Mining Project: Development of Engineering Guidelines for Shale Gas Wells Influenced by Longwall Mining

Keywords: Wells

Principal Investigator	Wen (Daniel) Su - Request information
Start Date	10/1/2020
Objective	To address the lack of information on longwall-induced subsurface strata deformations and stresses and their effect on shale gas well casings by conducting detailed analysis and instrumentation on unconventional shale gas wells that are scheduled to be mined-by within the time frame of the project research.
Topic Area	Geologic Characterization

Research Summary

Due to a recent shale gas boom, which may enable the United States to become more energy-independent, over the past 15 years more than 1,500 unconventional shale gas wells have been drilled through current and future coal reserves in Pennsylvania, West Virginia, and Ohio. These shale gas wells have penetrated many coal seams such as the Sewickley, Pittsburgh, Freeport, and Kittanning, which are either currently mined or will be mined in the future. Without adequate protection, the mechanical integrity of these shale gas wells may be compromised by longwall-induced deformations and stresses under certain scenarios, 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 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mining, the Pennsylvania Department of Environmental Protection (PADEP) initiated a call for research to update the outdated regulation. Importantly, 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 seven other states to govern gas well pillar stability issues over the past 60 years. In order to protect miner health and safety, five important questions need to be answered over the next four years:

(a) What are the critical parameters affecting longwall-induced subsurface deformations and stresses above, within, and below the pillars under a given overburden depth, mining geology, and mining geometry?

(b) What are the gas well setback distance and gas well casing and cementing alternatives for mitigating longwall-induced deformations and stresses?

(d) With damaged shale gas well casings, what are the migration characteristics of high-pressure shale gas and their contribution to gob gas?

(e) What are the appropriate engineering guidelines and recommendations?

In this four-year project, critical engineering guidelines and recommendations, which are unconventional and currently not available elsewhere, will address three gas well stability issues: the parameters affecting a gas well’s mechanical integrity, a gas well’s safeguard distance, and the optimal casing and cementing alternatives. A gas well’s safeguard distance, which is usually vague and poorly understood, is the distance from the edge of longwall extractions where subsurface deformation is too small to have any impact on a gas well’s mechanical integrity under a given overburden depth and geology. Unfortunately, it is a critical number for the coal and gas industries, since misuse of the safeguard distance could jeopardize miner safety or sterilize coal and gas reserves. Employing the optimal well casing and cementing alternatives may partially or totally absorb longwall-induced deformations to allow mine operators to safely mine-by live gas wells within the safeguard distance. In addition, under certain scenarios, longwall mining may potentially damage unconventional shale gas well casings enclosed within abutment or barrier pillars, posing a threat of a high-pressure and high-volume gas release into the mine. As the need for coexistence between shale gas production and coal production arises, proactive ventilation and methane control practices in longwall mines need to adapt to different mining depths, geological conditions, and mining geometries.

In this project, the above issues will be addressed by way of five research aims, as follows:

Evaluate longwall mine-by cases in Pennsylvania, West Virginia, and Ohio.
Employ field instrumentation results and 3D numerical modeling to identify critical parameters affecting subsurface overburden deformations and shale gas well casing stresses above, within, and below longwall abutment and barrier pillars.
Conduct potential shale gas inflow and proactive remedial ventilation control investigations using computational fluid dynamics (CFD), discrete fracture network (DFN), Longwall Instrumented Aerodynamic Model (LIAM), and VentSim modeling.
Experimentally evaluate the composition of coalbed gas, storage field gas, and shale gas, developing acceptable ranges for each in the Pittsburgh Seam.
Develop and propose engineering guidelines and recommendations for gas well safeguard distance and alternative casing and cementing design.

Accomplishment of these five primary objectives will provide a clear picture of gas well casing-rock interaction under a variety of cover depths, geological conditions, and mining geometries; identify critical parameters affecting a gas well’s mechanical integrity; define a gas well’s safeguard distance; develop optimal casing and cementing alternatives; characterize longwall-induced permeability changes within the abutment and barrier pillars; and identify ventilation challenges and remediation options.

MSHA, the PADEP, the West Virginia Department of Natural Resources (WVDNR), and the Ohio Department of Natural Resources (OHDNR) are at a crossroads on updating the current gas well pillar regulation due to a lack of relevant scientific data and research. Without such data and research, federal and state regulatory agencies often tend to approve a gas well pillar permit if it meets the 1957 PADEP regulations, which in theory would allow longwall extractions to mine-by live gas wells. The proposed four-year research will provide critical engineering guidelines and recommendations to help MSHA, the PADEP, the WVDNR, and the OHDNR make the necessary updates to the current gas well pillar regulations to safeguard miner and public safety.