Underground Testing to Determine Reservoir Characteristics
Underground tests to determine permeability and pressure are generally conducted for water in all mines, or gas in coal mines.
Being underground provides a great opportunity to understand the geology and in particular the structural features that control permeability. In the case of coal the ability exists to map the seam in great detail. The salient features are the coal bedding, including potential sealing bands and most importantly the cleating. The cleats are the sub vertical fractures within the coal seam. The cleat spacing defines the distance over which diffusion must take place before Darcy flow in the cleats can take place. The nature of the cleats themselves is also most important – do they have infill of clays or carbonates? Such infill can seal a seam very effectively. The hardness variation of the coal is also important as the coal’s mechanical properties will strongly affect how the coal seam responds to changes in effective stress brought about by drainage. Such changes are caused by a lowering of fluid pressure and the shrinkage of the coal as it gives up gas and dries.
Underground Test Methods
Incremental Flow Testing to Determine Homogeneity of the Seam
Coals are very inhomogeneous and the flow measured from a borehole usually is not released evenly over its length. The flow depends on many factors such as the coal ply in which the hole is drilled, cleat direction and intensity, stresses within the coal, direction of the hole and major jointing within the seam.
It is therefore desirable to measure the flow coming not only from a borehole but from each section of a borehole. The process shown in Figure 1 can be used to determine the flow at various locations along shorter holes. Here a single packer is pushed to the back of the hole and then inflated, before the flow is measured. The packer is then deflated and moved out by an incremental length, which is typically one drill rod, before being re-inflated and the flow measured again. The flow per unit length is therefore the difference between measurements, or the slope of the flow versus distance curve. This process may be repeated over the length of the hole to get a complete flow profile.
In longer holes this approach cannot be used because the change in flow over a useful change in measurement distance is small compared to the total test section flow. The system shown in Figure 2 is therefore used. In this system, straddle packers are inflated to isolate a section of hole and measure its flow. The flow from the hole behind is bypassed past the packer system.
The quickest experimental method to determine the permeability of gassy coal seams is to use the system shown in Figure 3. Here a hole is drilled and a single packer is inserted beyond the rib side zone and used to seal the hole, with all flow coming down the test tubing, which is connected to the packer. The flow of water and gas is measured over a period and then the valve on the tubing is closed and the pressure build-up is monitored. It is usual to repeat this for a second flow and build up period. The test technique can be conducted in holes of several orientations so as to determine the directional characteristics of permeability. If the flow is just water semi-analytical techniques can be used. If however a coal seam is being tested the exact nature of the permeability must be approximately determined by history matching, using a simulator, because of the complex behaviour of coal seams.
Long Term Permeability and Material Balance
Because of the variations in permeability throughout production, and the need to maintain a check on material balance, it is very useful to conduct longer term drainage tests. The essence of these is to compare the gas produced from the seam with the change in seam pressure, using the latter to arrive at a gas content through the sorption isotherm. Thus the material balance equation may be applied.
Gas in Place = Gas Initially in Place – Gas Drained + Other Sources
The Gas in Place is determined by pressure sensing and the use of the sorption isotherm.
The Gas initially in Place is either measured directly, or via pressure.
The Gas Drained is obtained by flow measurement.
The Other Sources are gas gained or lost through the ribside or entering the seam from the roof and floor, frequently via major joints.
The ideal embodiment of the test layout is shown in Figure 3. Here a central hole is drilled and fitted with pressure sensing points, either through the use of a multiple packer assembly (Figure 4), or by grouting the hole with either pressure sensors or sensing lines (Figure 5). Two flanking drain holes are then drilled on either side of the pressure sensing hole and the flow from these is measured.
An example of the correlation between the gas content derived from seam pressure measurement with sorption isotherm, and the content derived from the material balance calculation is shown in Figure 6. This example shows a very good match, which indicates that an understanding of the coal seam drainage characteristic has been achieved. The bulk permeability of the coal may be determined through history matching. Note that not all cases show such a good match and in these cases the Other Sources term needs to be investigated.