HOW IS A GAS WELL DRILLED?
Once the gas trap has been mapped using seismic measurements drilling might be planned to test the presence of gas and produce the gas. Before the drilling can start a suitable drilling location must be selected and the drilling site prepared. A drill site is typically the size of a football field, and is normally selected a good distance from any neighbours to cause minimum disturbance. The wellbore can be steered during drilling so there is flexibility in selecting the surface drilling site. After the selection the site will be prepared. This includes removing the top spoil and putting out a water tight membrane to ensure that no fluids can escape from the drilling location into the ground water. On top of the membrane a gravel layer is put to create a base for working. Water and power supply as well as good access road may also be established before the site is ready for the drilling rig.
The natural gas in the Zehdenick area is located at approximately 4,000 meters depth. A drilling rig to drill a well to more than 4000 m is about 35-50 m high or about half the height of a wind turbine (picture to the left). Drilling a well to more than 4000 meter would take 2 to 4 month and once the well in completed the drilling rig will be removed again, the gravel and membrane removed and the top soil but back to resort the site back to original again.
But how is the drilling done and is it safe? Drilling rigs used for gas wells normally use the Rotary process. This mean the whole drilling string or pipe from the rig at the surface to the drilling bit is rotated. The drilling bit at the end of the pipe works like a chisel braking through rock and thus deepened the well when rotated. Drilling mud is pumped inside the drilling pipe into the well to remove the rock that has been broke away by the bit and returns to surface outside the drilling string where the mud is separated from the rock and pumped down the hole again.The mud acts not only to remove the drilled rocks but also for cools the drilling bit and balance the pressure in the well. At 4,000 meters temperatures of 100-120° C and pressures of 400 bars are expected (Car tyres have approx. 3.5 bar pressure).
To protect ground water and other water bearing layers as well as provide safety during drilling and production, different pipe strings called casing are introduced into the borehole and cemented tight (picture to the right). At the start of the well a large diameter (75 cm+) drill bit is used. A hole down to approximately 100 m is drilled and the first pipe with a slightly smaller diameter is put into the drilled hole. Once this pipe has been put in the ground and cemented tight the next hole section is drilled though this first pipe. This process is repeated several times down toward the gas reservoir, and each time a pipe is set the hole size gets smaller. Each pipe section goes back to the surface and is pressure tested to a large pressure to make sure they are tight. In the end when the well reach the gas reservoir there is several pipes, at least 5, inside each other from large diameter to small diameter and all a connected back to the safety valve at the surface. This will ensure that there is no possibility for fluids or gas from inside the well to be mixed with fluids in layers outside the well. The many pipes however is not only a protection of the ground water or any other water layers between the surface to the gas reservoir but also necessary for a safe and controlled production of the gas to surface. The pipes cemented into the ground will not be visible on the surface but are all tie back into the safety valves that a located on the surface and will remain in the well also after the drilling rig has left. The safety valve is typically a couple of meters high.
After drilling and testing of the well the drilling rig will be dismantled and removed. If the well is used as a gas producer a site of up to 10 by 10 meters will remain with some 1-2 m high pipes and safety valves called a Christmas tree installed over the gas well, and a pipeline buried in the ground that lead the gas away.