Shales Closing 'Conventional' Gap

At what point do unconventional resources become conventional resources?

If this is possible, gas shales are certainly closing the gap.

Even though operators have produced gas from shales since the first shale-gas well in the organic-rich Dunkirk Shale (Devonian) in New York in 1821, the recent interest in gas shales began in 1981 with the first Barnett Shale well, drilled in the Fort Worth Basin in northern Texas by Mitchell Energy Corporation. Gas shales have advanced to an economic gas play since the year 2000 thanks to a combination of high gas prices, shale reservoir characterization and advances in drilling and completion technology.

The sheer number of articles and conferences describing gas-shale plays in the United States and Canada attests to the high interest level.

As for importance, the Barnett Shale alone supplies about 7 percent of the United States' annual dry-gas production.

Note the accompanying alphabetical list of many of the United States and Canada gas-shale plays in the news.

Gas shales span reservoir ages of Cambrian to Miocene, and shale formations such as the Barnett, Fayetteville and Woodford have become household names.

Some long-producing shales such as the Antrim, Huron and New Albany are seeing renewed interest – and the Haynesville Shale in Louisiana and Marcellus Shale in the Appalachian states are being compared to the Barnett Shale as the next big plays.

(The Late Devonian-Early Mississippian Bakken Shale in the Williston Basin is not on the list because it is primarily an oil play.)

Early lessons learned and shared concerning how to produce gas from shales have drastically shortened the learning curve when exporting the technology to new shale-gas plays.

One of these lessons was the importance of fractures (natural and induced) for gas production while avoiding fracture propagation into water-bearing formations with the occurrence of shale or carbonate fracture barriers.

Hydraulic slick-water fracturing and re-fracturing were the initial keys to completing the tight-shale reservoir. Today, horizontal wells – first applied to shales in 2003 – are now routinely applied to expose more of the shale to the well bore and use the shale boundaries as fracture barriers.

Recent technological advances applied to horizontal wells in shales include:

• ♦ 3-D seismic in determining lateral length and placement.
• ♦ Multilaterals (drilling several laterals from a single well pad).
• ♦ Multiple frac stages within a lateral.
• ♦ Real-time microseismic monitoring to image hydraulic-fracture treatments.
• ♦ Simul-fracs (simultaneous hydraulic fracturing of offset parallel horizontal wells about 1,000 feet apart).
• ♦ Seismic is playing an increasing role in shale gas appraisal and development. In addition to imaging the basic structural geometry and faults, multi-trace attributes such as coherence and volumetric curvature are being used to detect smaller-scale faults, areas of more intense fracturing and collapse features such as sinkholes.

Of course, not all organic-rich shales will be economic gas shales, even with the application of innovative completion technology. What was once thought of as a hydrocarbon source rock or cap rock must now be evaluated as a gas reservoir.

Also, it is not enough to have a thick, organic-carbon rich black shale in the gas window. Other factors such as mineralogy (e.g., clay content and types) and petrographic properties (e.g., silt stringers, laminae, bitumen network) are important to produce gas from shales.

Fortunately, research on petrophysics, geomechanics and geochemistry conducted at universities, service companies and consortia is advancing our understanding of shales as reservoirs.

Much has been learned about how to evaluate shale as a gas source rock and reservoir. Some remaining questions include:

• ♦ What is the optimum range of thermal maturity?
• ♦ How low or high of thermal maturity is too low or too high?
• ♦ What depth is too shallow or too deep to be economic?
• ♦ How important is reservoir pressure?

Energy Minerals Division members have access to the EMD members-only Web site, which has semiannual gas-shale reports, a calendar of gas-shale conferences, an extensive list of published gas-shale literature, presentations, online reports, Web links and short course notes.

Check it out – and please let me know if you have suggestions on additions to the EMD Gas Shales Committee Web site.