To perform heavy interventions, or workovers, rig crews remove the wellhead and other pressure barriers to allow full access to the well. Crews fill the well with kill weight mud to contain the formation pressure during an intervention. Kill weight mud is a dense fluid that creates a hydrostatic pressure at the formation that is greater than the formation's pore pressure.
Heavy interventions require a rig to remove and reinstall the completion hardware. In many cases, the operator's objective is to replace leaking or worn-out parts. Typically, this requires replacing only the failed parts and running the completion equipment back into the hole. In some cases, however, operators perform workovers to adapt the completion to reservoir conditions that have changed as a result of production. These changes may include the onset of water and sand production or formation pressure that has fallen too low to push fluids to the surface. Assuming the formation has reserves with economic potential, an operator may make adjustments that shut off water production, deploy sand control equipment or run artificial lift systems into the well.
Operators may perform a special type of workover option—a recompletion—to abandon one zone and open and complete another zone that was tested and left behind pipe when the well was drilled. In some cases, slickline may be used to shut off the first zone by running in the hole with a special tool to close a sliding sleeve that had been placed across the perforations as part of the original completion. Slickline is then used to open a sleeve to allow production from a secondary zone.
Typically, however, because of initial well conditions, sleeves are not a viable completion option and operators must first abandon the primary producing zone by placing a cement plug across it. They then install new completion equipment with which to produce from the secondary zone.
Operators are sometimes reluctant to use kill weight mud to perform heavy interventions because the dense fluid may permanently damage pressure-depleted formations. One option is to perform the heavy intervention with the well under pressure, as in light interventions, using a snubbing unit. Snubbing operations use a hydraulic jack to snub, or push, joints of pipe into a live well against well pressure. Although snubbing operations are similar to coiled tubing operations, the former use joints of stiff tubing or casing and can be performed in wells with significantly higher pressures than are possible with coiled tubing. Because snubbing equipment is more robust than that used in coiled tubing operations, it can be employed to perform nearly all operations that typically require use of a drilling rig.
Since the introduction of subsea wells in the 1970s, service companies have been developing methods to perform light interventions without costly off-shore drilling units. Using specially designed vessels, service companies perform slickline, wireline and coiled tubing operations through subsea wellheads using riserless or riser-based methods (Figure 3).