Actually, it’s unusual to drill a well that’s either 100% oil or 100% gas. Most oil wells produce at least a little gas and most gas wells produce at least a little oil.
DIFFERENCES BETWEEN DRILLING OIL AND GAS WELL
There are some practical well control differences between drilling oil and gas wells.
GAS KICK
A gas kick, an influx of gas into the wellbore is more dangerous and difficult to kill than an oil kick because gas expands to many times its original volume as it migrates up the wellbore. When large amounts of gas are present you might have to route your drilling mud through a degasser to maintain the proper density.
In drilling horizontal gas wells in porous formations you might try to keep the wellbore above the fluid contact depth. When drilling oil wells in porous formations you might try to keep the wellbore below the fluid contact depth but, above the oil/water contact depth.
Once the well is drilled and cased different production equipment is required, depending on whether the well produces only gas, primarily gas, a mixture of the two, or only oil.
YOUR TARGET PRODUCTS
Whether your target is gas or oil mainly. There can be some differences but most of those are not required. After all, you would not drill an oil well on air while that, in many cases, is the best method for drilling gas.
Most of the differences in drilling techniques are in regard to the reservoir and its properties. High porosity, high permeability requires significant mud loss control. Drilling into differentially stressed formations needs strong wellbore stability technology. The drilling string itself is changed out as a wellbore turns from vertical to horizontal/highly deviated.
There are a multitude of mechanical, mud mixing, bit selection and many other factors to be considered in designing and drilling a well. The target fluid is not all that high on the list of limiting factors.
An oil well is designed to bring petroleum oil hydrocarbons to the surface & Drilling gas a deposit of natural gas is found deep beneath the Earth’s surface. Once a likely deposit of natural gas is located, the energy company then drills down to reach it.
SPACING
One factor that’s not been mentioned is spacing. A gas molecule is much smaller than oil so can migrate farther to a wellbore. In general, gas wells can be twice as far apart. Another way of saying that is that a gas well might drain 4 times the acreage of an oil well at the same depth, pressure, etc.