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Página 1 de 3Just How Much Gas Is There, Really? :: POWER Magazine

20/06/2012http://www.powermag.com/gas/gas_power_direct/Just-How-Much-Gas-Is-There-Really_...

• Contingent resources, those potentially recoverable from known accumulations, but not mature enough for development.

• Prospective resources, those estimated to be potentially recoverable from undiscovered accumulations.

To make matters more confusing, the U.S. Geological Survey (USGS) uses some additional terminology when releasing its resource assessments:

• Technically recoverable resources: all resources that may be recoverable using current techniques without regard to cost.

• Economically recoverable resources: those technically recoverable resources for which the costs of development, including profit, can be recovered.

Although these definitions are not terribly challenging, a lot of pundits get lost when trying to apply the various estimates that go with them.

The first thing to understand is that a resource assessment is a moving target. This is especially true on a national scale. Take a look at how much the Energy Information Administration’s (EIA's) proven dry gas reserves estimates (derived from USGS data) changed over 2004-2009:

1. Despite growth in natural gas consumption, U.S. gas reserves have grown steadily since 2005. Source: EIA

Proven reserves grew an average of 16 Tcf every year, even though the U.S. consumed an average of 22.5 Tcf per year over the same period. This is why one cannot simply take current proven reserves, divide by current consumption, and come up with a meaningful estimate of "how much gas we have left." Yet it is not hard to find examples of energy reporters doing exactly that.

What's going on here? It's not just a matter of new gas being discovered. When a known but unexplored gas play is sufficiently mapped, it moves from the unproven portion of technically recoverable reserves to proven reserves. So even though the U.S. burned 135 Tcf of gas from 2004-2009—a figure that represents more than 70% of the 2004 estimate—almost 80 Tcf was added to proven reserves, both newly discovered gas and known gas that was re-assessed to have a 90+% chance of recovery.

This is a process that goes on continuously as gas plays are discovered, drilled, taken in and out of production, and retired. So while it's tempting, if one wants to be conservative, to dismiss much of the technically recoverable estimate as speculative, decades of experience have shown that most of it represents gas that will likely be produced at some point.

There's another point to consider. The technically recoverable estimate is based on current techniques. The importance of this cannot be overemphasized, since it's a safe bet that gas drillers in 2050 will not still be using 2012 technology. Going back just a decade shows how fast things can change.

Geologists, for example, have long known that shale formations held quantities of natural gas, but very little shale gas was included in USGS estimates until recently because it was considered unrecoverable with existing techniques. In 2002, the USGS estimated that the Marcellus shale formation contained a mere 2 Tcf of technically recoverable gas. Last fall, it upped the estimate to 84 Tcf. The gas had always been there; the only real change was improvements in drilling technology.

So where are we today? Current EIA estimates of technically recoverable gas resources are 2,214 Tcf. If one insists on dividing that by current consumption (22 Tcf in 2011), we get President Obama's much-bandied-about 100-year figure.

Of course, not all of that gas will be produced. It's impossible to know where gas prices will go in the future, and the price of gas has a key influence on gas exploration and development. Most indications, and the EIA, suggest gas prices are going to rebound through the rest of this decade—the general consensus is that $2.50/MMBtu gas is just not sustainable. Many factors, particularly liquefied natural gas exports are



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likely to push the price up.

Too, it's hard to predict what future gas consumption will look like. All sorts of developments could push it up or down, perhaps substantially.

But it's also certain that there is a lot of gas out there that is not included in current estimates, either because it hasn't been discovered yet, or—as in the case of methane hydrates—because commercial methods of recovery have yet to be developed. The DOE, for example, gingerly muses that "the energy content of methane occurring in hydrate form [may exceed] the combined energy content of all other known fossil fuels."

How does all that add up? Basically, you ain't seen nothing yet.

—Thomas W. Overton, JD is POWER's gas technology editor

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