The Devonian/Mississippian Age clastics continue to be a major source of natural gas production in the Appalachian Basin. However, the generally low productivity of these sands and shales calls for productivity of these sands and shales calls for innovative development strategies, particularly during this period of low gas prices. This paper examines a series of such alternative development strategies including drilling for deeper pay, using improved well logging to better identify productive net pay, and larger stimulation treatments.
The plateau region of Southwest Virginia was selected as a case study for three reasons. First, gas production in this area has increased four-fold over the past five years (4 Bcf in 1983 vs. 20 Bcf in 1987). Second, drilling activity was up 50% in 1986 counter to a sharp drop for the basin as a whole. Third, by multiply completing and properly stimulating wells in the full tight gas/Devonian shale sequence, this area offers the promise of attractive gas recoveries per well.
Average reservoir parameters were derived for the study area to enable simulations to be run. Past and current drilling, completion, and operating practices were then analyzed to determine average cumulative 20-year productions in the region. After representative reservoir properties and completion practices were established, the effects of larger stimulations and improved pay selection were simulated. The results indicate that current reserves per well can be almost doubled using improved pay selection, larger stimulations and multiple well completions.
The Appalachian Basin, the birth place of oil and gas drilling in the United States, has been through numerous "boom and bust" cycles. Appalachian Basin oil launched the era of low-cost energy, fueled the two world wars, and paced the speculative drilling of the 70s and 80s. The fall in oil prices and the abandoned drilling partnerships heralded the collapse of activity in the past few years. At present, Virginia is the "bright spot" in an otherwise difficult drilling and gas production time for the Appalachian Basin:
* Gas production or Virginia is estimated to reach nearly 20 Bcf in 1987, up from 15 Bcf in 1986 and up from less than 5 Bcf in 1983.
* Drilling activity for Virginia was up by 50% in 1986, counter to a sharp drop for the Basin as a whole. A record level of 147 wells (139 gas wells) was drilled in the state in 1986, although 1987 drilling is lower, based on initial data.
Recent geologic studies and well performance data lend support to the theory that the plateau region of Virginia (Buchanan, Dickenson and Wise counties) is an attractive area for expanded tight sands and Devonian shale gas drilling. A recent publication on the Middlesboro Syncline (Potter and others, 1986) suggests the potential for tectonic alteration and the presence of naturally fractured Devonian-Mississippian presence of naturally fractured Devonian-Mississippian sediments in the plateau region. Initial potentials (IP/AOF) for wells drilled in 1986 show average values in excess or one million cubic feet per day. Most of the wells were multiply completed into the tight Berea sandstone and the Devonian shale. Two of the gas wells completed in only the Devonian had IPs of 262 and 420 Mcf/D, while the remaining 137 multiply completed Devonian-Mississippian gas wells averaged 1.2 MMcf/D.
The recent success in tight sands/Devonian shale drilling in Virginia brings to light three key issues which will be addressed within this case study.