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Dewatering Process and the Lydia Pilot Project
De-watering coal seam gas wells – During the month of May we will start de-watering the newly completed LP-2, LP-3 and LP-4 wells, which are to be completed for production during the month. The de-watering process can take several months, although we have minimised the distances between the pilot production wells to provide for accelerated de-watering of the Walloon Coal Seams. Normal well spacings for coal seam gas wells tends to be about three quarters of a kilometre between wells, in the Lydia Pilot area this well spacing has been reduced to 200 meters, See figure 1 below: Figure 1 – Layout of the Lydia Pilot
The Monitor well, Lydia -1 will have three transducers designed to monitor the individual pressures in the each of three major Walloon coal beds. The reductions in pressure that we will observe in the monitor well will correspond to water being removed from each group of coal measures. As water production will give an indication of permeability for each major lobe we can then estimate in a gross sense, the pressure declines that we will observe as we dewater the Walloon Coal measures. We must insure that the drawdown of the individual Walloon Coal measures is gradual to prevent foiling or self - cavitation of the coals so that the downhole PCP pumps which are used to remove the water from the Coals are not mechanically damaged by cavitation. De-watering and de-pressurising the coals allows for the coals to begin to release methane which is trapped within the coal matrix and also present in the cleat structures of the coals. The cleats or tiny fissures in the coals allow for permeability of the coals and fluid movement both water and gas from the coal matrix into the well bore with water transferred to a holding pond. See Figure 2 below illustrating the cleat structure of Coals and face and butt cleats. The downhole configuration of the coal seam gas production wells is similar to that configuration being used by other major coal seam gas companies with wells being underreamed in the coal sections and completed with a down hole progressive cavity pump (PCP). The pilot production wells will have continuous monitoring production test skids to monitor pressures and water and gas production rates. Water will be produced up the tubing and the gas will flow up the casing annulus) to a gas line which will be connected to a flare for the pilot area and burned during the testing period. Timing for de-watering can take a few months to a year depending on a number of variables including the permeability of the coals, the strength of the water drive in the coals and any re-charging of water from the surface. We are anticipating that due to the close spacing of the wells and with the information that we have: initial gas may appear in the pilot production wells in a rather short period of time (after two to three months of sustained production), again depending on the factors above.  Figure 2 – Generalised diagram of a coal seam gas completion.
Fluid containing both water and gas, flow through the coals and any permeable beds on top or bottom of the coals into the under-reamed area of the completion interval, and through the slotted liner above the pcp pump. Gas will breakout in the tubing /casing annulus and due to the density differences move up in the annular space between the tubing and the casing, while water will continue to move through the bottom of the PCP pump and be produced to the holding pond for evaporation and storage. First water production will move through the cleat geometry in the coal matrix and eventually as pressure drops within the coal matrix gas begins to move through the coal cleat matrix into the well bore.  Figure 3 –Cleat Structure of Coals
Water and gas flow out of coal cleat structure De-watering the coals can cause the coals to shrink, crack, and often leads to changes in the apparent permeablities of the coals. Methane is liberated from both the cleat structure and the matrix of the coal itself. As can be observed by the attached figure called the Langmuir curve, more gas is liberated from the coals as pressure of the coal measure continues to drop. See Figure 3 Above. As pressure in the Coals decreases more gas (methane) is liberated. A core sample is planned for LP-2 in which the cores will be sent to a laboratory and a curve such as this one below will be constructed for the Lydia Pilot area. Gas evolves from the matrix of the coals and migrates through the cleat structure and move towards the pressure sink created by depressurising the coal seams by the removal of water in the Pilot production wells. The behaviour of the generation of gas by reductions is often characterised by laboratory measurements by taking a sample of the core and allowing the coal core to “desorb” or liberate gas at standard conditions See figure 4 for a typical Langmuire curve for a coal. The Vertical axis represents the amount of gas liberated at standard conditions over time. In the Lydia Pilot area we will be obtaining a core from the Walloon Coal Measures to construct this curve for our use in determining recoverable gas and more accurately refining original gas in place (OGIP) numbers. The first appearance of gas in the production stream occurs when desorption pressure occurs and gas begins to liberate from the coals.  Figure 4 – Langmuir Curve- Stitch -1
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Dewatering Process
Dewatering Process
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