Peter Zhang and Daniel Su, NIOSH, Pittsburgh Mining Research Division, and Jun Lu, CONSOL Energy
The influence of longwall mining on unconventional shale gas wells located in longwall pillars has been a continuing concern for mining. Over the past decade, many shale gas wells have been drilled through the Pittsburgh coal seam to the Marcellus shale formation. The shale gas wells in longwall chain pillars are subjected to longwall abutment pressure and subsurface deformation. The stability of those gas wells in the chain pillars is primarily influenced by the stability and deformational behavior of the chain pillars as well as subsurface ground movement. In another situation, the gas wells located in the barrier pillars are less influenced by longwall mining as the pillars are larger and the gas wells are farther away from the gob. However, when longwall mining occurs on both sides of the barrier pillars, the influence on the gas wells can be pronounced and they could be potentially compromised. This study involved a case in which a shale gas well was located in a barrier pillar between two longwall bleeders in the Pittsburgh coal seam. The first longwall panel was mined before the gas well was drilled and installed in the barrier pillar adjacent to the longwall setup bleeders. The second longwall setup bleeders were developed on the other side of the barrier pillar and the second longwall panel was subsequently mined. The stability of the gas well was evaluated using numerical modeling approach. The model was set up based on site-specific mining conditions and overburden geology and included details to simulate the gas well casings, mining sequence and longwall retreating. The modeling procedures produced realistic results of stresses and deformations around the barrier pillar where gas well was drilled. The induced deformations and stresses in the gas well casings were also obtained from the model. The modeling results were compared to the actual performance of the gas well during and after the second longwall was mined. There will also be discussion about the effect of longwall mining on stability of shale gas wells in the barrier pillars adjacent to longwall gob.