Arrival: Definition, Drilling Milestones, and Total Depth Events
In oil and gas drilling, arrival refers to the moment a drill bit reaches a specific target — whether a predetermined depth, a target formation, a stratigraphic marker, or the planned landing zone in a horizontal wellbore — and encompasses the procedural, geological, and wellbore integrity events that occur at or immediately following that milestone. In a vertical exploration well, arrival at a target formation triggers the suspension of drilling activity, circulation of the wellbore to condition drilling fluid, notification to geological and production engineering staff that the objective horizon has been reached, and the commencement of open-hole log runs and formation tests. In a horizontal or directional development well, the term "arrival" in the lateral sense refers specifically to the bit crossing from the build section into the horizontal pay zone at the landing depth, commonly called "arriving at the formation" or "arriving at total depth of the build," after which the well trajectory is held at the target inclination (typically 88 to 92 degrees from vertical) for the remainder of the horizontal section. In seismic and acoustic logging contexts the term carries a different, narrower meaning: an arrival is any distinct wave event detected at a receiver — compressional first arrival, shear arrival, reflected wave, or refracted head wave — and the arrival time of each event is the foundational measurement from which interval velocities, depth conversion functions, and formation slowness values are derived. The dual usage of "arrival" as both a drilling program milestone and a geophysical wave detection event is context-dependent, but in field operations the drilling context dominates — "arrival at zone" is the language used in drilling reports, well status summaries, operator notifications, and regulatory filings with the Alberta Energy Regulator (AER) and British Columbia Energy Regulator (BCER) in the Western Canada Sedimentary Basin.
Key Takeaways
- Arrival in vertical drilling — formation tops, total depth, and geological confirmation: In a vertical well designed to test a specific stratigraphic objective, "arrival at zone" is confirmed by a combination of real-time indicators: the mud logger monitors drill cuttings at surface and identifies the first appearance of the formation's characteristic mineralogy, colour, and fluorescence under UV light; the MWD gamma ray log detects the characteristic gamma ray signature of the target (e.g. the Viking B sand at approximately 40 to 55 API in freshwater-mud wells, contrasted with the 80 to 120 API of overlying Colorado Group shale); and the mud weight and gas readings confirm whether formation pressure is as predicted. On the well schematic, arrival at zone is marked with the measured depth (MD) and corresponding true vertical depth (TVD) from the directional survey, and the driller's report records the time of arrival, the formations drilled above, any unexpected markers encountered, and the decision to continue or stop drilling. The AER requires completion reports (AER Form B) to specify formation tops and pay zone depths for all wells drilled in Alberta, and the arrival depth forms the starting point of all volumetric calculations submitted in the completion report. "Total depth" (TD) is the most common single-point arrival event: the MD at which the well is plugged back or logged before any casing is set to that level.
- Arrival in horizontal drilling — landing zone entry and pay zone contact: Horizontal well drilling introduces the concept of a "landing zone" — the specific formation interval, typically 2 to 8 metres thick, within which the well is designed to arrive at 88 to 92 degrees inclination before commencing the horizontal production interval. Arriving in the landing zone correctly is critical: a landing that is too deep puts the wellbore in the water leg or a tight non-reservoir zone, and a landing that is too shallow places the wellbore in the cap rock or overlying shale where it cannot produce. The geosteering geologist confirms arrival in zone by monitoring the LWD gamma ray, resistivity, and density log signatures against the planned zone log window: in Montney horizontal wells at Dawson Creek, a clean arrival in the Upper Montney B siltstone is confirmed by LWD gamma ray below 60 API, resistivity above 35 ohm-m, and neutron porosity of 6 to 10 per cent NPHI all simultaneously within the planned 3 to 5-metre target window at the planned inclination. If the well arrives outside the window, the directional driller is instructed to build or drop inclination to steer into the target zone.
- Seismic arrival — first-break picking and wave detection: In exploration geophysics, "arrival" describes the detection of any distinct wave event at a receiver in a seismic survey or a downhole check-shot or VSP acquisition. The compressional first arrival is the event that reaches the receiver earliest, having traveled at P-wave velocity through the fastest available ray path (typically via critically refracted waves along high-velocity basement or carbonate formations). The first arrival is picked from the raw seismic trace as the first deviation from the pre-signal noise baseline, and the corresponding first-arrival time is used for static corrections, refraction tomography, and near-surface velocity model construction. In check-shot surveys (geophone clamped at discrete depths in the wellbore), the arrival of the surface source's compressional wave at the downhole geophone provides one-way travel time at that depth, calibrating the integrated sonic log travel time against the seismic time domain and anchoring the time-depth conversion function. The distinction between seismic and acoustic logging arrivals is primarily one of frequency and path length: seismic arrivals at surface geophones travel kilometres through the Earth at 10 to 200 Hz, while borehole array sonic arrivals travel centimetres through the near-borehole formation at 1 to 20 kHz.
- Post-arrival wellbore integrity procedures and circulating operations: When a drilling well arrives at its planned total depth, a defined sequence of wellbore integrity procedures is initiated before any open-hole logging or formation testing begins. In WCSB operations, the standard post-arrival procedure includes: (1) circulating the wellbore for at least one full bottoms-up cycle (time required for cuttings at total depth to reach surface, typically 1.5 to 3 hours for wells below 3,000 metres MD) to remove drill cuttings from the annulus and condition the borehole geometry for logging; (2) performing a final formation integrity test or leak-off test at a clean formation to confirm mud weight adequacy for the open-hole section; (3) pulling the drill string to a safe logging depth (typically the top of the open-hole section) and laying down the drill bit; (4) rigging up the wireline logging unit; and (5) running the logging suite in planned order (usually resistivity and nuclear first, then sonic, then any specialised tools). The wellbore circulation time between arrival at TD and the start of logging is one of the key scheduling parameters in daily cost control: idle time on a CAD 25,000 per day drilling rig while circulating and logging represents a direct cost that drilling engineers minimise by pre-positioning the logging crew and equipment at the wellsite before arrival.
- Casing point arrivals and intermediate formation evaluation decisions: For deep wells drilled through multiple formations with different pressure regimes or formation stability concerns, the total well programme specifies multiple intermediate casing points — depths at which drilling is paused to run and cement intermediate or production casing before drilling ahead through the next formation section. Each casing point is a form of arrival: "arriving at surface casing shoe" at 120 to 200 metres TVD in WCSB wells confirms that the conductor casing is set below the base of groundwater protection and that the surface casing can be set; "arriving at intermediate casing shoe" at the top of an over-pressured or mechanically unstable zone triggers a casing integrity test and cement job before the next section begins; "arriving at production casing shoe" at a designated depth above the producing formation marks the last intermediate casing point before the open-hole production section is drilled. Each casing point arrival requires formal geological confirmation of formation top depth (from cuttings and LWD gamma ray), a directional survey with TVD and inclination data, and an AER or BCER regulatory notification under applicable well-specific drilling approvals (WCSB: AER Directive 056, Resource Applications for Conventional Oil and Gas Reservoirs, specifies notification protocols for formation arrivals that differ from prognosis by more than 5 per cent in TVD).
Arrival Events in the Drilling Programme and Daily Operations
Drilling programmes approved by operators in the WCSB specify "prognosed tops" — the expected TVD of each formation boundary, typically within plus or minus 5 per cent of the actual value based on offset well data and seismic interpretation. As the well approaches each prognosed top, the drilling contractor alerts the mud logger to begin sampling cuttings at the wash interval (the shallower depth at which samples arrive at surface after accounting for lag time) and the wellsite geologist begins comparing real-time cuttings descriptions against the formation top prediction. Lag time is computed as the volume from total depth to surface divided by the pump output rate: for a 12.25-inch hole section at 2,000 metres with 100 litres per minute pump rate and 4.5-inch drill pipe, the lag volume is approximately 3,800 litres and the lag time is 38 minutes. The wellsite geologist marks the actual formation arrival on the mud log at the depth corrected for lag time and compares it to the prognosis — a procedure codified in CAPP (Canadian Association of Petroleum Producers) Recommended Practice 6.1 for mudlogging and geological operations.
In horizontal wells, the transition between the build section and the horizontal section is called the "kick-off point" (KOP) at the bottom of the build, and the "landing" is the specific depth where inclination reaches 88 to 92 degrees. The landing is not simply an inclination milestone but a geological confirmation event: the LWD suite must simultaneously show the target formation gamma ray and resistivity signatures at the planned inclination and TVD. In Cardium horizontal wells at Pembina, landing typically occurs at 1,620 to 1,660 metres TVD in the Cardium A sand, and the geosteering geologist confirms arrival by the characteristic log response: GR 30 to 55 API, resistivity 30 to 180 ohm-m, density porosity 12 to 18 per cent. If all three criteria are met simultaneously at the planned TVD and inclination, the landing is confirmed and the lateral section continues at that inclination. If the log response does not match the expected Cardium A signature at the planned TVD, the directional driller continues the build or adjusts azimuth while the geosteering geologist reconciles the updated depth against the seismic depth model.
From a rig cost perspective, the efficiency of arrival management is one of the key metrics in drilling performance benchmarking. The "arrival depth" versus "prognosed depth" comparison feeds the "geological prognosis accuracy" metric tracked in operator well delivery scorecards. Wells that arrive at zone more than 3 per cent above or below prognosis flag a discrepancy between the pre-drill seismic depth conversion and the actual subsurface, triggering a review of the seismic velocity model and potentially modifying the drilling programmes for subsequent wells on the same pad. In the Montney play at Groundbirch, systematic early arrival (wells arriving at zone 2 to 4 per cent shallower than prognosis) in a 16-well pad campaign led the operator to update the seismic interval velocity in the overlying Doig Formation and reduced arrival depth uncertainty from plus or minus 25 metres to plus or minus 8 metres for the following pad, saving an average of 1.2 hours of unnecessary drilling (CAD 30,000) per well.