Gas exploration and production is a complex process, but can be broken down into a number of distinct stages. The initial geological studies are followed by drilling, well testing and, if an economically producible amount of gas is discovered, the construction of a gas production installation.
Gas is held in strata of porous rock capped by a non-permeable formation. In the Netherlands, this kind of rock formation is found between 2,000 and 4,000 metres beneath the earth’s surface. Rock formations can be studied by seismic survey, which involves firing a shockwave through the earth’s crust and collecting the data reflected back. On land, explosives can be used to create the vibrations. At sea, specially equipped seismic survey vessels are used.
Different types of rock have a unique seismic fingerprint. The depth of the rock is deduced from the time between the initial shock and the reception of the return signal. Computers can translate the seismic data into human-readable maps of the subsurface.
Total E&P Nederland’s geophysicists analyse these maps to identify the formations which may be gas-bearing. Certainty on whether a formation does in fact contain gas can only be obtained by drilling.
A drilling rig is installed temporarily to drill a test or exploration well. A rotating drill bit bores into the subsurface and the cuttings are removed by the continuous pumping of a fluid (‘drilling mud’) into the borehole. The mud can be reused after cleaning. A steel pipe is inserted into the borehole to ensure that the well does not collapse.
Depending on the drilling depth and the formation type, it can take from three weeks to three months to drill a well. Once the target layer of rock has been reached, a special drill bit is used to take samples of the rock. Electrical measurements are taken in the borehole, and laboratory analysis of the rock samples will confirm the presence of gas and provide information on its quality.
A production test provides a clear idea of the production capacity and the size of the reservoir. This is done by allowing the gas to exit freely from the well and taking measurements. For safety reasons, this gas is immediately flared off. It generally takes one week to test an exploration well.
If an economically producible amount of gas is discovered, more wells will be drilled (‘development wells’) to ensure optimal production of the reserves. In the North Sea, the wells are connected to production platforms which in turn are connected by pipeline to the gas treatment platforms.
The gas is treated to remove impurities, such as other hydrocarbons and water. It is then brought to the required pressure for injection into the pipeline network for transport to shore.
A gas field normally has a lifetime of between ten and thirty years. At the end of production, the offshore production site has to be removed and returned to its original condition. The ability to recycle various parts of a platform is taken into account at the design stage. The bottom of the borehole and the pipes are sealed using cement plugs.