ABSTRACT
A detailed underground mapping survey to determine the impact of foam stimulation treatments on the mining environment was conducted at an underground mine in Indiana County, Pennsylvania. Three vertical boreholes intercepted unmined coal south of active workings. The boreholes were geophysically logged, and cased to the top of the Upper Freeport Seam. Nitrogen-generated foam was injected to fracture the seam, and 20/40 mesh sand was used to prop open the fractures. The foam was tagged with fluorescing, ultraviolet paint pigment to enable its pathway to be mapped with a short wave ultraviolet light.
As mining advanced toward the holes, the lithologic characteristics of the coal seam, roof, and floor were mapped, along with the location of roof falls and deformational features. The roof strata consisted of thin, irregular pods of gray shale and siltstone and of an overlying sandstone, which had an erosional contact with the seam in many areas. Roof falls and “slips” occur where the roof’s lithologic character is transitional from shale to sandstone. The falls occur with time as the roof weakens along slip planes and bedding planes where individual lithologic characteristics are not thick enough to support themselves.
Paint pigment from the treatment fluid was distributed in horizontal planes at the coal-roof interface and along the top of an in-seam rock binder up to 225 ft (70 m) from an individual borehole. Propping sand was found only on the top of the rock binder and in vertical fractures in the lower bench of the seam, near the boreholes. Fluorescing vertical fractures occurred predominantly in the friable lower bench and extended outward for a distance of up to 160 ft (50 m) from the boreholes. No fractures penetrated the roof or floor strata. No roof falls occurred near the well bores.
