Deviated Hole: Directional Drilling, Dogleg Severity, and WCSB Multi-Well Pad Development

A deviated hole is a wellbore that departs from vertical, usually because the operator intentionally steered it along a planned trajectory to reach a target that does not lie directly beneath the surface location. The term covers everything from a gently slanted well that corrects for a surface obstruction to a highly deviated or horizontal well that turns the bit until it runs parallel to a target formation, and it is the everyday product of directional drilling. Deviation is measured by two survey angles taken at intervals down the hole: inclination, the angle away from vertical in degrees, and azimuth, the compass direction of the wellbore. A well at 0 degrees inclination is vertical, one held at 90 degrees is horizontal, and anything between is a deviated or slant hole. The rate at which the wellbore changes angle is the build rate, and the combined three-dimensional change in inclination and azimuth over a standard interval is the dogleg severity, reported in degrees per 30 metres in the Western Canadian Sedimentary Basin or degrees per 100 feet in imperial practice. Dogleg severity matters because a sharp bend increases drag on the drillstring, concentrates casing wear, and can make it impossible to run production tubing or a pump, so drillers plan smooth curves rather than abrupt turns. Deviated holes are drilled with a bottomhole assembly built around either a steerable mud motor with a bent housing or a rotary steerable system, and the toolface orientation is adjusted from surface so the bit walks along the design path. In the WCSB the deviated hole is the foundation of modern Montney, Duvernay, and Clearwater development, where multiple horizontal wells are drilled from a single pad, each one kicking off from vertical, building angle through a curve section, then holding a long horizontal lateral through the reservoir. Operators such as Tourmaline and ARC Resources routinely place a dozen or more deviated wells on one pad, fanning the laterals out underground to drain a large area from a compact surface footprint. Deviating the hole is also the standard remedy when a vertical target is unreachable, such as when the reservoir sits under a lake, a town, a lease boundary, or an existing surface facility, or when a relief well must intersect a blowing wellbore. The deviated hole therefore is not an accident to be corrected but a deliberate engineering tool that lets a rig reach reserves a vertical well never could, and it underpins the pad drilling economics that define shale and tight-oil development across Alberta and British Columbia.

Building Angle: Motors, Rotary Steerables, and Toolface Control

A deviated hole is created by pointing the bit slightly off the current wellbore axis and drilling ahead. With a steerable mud motor the driller stops rotating the string and slides, letting the bent housing point the bit in the desired toolface direction to build or turn angle, then resumes rotating to hold a straight tangent. A rotary steerable system builds angle while the entire string rotates, using either a push-the-bit pad mechanism or a point-the-bit mandrel, which produces a smoother borehole and higher rates of penetration. On a typical Alberta Montney well the curve section might build from vertical to 90 degrees over 250 to 300 m of measured depth at roughly 9 to 12 degrees per 30 m, then hold horizontal through a 3,000 m lateral. Smooth doglegs here protect the casing and let the completion crew run the frac string cleanly.

Why Deviation Enables Reserves a Vertical Well Cannot Reach

The commercial value of a deviated hole is access. A vertical well drains only the rock directly beneath it, but a deviated or horizontal wellbore exposes hundreds or thousands of metres of reservoir, multiplying contact with tight formations like the Montney or Duvernay that have almost no productivity vertically. Deviation also solves surface conflicts: a target under the Bow River, under a First Nations reserve requiring setback, or beyond a lease boundary can be reached by kicking the hole off from a permitted surface location. Relief wells that kill a blowout are precisely deviated to intersect the runaway wellbore thousands of metres down. In each case the deviated hole converts an otherwise stranded or inaccessible target into a producible, permittable well.

Related Terms

A deviated hole is the physical result of Directional Drilling, the discipline of planning and steering non-vertical wellbores. Its curvature is quantified by Dogleg Severity, which limits drag and casing wear along the path. The extreme case is the Horizontal Well, a deviated hole turned fully to 90 degrees to run along the reservoir, and the whole trajectory is mapped by Measurement While Drilling surveys that report inclination and azimuth in near real time so the actual path can be corrected against the plan.

Montney Multi-Well Pad Scenario near Dawson Creek

An operator developing the Montney near Dawson Creek in British Columbia permits an eight-well pad, each wellbore a deviated hole kicking off from vertical, building to 90 degrees over a 280 m curve, then holding a 3,200 m horizontal lateral. The laterals fan out on staggered azimuths so the eight wells drain a section of reservoir from one 4-hectare surface lease. Each deviated well costs roughly CAD 7.5 million drilled and cased, and the shared pad, road, and pipeline save an estimated CAD 6 million versus eight separate single-well leases while concentrating surface disturbance under a single BC Energy Regulator area-based permit.

Anti-collision planning keeps the closely spaced kickoffs from intersecting, and rotary steerable assemblies hold dogleg severity under 3 degrees per 30 m in the laterals so the frac strings run without hanging up. The result is eight producing wells draining more than 6,000 hectares of Montney from a footprint smaller than a soccer field.