Mining Project: Stability Evaluation of Active Gas Wells in Longwall Abutment Pillars

Principal Investigator	Wen (Daniel) Su - Request information
Start Date	10/1/2016
Objective	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.
Topic Area	Geologic Characterization

Research Summary

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.

In order to protect miner and public safety and health, five important questions need to be answered by comprehensive geotechnical research efforts over the next few years. These questions are:

• What are the longwall-induced deformations and stresses in the gate road pillars under a given overburden depth and geology?
• What are the critical parameters affecting subsurface overburden deformations?
• What are the parameters affecting a gas well’s mechanical integrity?
• What is the safeguard distance of a gas well—i.e., the distance from the edge of longwall extractions where subsurface deformation is small enough that it has no impact on a gas well’s mechanical integrity—under a given overburden depth and geology to allow mine operators to safely mine by live gas wells?
• What are the proper well casing and cementing options for a variety of mining depths and geologies to allow mine operators to safely mine by live gas wells within the safeguard distance?

A few hundred shale gas wells are scheduled to be mined by in Pennsylvania, Ohio, and West Virginia within the next few years. These shale gas wells, whether tapped into the Marcellus or Utica Formations, contain very high (2000~3000 psi) gas pressure. Longwall excavations could induce high stresses and deformations in these gas wells, which may seriously compromise the mechanical integrity of the production, intermediate, and coal protection casings. Such a compromise of casing integrity not only could introduce high-pressure, explosive gas into underground mine workings to trigger mine fire and explosion, but also could introduce stray gas to surface dwellings and water wells, which would jeopardize public safety.

This project has three research aims, summarized as follows:

Research Aim 1. To evaluate and quantify subsurface overburden deformations in longwall abutment pillars: Such deformations are site-specific, depending on the mining depth, overburden geology, near seam geology, floor geology, mine geometry, and horizontal stresses. It is imperative to understand longwall-induced strata deformations in longwall abutment pillars under different overburden depths and geologies, even if the gas well pillar satisfies the current Pennsylvania Gas Well Pillar Regulation. The lack of information on the resulting strata deformations at, below, and above the coal seam will be addressed by analyzing available case histories and conducting one shallow (<500 feet) and one deep (>900 feet) inclinometer well monitorings. Data obtained from the inclinometer well monitoring program will address the gap in knowledge of longwall-induced subsurface deformations under shallow and deep covers, and will provide essential data for calibrating numerical models.

Research Aim 2. To identify critical parameters affecting subsurface overburden deformations. Many parameters can affect subsurface overburden deformation, such as geology, mining depth, mine geometry, and distance to edge of high extraction. These critical parameters will be investigated via the application of a 3D numerical model, which will be calibrated with results from the inclinometer well monitoring program. Results from the modeling analyses will provide the necessary framework and understanding for defining the safeguard distance and for developing gas well casing and cementing design guidelines for safeguarding miner, gas worker, and public safety in future project proposals.

Research Aim 3. To evaluate the geometric and geotechnical parameters of the 1957 PADEP gas well pillar regulation for their adequacy and deficiency. The adequacy and deficiency of these parameters employed in the 1957 PADEP gas well pillar regulation will be examined. Tasks that will be conducted to meet this objective include: (a) employing the calibrated 3D models to evaluate the geometric and geotechnical parameters used in the 1957 PADEP gas well pillar regulation, and (b) employing the calibrated 3D models to evaluate the 1957 PADEP safeguard distance and pillar stability under a given overburden depth and geology.

Regulatory agencies as well as mining and gas industries have significant interest in the outcome of this project. Specifically, even after the completion of a 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. Accordingly, the outcome of this two-year project will be the quantification of subsurface overburden deformation in longwall abutment pillars under shallow (<500 feet) and deep (>900 feet) covers, and the identification of critical parameters affecting subsurface overburden deformations in longwall abutment pillars. 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 depths and geologies. 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.

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