Surveying the Surface Before Drilling

When geologists identify a potential source of hydrocarbons, they use local and regional studies to assess whether it is worth drilling. They study the subsurface to more accurately determine if a drilling operation would be profitable. Once they have decided to drill, technical teams analyze the site to find the best location to install the necessary equipment.

Understanding the Drilling Area with Geology and Tectonics

Before drilling, seismic reflection (a technique that provides 3D mapping of the subsurface) is used to take images of the subsurface. Geological
cross-sections are also taken - these provide a graphical representation of rocky layers deep underground. However, all this data is not enough to help decide if it is worth drilling. Further extensive surveys must be carried out.

Many surveys have to be carried out before we decide whether to drill.

First, geologists carry out regional surveys.

   • They examine the structure and type of surface rock in the relevant region.
By extrapolating this information to rocky layers deep underground, they improve their understanding of the drilling site. Geologists also retrace the sedimentology of the area, i.e. they reconstruct the conditions under which the rocks were deposited over time and the evolution of the rocky layers that potentially contain hydrocarbons.

   • They also use the results from neighboring drilling operations carried out underground or under the seabed. If the drilling operations pass through rocks likely to contain hydrocarbons, and if they reveal the presence of oil or gas, geologists check to see if these results can be extrapolated to the new drilling area. For example, they try to find out if a reservoir rock (i.e. a rock that has the potential to contain hydrocarbons) extends as far as the area of interest intact. If not, they try to understand why previous drilling operations have not provided any results and what impact this could have on future drilling operations.

   • Geologists also survey the region's tectonics, i.e. the movements of the earth's crust that may have affected the rocks. They do this to trace the conditions under which any hydrocarbon traps may have formed.

After the regional surveys, geologists carry out local surveys, interpreting large volumes of seismic images and in-depth maps. Thus they can identify potential oil and gas reservoirs, and possibly ascertain their configuration and the amount of hydrocarbons they contain. These potential traps are known as prospects.

Other local surveys may also be carried out, such as looking for hydrocarbon traces on the surface to confirm whether there are oil or gas reserves underground.

Pre-Drilling Assessments- A Tricky but Essential Stage

Exploratory drilling costs at least € 3-4 million on land and € 20-€ 60 million at sea. In the case of very deep or difficult drilling operations, this cost can rise to € 100 million.

If the exploration results in the discovery of extractable hydrocarbons, the investment pays off in terms of oil and gas production. But if it does not reveal any commercially viable oil or gas reserves, the company loses its investment. This is why the decision to drill is not taken lightly. On the contrary, it requires precise assessment that takes the uncertainties and risks of the operation into account.

Technical information from regional and local surveys is collated to help make the best possible choice. Therefore, for each prospect, geologists and geophysicists calculate:

   • The estimated amount of accumulated oil and gas

   • The estimated amount of hydrocarbon reserves representing the proportion of accumulated oil and gas that can be extracted and brought to the surface for further use

These amounts are assessed in terms of a range providing a minimum and maximum value. This is because prior to drilling, there is not enough data available to accurately calculate accumulations and reserves.

Each oil company owns many prospects throughout the world. However, annual exploration budgets are limited and not all prospects can be drilled at the same time. This is why companies have teams to manage all their prospects. These teams are made up of experienced engineers and economists who allocate the exploration budgets to the company's various subsidiaries all over the world. These subsidiaries then assess their technicians' drilling proposals.

Setting Up the Infrastructure- Installation Logistics

Once a company has decided to drill an area, the necessary infrastructure and equipment have to be set up on site. A set of extremely precise location coordinates are displayed on the prospect map. However, a site survey has to be carried out first to determine whether the location is suitable. There are a number of stages to this survey:

   • If the site is on land, any obstacles (such as populated areas, or marshy or uneven ground) must be taken into account. 

   • If the site is at sea, the water's exact depth is measured and a seabed survey is carried out to determine if it can support platform foundations. Wind force, wave height, and current strength are all considered, as are the risks of severe storms in the region.

  • Whatever the site type, the environmental impact of the planned operations must be assessed in order to take necessary measures. For example, in some enclosed seas, drilling platforms are surrounded by a floating boom that prevents any contamination of the sea from waste or pollutants.

After this survey, geologists and geophysicists decide on the the safest and closest location possible to the initial coordinates.

It is at this stage that the actual drilling infrastructure proper is put in place:

   • Roads are built to bring in and take away drilling equipment and to bring supplies in to the personnel working on-site. At sea, tugboats or drill ships bring in the drilling platform.

   • On site, the areas where the infrastructure is to be set up are cleared of trees, leveled and cleaned up.

   • Drill rigs, drill rods, and drive pipes are set up and technical rooms and accommodation areas are built. This operation can take several days.

   • Finally, a shuttle system is set up to carry personnel to the drill site every day - buses on land and helicopters at sea.

When drilling operations are over, the oil company's teams leave the site as soon as the equipment is removed.

False. After drilling is over, the teams have to restore the site to its original state. This involves carefully eliminating any pollutants so that they do not spread into the surrounding countryside, starting with the mud pit (residual drill mud). If necessary, vegetation is replanted so that the site looks like it did before drilling.