Additional Post-Drilling Surveys 

A single drilling operation does not provide all the information needed to decide whether a deposit should be put into production. More tests, measurements, and drilling operations need to be carried out to ensure that the reservoir is useable. Geologists use a variety of techniques to identify hydrocarbons in the subsurface and to define the site features.

A close-up of trepans- mechanical tools used by drillers to break up rocks to go progressively deeper underground (Africa, April 2004).
© Total / Zylberman Laurent

How Do We Know if Gas or Oil Has Been Found?

While a site is being drilled, geologists use a variety of techniques to detect the presence of hydrocarbons.

The first of these are called direct methods.

   • As the drill rod drives through the rock, small quantities of methane gas are constantly released - this is background gas. This gas is dissolved in the drilling mud- a mixture of water and clay injected in the borehole to facilitate drilling. The background gas is analyzed throughout the operation. If it is stable or if the quantity of gas released remains proportional to drilling speed, there is nothing to report. However, if the quantity of gas increases suddenly, this means that the trepan (the three-section drilling tool with steel teeth) may have entered a hydrocarbon reservoir.

Geologists use a variety of direct and indirect methods to find out whether a deposit will be useable and profitable.

   • Similarly, geologists examine drilling debris carefully - i.e. the bits of crushed rock that rise to the surface inside the drive pipe when the rock is drilled. They remove the mud that covers this debris and study it under ultraviolet light. If the debris contains hydrocarbons, these will emit a fluorescent orange light.

   • If the geologists believe that the drilling tool has encountered a reservoir, they may decide to core drill. To do this, the drill string is raised and the drill head is replaced with a core barrel. This hollow pipe-shaped tool cuts up the rock gently, without crushing it, and brings back up a cylinder-shaped rock sample. The cores are then analyzed by geologists, who determine whether they contain oil or gas.

   • Finally, geologists may check for hydrocarbons in the rock by reducing pressure within the drive pipe. This makes the liquid in the rock surge into the borehole and, in some cases, rise to the surface. This liquid can then be recovered to determine its exact composition.

Once hydrocarbons have been detected underground using direct methods, the gas or oil can be extracted.

False. Direct methods provide information on whether the rocks drilled through contain oil or gas.

However, they do not provide enough information on the quantities of hydrocarbons trapped in these rocks or on the reservoir's physical and chemical properties. This information is vital to determine whether the deposit can be profitably used. To access this information, "indirect" exploration methods are used.

Indirect exploration methods all come under the term "wireline logging". Wireline logging refers to measurements taken with probes inserted into the borehole after drilling. These measurements provide information about:

   • The reservoir rock's porosity i.e. the proportion of empty space within the rock (this information is key because it is the spaces in the reservoir that contain oil and gas)

   • The temperature of the liquid contained in the trap

   • The reservoir's geometry and volume 

   • The rock's natural radioactivity 

These measurements provide information on the possible conditions of the deposit's use.

The Appraisal Process- the Final Stage before Extraction

When an exploratory well is drilled, there are many uncertainties about the extractability of the hydrocarbons potentially present underground. Previously only one well out of seven led to setting up a deposit's production. Nowadays, assessment methods are more advanced and most areas of interest have been prospected. This means that one well out of three or four leads to setting up production.

If the exploratory drilling is successful, the presence of a reservoir of oil or gas is confirmed. However, several wells need to be drilled in various locations above the deposit to gain better understanding of its features. These additional wells provide information that goes into the deposit's appraisal process. This is a full survey which is carried out. The decision to continue using or abandon a site is taken on the basis of this study.