If the initial phase delivers some positive results then further exploration to discover and define potential oil and gas deposits may take place. Exploration typically focuses on a smaller spatial area of interest than prospecting, which then becomes the focus of more intense investigation. Activities authorised by an exploration permit can also include further more detailed seismic surveying (usually 3D).
Drilling rigs range from small drills which can be carried in a backpack through to rigs which can reach several thousand metres depth. The material that is brought to the surface is examined and information about any minerals it contains is recorded.
In the marine environment, a mobile offshore drilling unit (MODU) is used to drill a well in order to assess whether oil and/or gas is located in a specific geological zone, and in the specific location, that has been identified as a “prospect” based on seismic surveying and other analysis. The type of rig used will depend on the depth of water, ocean conditions and other factors. The three main types of rigs used for exploration are:
- Jack up rigs – used in shallower water up to around 120 metres deep. The rig is towed to the location, and its legs are lowered to the bottom, to enable the rig to be raised above the water surface.
- Semi-submersibles – used in depths of 100 to 3000 metres. These consist of a floating, column-stabilised vessel, with a submerged hull. The rigs can be towed and anchored, or are able to maintain their own position with thrusters.
- Drillships – used in depths of up to 3000 metres. These are purpose-built ships with drilling and other equipment installed on-board. They function as a ship until they reach the drill site, where they are either anchored or maintain their own position with thrusters, while undertaking drilling operations.
Drilling a single well, may take weeks to a few months to reach the target depth, depending on various factors including the depth of the target, the rock types encountered and weather conditions. One or more wells may be drilled under a single exploration permit. Exploration permits require a certain number of wells to be drilled within a specified time frame.
In order to create an offshore well, an oil rig drills through the seabed and into the subsurface reservoir. There are a number of steps that are needed for drilling a well which are described below:
- Spudding – A metal tube is forced into the ground (this tube can be up to 100 metres long by 92 centimetres in diameter). This provides a casing which is the backbone of the well. The casing is lowered onto the seafloor by a “drill pipe”. More drill pipe is added over time to force the casing into the seabed until it reaches the targeted petroleum prospect. Some rigs use large hammers to push the casing deeper. Once the right depth is reached the drill pipes are pulled back to the surface.
- Drilling a hole for a second string of casing – A drill bit is lowered inside the casing. This is generally drilled between 300 metres and 1000 metres into the seabed. High pressure drilling fluids called “mud” is pumped down the centre as the drill is rotated. This fluid helps to keep the drill cool, provides lubrication, and carries chipped rock pieces away from the well. This “mud” is also the primary barrier for keeping hydrocarbons in the well during drilling. A second smaller casing (often 56 centimetres in diameter) is lowered down the hole.
- Cementing the casings in place – Cement is pumped down the drill pipe and back up between the two casings. This helps to cement the casing to the rock in the seafloor and is a vertical barrier on the outside of the casing.
- Connecting the Blowout Preventer and Marine Riser – A large underwater control valve known as the Blowout Preventer is placed on top of the wellhead. It is a critical piece of equipment that is generally fixed directly on top of the wellhead on the ocean floor and functions as the secondary barrier to controlling hydrocarbon pressures. The Blowout Preventer is connected by a Marine Riser to the drilling rig or drillship. Once these are installed, drill pipe is lowered through the Marine Riser and the Blowout Preventer. The Marine Riser allows the “mud” and rock cuttings to be pumped back to the surface and onto the drilling rig for processing.
- Drilling the remaining sections of the well – Steps 2 and 3 can be repeated to drill the remaining well sections. The rock cuttings and “mud” are circulated back up to the drilling rig.
- Conducting test logs – Once the correct depth has been reached, tests are conducted to see whether oil and gas is present, and if so the nature and extent of it. The information is logged. In some cases hydrocarbons will be brought to the surface to flow test the well.
- Once an exploration well is completed, it will usually be plugged with a cement plug and abandoned. In some situations, it may be temporarily abandoned so that it can be subsequently tested or produced from. Once the well is abandoned, the drilling rig will leave the drill site.
If the exploratory phase is successful then the operator will usually return later to drill appraisal wells to further understand the nature and extent of the field that has been discovered. In parallel to this, the operator will be working on development scenarios for the field, which will involve considering engineering, commercial, environmental and other factors.
Last updated at 1:41PM on February 25, 2015