How is oil produced?

Completing a well

Once a well has been tested and found to be economical to produce, the first step in the completion process is to install casing or tubular steel pipe that lines the hole to prevent water and rock from entering the wellbore and to ensure control of the production. The casing is sealed against the side of the well by cement pumped down the inside of the casing and up the outside, between the casing and the wellbore. Production tubing is then hung inside the casing and kept in place by inflatable rubber packers. The production tubing is connected to the wellhead, a device that contains valves and chokes which control production rates.

The next step in completing an oil well is to perforate the casing so oil can flow into the production tubing. This is accomplished by lowering a perforating gun, a device with many explosive charges which fires metal rods through the casing and into the producing reservoir.

Some wells may require stimulation either as part of the completion process or later on in the life of the well. Stimulation includes two processes. In acidizing a well, acids such as hydrochloric acid in carbonate reservoirs and hydrofluoric acid in sandstone reservoirs, are pumped into the producing reservoir under pressure to dissolve reservoir rock and increase the number and size of channels carrying gas to the wellbore. Another type of stimulation is fracturing, where fluids such as water or carbon dioxide are pumped into the reservoir at sufficient pressure to fracture the rock. To prevent the fractures from closing, proppant is then introduced into the reservoir. Proppant comprises sand, ceramic beads or resin-coated material which acts to prop open the new fractures and enhance hydrocarbon flow to the wellbore.

Until recently, most wells were drilled vertically. New technology has allowed for wells to be drilled directionally or horizontally to intersect more of the producing reservoir.

Unlike natural gas reservoirs, oil reservoirs seldom contain enough pressure to force the oil to surface. Due to its denser and more viscous nature, oil usually requires pumping. The most common method of pumping uses a pump lowered down the wellbore on a string of rods, and operated by an up and down motion created by a pumpjack at the surface. Other pumps use a rotary motion.

Developing an oilfield

Generally, more than one well is required to produce the recoverable oil from a reservoir. Oil wells in Canada are regularly spaced to conserve resources. In Alberta, the spacing is one oil well per quarter section with the well drilled in the centre of the quarter section. A section of land is roughly 256 acres.

There are several ways to produce oil. Primary recovery uses existing reservoir pressure or simple pumps such as those described previously to bring the oil to surface. Depending upon reservoir characteristics such as porosity and permeability and on the gravity and viscosity of the oil, primary methods can recover up to 60 per cent of the recoverable oil in a reservoir.

Secondary recovery involves injecting water or natural gas to maintain reservoir pressure. This is necessary because the reservoir pressure drops as the resource is produced.

Tertiary recovery involves more complicated methods. In miscible flooding, natural gas liquids such as ethane, propane and butane are injected into the reservoir to act as solvents that reduce surface tension and viscosity, making it easier for the oil to flow. Carbon dioxide has also been used in miscible flooding.

Using these techniques, anywhere from 10 to 60 per cent of the original oil in a reservoir can be recovered.

For heavy oil, steam is often used to facilitate production by softening the bitumen, diluting and separating it from sand grains, and enlarging or creating channels and cracks through which the diluted oil can flow. The two most successful methods are cyclic steam stimulation and steam-assisted gravity drainage (SAGD).

• Cyclic steam stimulation: a three-stage process involving several weeks of steam injection, followed by several weeks of “soaking,” followed by a production phase where the oil is produced by the same wells in which the steam was injected. As production declines, the injection phase is restarted.
• SAGD: a method involving two horizontal wells drilled one above the other. Steam is continuously injected through the upper wellbore, softening the bitumen which then drains into the lower wellbore and is pumped to the surface.

Once the oil is pumped to the surface, gathering systems bring it from individual wells to processing plants, called batteries, where natural gas, natural gas liquids, water and sand are removed. The natural gas is either recovered for use as fuel, piped to processing plants, re-injected into the reservoir for pressure maintenance, used as a fuel for generating thermal electricity or flared or incinerated. Water is also re-injected for pressure maintenance.