Shale Gas Content
This program uses data from my M.S. thesis on the Mississippian
Caney shale. It takes advantage of the fact that logged bulk
density is a superb proxy for a shale's total organic carbon
content. The data behind the calculations come from 40+ intervals of labmeasured
TOC and bulk density data, sliced from continuous core. The gas
content numbers are based on desorption canister measurements,
cut while continously coring. The core transit time to the surface
was rapid (15 minutes from 4,000 feet), minimizing lost gas.
My source data came from a shale focus area
with a thermal maturity of Ro=2, so expect the gas content to
drop for lower thermal maturities. Example: I also had data for
wells in a thermal maturity window of Ro<=1, which cut
the gas content in half.
As to producible gasI use a 10% recovery factor and am
happy to get that! For the sample calculation:
Thickness of shale: 100 feet
Area of shale: 640 acres
Shale bulk density: 2.5
Total Organic Carbon, TOC%= 4.95
Gas content in scf/ton= 108.855
Total Gas in place= 23.682 BCF
You'll be lucky to get:
Producible gas= 2.3682 BCF (using 10% rec factor)
and that's with a noincident frac job and most of the gas
coming back within the first year of production (before
desorption becomes the primary production mechanism).
After that, gas molecules will move through the shale
and toward your induced fractures at the blinding rate of a
centimeter per year.
This calculation gives you the adsorbed gas in the shale. The
usual followup is to assume both the porosity and resistivity curves
through your shale are valid, calculate a water saturation from
them, and then a volume that (you hope) represents total gas
in place. Subtract the adsorbed gas from the total gas and
you theoretically get the free gas in place. Shale gas production
curves typically look like a big hockey stick, and the handle
is the contribution from free gas. If you haven't paid out your
well by the time you reach the bottom of the handle...you've
got a long wait ahead.
Your Mississippianage
shale will have its own thermal history and composition,
yet there are few geologic depositional environments as boring
and monotonous as organic shales. You might find these
equations surprisingly accurate for estimating gas
content.
