6) provides permeability results for several wells in the Upper and Lower Lance zones.

Drilling and Field Expansion History. The field discovery well and two offset wells were drilled during the 1970s and 1980s. These wells were never produced because of low production rates, depressed gas prices, and lack of pipeline access. A production company purchased the wells in 1992 and installed a 4-in. gas line to a gathering point approximately 10 miles from the field. Gas sales began in September 1992. In December 1992, the production company drilled another well. This well encountered multiple gas sands in the Lance formation and was successfully stimulated with a nitrogen foam fracture system that included 690,000 lb of 20/40-mesh ceramic proppant. The initial production for this well was 4,000 Mcf/D and 80 Bcf/D from 140 ft of net pay. Another 195 ft of pay was left undisturbed behind pipe.

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