Borehole Permeability Damage and Its Impacts on Gas Drainage

Drainage of gassy coal seams using boreholes is essential to many coal mining operations. The effectiveness with which boreholes drain the coal gas is a complex function of seam and borehole properties. A complication to the drainage process occurs when the fluid permeability is lowered in the coal near the borehole. This zone of low permeability around the borehole, known as skin in well testing terms, acts to reduce the rates of gas and water drainage from the seam. A positive skin represents a reduction and a negative skin an enhancement to permeability near the borehole.  A positive borehole skin could result from a number of mechanisms associated with the drilling of the hole or its subsequent operation. While there is extensive information on the role of and mechanisms responsible for skin in conventional oil and gas boreholes, little is known of skin with coal gas drainage boreholes.

This project has investigated borehole permeability damage in gas drainage boreholes for coal mining. The potential for damage is site specific, a result of a number of potential mechanisms some of which may result from the process of drilling the boreholes, others associated with the actual drainage of gas and water from the coal seam. There is evidence that damage could occur with gas drainage boreholes and that it could have a significant impact on the drainage process. In particular the study of Jeffrey et al. (2005) found significant damage existed in drainage boreholes at Dartbrook and that this could be overcome through fracture stimulation. It was found that a significant proportion of vertical wells drilled from surface for coal seam methane characterisation were damaged. In contrast a detailed set of well tests carried out by Wold et al. (2007) on underground in-seam boreholes at West Cliff found little evidence of damage; however these boreholes had been drilled in a coal seam drained of gas and water.

The following damage mechanisms were identified in this study as being particularly relevant for coal;

For drilling fines and fluid migration to significantly impact the near borehole permeability overbalanced conditions are required within the borehole during drilling. This is less likely to occur with underground in-seam boreholes since these are drilled with the collar of the borehole open or subject to only low pressure. Blockages around the rod string may increase the pressure of the returning fluid over portions of the hole. In addition, during normal drilling operations, gas is produced from the coal around the borehole which, in turn, lowers the local gas and water pressure in the seam. Then, higher drilling pressures generated for short perioids would be sufficient to force water and fines into the near borehole zone, perhaps creating a damage zone. For medium radius surface to in-seam boreholes, overbalanced conditions can occur and efforts are required to reduce this during the drilling process. This is particularly relevant for these boreholes since they take longer to drill than vertical boreholes, with long periods spent drilling in the coal seam and thus there is more time for fines migration into the coal to occur. However field evidence of damage for medium radius surface to in-seam boreholes was not available to the authors of this report. It is recommended that greater monitoring is required of pressures along the borehole during drilling and that the reservoir pressure be ascertained prior to the start of drilling.  

Mineral precipitation can occur when certain combinations of solutes are supersaturated. Calcium carbonate precipitation could occur as groundwater flows towards a borehole where the pressure drops and CO2 comes out of solution, resulting in precipitation. For a number of mines, with mineralised cleats and high CO2 gas contents, the potential exists for precipitation. While difficult to prevent happening it can be remediated through the use of acid treatments.

Gas and water blocking is a result of the presence of free gas within the cleats acting to impede the flow of water into the borehole, or vice versa, water impeding the flow of gas. Since gas drainage requires the combined flow of water and gas to lower the pressure, at least in the early stages of draining a virgin coal seam, there is an optimal state where maximum gas drainage can be achieved over the required drainage lead time. However to identify this would require a meaningful characterisation of the gas and water relative permeability relationships for the target coal seam.  

Initial drilling of the borehole results in a redistribution of stress around it, generally leading to higher compressive stresses in a zone extending out to about two borehole diameters. During drainage, drawdown of the pressure will act to further increase the effective stress in the vicinity of the borehole. However gas desorption also occurs which counteracts this effect through matrix shrinkage. It was found that the permeability decline will be greatest in deeper coal seams where initial pore pressures are higher, where the beneficial effect of gas desorption is outweighed by the effects of pressure.  This behaviour is dependent on a range of coal properties but in particular the magnitude of the sorption strain and the geomechanical properties.  For a coal seam where the gas content is undersaturated and pressure drawdown does not lead to gas desorption (at least until the desorption pressure is reached), the permeability decline due to effective stress will be greatest.