Survey: Wireline Logging, Directional MWD and Gyro Stations, and Wellbore Position Uncertainty

In oilfield usage a survey is the act of recording a measurement of one or more physical quantities versus depth, versus time, or versus both in or around a well. The word is deliberately broad. In the earliest decades of the industry, before the term log became dominant, a survey meant any systematic downhole recording, and that older meaning survives in expressions such as electrical survey, temperature survey, cement bond survey, and directional survey. Two large families of survey work dominate modern Western Canadian operations. The first is the petrophysical or wireline survey, in which a sonde is lowered on cable and pulled back uphole while it records formation properties such as natural gamma radiation, bulk density, neutron porosity, acoustic slowness, and resistivity against measured depth, building the curves that drive porosity, saturation, and net pay calculations through a Montney, Duvernay, Cardium, or Viking target. The second is the directional survey, in which inclination and azimuth are measured at discrete stations down the borehole so the three dimensional path of the well can be computed and the bottomhole location placed accurately in the subsurface. Directional surveys are acquired either by measurement while drilling tools that send inclination and azimuth uphole in near real time through mud pulse telemetry, or by gyroscopic instruments run on wireline or drill pipe where magnetic interference from casing or nearby wells makes magnetic sensors unreliable. Both magnetic MWD and gyro surveys feed a position computation, almost always the minimum curvature method, which fits a circular arc between adjacent stations rather than assuming straight tangents, and the result is a list of measured depth, inclination, azimuth, true vertical depth, northing, easting, and dogleg severity. The quality of a survey is never absolute. Every station carries an uncertainty that grows with depth and accumulates along the path, so survey programs are described by an error model such as the industry standard ISCWSA model that produces an ellipsoid of uncertainty around the computed position. In Alberta, AER Directive 059 governs well drilling and completion data reporting and requires directional survey submission, and accurate surveys underpin anti-collision analysis in the crowded multi-well pads of the Montney and Duvernay, where a dozen horizontals can be drilled from a single surface lease with as little as a few metres of planned separation between adjacent laterals. A survey also connects to pressure and reservoir work, because the true vertical depth it produces is exactly what allows a measured pressure to be corrected to a common datum level for comparison between wells. Whether the goal is formation evaluation, geosteering, anti-collision, or reserves booking, the survey is the measured backbone on which nearly every downhole interpretation is built.

Geosteering a Montney Lateral With MWD Surveys

On a 2,800 m Montney lateral near Dawson Creek, an MWD tool takes a directional survey station roughly every 30 m as drilling progresses, sending inclination and azimuth uphole by mud pulse within minutes of each connection. The geosteering team merges these stations with the projected true vertical depth and a gamma image to keep the wellbore inside a 3 to 5 m thick target sublayer. A single station showing inclination drifting from 90.5 to 89.2 degrees flags the bit dropping out of zone, prompting a steering correction before the lateral exits the pay. The cumulative survey also feeds the as-drilled trajectory used for completion stage spacing.

Gyro Surveys for Anti-Collision on Dense Pads

When a Duvernay pad near Fox Creek hosts eight to sixteen wells from one surface location, magnetic MWD is unreliable in the shallow cased sections where wellbores are only metres apart. A gyro survey run through the build section provides azimuth free of magnetic distortion from adjacent casing strings. The resulting positions feed an anti-collision calculation that reports a separation factor; values below the operator threshold, often 1.5, halt drilling until the geometry is re-evaluated. A gyro run costs roughly CAD 25,000 to 45,000 but is far cheaper than a wellbore intersection, which can sterilize multiple wells.

Related Terms

A survey ties into several other glossary concepts. The true vertical depth produced by a directional survey is what permits a measured pressure to be normalized to a common datum level so wells across a pool can be compared on equal footing. Every survey station carries statistical scatter, a form of random error that survey programs reduce by repeating and averaging stations. And a wireline resistivity survey is precisely the measurement most distorted by fluid invasion, since mud filtrate alters the very formation resistivity the survey is trying to record.

Real-World WCSB Scenario: Survey Tie-On Error on a Twinned Well

An operator re-entering an older Cardium well near Pembina to drill a twinned horizontal discovered that the original 1990s magnetic survey had been run inside a string influenced by nearby casing, introducing an azimuth bias of nearly 2 degrees. Re-running a high-grade gyro from surface to the kickoff point at a cost of about CAD 38,000 revealed the bottomhole location was roughly 14 m from where the legacy survey placed it. The corrected position shifted the planned lateral azimuth and avoided steering the new well toward an offset producer.

Had the team trusted the legacy survey, the twinned lateral risked drilling outside the leased spacing unit and toward a collision corridor. The gyro re-survey, a fraction of the CAD 6 to 9 million well cost, protected both the offset well and the operator's regulatory standing under Directive 059.