Borehole Instability in Malmberget Underground Mine

Excerpt

Borehole instability causes borehole failure, presenting a challenge to the drilling industry (Muller et al. 2009; Zhang 2013). The borehole stability of Marcellus shale wells in long wall mining areas in southeast Pennsylvania, West Virginia, and eastern Ohio has been evaluated by Wang et al. (2014). Here, the ground deformation caused by coal mining generates large ground movement and creates complex stress changes in subsurface rock. This, in turn, triggers interruptions in the operation of the borehole well causing safety and environmental concerns. Borehole walls may fail when the surrounding stress exceeds the tensile, the compressive, or the shear strengths of the rock formation, whichever is reached first (Zhang et al. 2003). Horizontal well borehole stability is analyzed by the new in situ stress prediction model in shale gas reservoirs (Yuan et al. 2013). Besides this, in hard rock underground mining, such as sublevel caving (SLC), uncharged and undetonated blast holes reduce specific charge, result in poor fragmentation and may even lower ore recovery (Zhang 2005). It is often found that boreholes are blocked by stones and the charge work has to stop in the field. In many SLC rings two or three boreholes in each ring are often broken or blocked by either stones or pieces of concrete according to the mine’s production archives that contain daily notes on various production problems met by the miners. In order to get detailed information on the borehole problems occurring in LKAB’s two mines, a preliminary investigation was carried out by Kangas (2007) with a mini-video camera, showing that typical borehole problems were borehole deformation and boreholes jammed by stones. Due to various reasons, the above investigation rested for a while. This study is a follow-up of the previous investigation made by Kangas (2007) in more ore bodies and more boreholes. The main objective of this study is to identify various instability problems of boreholes by field filming in different ore bodies in the mine. …