Abstract
This paper presents a study on the impact of well integrity failure on coalbed methane (also known as coal seam gas) production and potential shallow water contamination using numerical simulations with a finite-difference method. Two connection types and 12 cases were simulated: Type A – leakage through cement sheath and Type B – impaired zonal isolation at the aquifer interval. The effect of the distance between the aquifer and the coal seam, drainage area and desorption time on gas and water production was also inspected. Results show that both connection types have strong effects on the gas and water production; the cumulative water and gas production increases with increasing drainage radius; the distance between aquifer and the coal seam has a negative effect on the water production but a negligible effect on the gas production; desorption time, ranging from 5 to 30 days, has a negligible effect on the water and gas production. Connection Type A yields a potential water contamination but connection Type B does not. Gas concentration in the shallow aquifer decreases sharply with an increase of distance away from the producer and the unsafe area are within an area with a radius ranging from approximately 50 m to 90 m away from the producer in this study.
Highlights
A wellbore is a pathway through which water and coalbed methane (CBM) are produced
Cased-hole completion is used, the shallow aquifer may still be connected with coal seams from the leakage through cement sheath which may be caused by poor well completion practices, corrosion of steel casing, the deterioration of cement and shear failure or de-bonding of cement sheath (De Andrade et al, 2015; Davies et al, 2014; Feng et al, 2016)
Analysing a real field data to verify the methods used in this study need to be done in the future. Note that this is a conceptual study to address the consequence if there is a well integrity issue
Summary
A wellbore is a pathway through which water and coalbed methane (CBM) are produced. It is usually 200–1000 m in depth (APLNG, 2015) for a CBM well. Cased-hole completion is used, the shallow aquifer may still be connected with coal seams from the leakage through cement sheath which may be caused by poor well completion practices, corrosion of steel casing, the deterioration of cement and shear failure or de-bonding of cement sheath (De Andrade et al, 2015; Davies et al, 2014; Feng et al, 2016). There are few studies on the impact of well integrity failures on CBM production and shallow aquifer’s contamination using numerical simulation. Effect of the distance between the aquifer and coal seam, the drainage area and the desorption time on CBM production and shallow aquifer’s contamination is inspected. All simulations were run for 10 years with a closed boundary which mimics a circular drainage area with a radius of 300 m
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