Study of Borehole Stability of Marcellus Shale Wells in Longwall Mining Areas

Open Access
Author:
Wang, Yi
Graduate Program:
Petroleum and Natural Gas Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
September 11, 2012
Committee Members:
  • Yilin Wang, Thesis Advisor
  • Jamal Rostami, Thesis Advisor
  • Robert W Watson, Thesis Advisor
Keywords:
  • Marcellus shale
  • longwall mining
  • horizontal displacement
  • casing design
Abstract:
Gas wells drilled and completed in longwall mining areas face the potential for wellbore instability problems due to substantial ground movements in overburden strata following the mining activity. The ground deformation, caused by caving of the ground behind longwall panel can generate large horizontal displacements and complex stress change in subsurface rock above and to some extent below the coal seam. The deformation and variation of the stresses in subsurface rocks can trigger ground movement which causes casing failure, and thus interruption in operation of the well, need for shutting down the well for repair, or permanent abandonment. Either way, the wellbore failure imposes economic losses to gas well operators while posing a safety risk to miners and mining operations as the gas could be released in underground workings. Thus, it is critical to study the ground movements caused by longwall mining and characterize the parameters related to the mining process to evaluate the changes in ground stresses / strains in different layers to evaluate the wellbore stability under such conditions. The main propose of this study is to develop a general casing design guideline in such areas that can withstand the potential deformations and assure wellbore stability. To estimate the variation of stresses and strain in the ground surrounding the longwall panel finite element analysis modeling was used. The model included the geometrical design of the longwall panel including the depth, length of the panel, proper size pillars for a three entry system that is comparable to the current and future practice in south west Pennsylvania as well as West Virginia and Ohio. The model simulated the complex ground condition associated with longwall mining areas and quantified the factors affecting the wellbore stability, such as induced stresses and ground movements. The output of the numerical modeling of the longwall mining and related stress and strain calculation for the pillars and upper strata was used for developing a proper casing design according to API guidelines. The results indicate that the controlling parameter in wellbore stability is horizontal strain, and resulting deformation and ground displacement at certain depths above the coal seam. The modeling of the coal seam at depths of 100, 200, and 300 m shows that the maximum horizontal stress occur at X, Y, Z m above the coal seam, respectively. The magnitude of the maximum horizontal strain was estimated to be around 10-2, corresponding to displacement of about 100mm per 10 m long casing. Total deformation of about 250-350 mm was also estimated based on the modeling but the values used for design purposes was restricted to 100 mm due the lack of field observation of such high values of horizontal deformation. The review of literature shows that despite possibility of high horizontal displacement in such scenarios, the observed values in the field are typically within 150-200 mm even in extreme conditions and mostly below 100-150 mm. A casing design model based on spreadsheets was developed to carry out appropriate combination of casings. The design procedure was based on API guidelines and Pennsylvania department of environmental protection (PA-DEP). The wellbore completion was then performed using the values of strain and deformation obtained by the ground modeling. The output of the program included a wellbore completion design for 5 layer casing design, which included the collar, water, two layers of protection and the production casing to assure stability of the wellbore casing after mining activities. The results were compared to the current practice in wellbore design in south-west PA and the designs proved to be compatible. This study combined finite element analysis to develop an appropriate completion guide for Marcellus Shale gas wells in longwall mining areas.