Oil flow through a 100 um microannulus (green) is unabated in neat cement, but annular integrity is completely regained within hours when FUTUR technology (blue) is used.

Long known to be seismically active, the Alberta Foothills are characterized by tough drilling problems and post-completion damage to wells’ cement sheaths caused by changing stress fields. The stresses can cause loss of cement integrity that can result from cement debonding from the casing or formation interfaces, allowing gas to migrate to the surface. In the best case, the gas is trapped in the wellhead and shows up as sustained casing pressure. In cases where the surface casing vent is left open, such as in Alberta, gas can vent to the atmosphere. Possibly a worst case occurs when gas migrates into the earth surrounding the wellhead, affecting vegetation growth and contaminating groundwater.

Neither situation is good. When surface casing vent flows or sustained casing pressure are discovered, the well must be shut-in and the condition remedied. In some wells the cost of remediation is too high or the condition is deemed unrepairable and they are abandoned. However, the decision to abandon the well does not relieve the operator of the responsibility of fixing the leak before proceeding with abandonment. The problem can occur at any time during the life of the well, even after abandonment. This is not unique to this area. Reports of loss of cement integrity have been received from Central Europe, the Middle East and the Unites States. It has been estimated that thousands of producing wells are at risk.

A Canadian operator was concerned about the hydraulic integrity of its gas wells in the Alberta Foothills region. The engineers became aware of a new active set-cement formulation called FUTUR from Schlumberger that possesses properties that enable it to detect and seal hydrocarbon leaks. Pumped conventionally using no special techniques or equipment, the cement can be spotted above and below producing zones by staging it during the primary cement job. Placement can be targeted, with a recommendation that 500-ft (150-m) lengths of the active set-cement be placed above the zone to be isolated, as well as behind the previous casing (during the previous cementing operation). The cement sets up conventionally and performs the same function as a traditional cement sheath with respect to casing support and zonal isolation, with the added benefit of remaining active in the event of hydrocarbon leaks.

Later in the life of the well

At any time during the life of the well, changing stress fields around the well bore can cause de-bonding of the cement sheath from casing or formation or loss of cement sheath integrity. In either case the condition can create flow paths that would allow gas to migrate upwards. Slowly and steadily, the gas can build pressure in the casing head or vent to the atmosphere. Stress changes can come from natural tectonic or seismic activity, subsidence or active faults in the area. However, when the hydrocarbon reaches the FUTUR cement, a reaction occurs causing the cement to swell, thereby resealing the leak. With the leak sealed, the swelling activity stops, until further stress changes cause more cracks. The cement can seal and reseal against hydrocarbon leaks indefinitely, even after commercial gas production ceases and the well has been abandoned.

A perennial problem with cement jobs is microannulus. This is a gap that forms between the casing and cement in conjunction with curing. Microannulus can be caused if there is a thin coating of oil on the outside of the casing or if the casing is subject to internal pressure when it is cemented that is subsequently relieved while the cement is curing. The microscopic shrinking of the pipe can cause a microannulus. Microannuli are hard to identify and localize, and even harder to repair. But with active set-cement any hydrocarbon that tries to seep through the microannulus will cause the cement to swell, sealing off the leak permanently. Tested in a flow loop with oil, a sample of neat cement with a 100 ?m induced microannulus showed no change in oil leak rate after three days. A similar sample cemented using active set-cement sealed the leak completely within a few hours.

Reaction time is fast, and sealing is permanent. In laboratory testing to prove the self?healing effect under exposure to gaseous hydrocarbons, cement plugs were cured within a test cell and microannulus was created. The cell was then subjected to nitrogen flow to confirm the presence of a microannulus, indicating that the cement integrity had been compromised. The system was then switched to natural gas at 3,000 psi (21 MPa). Within 30 minutes the FUTUR cement had activated, reducing the gas flow to negligible quantities. Neat cement could not compare. Within five days gas flow around the annulus was almost as high as the original nitrogen leak.

When the operator used the new cement in one of its Foothills wells, in an area where sustained casing pressure and gas venting problems had been experienced previously, the job was conducted normally and without incident. More than a year later, there have been no signs of pressure buildup in the annulus or venting at the surface.

Not exclusive to Canada

While this operator’s application focused on sustained casing pressure and surface venting, others have found that crossflow to an adjacent formation through a leak in the cement sheath can be sealed off as well. In Europe, there are several gas storage wells that have benefited from the active set-cement technology. In Italy, gas storage wells are often located near densely populated urban settings where any leak poses an unacceptable risk. The use of FUTUR allowed the operator to achieve excellent results. In their first deployment of the active set-cement, the well had a deviation of 48° and there was high risk of gas leaking through the set-cement matrix because of cyclical casing stresses caused by alternating production and injection phases. The cement was used to seal around the 7-in. liner across the injection/production zone as well as the 7-in. casing tieback. The post-job Isolation Scanner cement evaluation log showed optimal cement distribution and bonding across the intervals treated.

Reliable hydraulic isolation is the goal of any cementing program. This objective can be met in many ways. Operational solutions focus on improving field techniques such as improving efficiency and quality by consistent mixing of slurries so the results are predictable and do not vary from job to job. Materials solutions focus on improved distribution, more uniform slurries, improved strength-to-weight ratios and reduced critical hydration times. The FUTUR cement formulation meets these objectives when it is first pumped and set, but in addition it maintains its ability to form a permanent seal forever, for the life of the well.