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 lab-measured 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 gas--I 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 no-incident 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 Mississippian-age 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.