Coal Failure and Consequences for Coalbed Methane Wells

Abstract We provide a summary of coal failure during the life of a typical coalbed methane (CBM) well, and some consequences for the CBM industry. Because coals are weak in general, these consequences are often important, and can have a significant impact on drilling, completion, and production of CBM wells. Failure during drilling: horizontal wells are the preferred completion in the Arkoma basin, for instance. Drilling horizontal wells is sometimes hindered by hole collapse, especially when drilling underbalanced, or drilling through a depleted coal. We demonstrate the relative importance of this by comparing drilling stability in coals of various ranks and depths. Failure during hydraulic fracturing: in weak coals the increased pore pressure around a fracture can cause shear failure. The likelihood of shear failure is charted against coal rank, for given in situ stress, and fracturing pressure. When coal fails in shear, coal fines are produced in the reservoir, as has been demonstrated in the lab. The creation and movement of such fines can reduce the coal permeability, and the frac conductivity. However, dilatancy accompanying shear failure can also lead to permeability increases. Failure during production: when a CBM well is put on production, by applying a drawdown, the lowering of BHFP can cause failure of the coal (or collapse of a horizontal openhole) if it is weak enough. The critical drawdown for failure and fines production can be predicted by using advanced technology borrowed from the sand production arena. We can characterize the probability of failure by coal rank, drawdown, depletion, as well as other parameters like in situ stress. Careful assessment of the likelihood of coal failure during drilling, fracturing, or production, as a function of coal rank and in situ stress, can lead to mitigating efforts to avoid the bad consequences. For example, applying a smaller drawdown in a producing CBM well may prevent or defer failure in weak coals and mitigate fines plugging of the formation and fracture, fill in the wellbore, and pump or compressor problems.