Single-Shot Survey: Magnetic Compass Recording, Slickline Deployment, and WCSB Directional Control

A single-shot survey is a downhole measurement technique that records the inclination and azimuth of a borehole at one specific depth, capturing the wellbore's orientation on a single photographic film disc or, in modern tools, a single electronic snapshot. The instrument is run on a slickline or wireline inside the drillpipe and positioned just above the bit or near total depth, where it sits still long enough for a magnetic compass card and a pendulum or plumb assembly to settle. The compass aligns the tool to magnetic north while the pendulum registers the angle of deviation from vertical, and a timer trips a small light that exposes the film, freezing both readings together as a single image of a reticle superimposed over a graduated compass face. When the tool is recovered, the disc is developed and read under a magnifier, yielding inclination in degrees and azimuth as a magnetic bearing that is later corrected for local declination to give true or grid north. The technique earns its name because it records exactly one station per trip, in contrast to a multishot survey that exposes a series of stations on a film strip as the tool is pulled out of the hole. In Western Canadian Sedimentary Basin operations, single-shot surveys were the backbone of directional control on conventional vertical and slant wells through the Cardium, Viking, and Mannville before measurement-while-drilling became standard, and they remain a low-cost contingency when an MWD tool fails or when a slimhole or shallow gas well does not justify the cost of a full logging-while-drilling string. A typical reading is taken every drilled stand or every 30 to 90 m of new hole, building a deviation profile that the directional driller uses to keep the wellbore inside the licensed surface and bottomhole tolerance. Because the magnetic compass is the heart of the tool, single-shot surveys must be run in a nonmagnetic drill collar, usually a monel or K-monel sleeve, to keep the steel drillstring from deflecting the compass and corrupting the azimuth. Inclination accuracy is generally within 0.25 degrees and azimuth within 1 to 2 degrees under good conditions, which is adequate for the gentle build rates of slant-hole heavy oil and shallow gas drilling but insufficient for the tight anti-collision tolerances of modern pad development, where electronic gyroscopic and MWD surveys have largely replaced the photographic single shot.

Photographic Compass Mechanics and Film Reading

The classic mechanical single-shot tool stacks four parts inside a pressure barrel: a battery and timer, a small lamp, a compass and pendulum assembly floating in a damping fluid, and a film disc in a light-tight chamber. The directional driller sets the timer for the estimated run-in time, drops or lowers the tool, and waits at the survey depth while the compass settles and the pendulum centres a hanging reticle over the graduated card. When the timer fires, the lamp flashes and exposes the disc, recording the reticle crosshair against the compass ring and an inclination scale of concentric circles. A reading of the centre crosshair against the rings gives inclination, and its bearing against the card gives magnetic azimuth. Film handling discipline matters because a fogged or double-exposed disc means a wasted trip and lost rig time.

Electronic Single Shot and the MWD Transition

Modern single-shot surveys replace film with solid-state accelerometers and magnetometers that compute inclination from the gravity vector and azimuth from the Earth's magnetic field, writing the result to memory or transmitting it up a slickline conductor. These electronic kits remove the develop-and-read delay, deliver digital accuracy, and can be dropped through the bit on a go-devil. On most WCSB directional and horizontal wells the same accelerometer and magnetometer physics now lives permanently in the MWD collar, sending continuous surveys to surface by mud-pulse telemetry, so the standalone single shot has shifted from primary survey method to backup and audit tool. It still anchors tie-in surveys and verifies MWD readings when an operator or the AER requires an independent check of the as-drilled wellbore position.

Related Terms

A single-shot survey is one tool in a family of wellbore positioning methods. The multishot survey extends the same magnetic-compass physics to record many stations in one trip, which is why it supplants the single shot in high-angle and horizontal wells. A gyroscopic survey uses a spinning or rate gyro instead of a magnetic compass, allowing accurate azimuth readings inside steel casing where a magnetic tool would fail. The results of all three feed the deviation survey that documents the wellbore's true path, and they are typically deployed on slickline inside the drillstring.

WCSB Slant-Well Survey on a Lloydminster Heavy Oil Pad

On a shallow Sparky and Lloydminster heavy oil slant well near Lloydminster, an operator running conventional slant rigs at roughly 700 m measured depth chose single-shot surveys over a full MWD string to hold costs down. The directional crew ran a magnetic single shot inside a monel collar every 30 m, building from surface at a 25-degree slant angle, with each disc read and corrected for the local 14-degree east declination before plotting. A single-shot kit and operator ran near CAD 3,500 per day against roughly CAD 9,000 to 12,000 for an MWD crew, a meaningful saving across a multi-well pad of nearly identical slant geometries.

The program landed each bottomhole inside the AER Directive 056 licensed target window, and the saved surveying cost across eight wells funded most of the cost of an additional infill well on the same pad. When one well's readings drifted near the lease edge, the crew added intermediate stations to tighten control rather than escalating to a costlier real-time tool, demonstrating why the low-cost single shot still earns its place on shallow, repeatable WCSB programs.