Vertical Lift

Vertical lift is the true vertical distance between any two points in a wellbore, regardless of how far apart they are when measured along the actual path of the hole. In a straight vertical well, vertical lift equals the along-hole distance. In a deviated or horizontal well, the hole travels a longer path through the rock than the vertical distance between the same two points. Every pressure calculation and every pump sizing calculation uses the vertical lift, not the measured depth along the hole, because pressure depends on the height of the fluid column, not on how long or winding the path to get there is.

Key Takeaways

Vertical lift is the difference in true vertical depth (TVD) between two points in a wellbore. It is always less than or equal to the measured depth (MD) between those same points. The ratio of vertical lift to measured depth is the cosine of the inclination angle.
Hydrostatic pressure in a wellbore depends on vertical lift, not measured depth. A deviated well that reaches 3,000 metres TVD has the same bottomhole pressure as a vertical well at 3,000 metres TVD, even if the deviated well's total measured depth is 5,000 metres.
Artificial lift sizing, whether for electrical submersible pumps, rod pumps, or gas lift, is based on vertical lift because the pump works against the weight of the fluid column above it, which is determined by vertical height, not path length.
In horizontal wells, the lateral section contributes almost zero vertical lift. The bulk of the pump's pressure differential is used to lift fluid from the heel of the horizontal section up the deviated build section to surface, not to move fluid along the lateral.
Friction pressure losses add to the vertical lift pressure requirement and do depend on the full measured depth of the tubing string, not just the vertical component. Sizing calculations must include both the hydrostatic component (based on vertical lift) and the friction component (based on measured depth and flow rate).

What Is Vertical Lift?

Pour water into a curved straw and try to drink it. The effort you need depends on how high you have to lift the water, not on how long the straw is or how many bends are in it. If the straw is 30 centimetres long but curves around so the opening is only 10 centimetres above the water surface, you only have to work against 10 centimetres of water pressure, not 30.

Vertical lift in a wellbore works exactly the same way. The pressure that a pump or the reservoir must overcome to push fluid to surface depends on the vertical height of the fluid column, not on how many metres of measured depth the wellbore travels through. A horizontal well that kicks off at 500 metres TVD, builds to horizontal over the next 1,000 metres of hole, and then goes sideways for 2,000 metres has a total measured depth of 3,500 metres, but a total vertical lift of only about 1,500 metres (from the pump setting depth to surface). The pump is only fighting 1,500 metres worth of hydrostatic pressure.

Fast Facts

In extended-reach drilling (ERD) wells, measured depths can reach 10,000 to 15,000 metres while the vertical depth is only 2,000 to 4,000 metres. The world's longest wellbore by measured depth is the Odoptu OP-11 well drilled by ExxonMobil in Sakhalin, Russia, which reached a measured depth of 15,000 metres with a horizontal reach of 12,033 metres. In that well, friction is the dominant pressure challenge during drilling, but vertical lift is still the primary factor governing the hydrostatic pressure at any point downhole.

Vertical Lift in Pressure Calculations

Any static pressure calculation in a wellbore starts with the hydrostatic pressure equation: pressure equals fluid density multiplied by the gravitational constant multiplied by the vertical height of the fluid column. Vertical height in this equation is the vertical lift between the surface and the point of interest.

For a well with a mud weight of 1,600 kg/m³ and a TVD of 3,000 metres, the hydrostatic pressure at bottom is 1,600 × 9.81 × 3,000 = 47,088 kPa (about 6,828 psi). It does not matter whether the measured depth is 3,000 metres (vertical well) or 5,500 metres (deviated well). TVD is what the pressure equation uses.

When fluid is flowing, friction pressure losses are added to the hydrostatic component. Friction does depend on measured depth because it comes from fluid in contact with tubing walls along the full length of the path, not just the vertical component. For high-rate wells or long lateral sections, friction can be a substantial part of the total pressure requirement and must be calculated separately from the vertical lift pressure.

Vertical Lift and Artificial Lift Design

An electrical submersible pump sitting at 1,800 metres TVD in a Cardium horizontal well in west-central Alberta needs to generate enough differential pressure to lift fluid from the pump to surface. The vertical lift from pump to surface is 1,800 metres. At a producing fluid gravity of 0.88 (a mix of oil and water), the hydrostatic pressure the pump must overcome is 0.88 × 1,000 × 9.81 × 1,800 = 15,500 kPa, roughly 2,248 psi. Friction adds perhaps 500 kPa on top of that for a typical flow rate. The pump must be capable of at least 16,000 kPa differential, and the engineer adds a design margin.

If the same calculation mistakenly used the measured depth of 4,200 metres instead of 1,800 metres TVD, the required differential would calculate to 36,100 kPa, more than double. The pump would be massively oversized, the motor would run well below its rated load, and the operator would have paid two to three times too much for the equipment.

Vertical lift is used interchangeably with TVD difference, vertical height, and static head in different engineering contexts. Related terms include true vertical depth (TVD, the vertical distance from the surface datum to any point in a wellbore, measured straight down regardless of the wellbore path; the fundamental depth reference for pressure calculations), measured depth (MD, the distance along the actual path of the wellbore from the surface reference point to any downhole point; always greater than or equal to TVD), hydrostatic pressure (the pressure exerted by a static column of fluid, equal to fluid density times gravitational constant times vertical height; the starting point for all wellbore pressure calculations), artificial lift (the family of methods used to bring fluid from a reservoir to surface when reservoir pressure alone is insufficient; all artificial lift sizing relies on vertical lift for the hydrostatic pressure component), and deviated well (a wellbore drilled at an angle from vertical; requires directional survey data to calculate TVD and vertical lift at any point along the hole).

Why Confusing MD and TVD Halted a Gas Lift Installation

A production engineer at a mid-size Alberta operator was sizing a gas lift system for a newly drilled deviated well in the Swan Hills area. The well had a measured depth of 3,800 metres but a TVD of only 2,200 metres. The engineer, working from field notes that listed measured depth without a clear TVD annotation, entered 3,800 metres into the gas lift design software.

The design called for an injection gas pressure of 19,500 kPa at the surface compressor, high enough to require a compression upgrade at the battery. The capital approval was filed, the compressor order was placed, and the well was completed.

During commissioning, the well unloaded and started flowing at a fraction of the predicted injection pressure. A review found the TVD error. The correct design required only 11,500 kPa. The compressor that had been ordered was oversized by 70 percent. Fortunately, the equipment was on a five-week lead time and the order could be revised before fabrication was complete. The cost of the revision was CAD 12,000 in engineering change fees. Had the compressor been built and shipped, the cost of returning and resizing would have exceeded CAD 180,000.

Vertical lift is not a detail. It is the physical basis for every pressure number in the well.