Guntis Moritis
Production Editor
The recent estimates from the US Energy Information Administration on world shale gas show that these sediments contain considerable volumes of potential gas resources.
The estimates are discussed in the April report, World Shale Gas Resources: An Initial Assessment of 14 Regions Outside the United States, commissioned by EIA from Advanced Resources International Inc. The report said risked shale gas in place in the world amounts to 25,300 tcf. This estimate includes 3,284 tcf in the US but excludes such areas as Russia and the Middle East that contain considerable amounts of conventional gas.
The report's current estimate is considerably greater than the 1997 publically available estimate by H-H Rogner of 13,897 tcf, excluding areas not covered in the EIA report, or 16,112 tcf with those areas considered.
The largest difference in the assessments are for Europe, Africa, and North America.
Broken down by region, the assessment by EIA compares with those of Rogner as follows: North America, increasing to 7,140 tcf from 3,842 tcf; South America, up to 4,569 tcf from 2,117 tcf; Europe, up to 2,587 tcf from 549 tcf; Africa, up to 3,962 tcf from 1,548 tcf; Asia, up to 5,661 tcf from 3,528 tcf; and Australia, down to 1,381 tcf from 2,313 tcf.
Rogner in his Africa estimate includes 1,274 tcf from one half of Middle East and North Africa (MENA) and 274 tcf from sub-Saharan Africa.
Rogner's 16,112 tcf estimate also includes areas not covered in EIA's report such as 627 tcf in the FSU, 314 tcf in Asia-Pacific, and 1,274 tcf in one half of MENA. Even though EIA's estimated Australia in place resources are lower than those of Rogner, the report notes that Australia's geologic and industry conditions resemble those of the US and Canada and the country appears poised to commercialize its gas shale resources on a large scale. For example, it notes that such companies as Santos and Beach Energy are testing shale reservoirs with core wells and initial frac production tests are planned for later this year.
Resource calculations
For determining prospective areas, EIA's report said the shales need to have a vitrinite reflectance (Ro) greater than 1% and a 1.3% Ro for higher quality prospects. The report noted that higher thermal maturity shales may contain nanopores that contribute to additional porosity in the shale matrix.
The report also said marine deposited shales tend to have less clay and contain more brittle minerals such as quartz, feldspar, and carbonates that respond more favorably to hydraulic fracturing. On the other hand, shales deposited in a nonmarine setting tend to have more clay, be more ductile, and respond less to fracing.
Reservoir depth is another consideration. The report notes that the prospective areas are between 3,300 to 16,500 ft. It explains that shales less than 3,300 ft have lower pressure, less gas concentration, and risk higher water content in natural fractures, while shales deeper than 16,500 ft may have less permeability and higher drilling and development costs.
The assessment includes both free and adsorbed gas in place. The report explains that adsorbed gas can be the dominant resource for shallow and highly organically rich gas, while free gas is dominant in deeper, higher clastic content shales. Total organic content in prospective shales should be at least 2%, the report said.
To calculate recoverable resources, the assessment used several recovery factors:
• 30% for shales with low clay content, low to moderate geological complexity, and favorable reservoir properties such as overpressure and high gas-filled porosity.
• 25% for shales with a medium clay content, moderate geologic complexity, and average reservoir pressure and properties.
• 20% for shales with medium to high clay content, moderate to high geologic complexity, and below average reservoir properties.
• 35% for a few cases with established wells with high production rates.
• 15% for a few cases of severe under-pressure and reservoir complexity.
Recoverable resources
The report concluded that the 14 regions outside of the US studied contain 5,760 tcf of technically recoverable shale gas resources. By area, North America has 1,069 tcf; South America, 1,225 tcf; Europe, 624 tcf; Africa, 1,042 tcf; Asia, 1,404 tcf; and Australia, 396 tcf.
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