The Lance formation in the Jonah field is an over pressured, tight-gas sand that requires hydraulic fracturing for economic production. Because of large gross intervals containing several individual sands, limited entry has been the typical fracturing technique. Wells can have more than 30 individual sands that are completed with multiple fracture treatments; however, production log data indicate that only 58% of the perforated sands contribute to production. Production optimization is dependent on improving the percentage of completed pay contributing to production.
A detailed field study was conducted to determine pay identification and best practices for completions. In the study, 44 wells in the Stud Horse Butte area were analyzed. The study comprised detailed log analysis, fracture treatment data, production data, reservoir analysis, and completion practices. Some of the processes used were log normalization, traditional statistical analysis, and reservoir modeling. Log and treatment data were also analyzed with an artificial neural network (ANN).
Discovered in 1975, Jonah field is located in the Hoback sub-basin in the far northwest corner of the Green River foreland basin (T28 - 29N, R108W) (Fig. 1). The Hoback Basin lies between the Wyoming Thrust Belt to the west and the Wind River Mountains to the east. The field is 60 miles north of Rock Springs, WY. By mid-1999, more than 100 commercial wells had been drilled on 80-acre spacing. Field boundaries to the north, south, and west have been well defined. The eastern (downdip) edge of the field is still being extended. The known limits of the field exceed 33 square miles.
Jonah Field is bounded on the south and northwest by two wrench faults. Faults within the field boundaries add to the complexity of the reservoirs. Wells within the field encounter overpressured gas at 8,100 to 9,300 ft (0.58 to 0.65 psi/ft gradient), whereas nearby wells drilled across the bounding faults find normal pressure gradients at similar depths.
The Lance formation is from the Upper Cretaceous age and consists of 2,000 to 3,000 ft of interbedded fluvial sands, mudstones, and coals. A log section of the Lance is shown in Fig. 2. Individual sandstone units range from 5 ft to more than 50 ft in thickness, and have areal extents ranging from a few acres to 100 acres. Individual sands are geologically heterogeneous reservoirs because of their depositional shapes, but certain stratigraphic intervals consistently have sands developed. Sand-rich intervals are locally called the Upper Lance, Middle Lance, Jonah, Yellow Point, Wardell, and Upper Mesaverde (or Rock Springs). Total net sand in the field ranges from 300 to 600 ft of stacked net pay. Drilling depths range from 11,000 to 12,500 ft, depending on the number of sand packages an operator believes to be economically feasible to develop. More specific geological descriptions can be found in references.1,2
Sand porosity ranges from 5 to 14%, with relative gas permeability ranging from 0.001 to 0.02 md. Water saturation varies from 30 to 60%. Currently, there is no significant water production in the field. The producing condensate yield is between 8 and 10 bbl/MMscf, with an API gravity of 52°. Pressure-volume-temperature (PVT) fluid data are scarce, although it appears that the fluid composition is similar throughout the entire productive section.