Mining Project: Stability Evaluation of Active Gas Wells in Longwall Abutment Pillars
To evaluate and quantify subsurface overburden deformations in longwall abutment pillars under shallow (<500 feet) and deep (>900 feet) overburden depths, and to employ field instrumentation results and 3D numerical modeling to identify critical parameters affecting subsurface overburden deformations in longwall abutment pillars.
Over the past 10 years, more than 800 shale gas wells have been drilled through current and future coal reserve sites in Pennsylvania, West Virginia, Ohio, Virginia, and Tennessee. These shale gas wells have penetrated many coal seams such as the Sewickley, Pittsburgh, Upper/Lower Freeport, and Upper/Middle/Lower Kittanning seams, which are either currently mined or will be mined in the near future. The mechanical integrity of these shale gas wells may be compromised by longwall-induced deformations and stresses, which may result in unexpected, dangerous inflow of high-pressure gas into underground mine workings.
Recently, due to the interest of shale well drillers and operators in drilling wells into coal mine gobs, a coal gas working group led by Pennsylvania Department of Environmental Protection (PADEP) managers has recognized the importance of regulatory input for producing wells drilled into longwall gobs. This working group has just begun to identify key technical issues since this process has never been completed in an active mine gob. Only anecdotal evidence from a limited number of shale wells drilled in abandoned mine gobs is currently available. The shale gas industry has expressed a strong interest in drilling wells at these locations and since these wells will not be subject to any longwall-induced subsurface deformation, coal operators are also engaged in discussions and planning of these drill sites. Drilling activities in longwall gobs at active mines have potential safety effects on all rig workers and all underground coal mine workers due to the potential gas explosion hazard.
The research aims of this project in fiscal year 2017 through fiscal year 2020 are as follows:
(1) Evaluating and quantifying subsurface overburden deformations above, within, and below longwall abutment pillars under shallow (<500 feet) and deep (>900 feet) overburden depths.
(2) Employing field instrumentation results and 3D numerical modeling to identify critical parameters affecting subsurface overburden deformations above, within, and below longwall abutment pillars.
(3) Evaluating the geometric and geotechnical parameters of the 1957 PADEP gas well pillar regulation for their adequacy and deficiency.
(4) Characterizing longwall-induced permeability changes.
(5) Conducting ventilation investigations using physical and numerical models to investigate potential shale gas inflow and impact on longwall ventilation.
(6) Determining the height of longwall-induced fracture zone under a given mining and geologic condition and employing FLAC3D model to predict fracture zone heights under different mining and geologic conditions.
(7) Determining gas content and reservoir parameters in a longwall gob, assessing potential for coalbed gas flow to the well bore, and defining the potential effects of shale well drilling in longwall gobs on the gob gas reservoirs.
Accomplishment of these aims will provide the input and justification for future research projects aimed at quantifying gas well casing-rock interaction, identifying critical parameters affecting a gas well’s mechanical integrity, defining a gas well’s safeguard distance, developing optimal casing and cementing alternatives, defining longwall-induced permeability change and its impact on longwall ventilation, defining the fracture zone height under different mining and geologic conditions, and defining the potential effects of shale well drilling in longwall gobs on the gob gas reservoir.
Regulatory agencies as well as coal and gas industries have significant interest in the outcome of this project. Specifically, even after the completion of the 2014 gas well pillar study by the coal and gas industries, the Pennsylvania Department of Environmental Protection (PADEP) and the Mine Safety and Health Administration (MSHA) have expressed strong interest in more field data and research before any new gas well pillar regulation can be published. The intermediate outcomes of this four-year project will be the quantification of subsurface overburden deformation in longwall abutment pillars under shallow (<500 feet) and deep (>900 feet) covers, the identification of critical parameters affecting subsurface overburden deformations in longwall abutment pillars, the adequacy of the 1957 PADEP regulation, the longwall-induced permeability changes and gas migration pattern, and effects of shale gas well drilling in longwall gobs on gob gas reservoir.
These intermediate outcomes will provide the relevant scientific data and justification for future research projects that aim at understanding the complex gas well casing-rock interaction, defining the intrinsic gas well safeguard distance, developing innovative casing and cementing alternatives, understanding longwall-induced permeability changes and gas migration pattern, and defining the effects of shale gas well drilling in longwall gobs on gob gas reservoir. Employing the safeguard distance and optimal casing and cementing alternatives could eliminate the effects of longwall-induced subsurface overburden deformations that may induce global as well as local well casing instabilities. Specifically, the safeguard distance and the casing and cementing alternatives will be used by MSHA and state regulatory agencies as well as the coal and gas industries to protect miner, gas worker, and public safety. In addition, specific guidelines developed from future projects will help mine operators understand the risk of mining by live gas wells and guide gas operators to proactively employ the best casing and cementing alternatives to ensure mechanical integrity of well casings for a variety of mining depth and geology. The ultimate outcome of this research will safeguard miner, gas worker, and public safety, while allowing coal operators to safely mine by live gas wells without costly well plugging and enabling gas operators to avoid costly loss of income.
This research represents a very important initiative in utilizing the knowledge and science obtained from mining research to improve not only miner and public safety and health, but also the safety and health of the people employed in the oil and gas industries.
Zhang Y, Su W, LuSu J (2019). Evaluating the Stability of Shale Gas Wells in Longwall Barrier Pillars. Coal Age, Vol 124, No. 8, pp. 34-38.
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