Shallow gas migration through cement, in which gas from a shallow zone migrates to the surface before the cement can properly harden, affects zonal isolation and has been a major source of problems for many years. It is not easy to prevent the micro-annular gas fissures or permanent flow channels to the surface created due to this migration. Because they occur at shallow to medium depths, it is difficult to maintain sufficient hydrostatic pressure while the cement is still in a liquid state, and lower temperatures near the surface extend the transition period from the cement’s liquid to elastic-solid state. In applications where the well is going to be fractured, the problem can be further compounded due to ballooning and contraction of the pipe from the fracing pressure cycles.

This is becoming a particularly difficult problem as operators continue to push into unconventional shale plays characterized by a large number of natural fractures. Solutions that can effectively, economically, and permanently isolate these zones – and allow the cement to harden thoroughly prior to the onset of gas migration – are becoming increasingly critical to the long-term viability of the well.

Weatherford’s swellable Micro-Seal isolation system (MSIS) prevents fluid and pressure migration because of its proprietary hybrid swellable elastomer that conforms to and seals against micro-annular voids between the cement sheath inner diameter and the casing outer diameter.

Conforming to the problem

While swellable elastomers have demonstrated an ability to isolate wellbores in many types of wells, a range of construction challenges complicates isolation performance when conventional elastomers are used. These challenges range from short-term stimulation concerns to long-term micro-annulus isolation. Planning for these problems is complicated by differences in wellbore fluids and downhole temperatures, which affect swelling and compatibility.

Conventional elastomers tend to swell only in the presence of a single fluid phase – oil or water – and, depending on the downhole temperature, may swell too slowly to isolate the wellbore prior to gas migration. The proprietary hybrid elastomers used in the isolation system demonstrate greater flexibility and are activated by oil, water, wet gas, or a combination of any of these to conform to and seal off the micro-annulus. The elastomers also are designed to swell only when they are in place to avoid rapid reaction with wellbore fluid during deployment on the pipe or with the mix water in the cement while it is curing. The MSIS-Unit is the central system component. When combined with other mechanical cementing products, the unit can provide excellent casing standoff and highly efficient displacement of mud during primary cementing operations for improved wellbore stability in both regular and irregular annular geometries. The system’s ultimate goal is to create at least one point in the well where total annular isolation is achieved. However, the traditional deployment scenario consists of strategically positioning multiple tools on cemented pipe to provide a series of response points to ensure that fluid migration is stopped wherever it may occur. The customizable system comprises of a family of elastomers and centralizer designs for deployment in a range of wellbore geometries and reservoir temperatures. The elastomer is available in a slow-swelling form for deployment in wells with ambient temperatures of 66°C up to 150°C (150°F to 300°F) to prevent premature swelling and sticking prior to arriving at the designated isolation point. For wells in the range of 38°C (100°F) or lower, another elastomer can be used to swell more quickly and provide faster micro-annular isolation.

The bow string, as part of the isolation system, helps to enhance annular isolation. (Images courtesy of Weatherford)

The suitable elastomer is selected for each well after comprehensive screening tests during the planning stage, in which the elastomer packer seals are tested in actual well fluids for criteria including fluid compatibility, life expectancy, bond strength, swell ratio and elongation, and shear. With this information, an optimal elastomer is chosen that not only enables swelling times to be properly managed during deployment but also maintains a high-integrity seal during its required operating life.

The MSIS-Unit consists of a slip-on seal unit that is effective as a standalone product. It also can be deployed with one of several centralizers, depending on the wellbore. The MSIS-Bow is a slip-on bow-spring centralizer designed to provide optimal casing standoff in appropriate cased-hole and openhole sections. Finally, the MSIS-Rigid is a slip-on rigid centralizer with axial or biased blades designed to optimize mud displacement in vertical, inclined, or horizontal wells. The ultimate result of each of these designs is better mud removal during primary cementing operations for improved wellbore stability.

The swellable isolation system has demonstrated improved wellbore quality while reducing the frequency of interventions and remedial cementing operations. Strategic positioning of multiple systems across the expected problem area also has been shown to prevent buildup of annular casing pressure, which reduces or eliminates the need to bleed off at the wellhead.

Field tests proving potential

To date, more than 1,000 of these systems have effectively and efficiently shut off gas migration along the micro-annuli in cemented liners, including use in many US shale gas plays like the Marcellus. Producers in this region needed a long-term solution to isolate micro-annuli created during or after cementing operations or following fracture stimulation.

The company’s swellable elastomers yield many benefits and are formulated to meet an operator’s specific operational environment objectives.

The isolation units are deployed with a bow centralizer design on 11 3/4 -in., 9 5/8 -in., 8 5/8 -in., and 5 1/2 -in. casing/liner strings in open holes ranging from 8 1/2 in. to 14 in. As of September 2012, the units have been deployed in more than 70 wells in the field and have consistently prevented gas migration to the surface.

The technology has been implemented elsewhere, including offshore Angola, for an oil producer looking for a way to shut off potential water migration along annular voids in an 8 1/2 -in. hole with 7-in. casing cement sheath. This operator deployed the MSIS-Rigid at 3,271 m (10,733 ft), providing isolation above and below the oil/water contact zones and the oil-bearing zones at an inclination angle of 28°. To date, the installed casing string and units have successfully isolated water migration per the client’s production program.

The MSIS-Bow also has been deployed for a gas producer in the Middle East who wanted to mitigate against sustained annulus pressure and shut off potential pressure migration in a well. Several systems were installed on the 9.63-in. casing string inside 13.38-in. casing, at key points along a 27° to 31° interval to provide a series of response points for potential gas migration. The system’s integrated centralizer provided maximum casing standoff for mud displacement during cementing operations, and the combined action of the strategically placed units prevented the need for the remedial cementing operations that are typically required due to micro-annular pressure migration. Since this initial deployment, the operator has deployed similar MSIS-Bow units on six other wells in the region.

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