Development of Numerical Models to Investigate Permeability Changes and Gas Emission around Longwall Mining Panel

Underground longwall mining of coal causes large scale disturbance of the surrounding rock mass. The disturbance can increase the rock mass permeability through a reduction on the stress as well as formation of new fractures in the rock. Methane gas contained in the disturbed rock mass can migrate towards the low pressure mine workings and present an explosion hazard. This paper describes the application of a finite difference program to develop a geomechanical model that predicts permeability changes within the rock mass. The calculated permeabilities are used as input to a reservoir simulator that models methane desorption from the coal matrix, methane release from the rock layers and flow towards the mine excavations. The model also considers the basic characteristics of the mine ventilation network. The geomechanical model uses empirical relationships between fracture permeability and stress to calculate permeability changes around a longwall face. The extent of rock failure is determined using a strain softening model that considers both rock matrix and bedding plane failure. The cave rock (gob) is modeled as a compressible, granulated material. The calculated horizontal and vertical permeabilities around the longwall face are averaged and used as one of the inputs to the reservoir model. The reservoir model was developed and calibrated against records of methane flow at a study mine in southwestern Pennsylvania. Good correlation between actual gas production and model outputs has been achieved. The modeling approach provides a basis for estimating methane inflow and optimizing control measures. control techniques employed in the coal mines of the Eastern United States. A two staged approach has been followed to develop models of methane emissions and flow around longwall mines. The first stage has been to make use of the FLAC2D [2] finite difference code to simulate the geomechanical response of the rock mass to longwall mining. The program was used to calculate the stress changes, extent of rock fracturing and bedding plane shear. The output of the FLAC models was used to calculate likely permeability changes, based on empirical relationships. The permeability distribution was then used to develop inputs for the 3-D compositional reservoir simulator (GEM) by Computer Modeling Group [3]. The reservoir simulator was used to develop a model of several longwall panels in a study mine against which the model was calibrated by history matching of gob vent borehole (GVB) methane production. The model has been used to evaluate the relative merits of methane drainage options. 2. PERMEABILITY OF COAL MEASURE ROCKS Coal measure rocks typically consist of interbedded shale, siltstone, sandstone, claystone and limestone. The permeability is anisotropic because of interlayered low and high permeability strata [4]. Sandstone and limestone beds have the highest permeabilities and act as aquifers, they promote horizontal flow and have relatively high storage capacity. The shales act as aquitards, having low permeability, but may contain fractures and bedding planes that enhance permeability. Thin clay bands can exist within the measures that act as aquicludes. Coal is highly transmissive, but typically has poor storage. The overall permeability of the strata tends to decrease with depth, a decrease of 1 order of magnitude of the permeability of coal for every 25 MPa increase in overburden loading was reported by Sparks [5]. Permeabilities measured in the field can be dominated by fracture flow and are typically highly variable. Tests are usually performed in vertical wells, which provide a better indication of horizontal permeability than vertical. Test results published by Hasenfus et al. [6] in strata above the Pittsburgh coalbed showed that the permeability can vary by several orders of magnitude in different sections of a vertical borehole. They measured hydraulic conductivties of 7x10cm/s in sandstone near the ground surface. Brutcher et al. [7] tested the conductivity of a sandstone aquifer and found the values to vary between 10 and 10 cm/s while shale conductivity was one order of magnitude lower. Booth and Spande [8] reported hydraulic conductivities of 9x10 cm/s for sandstone in Southern Illinois. Matetic et al. [9] reported permeabilities of 7x10 cm/s in shale materials near surface and 7x10 cm/s for sandstone in southeastern Ohio. The field measured hydraulic conductivities all fall within published ranges for sandstone and shales and the upper limits fall in the range that one would expect for jointed rock. Coal measure rocks in the eastern United States are typically poorly jointed, but contain bedding planes that act as discontinuities which allow horizontal flow. Vertical flow is constrained, especially by thin clay layers. 3. EFFECT OF LONGWALL MINING ON PERMEABILITY Longwall mining induces both stress increases and stress reductions in the surrounding rock. Around the edges of the panel, the stresses will increase, while the rock directly above and below the extracted panel will experience significant stress relief. In addition, the increased stresses can cause fracturing of the rock mass. These changes can have a profound effect on the rock mass permeability. Field studies have shown both increasing and decreasing changes of approximately one order of magnitude in the hydraulic conductivity of the rock mass above a longwall panel in Pennsylvania [6]. 3.1. The effect of stress changes Changes in stress can produce large variations in the permeability of laboratory and field scale rock. In the laboratory, the permeability will initially decrease as rock is subject to increasing loads, but the permeability will increase as the rock reaches its peak strength, and permeability attains a maximum during the post failure stage [10]. The permeability of the field scale rock mass is affected by the closure or opening of fractures under changing stresses. The equivalent permeability can be related to the fracture aperture and fracture spacing [11, 12, 13]. Some researchers have related changes in permeability directly to changes in the confining stress, and found an exponential relationship between permeability and stress [14]. 3.2. The effect of fracturing In addition to stress changes, fracturing occurs in the rock mass in the vicinity of a longwall panel. Three zones can be distinguished in the roof rocks, shown in figure 1: the caved zone, fractured zone and the bending zone [15, 16]. The caved zone is created as the mining face advances and the immediate overburden falls and fills the void created by the extraction of the coal. The caved zone extends upwards, 3-6 times the extraction thickness. The caved zone is characterized by irregular rock fragments that may have rotated relative to their initial locations, resulting in relatively high void ratios and permeability. Laboratory tests have shown that the void ratio can be in the order of 30%-45% [17]. As the face advances, the caved rock (gob) is re-compacted by the weight of the overburden. The amount of recompaction depends on the depth of overburden and the strength of the gob material. The permeability of the caved zone can be expected to be high but will vary as the compaction of the gob varies. The fractured zone is located above and around the caved zone and is characterized by near vertical fractures and bedding plane shearing caused by the passage of the longwall face [6]. Bed separation can occur in this zone. The fractured zone can extend 30 to 60 times the extraction thickness. In this zone, water drains directly to the caved zone and into the mine workings. Measurements of permeability in the fractured rock have shown up to forty fold increases in permeability [18]. Above the fractured zone is the bending zone. The rock is essentially un-fractured, but can experience shearing along bedding planes as they are deflected over the edges of the extracted longwall panel [6]. Bedding plane shear will affect the horizontal conductivity of the rock. Field observations have shown that water levels and ground movements occur up to 60 m (200 ft) ahead of an advancing longwall face. These movements can be associated with shear along weak clay filled bedding planes. Rock mass disturbance also extends into the floor of a longwall panel. The floor experiences stress relief as the longwall face passes overhead. The stress relief will be partially reversed as the gob is recompacted by the weight of the overburden. In addition, the elevated abutment stresses can cause deep seated fracturing of the floor rocks at the advancing face and around the stationary abutments. Floor gas emissions are not uncommon in longwall mines and can be explained by the failure and stress relief in the floor. 4. METHANE SOURCES AND CONTROL The sources of methane in longwall operations are likely to be: the coalbed being mined, overlying or underlying coal coalbeds and to a lesser extent the methane contained within the surrounding rock strata. Porous rocks such as sandstones are likely candidates for gas storage. However, Diamond et al, [19] reported that the primary sources of longwall gob gas resulting from the mining of the lower Kittanning were the coalbeds in the overlying strata. As much as 91% of the longwall gob gas originated in the overlying coalbeds, with those as high as 200 ft above the mined coalbed contributing gas to the gob.. Prior to mining, the gas and groundwater are in equilibrium and are contained by various clay rich layers. The disturbance caused by the longwall mining first dewaters the rock within the caved and fractured zones [4], this is followed by gas liberation mainly from the coalbeds, as the pore pressure drops. The stress relief and mining related fracturing can provide new pathways for the flow of methane to the mined excavations. Material balance calculations indicated that the volume of the gas drained from the strata directly overlying the longwall panel could only account for 40% of the total volume of gas vented by the ventilation system and gob vent boreholes [19]. The rest of the methane migrates from the adjacent formations Bending