Since the end of the era of “easy oil”, the oil industry has begun the search for hydrocarbon deposits in areas where until a few decades ago was not profitable due to its high cost and risk: offshore. The search for oil (exploration or prospecting) below the seabed research involves three phases of research:
The first phase consists in the acquisition of bibliographic, seismic, bathymetric, etc. information about the area for which it has been obtained the exploration permit.
In the second phase of exploration, the seismic acquisition,
you start by searching for the existence of underground geological formations in which potentially has been trapped gas and / or oil (geological traps) by sampling seafloor sediments and the technique known as “seismic acquisition” or “sonar exploration”.
To do this airguns are used, towed by a vessel, firing into marine waters “bubbles” of air at high pressure are used. The bubble burst produces a loud sound that penetrates up to 7,000 meters into the ocean ground. The sound waves emitted have a sound level of 249-265 decibels (dB) and a frequency between 10 and 250Hz. A comparison is the sound emitted are 10,000 to 100,000 times louder than the engine of a jet, which emits a noise of 140dB. Part of the acoustic energy is reflected and travels back to the surface where it is recorded by hydrophones called “streamers”. This information provides geologists details of rock formations situated below the seabed, as if it were an ultrasound.
Away-Explosions occur continuously over 24 hours a day, seven days a week, for a total period of about four months.
The world fleet of seismic vessels currently consists of about 100 vessels.
|Audio of a compressed air gun as those used in the tests|
|Airguns fired every 10-15 seconds. They can be heard thousands of miles away|
If the polls reveal the existence of geological traps, at this early stage it’s not possible to ascertain whether they contain oil, what kind, in what quantity and whether they have enough quality to be commercially exploited. To find out for sure, it then passes to the second phase of exploration: drilling boreholes for oil tasting.
Third phase of exploration: perforation
Drilling of boreholes for a tasting of hydrocarbons is not limited to a mere sampling. In many cases you drill (in the case of Cairn Energy Project at depths vary between 50 and 2000 meters below sea level) the same well, which is later used for extraction. Normally, kilometres of rocky substrate are drilled under the seabed, to determine the presence, quality, quantity and type of hydrocarbons trapped in potentially exploitable geological formation detected in the preliminary investigation.
The result of this type of poll is therefore, a full well that would connect a drainage system and exploitation of oil or gas. Depending on the results of the tasting, the well is sealed and left or temporarily sealed meanwhile waiting for the exploitation permits.
This is also the most dangerous phase, most expensive research and highest risk to a possible catastrophic spill. Between 1959 and 2011 there have been drilled 688 exploration hydrocarbon wells in Spain. Only 25 of them have been exploited and of these, only 14 are still in use.
Examples of waste in case of drilling exploratory wells are the cases of BP platform in the Gulf of Mexico in April 2010 and spills in 2009 by Repsol with Bass -1 -D5 Montanazo well near the coast of Tarragona. The three wholes were explored to depths more than 600 meters below sea level, which is in depth water.
Should get interesting results for the company, the next step would be the commercial exploitation.
For this the company will need new licenses, as those currently relate exclusively to prospecting. If obtained, the wellhead (diameter between 50 cm and 2 m) would be connected to a marine platform which must evacuate gas or oil with the help of an underwater vessels or piping connected to the coast during the life of the well, which is usually between 10 and 20 years.
During the operation of a well, the risk of spills is high and systematic oil pollution occurs. Proper maintenance of the facility over its lifetime is essential to minimize spills and prevent the arrival of oil slicks to the coast. But this maintenance is very expensive, and hence on the Casablanca oil-rig off Tarragona have already produced several discharges caused by poor maintenance of facilities.