Introduction to Oil and Gas Operations
Modern oil and gas exploration and production is a highly regulated activity in most regions of the world, particularly within the EU. Regulation of the industry is designed to ensure that the risk of oil and gas operations causing harm to either human health or the natural environment is kept to the absolute minimum. The following is an overview of the key elements involved in siting and drilling a shallow (i.e. less than 2,000 metres) onshore oil or gas well, within the EU.
Selecting a Well Location
Until recent decades, the lack of good quality subsurface data meant that many oil and gas wells missed their target. Consequently, much larger numbers of wells were required to develop a single deposit than would be required today.
Modern wells are placed using good quality seismic, thus significantly reducing the number of poorly located wells. Additional survey tools, such as gravity and aeromagnetic surveys, can be used to complement the seismic data, enabling the risk of error to be yet further reduced.
Directional drilling, whereby wells are drilled away from the vertical axis, gives greater flexibility in the siting of well locations so as to enable sensitive surface areas to be avoided. Drilling more than one well from the same well location means that several targets, spread over a wide area, can be accessed, thus minimising the surface impact.
Regulations and industry best practice are designed to ensure that the drilling operation itself is conducted so as to cause minimum disruption to the surrounding environment. Wells are drilled using rotary drilling. Drilling fluid or drilling mud is pumped down the drill string to lubricate the drill bit and carry away rock cuttings from the bottom of the hole. The drilling mud is typically a water-based mud made with bentonite, which is a widely-available and naturally occurring clay.
The first stage in drilling is called setting the conductor pipe. The hole is drilled to a depth of approximately 30 metres and steel pipe, called conductor pipe is set from the bottom of the hole up to the surface. The conductor pipe prevents any surrounding soil falling into the hole. The well is then drilled to a depth of approximately 200 metres and a second set of casing, called surface casing, is set in the hole inside the conductor pipe and again cemented in place from the bottom of the hole up to the top. The surface casing is sufficient to isolate the surrounding rock, and any aquifers within the rock, from the interior of the wellbore, and from operations conducted lower down in the wellbore. A blow-out preventer is then installed at the top of the well to prevent any danger from an increase in pressure lower down the well.
When the well has been drilled to its target depth, the drill bit is again pulled out of the hole and production casing is set in the hole, from the bottom up to the surface (inside the surface casing), and cemented in place in the same way as the surface casing. In deeper wells, intermediate casing may be set and cemented in the same way, after the surface casing is set but before the production casing (which in this instance is set inside the intermediate casing).
This system thus ensures that all hydrocarbons flowing from the productive zone, also known as the payzone, are safely and securely isolated from the surrounding rock and can only exit at the surface. It is also designed to ensure that there is no escape of fluid up the outside of the wellbore, nor is there any compromise of the water-bearing zones.
Completion and Putting into Production
Completion of the well occurs when the production casing is perforated in the area of the payzone, typically using very small explosive charges. This enables hydrocarbons to flow into the production casing and up to the surface. The well may then be put on test to determine if the flow rate is sufficient to justify putting the well into commercial production.
Following the conclusion of drilling operations, drilling mud will be recycled and reused where possible. Any waste materials such as drilling cuttings are removed from the location by licensed contractors and disposed of in approved facilities in accordance with applicable regulations. The drilling rig is removed as soon as practicable in order to minimise expensive rig hire charges.
If a well is considered successful enough to be put into production, then additional facilities may be installed if required, for example to separate natural gas liquids from natural gas. In the case of a gas well, a pipe to transport the gas will be laid underground, to connect into the local gas transmission network. In the case of an oil well, oil will typically (in the case of shallow wells, which generally produce lower volumes of hydrocarbons) be removed to a refinery by truck. Any part of the well location which is no longer required following putting the well into production will be reinstated to its original condition.
Plugging and Abandoning
At the end of the productive life of a well, or if a well is drilled which proves unsuccessful, the well will be plugged to make it permanently safe, in accordance with applicable regulations, and abandoned. Any remaining equipment on the well location will be removed and the remaining well location will be reinstated. Topsoil which was set aside when the location was first constructed will be replaced and the land will be reseeded or replanted. Typically, with modern techniques, there will quite quickly be almost no visible indication that well operations had ever even been conducted.