Abstract

A combination of geophysical log analysis, pressure buildup testing, decline curve analysis, and pressure buildup testing, decline curve analysis, and production logging was used to obtain a consistent production logging was used to obtain a consistent picture of reservoir performance for the Barrel picture of reservoir performance for the Barrel Springs Field in the Washakie Basin, Wyoming. Production is from the lenticular Almond and Ericson Production is from the lenticular Almond and Ericson sands in the upper Mesaverde. Geophysical log interpretations indicate an average of 110 feet (33 m) of net pay within a gross interval of 645 feet (197 m).

After fracturing, wells initial produce from 100 to 2000 Mcf per day (2500 to 60000 m3/d) with an initial decline of 4% to 10% per month. This behavior could be due to small reservoirs, extremely low permeability, or inadequate fracture treatments. Pressure buildup interpretations indicate an average Pressure buildup interpretations indicate an average permeability of about 0.1 md and show strong linear permeability of about 0.1 md and show strong linear flow behavior at early times. Hence, both permeability and fracture length appear adequate. Material permeability and fracture length appear adequate. Material balance computations and decline curve extrapolations substantiate the concept of small reservoirs. The estimated average ultimate production is about 600 MMcf (17 × 10(6) m3) per well.

Based on the data now available, the various sand bodies in the Barrel Springs field are relatively small (40 to 80 Ac or 0.16 to 0.32 × 10(6) m3) and tight (about 0.1 md). Sands currently contributing production are effectively stimulated, but these sands production are effectively stimulated, but these sands form only a fraction of the total net pay indicated by logs. This suggests that the key to successful exploitation of these sands lies in the use of relatively small fracture treatments for each sand, rather than massive fracturing of a thick gross section.

Introduction

The Barrel Springs Field is located in the south-eastern portion of the Green River Basin in south-western Wyoming (Fig. 1). Gas-bearing sandstones of the Upper Cretaceous Mesaverde formation are encountered at depths ranging from 8000 ft (2400 m) on the eastern edge of the Basin to 12000 ft (36 m) in the central portion. The low permeability of these sandstones is attributed to the high degree of cementation and clay filling observed in cores and drill cuttings. Cross sections (Fig. 2) show that coal beds in the Mesaverde interval can be correlated over large distances, while individual sands are often not correlative even between adjacent wells. This combination of low permeability and limited reservoir area presents problems in the economic development of these gas resources.

In spite of active exploration and development drilling in the area, production history is limited because most of the recent wells are not yet connected to pipelines. Because Barrel Springs has been producing since Oct., 1976, it provides one of the few producing since Oct., 1976, it provides one of the few examples available for quantitatively assigning reserves to the Mesaverde in southwestern Wyoming. The problems in evaluation, stimulation, and reservoir analysis encountered at Barrel Springs are common to most tight gas sands. By presenting a case history for this field, the authors hope to stimulate work leading to better methods for producing gas from similar reservoirs.

GEOLOGY

The eastern and southeastern flank of the Greater Green River Basin is bounded to the south by the Wyoming-Colorado border, to the east by the Sierra Madre uplift, to the west by the Rock Springs uplift, and to the north by the Sweetwater arch. Locally, the northern part of this area is known as the Great Divide Basin, and the southern part, containing the Barrel Springs Field, is known as the Washakie Basin.

Surface rocks over most of the area comprise the Tertiary Wasatch and Green River formations. These are underlain by the Upper Cretaceous Fort Union, Lance, Lewis, and Mesaverde formations, which crop out on the eastern and northern flanks. Although most Cretaceous oil and gas production in older fields is from structural traps, current exploration emphasis is on stratigraphically trapped gas in the Mesaverde.

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