A well is said to be dead when it stops flowing. A well can die even when it still contains hydrocarbons in commercial quantities. Artificial lift systems are man-made techniques to bring the well back to life. Wells only flow when the reservoir pressure is enough to force the hydrocarbons through the pore spaces of the reservoir, into the production string up to the surface and through the surface facilities into the storage tank. Where we want the hydrocarbon fluids to be is in the storage tanks, not in the pore spaces of rocks. (Learn more about pore spaces in the article The Significance of Porosity to Original Hydrocarbon in Place.)
The Concept of Pressure Equilibrium
Due to the overburden pressure from overlying rocks, hydrocarbon fluids in reservoirs are usually found under pressure, which over time reaches equilibrium. The moment a well is drilled into the reservoir, this equilibrium pressure is disturbed and the hydrocarbons in the reservoir tend to flow from the zone of high pressure to the zone of low pressure. (For more about this topic, see Understanding Reservoir Drive Mechanisms.)
A carbonated drink that is given a vigorous shake will have its pressure at equilibrium within the container after a while. But opening the drink creates a pressure disturbance that forces the fluids in the can out under pressure in a bid to return pressure back to equilibrium.
Similarly, a drilled well opens up the reservoir and creates a communication between the reservoir and the surface. So fluids in the reservoir will be forced to the surface as the well tries to achieve pressure equilibrium with the surface. As long as the reservoir pressure is greater than the surface pressure, the well will continue to flow. And as fluids flow from the reservoir to the surface, the fluid pressure will continue to drop. The moment the reservoir pressure equals the surface pressure, a pressure balance is achieved and the well will stop flowing.
Let’s look at an example for a well operating at a reservoir pressure of 5200 psi and a surface pressure of 15 psi. We can clearly see that the reservoir pressure is higher than the surface pressure. The pressure communication between the reservoir and the surface through the drilled well will tend towards equilibrium and will force pressurized fluids out of the reservoir. The well will continue to flow until the reservoir pressure drops to 15 psi. At this pressure, both the reservoir and the surface pressures are equal, right? False. The well might stop flowing when the reservoir pressure is at 400 psi. Do not forget that a well flows when the reservoir pressure is enough to force the reservoir fluids not only through the pore spaces of the reservoir rock, but also enough to push the reservoir fluids into the production string, up the production string to the surface, through surface facilities and into the storage tanks.
Now, as the well fluids move through each of these stages, some energy is lost as the fluid moves; this reservoir energy is the reservoir pressure. This is why a well could die even before the reservoir pressure approaches surface pressure.
Factors that Affect Fluid Flow
The denser the oil, the heavier it is. There is something called hydrostatic pressure, which is the pressure of the fluid column in a vertical string of pipe. This hydrostatic pressure increases with fluid density. Thus, more energy is needed to push heavier crude from the reservoir all the way through surface facilities to the storage tank. Do not forget that fluids from reservoirs have to flow vertically upwards through the production string. The denser the fluid the greater the hydrostatic pressure of the vertical fluid column that must be overcome before the fluid flows to the surface.
The natural life of a well is considerable increased when the reservoir contains light crude that can easily be pushed through the production string all the way to the surface. This is why some heavy crude and bitumen will not even flow at all. And something must be done to reduce the oil’s viscosity, making it lighter and easier to flow before even installing some type of artificial lift in the well.
Frictional drag in the reservoir and production string
Rock and fluid interaction in the reservoir can generate some frictional drag as the fluid makes contact with the walls of rock grains. This frictional drag consumes reservoir fluid energy and reduces the efficiency of the fluid flow from the reservoir into the production string. Even after the pressure drops in the reservoir, when the fluid nears the wellbore, they all have to squeeze through the small openings created by perforations. What this means is that the pressure drops while the fluid's molecules have to wait in a queue to pass through these small perforation openings.
As if this is not enough, when the fluid reaches the production string, there is further contact with the internal walls of the production string. Every pipe has some degree of roughness, so contact between the fluids and the inner walls of the production string will generate some frictional drag that must be overcome for the fluid to flow any further. Even for horizontal wells, the horizontal section still offers this frictional drag. Therefore, the frictional drag can reduce the natural life of a flowing well.
Length of the production string
This one is easy. The deeper a well is, the longer the length of the production string and the greater the pressure drop as the fluid flows from the well's bottom to the surface. So comparatively, the pressure drop for deeper wells is higher than the pressure drop for wells drilled into shallow reservoirs. The same reservoir pressure that could not push the fluids from the reservoir to the surface in deeper wells may be just enough if the reservoir depth was just a few hundred feet less than what it is.
Force needed to push fluid through surface facilities into storage tanks
It is not enough to flow to the surface. The reservoir pressure must be enough to move the fluids through surface facilities such as surface pipes and separators on the way to the storage tank. Once again, energy is lost in terms of a pressure drop. So for the well to flow the reservoir pressure must be enough to complete this flow all the way from the reservoir to the storage tank.
Artificial lift is needed when the natural reservoir energy needs some help to push the fluids in the reservoir pores all the way to the surface and into the storage tanks. No matter how much initial energy the reservoir has, there will come a point where the reservoir pressure drops to where it may not be economical to continue to produce at such a rate or the well dies. At this point, some form of artificial lift, such as sucker rod pumps, gas lifts, electrical submersible pumps (ESPs) or even progressive cavity pumps (PCPs) must be installed in the well to help lift the hydrocarbons to the surface.