Gas lifting technology utilizes the low density of gas to lower hydrostatic pressure of the fluid column in the wellbore and assist in lifting oil to the surface.
Gas lift is a type of artificial lift that involves pumping gas all the way from the surface down the annulus and into the production tubing for the purpose of reducing the flowing bottomhole pressure in order to make it easy for oil to flow to the surface. A well will stop flowing if the difference between the static reservoir pressure and the flowing bottomhole pressure is zero (i.e. static reservoir pressure – flowing bottomhole pressure = 0).
So gas lift works by making sure that the flowing bottom hole pressure is never equal to the static reservoir pressure by lowering the pressure that must be overcome before oil can flow to the surface. This pressure is the flowing bottomhole pressure. The greater the difference between the flowing bottomhole pressure and the static reservoir pressure, the greater the flow rate will be from the well.
Now, this is how gas lift achieves this: The gas used to lift the oil is normally pumped from surface compressors under high pressure, down the annular space between the production tubing and the casing. Gas lift valves are installed at strategic points within the production tubing.
These gas lift valves open-up to accept the gas into the production tubing. Since gas is lighter than oil and water, gas expansion begins the moment the gas gets into the tubing and the gas begins to move upwards and occupy space due to its negligible density. This leads to a reduction in overall fluid density.
Gas expansion reduces both the hydrostatic pressure throughout the fluid column and the flowing bottomhole pressure at the bottom of the well. By lowering the flowing bottomhole pressure, gas lift ensures that fluids can still flow upwards all the way to the surface even if the reservoir pressure is low . The expanding gas bubbles, themselves, also help in lifting oil in the tubing to the surface.
Continuous and Intermittent Gas Lift
There are two techniques to gas lift; the continuous flow and the intermittent flow technique. Continuous flow gas lift works by pumping high pressure gas into the production tubing continuously. Before this technique is selected, it is important to be sure that there is enough gas to ensure continuous injection.
Otherwise, pressure could fluctuate erratically or the well may even stop flowing altogether if there is not enough gas to sustain continuous gas lifting. Continuous gas lift is best suited for wells with high volume and relatively high static reservoir pressure.
Where continuous-flow gas lift requires a constant gas injection, intermittent gas lift involves injecting the gas periodically and only when enough reservoir fluids have accumulated within the tubing. Intermittent flow technique waits for reservoir fluids to accumulate in the production tubing before injecting the gas.
Intermittent gas lift works best at relatively low static reservoir pressure such as when the well has enough pressure to allow reservoir fluids flow into the wellbore but lacks the energy to push the oil from the wellbore all the way to the surface. Hence, gas is injected into the tubing to assist in “pushing” the slug of reservoir fluids all the way to the surface after enough fluids have gathered in the wellbore.
Intermittent gas lift technique will, however, present gas handling problems at the surface while waiting for the reservoir fluids to accumulate within the tubing.
Advantages and Disadvantages of Gas Lift Technology
Gas lift is a very attractive method of artificial lift especially when gas prices are low since the gas can then be put to good use instead of selling at a loss. Asides this, gas lifting is well suited for wells producing sand. With the exception of progressive cavity pumps (PCP), sand production can be a major problem for other artificial lift methods.
In recent times, drilling of deviated and horizontal wells is becoming increasingly popular. Other artificial lift methods may also fail to function effectively due to the bends and twists of deviated well and horizontal wells. This is not a problem with gas lift because gas can assume any shape as it travels down the annulus, through the gas lift valves and into the production tubing.
In spite of its advantages, one of the biggest disadvantage for gas lift technology, however, is the cooling effect gas has as it flows into the tubing. Gas cooling can lead to gas hydrate formation within the tubing or precipitation of paraffins from the oil. Both gas hydrate and paraffin accumulation within the tubing have the potential to restrict fluid flow or even totally prevent flow over time.
Again, if the gas used has corrosive components, then surface and downhole components are at risk of corrosion. An understanding of both the advantages and disadvantages of every artificial lift technique should be weighed side-by-side when selecting the right kind of technique for a poorly producing oil well.