Wayne Evans
Introduction
Measurements of methane leaks into the atmosphere show wide variations from gas basin to gas basin. These range from 1% to 17% of natural gas production. The highly variability measurements are taken with aircraft borne instruments in a top down methodology. Satellite measurements over some areas such as the Bakken basin agree with large values measured from aircraft. coal beds may be cracked by the drilling or they are already cracked by natural forces (i.e. such as earth quakes and they can leak.)
In this work we propose that Leaks in Gas Fields are due to over-lying Coal Beds
Table 1 illustrates the Gas fields in Basins in the USA with no overlying coal beds.
Table 2 illustrates the overlap of Coal beds with the Gas Fields in USA
Table 1 illustrates Non-Coincidence of Coal Beds with Gas Fields with Low Leakage Rates Barnett : no coal bed, Fayetteville: no coal bed, Haynesville: no coal bed, NE Marcellus: indeterminate.
Table 2 the coincidence of Coal Beds with High Leakage Gas Field Rates ( Low Leakage or high Leakage Source)
Bakken 10 %---Schneising et al,2014 LA Basin 17%---Peischl,2013 4 corners 9%---Dubey et al,2014 Pennsylvania 7%---Caulton et al,2014 Bartlett 1 to 2%---Peischl et al 2014 Denver-Julesburg 4%---Petron et al,2014 Unitah 9%---Petron et al,2013 Marcellus shale 1 %---Peischl et al, 2015 Texas 1%---Peischl et al, 2015 Louisiana 1%---Peischl et al 2015 Arkansas 1%---Peischl et al 2015.
The postulate is further supported by the measurements of large leak rates during the drilling of a new gas well in PA as reported by Caulton et al (1914). Some of the Marcellus and Pennsylvania fields leak little and some leak a lot. This could be explained if wells with shallow overlying coal beds leak a lot and wells with deep overlying coal beds do not leak much. This could be explained if wells with overlying coal beds leak badly (4 to 17%) while gas fields with no overlying coal beds leak only a small amount (1 to 2%). The leaks occur in the coal beds since the coal beds are porous to methane. An outstanding question is; are the coal beds leaking methane because of drilling damage or is natural gas from the well escaping into the coal bed? The measured ratio of methane to ethane could potentially answer this question. Coal gas has low methane content whereas natural gas has a high ratio of ethane to methane. Large methane leaks during drilling phase of a new well in the Marcellus gas field reported by Caulton et al (2014).Are coal beds naturally cracked along the horizontal layers? The Flat Irons near Boulder are tilted at 20 degrees.
Table 3 OVERALL VIEW OF METHANE MEASUREMENTS PARADIGM
AMOUNT MEASURED TOTAL COAL BED WELLHEAD GAS WELL LEAKS TROPOSPHERE
Satellite view all 10.0 % 4.5% 1.5% 5% 11.0 %
Aircraft view typical 5 % 4.5 % 1.5% 11.0%
Aircraft over no coal 1.5 % 0 % 1.5% 0% 1.5%
Wellhead view 1.5% 0% 1.5% 1.5 % 0%
Measured values 1.5 % 0% 1.5% 1.5 % 0%
Typical measured 1.5 % 4.5% 1.5% 5 % 11. %
Conclusions1. If gas wells lie below shallow coal beds, drilling may cause leaks of methane from the coal deposits. This is verified by six gas field cases with large leakage rates with shallow coal deposits.
2. If gas wells lie below deep coal beds, drilling does not cause leaks of methane from the coal deposits. This is verified by four gas field cases with low leakage rates with deep coal deposits.
3. Oil field drilling may cause leaks from shallow overlying coal beds as well just as the gas drilling does.
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