In a previous study (Clarkson 2009), advanced production analysis techniques, including production type-curves and flowing material balance (FMB) were applied to Horseshoe Canyon (HSC) coal wells to establish the applicability of these techniques and to resolve the importance of multi-layer behavior for production analysis. The field examples chosen for analysis included single wells that exhibited production declines analogous to shallow gas wells, with decline in production occurring from day one, and in one case exhibiting transient flow characteristics. There are many more HSC wells that exhibit less straight-forward production characteristics, including flat or even inclining production. These wells have production profiles qualitatively similar to 2-phase CBM wells, yet lack water production. There are several possible explanations for the flat or inclining production behavior, including changing skin associated with near-wellbore cleanup of drilling fluids over time, and increase in absolute permeability associated with matrix shrinkage. Regardless of the cause, these effects need to be accounted for in quantitative production analysis.

In this follow-up paper, we continue to perform comparisons between multi-layer and single-layer-equivalent production analysis of HSC wells, but focus on wells that exhibit inclining gas production. We develop a methodology to correct for changing skin in both type-curve and FMB analysis that allows this more complex well behavior to be analyzed. We validate the new methodology using a simulated example, and then apply it to actual field cases. Changing skin and absolute permeability by layer/coal zone was quantified through periodic shut-in/buildup testing of isolated coals throughout the life of the wells, and these dynamic properties were used in production data analysis and analytical simulation. Comparing single-layer-equivalent and multi-layer analysis, we have found differences in EUR estimates of < 15 %, due to the existence of one or two dominant (high kh) coal zones. Additionally, we investigated the impact of free gas storage on the results of production data analysis of HSC wells.

The production analysis methodology introduced in this work, while useful for analyzing HSC coal wells with dynamic skin/permeability, is believed to be applicable to a broader range of reservoir types that exhibit this complex behavior.

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