Mining Project: Stability Evaluation of Active Gas Wells in Longwall Abutment Pillars
To evaluate and quantify subsurface overburden deformations and permeability changes 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 and evaluate migration pattern of hypothetical shale gas intrusion.
Due to a recent shale gas boom, which may enable the United States to become more energy independent, over the past 10 years, more than 800 shale gas wells have been drilled through current and future coal reserves 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.
In 2012, upon recognizing that the 1957 Pennsylvania Gas Well Pillar Regulation (Commonwealth of Pennsylvania, 1957) was formulated without data from modern day longwall mining, the Pennsylvania Department of Environmental Protection (PADEP) initiated a call for research to update the outdated regulation. It is important to note that the 1957 Pennsylvania Gas Well Pillar Regulation has been widely used by the Mine Safety and Health Administration (MSHA) in conjunction with CFR 75.1700 and other states to govern gas well pillar stability issues over the past 60 years. In order to protect miner and public safety, this four-year research project was proposed to address four important questions concerning gas well stability and high-pressure shale gas migration above, within, and below longwall abutment pillars. These questions are as follows:
(a) What are the longwall-induced deformations and stresses above, within, and below the abutment pillars under a given overburden depth and geology?
(b) What are the critical parameters affecting subsurface overburden deformations above, within and below the abutment pillars?
(c) Is the 1957 PADEP gas well pillar regulation or CFR 75.1700 adequate for protecting miner and public safety?
(d) With damaged shale gas well casings, what are the migration characteristics of high-pressure shale gas and its contribution to gab gas?
The research aims of this project in fiscal year 2017 through fiscal year 2020 were 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 critical scientific data for 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 outcomes from this project indicate that under shallow and medium covers, the measured horizontal displacements within the abutment pillar are about one order of magnitude higher than those measured under deep cover. In addition, the P-110 production casings in all NIOSH case studies remain at least 15% below their rated yield strength, which has serious implications for unconventional shale gas wells drilled ahead of longwall mining in the Pittsburgh Coalfield. Limitations to the 1957 PADEP Gas Well Pillar Regulations have been identified, and preliminary gas well setback distances under barrier and chain pillar scenarios have been proposed. Longwall-induced permeability changes under shallow cover were characterized and used to estimate potential shale gas inflow. These outcomes have been presented to the stakeholder group, which includes MSHA, PADEP, WVDNR, OHDNR, the coal industry, and the gas industry. Regulatory agencies and coal and gas industries are expected to use these outcomes to update existing risk matrix and technical guidance documents to safeguard miner safety and health.
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.
Su, W.H. (2018). Stability of Shale Gas Well Casing under the Influence of Longwall-induced Subsurface Deformations. Presentation at the 2nd International Conference on Gas, Oil, and Petroleum Engineering, Houston, Texas.
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T. (2018a). Effects of Cover Depth on Longwall-induced Subsurface Deformations and Shale Gas Well Casing Stability. In Proceedings of the 52nd ARMA Conference, Seattle, WA, 20 pp.
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T. (2018b). Effects of Longwall-induced Subsurface Deformations on Shale Gas Well Casing Stability under Deep Covers. In Proceedings of the 37th International Conference on Ground Control in Mining. Morgantown, West Virginia, 63-70.
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T. (2019a). Effects of Longwall-Induced Subsurface Deformations and Permeability Changes on Shale Gas Well Integrity and Safety under Shallow Cover. In Proceedings of the 53rd ARMA Conference, New York, NY, 18 pp.
Su, W.H., Zhang, P., Van Dyke, M., and Minoski, T. (2019b). Longwall-induced Subsurface Deformations and Permeability Changes – Shale Gas Well Casing Integrity Implication. In Proceedings of the 38th International Conference on Ground Control in Mining. Morgantown, West Virginia, 49-59.
Su, W.H., Zhang, P., Van Dyke, M., and Kimutis R. (2020). Longwall Mining, Shale Gas Production, and Miner Safety and Health. In Proceedings of the 39th International Conference on Ground Control in Mining. Morgantown, West Virginia.
Zhang Y, Su W, Lu J (2019). Evaluating the Stability of Shale Gas Wells in Longwall Barrier Pillars. Coal Age, Vol 124, No. 8, pp. 34-38.
Zhang, P., Ramburg, G., Mark, C., and Su, W.H. (2020). The Current Perspective of the 1957 PADEP Gas Well Pillar Study and its Implications for Longwall Gas Well Pillars. In Proceedings of the 39th International Conference on Ground Control in Mining. Morgantown, West Virginia.
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