total depth, Oil & Gas Glossary

Total depth, almost always abbreviated TD, is the bottom of a particular hole section, the point at which drilling is stopped so that logs can be run and casing set and cemented before a smaller-diameter section is drilled ahead. In everyday rig language TD is also used for the final bottom of the entire well, the deepest point reached, but in precise drilling-engineering usage every hole section has its own section TD where the bit is pulled and the well is secured. Reaching TD is a defining milestone in a well's life: it marks the end of active drilling for that interval and the start of evaluation and zonal isolation. TD is fundamentally a depth, and because modern wells are rarely straight, it is essential to distinguish how that depth is expressed. Measured depth (MD) is the total length of the wellbore measured along the actual drilled path from the surface reference, regardless of how the hole deviates or curves. True vertical depth (TVD) is the straight-line vertical distance from the reference datum down to that same point. In a perfectly vertical well MD and TVD are equal, but in the long horizontal wells that dominate WCSB development the two diverge dramatically: a Montney well might reach a TD of 5,500 m MD while its TVD is only 2,400 m, because roughly 3,000 m of that length is a near-horizontal lateral. This distinction is not academic. TVD controls the hydrostatic pressure exerted by the mud column and therefore the pore-pressure and fracture-gradient calculations that set safe mud weight and casing-shoe depths, while MD governs the actual lengths of drill pipe, casing, tubing, and wireline needed to reach bottom. The decision of where to call TD on a section, known as picking the casing point, is driven by geology and pressure: a section is drilled to the deepest point where the open hole remains stable and the next formation can be safely penetrated only behind set casing, often just above an overpressured zone, a lost-circulation interval, or the top of the reservoir. At section TD the crew typically circulates the hole clean, runs an openhole logging suite or logging-while-drilling data to evaluate the formations, then runs and cements casing to isolate the drilled interval before drilling out the shoe into the next section. Final-well TD is where the production casing or liner is set across the reservoir, after which the well moves to completion. AER Directive 008 surface-casing depth requirements and Directive 009 casing-cementing minimums frame how casing points and TD are planned for Alberta wells, tying the engineering choice of total depth directly to regulatory groundwater-protection and well-integrity rules.

Key Takeaways

Section TD versus well TD: Each hole section has its own total depth, the casing point where drilling stops to log and set casing before the next smaller section. The final-well TD is the deepest point of the whole well, where production casing or a liner is set across the reservoir ahead of completion. Rig language uses TD for both, so context matters.
MD and TVD diverge: Measured depth is length along the actual wellbore path; true vertical depth is the straight-line vertical drop to the same point. They match only in a vertical well. A WCSB Montney horizontal might TD at 5,500 m MD but just 2,400 m TVD, the difference being the near-horizontal lateral.
TVD drives pressure: Hydrostatic head, pore pressure, and fracture gradient all scale with TVD, not MD, so TVD sets safe mud weight and casing-shoe depths. Using MD by mistake in a long horizontal would badly overstate downhole pressure and could lead to a kick or a fractured formation.
Casing point selection: TD on a section is picked at the deepest point the open hole stays stable and the next formation can be entered safely only behind casing, commonly just above overpressure, lost circulation, or the reservoir top. The choice balances drilling fewer, longer sections against the risk of an unstable or unbalanced open hole.
Logging and isolation at TD: At section TD the hole is circulated clean, evaluated by openhole wireline or LWD, then cased and cemented to isolate the interval before the shoe is drilled out. AER Directive 008 and Directive 009 set surface-casing depth and cementing minimums that govern how Alberta casing points and TD are planned.

Picking the Casing Point at Section TD

Choosing where to call TD on an intermediate section is a pressure-and-stability decision. Drillers extend the section as deep as the open hole can be safely held with the current mud weight, then stop just above a problem zone, an overpressured shale, a depleted lost-circulation sand, or the top of the reservoir, so that casing can be set and cemented before drilling ahead. Setting too shallow wastes a casing string and adds cost; setting too deep risks exposing an incompatible pressure regime in open hole, inviting a kick or losses. In a WCSB Duvernay well, the intermediate casing point is often placed just above the overpressured target so the production casing can be run into a controlled, isolated reservoir section.

Logging and Evaluation When TD Is Reached

Reaching TD triggers formation evaluation. The crew circulates bottoms-up to clean the hole, then either runs an openhole wireline suite, gamma ray, resistivity, density-neutron porosity, and often sonic and image logs, or relies on logging-while-drilling data already recorded in the lateral. These logs confirm the reservoir quality, fluid contacts, and mechanical properties before casing is run, because once casing is cemented the openhole window is closed. In a Montney horizontal where wireline cannot easily reach the toe, LWD acquired during drilling becomes the primary TD evaluation, with the gamma and resistivity used to confirm the well stayed in the target landing zone the whole lateral.

Fast Facts

The longest wells on Earth now reach total depths that dwarf their vertical extent. Extended-reach wells drilled offshore Sakhalin and in the Middle East have passed 15,000 m of measured depth while their true vertical depth stays under 3,000 m, meaning more than 12,000 m of the hole is essentially horizontal reach. In the WCSB the same physics drives Montney and Duvernay laterals past 5,000 m MD at barely 2,500 m TVD, which is why a driller who quoted TD without specifying MD or TVD would leave the mud engineer unable to compute a single correct pressure.

Total depth ties together several drilling concepts. Measured Depth expresses TD as length along the wellbore and governs pipe and casing tallies, while True Vertical Depth expresses the same TD as vertical drop and drives all pressure calculations. Casing is run and cemented at each section TD to isolate the drilled interval, making the casing point and TD effectively the same decision. Kick-Off Point is where a deviated well begins to build angle, the start of the divergence between MD and TVD that makes a clear TD reference essential.

Real-World WCSB Scenario: Reaching TD on a Montney Horizontal

An operator drilling a Montney horizontal near Wembley, Alberta, plans a three-string design: surface casing set per AER Directive 008 to protect groundwater, intermediate casing at a section TD of 2,350 m MD just above the overpressured target, and a final-well TD at 5,450 m MD, roughly 2,420 m TVD, at the toe of a 3,000 m lateral. At the intermediate TD the crew circulates clean, runs an openhole log, and cements the casing before kicking off and steering the horizontal with LWD gamma to stay in the landing zone.

The well reaches final TD on plan after 18 drilling days. Because the team consistently worked pressures in TVD while tallying casing and tubing in MD, the 9.2 million CAD drilling program hit its casing points cleanly with no kicks or lost-circulation events, and the cemented production casing isolated the full reservoir for a 60-stage completion.