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 are man-made techniques of bringing 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.
The Concept of Pressure Equilibrium
Due to overburden pressure from overlying rocks, hydrocarbon fluids in reservoirs are usually found under pressure and over time, this pressure is now at equilibrium. The moment a well is drilled into the reservoir, this equilibrium pressure is disturbed and the hydrocarbons in the reservoir will tend to flow from the zone of high pressure to the zone of low pressure. A carbonated drink that is given a vigorous shake will have its pressure at equilibrium with the container after a while. But opening up the drink creates a pressure disturbance which 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 higher 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 5200psi, if the surface pressure is 15psi. We can clearly see that the reservoir pressure is higher than the surface pressure and the pressure communication between reservoir and surface through the drilled well will tend towards equilibrium, this equilibrium seeking condition will force pressurized fluids out of the reservoir. The well will continue to flow until the reservoir pressure drops to 15psi, at this pressure, both reservoir and surface pressures are equal, right? False. The well can even stop flowing when the reservoir pressure is at 400psi. 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
Oil density: The denser the oil, the heavier it is. There is something called hydrostatic pressure. This 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: Asides, the properties of the fluid like density considered, rock and fluid interaction in the reservoir can generate some frictional drag as the fluid makes contact with walls of the rock grains. This frictional drag consumes reservoir fluid energy and will reduce the efficiency of fluid flow from the reservoir into the production string. Even after pressure drops in the reservoir, when the fluid gets near to the wellbore, they all have to squeeze through the small openings created by perforations. What this means is that the fluids have to wait in a sort of a queue to pass through this small opening, pressure drops while fluid molecules are waiting and squeezing their way through the perforation openings. As if this is not enough, when the fluid gets into the production string, there is further contact of fluids with the internal walls of the production string. Every pipe has some degree of roughness, the contact between the fluids and the inner walls of the production string will generate some form of 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. So 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 more pressure drop for the fluid to flow from bottom of the well all the way to the surface. So comparatively, the pressure drop for deeper wells is higher than the pressure drop for wells drilled into shallow reservoirs because of the length of the production string the fluid has to pass through. The same reservoir pressure that could not push the fluids from the reservoir to the surface in deeper wells may have been enough to push the fluid if the reservoir depth was just a few hundred feet less than what it is.
Extra force needed to push fluid through surface facilities into storage tanks
Now, it is not enough to flow to the surface, the reservoir pressure must be enough to still move the fluids all the way through surface facilities like surface pipes and separators all the way to the storage tank. Energy is lost in terms of pressure drops in psi when this is done. So for the well to flow, the reservoir pressure must be enough to complete this flow all the way from the reservoir and into 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 storage tanks. No matter how much initial energy the reservoir has, there will come a point where the reservoir pressure drops to a point 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 like the sucker rod pump, gas lift, electrical submersible pumps or even progressive cavity pumps can be installed in the well to help lift the hydrocarbons to the surface.