When drilling for oil, how do the drillers know they have struck oil?

I will try to answer comprehensively and sequencially.


First if the oil/gas is blowing out to the surface, it is not a success but a failure. And very often, it will not give much indication on the RESERVES, the TYPE OF FLUIDS in the reservoir and the possible PRODUCTION FLOW.





Before drilling a well, a seismic is generally shot. It means that vibrations (through air guns in water or truck shaking on land) will be generated, the sound will propagate in the ground and will reflect at the interface of the various rock layers. It will give an idea whether there is trap to keep the oil and how big are these traps. Advanced analysis can even give some clues on the type of fluids in these traps (water, gas or oil). So we get the POSSIBLE/PROBABLE RESERVES.





When a well is drilled, there is inside the drilling assembly of most well a certain set of tools called MWD or LWD (Measurement While Drilling and Logging While Drilling which is one step further). The MWD/LWD is measuring downhole some parameters such as the well inclination and direction toward north, the resistivity of the rock, the natural radioactivity of the rock, the vibration and mechanical parameters (weight, torque etc...). The LWD can also give information on the porosity, the type of hydrocarbons etc... So we get the THICKNESS OF THE RESERVOIR ROCK, THE THICKNESS OF THE OIL/GAS ZONE AND THE POSSIBLE TYPE OF FLUID (OIL/GAS)





Once the well is drilled, the company will 'logged' the well with a set of tools which will give as well all parameters recorded with MWD/LWD plus some extras like permeability, hydrocarbon density etc... Samples can be taken as well and the time for taking a sample at a given pressure will give an idea on the permeability of the rock. The fluid taken downhole are brought and later are carried detailled analysis, called PVT and petrochemical analysis (for Pressure/Volume/Temperature). So we get the ACCURATE THICKNESS OF THE RESERVOIR AND THE HYDROCARBON ZONE, THE TYPE OF HYDROCARBONS DOWNHOLE, THE POSSIBLE PRODUCTION LEVEL OF THE WELL.





However, people in the oil company really like it big and practical, so they are carrying well test. Basically, they will allow the well to produce the oil or the gas, measure with various settings the amount of oil and gas produced and the quality of the oil, mainly the density and for the gas the various gas components (Methane, Ethane, Propane, Butane and iso-Butane, Pentane and iso pentane). The fluids produced by the well passes through a separator, separating oil from water and from gas and sand. Then the flow(quantity) is measured using various type of flowmeters. Now it is even possible to measure flow without separators by using multiphase flowmeters allowing the calculation of the relative flow of oil, gas and water. So we get the TYPE OF FLUID REALLY PRODUCED AT SURFACE AND THE AMOUNT IT IS POSSIBLE TO PRODUCE IN SAFE CONDITIONS.





I hope this answer your question.When drilling for oil, how do the drillers know they have struck oil?
It starts spraying out of the ground in most cases.When drilling for oil, how do the drillers know they have struck oil?
Oil wells are pressurised and there are blow out preventers. If it spurts up out of the ground these days it means someone has stuffed up. Its flammable and very dangerous for one thing. There are tools sometimes run behind the drill bit that can detect gas and oil. Otherwise, the tools are run down the hole after drilling has stopped. Also, you can sometimes see and smell it in the rock chips that come up in the drilling fluid. Its the geologists and mudloggers that look at all this. The drillers just drill the hole.
they don't right away. they drill to take core samples (basically a big long cylinder of rock) and find out when they take the core sample out.
alot of times they drill test holes and and check the debris outflow from a determined depth.they look for certain porous rock and look for the smell of oil and gas that the pores in the rock hold.then they know they are close.
because it comes to the top
The enormous amount of pressure forces the oil in all directions. Then once a hole is created, all the pressure is forced out that one way.
oil doesnt ';spurt'; out of the ground anymore. that was back in the days when it was abundant. now the oil reservoirs are deeper underground. we know we have hit ';pay'; (produceable hydrocarbon reservoirs) from the mud/cutting byproduct that flows back to the surface
When it spurts up in a spout and whacks them right in the face haha, i have no idea, sorry.