Abstract
The contribution to production of the gas stored within the coal and shale beds adjacent to the main coal seam in the Mannville Group, in which a lateral is drilled, was investigated through a series of numerical simulations. The results indicate that the added gas from the minor coal seams, with interbedded shales with no gas, results in 1.4 times (×) more produced gas and 3.0× more produced water after 25 years of production than when only the main Mannville coal seam is considered. Including gas in the shales results in 1.7× more produced gas and 2.5× more produced water after 25 years of production than when only the main coal seam is considered. The produced gas recovered from the shales exceeds the produced gas recovered from the coals after ~8.5 years, resulting in 2.1× more produced shale gas than coal gas after 25 years of production. Over half (56%) of the produced coal gas after 25 years of production is recovered from the main coal seam while a quarter (22%) is recovered from the L1 seam, which is the thickest and nearest minor coal seam to the horizontal wellbore located in the main seam. The results from the numerical simulations provide insights that are not intuitive or otherwise predictable in developing complex reservoirs. Although the results are specifically for the Mannville producing fairway, undoubtedly the production from minor coal seams and interbedded gas shales should be considered in other producing and potential coal gas reservoirs to identify higher producible reserves and optimize drilling and completions strategies.
Highlights
The main coalbed methane target in the Mannville Group of the Western Canadian Sedimentary Basin in south-central Alberta is the thickest coal seam in the Upper Mannville (Figure 1(a) and Figure 1(b)), which has an average thickness of 2.9 m (range 1.3 - 4.5 m; Figure 1(c))
Over half (56%) of the produced coal gas after 25 years of production is recovered from the main coal seam while a quarter (22%) is recovered from the L1 seam, which is the thickest and nearest minor coal seam to the horizontal wellbore located in the main seam
After 25 years of production, over half (56%) of the produced coal gas is recovered from the main coal seam, while almost a quarter (22%) is recovered from the L1 seam, which is the thickest and nearest minor coal seam to the wellbore, when shale gas is included in the model
Summary
The main coalbed methane target in the Mannville Group of the Western Canadian Sedimentary Basin in south-central Alberta is the thickest coal seam in the Upper Mannville (Figure 1(a) and Figure 1(b)), which has an average thickness of 2.9 m (range 1.3 - 4.5 m; Figure 1(c)). Two to three additional coal seams; are generally present both above and below the main coal seam (Figure 1(d)), giving an average net coal thickness of ~6.8 m (3.8 - 8.2 m). Reference [1] showed that 36% of the total calculated gas-in-place in the Upper Mannville coal measures (average of 5.9 BCF/mi2) is contained within the minor coal seams (Figure 1(e)). Significant gas is stored within the high organic content, fine-grained strata (referred to here as gas shales) interbedded as well as over- and under-lying the coal seams (Figure 1(d)). Assuming a ±20 m stratigraphic interval around the main seam [2], calculated that including the shales increases the average total gas-in-place in the Upper Mannville Group 1.7× (10.2 BCF/mi vs 5.9 BCF/mi; Figure 1(f))
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