Off-Pattern Well: Five-Spot Geometry, Sweep Efficiency, and WCSB Secondary Recovery

An off-pattern well is a production or injection well whose location is displaced laterally or diagonally from the idealized grid position it would occupy in a regular flood pattern, and that displacement changes how injected fluid moves through the reservoir, the volume of oil ultimately recovered, and the water/oil ratio (WOR) measured at offset producers. Secondary and tertiary recovery floods are designed around repeating geometric patterns such as the five-spot, the inverted five-spot, the line drive, the seven-spot, and the nine-spot, each of which fixes the spacing and angular relationship between injectors and producers so engineers can predict areal sweep efficiency, breakthrough time, and the fraction of the pattern that is contacted by the displacing front. In a normal five-spot, four injectors sit at the corners of a square with a producer at the centre; in an inverted five-spot the roles reverse, with a central injector surrounded by four corner producers. Real fields almost never honour these idealized geometries because wells were drilled on legal survey grids, on the best surface locations, or were inherited from an earlier primary-depletion campaign that predated any thought of waterflood. When a well sits off its theoretical pattern node, the streamlines between injector and producer become asymmetric: the displacing fluid travels a shorter distance along one flank and a longer distance along another, so water or gas breaks through early on the near side while bypassed oil is left in the far quadrant. The classic SPE work on the subject quantified this, showing that off-pattern wells in a five-spot tend to lower the expected oil recovery at the time of breakthrough and raise the WOR sooner than the symmetric base case. In the Western Canadian Sedimentary Basin (WCSB) the issue is pervasive because most pressure-maintenance floods in the Cardium, Viking, and Pembina pools were converted from wells drilled on a one-well-per-quarter-section pattern, then selectively converted to injection. Reservoir engineers compensate by reallocating injection volumes, throttling producers near the short streamline, adding infill wells to re-balance the pattern, or modelling the irregular geometry directly in a streamline or finite-difference simulator rather than relying on analytic pattern equations. Quantifying the recovery penalty of an off-pattern well, and deciding whether an infill drill or a rate change is the more economic fix, is a routine part of waterflood and CO2 flood surveillance across Alberta and Saskatchewan.

Streamline Distortion and Early Water Breakthrough

In a symmetric five-spot the distance from a corner injector to the central producer is equal along every flank, so the flood front advances uniformly and breakthrough occurs simultaneously around the producer. Shift that producer 80 m toward one injector and the streamline on that side shortens while the opposite streamline lengthens. Injected water races down the short path, breaks through, then continues to cycle water rather than displace oil, while the long-flank oil stays unswept. A Cardium pattern at Pembina with a producer offset 100 m from centre can see breakthrough months earlier on the near injector, pushing the producing WOR from roughly 2:1 to 6:1 well ahead of the symmetric forecast and trimming pattern recovery factor by several points.

Rebalancing Injection Versus Drilling Infill

The cheapest fix is usually rate reallocation: cut injection into the near injector and raise it on the far injectors to re-centre the pressure field around the off-pattern producer. This costs nothing but instrumentation and surveillance time, though it caps total pattern throughput. When the geometry is too distorted, an infill producer or a converted injector restores symmetry at a capital cost of roughly CAD 1.5 to 3.5 million per well in a shallow Viking or Cardium pool. Streamline simulation is used to compare the incremental recovery of an infill against the no-capital rate-balancing case, and the decision turns on oil price, well cost, and how much bypassed oil the simulator predicts in the long-flank quadrant.

Related Terms

An off-pattern well is best understood alongside the larger flood it sits in. A Waterflood is the secondary-recovery process whose regular pattern the well departs from, and its success is measured by Sweep Efficiency, the very quantity an off-pattern location degrades. The displacing fluid enters through an Injection Well, and the whole campaign falls under Secondary Recovery, the stage of field life after primary depletion when external energy is added to maintain reservoir pressure and push oil to producers.

Real-World WCSB Scenario: A Cardium Pattern at Pembina

A central Alberta operator runs an inverted five-spot CO2 and water flood in the Cardium at Pembina, with a central injector and four producers on a quarter-section grid. One producer was drilled 110 m off its pattern corner to avoid a surface lease conflict, so the streamline to the injector is roughly 20 percent shorter than the other three. Within 14 months CO2 and water broke through at that producer, lifting its WOR to over 7:1 while the other three held near 2.5:1 and the far quadrant logged residual oil saturation well above the swept-zone target. Voidage replacement and Hall-plot surveillance under the AER Directive 065 scheme approval flagged the conformance loss.

The engineering team cut injection allocation, re-centred the pressure field with a streamline model, and elected to drill a CAD 2.6 million infill producer in the bypassed quadrant rather than continue cycling CO2. The infill recovered an estimated 38,000 incremental barrels over five years, paying out inside two years at the prevailing price and confirming that the off-pattern penalty, not reservoir quality, had been stranding the oil.