Crude oil extraction begins after a reservoir is located and drilled. Oil is trapped within the tiny pores of subterranean rock formations and does not rush to the surface naturally. Recovery requires engineering techniques designed to overcome geological forces resisting fluid movement. These methods are systematically applied over a field’s lifetime to maximize the total volume of oil recovered.
Primary Recovery: Relying on Natural Pressure
Primary recovery is the initial phase of extraction, relying entirely on the natural energy present within the reservoir. This energy comes from natural drive mechanisms that push the oil toward the wellbore and up to the surface. Recovery during this phase can range from 5% to 60% of the oil originally in place, depending on the specific drive mechanism.
The solution gas drive occurs when gas dissolved in the oil comes out of solution as pressure drops, expanding to push the oil out. This is typically the least efficient mechanism, resulting in a low recovery factor because the expanding gas often flows preferentially, bypassing much oil. A more effective mechanism is the gas cap drive, where a pocket of free gas sits atop the oil zone. As oil is produced, the gas cap expands downward, maintaining pressure and sweeping the oil toward the production well.
The most efficient natural mechanism is the water drive, where a large, underlying aquifer exerts pressure on the oil column. This pressure forces the oil upward and into the wellbore, often resulting in a high percentage of oil recovery. Gravity drainage also contributes, particularly in steep reservoirs, allowing oil to trickle downward and collect near the wellbore base for easier extraction. When these natural pressures decline, the oil flow rate decreases, signaling the need for supplemental energy to maintain production.
Artificial Lift: Mechanical Pumping Systems
Once natural reservoir pressure falls below the level required to lift fluids to the surface, the well requires an artificial lift system to continue economic production. These systems use external mechanical or gas energy to reduce pressure at the bottom of the well, allowing the remaining oil to flow. Artificial lift is employed in over 90% of all producing oil wells at some point in their lifespan.
The sucker rod pump, commonly known as a pump jack, is one of the most recognizable systems. It uses a surface unit to generate reciprocating vertical motion. A motor drives a crank and walking beam, which lifts and lowers a string of steel sucker rods extending into the wellbore. This motion actuates a downhole pump assembly consisting of a plunger, a standing valve, and a traveling valve. During the upstroke, the pump lifts the fluid column, and on the downstroke, it captures a new volume of fluid from the reservoir.
For high-volume or deep wells, the electric submersible pump (ESP) is the preferred method. The ESP system uses a powerful downhole electric motor coupled to a multistage centrifugal pump. Submerged in the well fluid, the motor rotates a shaft connected to a series of impellers and diffusers. As fluid flows through each stage, impellers increase its velocity, and diffusers convert that kinetic energy into pressure, sequentially boosting the fluid to the surface.
Gas lift is an alternative method that does not require a downhole mechanical pump. Compressed gas is injected from the surface into the annulus between the production tubing and the well casing. The injected gas enters the production tubing through specialized valves, mixing with the produced fluids. This action lightens the fluid column’s overall density, reducing hydrostatic pressure and allowing the reservoir pressure to push the less dense mixture to the surface.
Enhanced Oil Recovery Techniques
When artificial lift methods become inefficient, a significant portion of the original oil, often 60% or more, remains trapped in the reservoir rock. Enhanced Oil Recovery (EOR) techniques mobilize this residual oil by altering the physical or chemical properties of the oil or the reservoir rock. EOR can increase the total recovery of a field to between 30% and 60% of the oil originally in place.
Thermal recovery methods are designed for extracting heavy, viscous crude oil that flows poorly under normal conditions. The most common technique is steam injection, where steam generated at the surface is injected into the reservoir. The heat dramatically reduces the oil’s viscosity, allowing it to flow more freely toward the production wells.
Another significant EOR approach is gas injection, most notably using carbon dioxide (CO2). The CO2 is injected into the reservoir where, at high pressure, it acts as a solvent, swelling the oil and reducing its interfacial tension with the rock. This process makes the oil miscible or near-miscible with the injected gas, allowing it to be effectively swept out of the rock pores. A common practice is Water-Alternating-Gas (WAG) injection, where slugs of CO2 are alternated with water to improve sweep efficiency and ensure the gas contacts more of the reservoir area.
Polymer Flooding
Chemical flooding involves injecting specific chemical solutions into the reservoir to change the fluid dynamics. Polymer flooding uses long-chain polymer molecules mixed with injected water to increase the water’s viscosity. This viscous fluid pushes the oil more uniformly through the reservoir. This improves sweep efficiency by preventing the water from taking easy, high-permeability paths and bypassing the oil.
Surfactant Flooding
Surfactant flooding is a complementary technique that introduces chemicals designed to lower the surface tension between the oil, water, and rock. This reduction in tension helps to free oil droplets trapped by capillary forces within the microscopic pore spaces of the reservoir rock.