Mining Publication: Reservoir Diagnosis of Longwall Gobs Through Drawdown Tests and Decline Curve Analyses of Gob Gas Venthole Productions
During longwall mining, fracturing and relaxation in the gob creates new and highly permeable flow paths. Methane inflow from the gob into the mining environment is influenced by the magnitude of fracturing and the extent to which the fractures stay open during mining. Singh and Kendorski evaluated the disturbance of rock strata resulting from mining and described a caved zone that extends from the mining level to 3-6 times the seam thickness,a fractured zone that extends from the mining level to 30-58 times the seam thickness, and a bending zone where there is no change in permeability that extends from 30 times the seam thickness to 50 ft below ground surface. The characteristics of fracturing and the subsidence of overburden were revealed through predictive techniques and field studies. It was concluded that rock failure leading to increased hydraulic conductivity in the gob was initiated by high compressive stresses ahead of the face with the fractures subsequently opened by tensile stresses behind the face. Gas, particularly methane that is contained within the gob, will be released over time as mining progresses and is a big contributor to ventilation emissions if not controlled. Relaxation of the roof rocks, ventilation pressure and the associated fracture connectivity allow gas to flow from all surrounding gas sources toward the mine workings, which eventually may create an unsafe condition for the underground workforce. As longwall mining continually progresses, the caved zone in the gob gradually consolidates to support large loads resulting from the overburden weight . This consolidation results in a reduction in the initial porosity and the associated permeability, where prevailing high permeability pathways in the consolidated gob still affect the flow of methane from surrounding sources into the gob and into the mine. Thus, an understanding of resultant reservoir properties of gob material is very important for developing adequate methane control strategies. One common technique to control methane emissions is to drill vertical gob gas ventholes (GGV) equipped with exhausters into each longwall panel to capture the methane within the overlying fractured strata before it enters the work environment. Gas production from GGVs may exhibit variable gas quality. In the early stages of production, the gas quality is generally high (>80%) after a hole is intercepted by the longwall. Relatively high production rates with high methane quality are usually sustained for few weeks. Later in time, especially towards the end of the panel mining or after the panel is completed, gob gas production may exhibit decreased methane levels as ventilation air is drawn from the active mine workings. When the methane concentration in the produced gas reaches 25%, the exhausters are commonly de-energized as a safety measure and the holes allowed to free flow. Some of the barriers toward effective management of methane in mines through use of gob gas ventholes are the complexity of the gob environment, the involvement and interdependence of multiple influential factors, and the lack of knowledge on interactions of the GGV with the gob reservoir. Improvements in venthole gas drainage evaluation and prediction capabilities for site-specific mining conditions and circumstances can address longwall gas emission issues, resulting in ventholes designed for improved gas capture. At this juncture, well test and production analyses methods, such as multi-rate drawdown  and decline curve analyses, which are applied in the petroleum and natural gas industry and in coal bed methane reservoirs, can be effective to understand the transport characteristics of the gob reservoir and to forecast production potential of gob gas ventholes. In this paper, the production rate-pressure behavior of a gob gas venthole (GGV) drilled over a longwall panel was analyzed by using multi-rate drawdown and decline curve analysis techniques. Analyses were performed for a pseudo-steady state (PSS) flow period, which started when mining of the panel was complete and manifested itself with the production decrease from gob gas ventholes. In PSS, both drawdown and decline curve analyses could be carried out simultaneously . Multi-rate production drawdown and decline curve analysis techniques were used to diagnose the properties of the gob reservoir and GGV such as skin, permeability, radius of investigation, flow efficiency, damage ratio and production decline rate of the venthole.