The purpose of drilling is to confirm the presence of oil and/or gas in a prospect. This can be done continuously, by using various qualitative and quantitative methods that range from direct observations of drill cuttings to more elaborate measurements.
While a site is being drilled, geologists use a variety of “direct” methods to detect the presence of preservation (hydrocarbons)The final phase in petroleum system formation, after a deposit has accumulated... :
- Analysis of background gas. During drillingThe process of boring a hole into the ground using special equipment... , small amounts of methane (ch4)The main component of natural gas deposits and oil deposit gas caps. Methane is produced naturally by landfills... gas are continually released: this is background gas, dissolved in the drilling mud. The mud is analyzed throughout the operation. If it is stable or if the amount released remains proportional to the drilling speed, there is nothing to indicate the presence of a hydrocarbonOrganic compound consisting of carbon and hydrogen. Hydrocarbons are the principal constituents of crude oil, natural gas and petroleum products. accumulation. However, if the amount increases steadily or suddenly, this means that the borehole may be close to or has already entered an oil or gas reservoir.
- Examining drill cuttings. Cuttings are bits of crushed rock that are brought back up with the mud during drilling. Geologists recover them using vibrating screens, wash off the mud covering them and examine them under ultraviolet light. If the cuttings contain hydrocarbons, they produce a white, yellow or orange fluorescent light. They can also be pulverized and placed in a test tube with an organic solvent before undergoing this test.
- Core drillingTechnique used to obtain a cylindrical sample of rock during drilling... . If geologists believe that the drilling tool has entered a reservoir, they may decide to take a core sample. To do this, the drill string is raised and the drill head is replaced with a core bit. This hollow, pipe-shaped tool, which can be 9, 18 or 27 meters long, cuts up the rock gently, without crushing it, and brings back up a cylinder-shaped rock sample. The core sample is then analyzed to determine whether it contains oil or gas. These samples are very useful to determine the properties of the reservoir rock and calibrating wireline logging.
- Formation testing. This method consists of isolating a portion of the borehole, either unlined or lined with a casingA well bore is inherently unstable due to the pressure...
tube and perforated, and reducing the pressure inside it. This causes fluid from the reservoir being tested to enter it. Samples of these fluids can then be recovered for analysis.
Indirect Measurements or Logs
Drilling involves an array of observations and measurements before a development decision is made.
But direct methods do not provide enough quantitative data on the hydrocarbons present in the rock or the physical properties of the reservoir, which are essential to determine whether the depositAn accumulation of natural resources, such as oil, natural gas, coal, uranium, metal ore or another commodity... can be developed. So-called indirect exploration methods are used to measure these parameters.
Indirect exploration is known as wireline logging and it involves taking measurements after each drilling phase, before the casing tube is inserted. These measurements provide information on:
- The type of rock encountered (lithology and speed at which sound travels through the rock).
- The porosity of the reservoir rock, i.e. the proportion of empty space within the rock. This information is important, because the oil and gas is contained within these spaces.
- The saturation of the reservoir rock encountered, in other words the relative proportion of water and hydrocarbons present in the reservoir (there is always a certain amount of residual water in a hydrocarbon reservoir).
- Fluid temperature in the different reservoirs and the natural radioactivity of the rock, which indicates the quality of some reservoirs.
All this data is essential to ascertain whether the deposit can be produced and make the decision to launch development.