Artificial Lift

Artificial lift refers to the process of pumping hydrocarbons from a reservoir at a higher rate than they would naturally flow. There are two scenarios in which artificial lift is used:

  1. Declining reservoir drive: Many reservoirs have enough natural pressure that, initially, hydrocarbons flow to the surface at an economic rate without assistance. As production continues, however, this natural reservoir drive begins to decline. Once the production rate is no longer economical, operators usually implement an artificial lift system, which will allow the well to continue producing. Eventually, even artificial lift isn’t enough, and the operators turn to secondary recovery techniques.
  2. Insufficient reservoir drive: Some reservoirs never have enough natural pressure to produce flowing hydrocarbons. This is often the case in reservoirs with heavier mixtures of hydrocarbons, as well as those that produce water along with the oil and gas.

There are a number of artificial lift methods. Each technique has strengths, weaknesses, and operational restraints. A plan for artificial lift is typically developed as part of the production plan for a well. Such a plan needs to take into account technique effectiveness, cost, and whether conditions are within the operational limits of the technique.

Let’s take a look at some of the more common artificial lift techniques.

Pumpjacks

The pumpjack, also known as a Nodding Donkey, Grasshopper, Horse-head, or Thirsty Bird, is one of the most iconic symbols of the oil industry. It’s hard to drive through an oil-producing region of the American West without coming across one or more pumpjacks gently bobbing up and down.

Pumpjacks are mechanical pumps that are capable of producing up to a few tens of liters of oil per cycle. Because of their modest production capabilities, they’re usually associated with low volume wells or wells near the end of their productive lives.

Here’s how they work:

A metal rod supported by an A-frame structure is moved up and down by a electric motor or a combustion engine. At the end of the rod is a metal block called the “horsehead”.  It is the orange part at the top of the pumpjack in the accompanying photo. Cables wrapped around the horsehead connect to a “polished rod” – a metal rod that extends into the well.

At the bottom of the well, the movement of the polished rod raises and lowers the pump itself – a series of valves and lifting vessels that push hydrocarbons further up the well with each up and down motion of the pumpjack. Attached to the mechanical axle of the motor is usually a series of counterweights designed to offset the fluid weight that the pump is pushing so that the motor requires very little power to continue spinning the wheel on the pumpjack that raises and lowers the back end of the jack.

Electrical Submersible Pumps

Electrical submersible pumps are efficient and suitable for a wide range of conditions and pumping rates. They are capable of pumping at a much higher rate than a pumpjack, and can operate at much greater depths.

An electric submersible pump is lowered to the bottom of the hole, where an electric motor turns a series of impellers. The presence of multiple impellers in series means that the pump is able to operate in high pressure environments where a pumpjack wouldn’t work.

Electricity is provided via a cable from the surface, where the pumping rate can be controlled. The production tubing is attached directly to the pump.

The main challenge facing electrical submersible pumps is that they must be designed to withstand the high temperatures and corrosive environment of a deep oil well. Depending on the conditions down-hole, a specialized pump or specialized components may be used.

Gas lift

Unlike the other artificial lift methods, gas lift doesn’t depend on a mechanical system to pump oil from the well. Instead, gas is injected into the wellbore partway down the well. The gas reduces the bottomhole pressure and lowers the density of the hydrocarbons, allowing the natural formation pressure to push them towards the surface.

Gas lift is specifically designed for wells where natural formation pressure is present but not quite enough to lift the oil all the way to the surface. It is ineffective if formation pressure is extremely low.

Hydraulic Pumping

A hydraulic pump is similar to a pumpjack in that reciprocating motion is ultimately used to lift oil using a series of valved containers that move up and down in the well. The key difference is that, rather than being powered by a surface-based pumpjack, the reciprocating motion in a hydraulic pump is powered by fluids, such as oil, pumped from the surface under high pressure.

A hydraulic jet pump is also powered by fluids pumped down from the surface. However, instead of powering a pump mechanism, the pumped fluids are mixed directly with the oil in the well. The energy imparted to the oil by the hydraulic pressure of the pumped fluids causes it to rise up the well.

Images: “Multiwell” by Jim Blecha; “Artificial Lift” by mysticenergy via iStock