Researchers evaluate the stability of shale gas wells in longwall barrier pillars
by peter zhang, daniel su, and jun lu
Unconventional shale gas development in longwall mining regions has given rise to safety concerns in longwall mines. With the recent shale gas boom, approximately 1,500 shale gas wells have been drilled through current and future coal reserves in Pennsylvania, West Virginia and Ohio over the past 15 years. Longwall mining removes coal from underground in large blocks and causes the surface and subsurface to move as overburden strata above longwall panels settle to fill the mined void.
When gas wells are located in longwall pillars, the longwall-induced subsurface movement can influence their stability, inducing stresses and deformations in gas well casings in the coal pillars. If gas well casings are damaged or ruptured by excessive stresses and deformations, natural gas could leak into active longwall mines, potentially causing a fire or explosion in underground workings. For these reasons, unconventional shale gas wells in longwall pillars not only present safety concerns in longwall mines, but also cause safety and economic concerns for the gas companies.
To address this issue, the National Institute for Occupational Safety and Health (NIOSH) has been conducting research on gas well stability in longwall pillars to provide technical guidance for state and federal regulatory agencies as well as the coal and gas industry. Researchers have studied the critical factors through field experiments and developed numerical models to evaluate the stability of shale gas wells in longwall barrier pillars, as described in this article.
Review of Current Gas Well Pillar Regulation
The current gas well pillar regulation is the PA 1957 gas well pillar study (commonwealth of Pennsylvania, 1957). This study was completed by the Joint Coal and Gas Committee based on gas well failures caused by coal mining in the state of Pennsylvania prior to 1957. The study included 77 gas well failure cases that occurred over a 25-year span in room-and-pillar mines with full or partial pillar recovery in the Pittsburgh and Freeport coal seams. The mining depth in those mines ranged from 55 feet to 750 ft. The 1957 study provided guidelines for pillar sizes around gas wells under different overburden depths up to 750 ft, which became a gas well pillar regulation in Pennsylvania as well as for other states.
Because the technical guidelines developed in the 1957 study were based on data from room-and-pillar mining under shallow cover, they have been found to be inadequate for longwall gas well pillars, especially under deep cover. In fact, gas well casing failures have occurred in longwall chain pillars even though the chain pillar sizes met the requirements by the 1957 study. Although barrier pillars are usually larger than required by the 1957 study, there is still no guarantee that the gas wells are stable in all circumstances, and other factors have to be taken into consideration when evaluating stability.
Critical Factors Influencing Gas Well Stability in Barrier Pillars
The stability of gas wells in barrier pillars is mainly influenced by overburden depth, gas well location relative to the gob, overburden geology and floor stability. First, overburden depth determines how much abutment pressure could be induced over the barrier pillars. The greater the overburden depth, the larger the induced abutment pressure in the barrier pillars and thus the greater the stresses in the gas well casings. In this respect, the gas wells in barrier pillars under deep cover are potentially subjected to higher induced stresses in the casings near coal seams depending on how far the wells are away from the gob.
Overburden depth also influences where gas well failures could occur. Figure 1 shows locations of gas well failures as a result of retreat mining. Because no cases of gas well failures in barrier pillars could be found, the failure cases from the PA 1957 study as well as two failure cases in longwall chain pillars in the Pittsburgh seam are used to show potential failure locations along the vertical axis of a gas well. Based on these available cases, gas well failure can occur in three locations: in the coal seam, within about 100 ft of the roof strata, and within 40 ft of the immediate floor. The figure also indicates that at greater overburden depth, the failures are more likely to occur either in the coal seam or in the floor.
Although barrier pillars relatively large in size generally have no stability issues, the location of the gas wells in barrier pillars — i.e., the distance of gas wells to the edge of the gob — still has an effect on gas well stability. This effect is shown in Figure 2 using the failure cases from the 1957 study and the cases in the longwall chain pillars in the Pittsburgh seam. The case history demonstrates that the majority of failures occurred when the gas wells were located within about 50 ft horizontally from the gob