Dispersive Natural Gas Discoveries
As a follow-up to the last post on the Dispersive Discovery model, I decided to apply the same technique to US Natural Gas discoveries.
As background, I found a few references which said "The United States has proved gas reserves estimated (as of January 2005) at about 192 trillion cubic feet (tcf)" and from NETL this:
U.S. natural gas produced to date (990 Tcf) and proved reserves currently being targeted by producers (183 Tcf) are just the tip of resources in place. Vast technically recoverable resources exist -- estimated at 1,400 trillion cubic feet -- yet most are currently too expensive to produce. This category includes deep gas, tight gas in low permeability sandstone formations, coal bed natural gas, and gas shales. In addition, methane hydrates represent enormous future potential, estimated at 200,000 trillion cubic feet.This together with the following reference indicate the current estimate of NG reserves lies between 1173 and 1190 TCF (Terra Cubic Foot = 1012 ft3).
How much Natural Gas is there? Depletion Risk and Supply Security Modelling
US NG Technically Recoverable Resources US NG Resources
(EIA, 1/1/2000, Trillion ft3) (NPC, 1/1/1999, Trillion ft3)
Non associated undiscovered gas 247.71 Old fields 305
Inferred reserves 232.70 New fields 847
Unconventional gas recovery 369.59 Unconventional 428
Associated-dissolved gas 140.89
Alaskan gas 32.32 Alaskan gas (old fields) 32
Proved reserves 167.41 Proved reserves 167
Total Natural Gas 1190.62 Total Natural Gas 1779
Hubbert originally plotted yearly discoveries per cumulative footage drilled for both oil and natural gas. I also found a curve for Natural Gas at this reference (may be the same one as found by McManus).
Interesting that if we fit the cumulative discovery data to the naive exponential that the curve matches very well on the upslope (see below) but that the asymptote arrives way too early, obviously missing all the dispersed discoveries covered by the alternative model. The dispersive discovery adds a good 20% extra reaching an asymptote of 1130, coming much closer to the value from NETL of 1190.
Although a bit unwieldy, one can linearize the dispersive discovery curves, similar to what oil analysts do with Hubbert Linearization. Although it swings wildly initially, I can easily see the linear agreement with a correlation coefficient very nearly one and a near zero extrapolated y-intercept.
As a comparison, I plotted the previously discussed fits for USA oil discoveries and found the same general agreement in linearization between NG and oil.
(note that the naive exponential that Hubbert used below overshoots the fit to better match the asymptote but still falls short of the alternative model's asymptote, and which also fits the bulk of the data points much better)
Every bit of data tends to corroborate that the dispersive discovery model works quite effectively in both providing an understanding on how we actually make discoveries in a reserve growth fashion and in mathematically describing the real data.
note: on some of the charts, the correlated fit does not show the decimal points in the linear equations due to a bug in the plotting program.