Three-dimensional Numerical Modeling Encompassing the Stability of a Vertical Gas Well Subjected to Longwall Mining Operation - A Case Study

摘要

Longwall mining has been one of the prominent underground mining methods for extracting coal in the United States for many years due to its production efficiency. However, the method itself can possibly cause problems to surface structures, i.e. gas wells, buildings, roads, etc., that are built or located nearby mining operations. In recent years, significant gas resources have been identified to be located underlying coal deposits, particularly in Southwestern Pennsylvania, West Virginia, and Ohio. As a result, within the time frame of the last 10 years, more than 800 gas well have been drilled through active and future underground longwall mining. Practically, strata deformation caused by longwall mining could induce high stresses and deformation, which may lead to the instability of a gas well. Therefore, it is imperative that gas well stability penetrating longwall mining operation should be investigated and studied in order to maintain the stability of the gas well, as well as to enhance the safety of underground longwall mining working face and it's operation.;In this study, various three-dimensional numerical models were developed using Finite Difference Method in FLAC3D program and calibrated against an actual gas failure case cited in Peng et al. (2003). Field observation of the case study indicated that the gas well failed after second panel mining. In the previous study, Peng et al. (2003) used the Finite Element Method in ABAQUS program to simulate the gas well failure in order to find out the reason behind the gas well failure. They found out that that the presence of clay layer between two strong strata induced high stresses on the gas well casing and causing the high compressive strain at the gas well failure horizon. For this study, the main objective is to assess the influence of overburden depth and the presence of soft strata on the stability of the vertical gas well subjected to longwall mining operation. In addition, the determination of suitable pillar size for protecting the vertical gas well from failure, as a result of longwall mining, will also be investigated.;The results of this study indicate that the overburden depth, soft strata presence, and the pillar size of a longwall mine plays an important role on the gas well stability penetrating longwall mining abutment pillar. It was found that the deeper the overburden depth of a mine, the higher possibility for gas well to fail under axial compression. The critical overburden depth for the gas well to survive is 600 ft. (183 m). However, it should be noted that the obtained critical overburden depth is specific to the given case study. This study revealed that the geological condition of a mine, such as the presence of soft strata, could induce stresses and deformation on the gas well, which will lead to the instability issue of the gas well. The gas well would typically face more instability issue when it is drilled through a horizon, where the presence of soft strata takes place in between two layers of strong strata. In addressing the need of suitable pillar size for protecting the gas well from failure as a result of longwall mining, it is determined in this study that in order to maintain the stability of the gas well for the case study, considering three entry system, the pillar size should be 80 ft (24.4 m) wide rib to rib. Whereas if the four-entry system is used, the size of the pillar should be kept at 48 ft (14.6 m) wide rib to rib.;The main significance of this study is that it not only proposes a numerical modelling procedure for assessing the stability of gas well subjected to longwall mining operation of the particular case study, but it can also be used for other similar projects. This study concludes that with the accurate data input of rock and casing properties, as well as the gob model simulation, the numerical modelling analysis approach presented in this thesis will be able to accurately predict the stability of the vertical gas well penetrating longwall mining abutment pillar.