Title: Identifying Productive Stratigraphic Intervals in the Piceance Basin
Speaker: David P. Craig, Halliburton Energy Services
Date: August 18, 2000
Publication: The Outcrop, August 2000, p. 4-5
A typical Mesaverde natural gas well in the Williams Fork formation of the Piceance Basin encounters a gross gas-saturated interval approximately 2,000-ft thick that consists of interbedded sandstones, siltstones, mudstones, shales, and coals. Within the gas-saturated interval, the average well contains 26 “target” pay sands dispersed throughout the Paludal, Coastal, Lower Fluvial, and Middle Fluvial stratigraphic intervals. The challenge has been to identify the productive sandstones.
Most Piceance Basin studies have concluded that well productivity is highly unpredictable using only conventional open hole log data. In 1997, however, Esphahanian and Storhaug compared production data to open hole and production logs for 13 wells in Mamm Creek Field. Their study demonstrated that well productivity is highly unpredictable, but also suggested that productivity within specific stratigraphic intervals is much more predictable.
Since December 1998, over 200 diagnostic fracture-injection tests have been pumped into isolated Piceance Basin Mesaverde sands in an effort to optimize well completions and to identify productive stratigraphic intervals. The injection tests were implemented to identify the presence of fractures/ fissures, to provide estimates of pore pressure and permeability, and to optimize the perforation scheme.
Results from 201 injection tests show extreme differences in reservoir quality between sandstones with very similar open hole log signatures. While approximately half of the injection tests indicate open fractures/ fissures, it is quite common to observe relatively high-permeability (kg > 0.050-md) without indications of open fractures/issures. Permeabilities of pay sandstones separated by less than 50 ft range from kg < 0.001-md to kg > 0.100-md.
Diagnostic fracture-injection tests also suggest that approximately 30% of Mesaverde sandstones in Mamm Creek Field are essentially nonproductive or only marginally productive with estimated gas permeability less than 0.001 md. Additionally, analysis by stratigraphic interval demonstrates that the Middle Fluvial zone should be the least productive, which supports the statistical analysis of Esphahanian and Storhaug.
Several examples of sandstones damaged by drilling mud invasion have also been verified with the injection tests, which can differentiate between reservoir fluid permeability and fracture-face damage. Since the differences in reservoir quality are identified with a “pre-frac” diagnostic injection test, the final perforation scheme can also be adjusted to ensure that a subsequent fracture treatment will divert to the “best” reservoir rock.