When specifying APB mitigation requirements the operator’s well design engineers will normally declare:
- The temperature and annular pressure range at which pressure mitigation is likely to take place
- The ‘collapse volume’ required. Collapse volume is the additional volume in the casing annulus required to be created by foam collapse and is based on the anticipated fluid thermal expansion and the residual annular pressure build-up that the casings are able to accommodate.
The volume of Balmoral Casing Guard required to achieve APB mitigation is dictated by the collapse volume for the particular annulus, based on the annular volume and temperature rise, and the collapse percentage of the selected foam for the specified collapse pressure band. Standard practice is to allow a modest excess of foam, typically 20-25%, as a safety margin should the collapse volume be underestimated.
In the majority of cases the APB is linked directly to annular fluid temperature increase upon well start-up. In this situation, the standard design practice for BCG is to select a foam grade where the matrix Tg is close to or less than the temperature at which the APB mitigation is to take place.
This ensures that BCG collapse pressure is highly sensitive to temperature change and collapse is guaranteed once the design APB mitigation temperature is reached.
The effect of temperature upon the volume compression behaviour of a BCG system based on a low pressure-rated HGMS and matrices of varying Tg is shown in Figure 4.
In certain circumstances APB may occur across a range of operating temperatures. In order to mitigate APB in these circumstances the foam performance is completely different, the requirement being for a system with a collapse pressure which is largely insensitive to temperatures across the range of interest.
This is achieved by selecting a foam matrix with a Tg far above the temperature range of interest. The only significant means of collapse pressure control for these systems is now through HGMS grade selection. The behaviour of such systems is illustrated in Figure 5.
Application and operation of Balmoral Casing Guard
Balmoral Casing Guard is supplied as an easy-to-install kit for site application. It is routinely supplied as foam quadrants, typically 0.5–1.5m in length. These quarter cylinders are bonded to the outside diameter to create a continuous cylindrical section or, where a complete cylinder could later result in restricted fluid flow, in two or three sections around the casing outside diameter. The effective outside diameter of the installed BCG quadrants is restricted to minimise damage to the foam during casing deployment.
The complete BCG kit comprises the quadrant sections, adhesive system, powered adhesive dispensing equipment, banding and full HSE and installation documentation.
Once installed and the coated casing run, BCG has no detectable effect upon normal well operations and remains passive. When the design collapse conditions are reached, BCG responds immediately to provide the degree of APB mitigation specified by the engineering design team.
Balmoral Casing Guard materials can be successfully adapted for wider use in downhole applications including radial cavity inserts for pressure deformation swaging where pressure/temperature combinations range from 35°C-130°C and 6000psi-10000psi.
Balmoral Offshore Engineering’s technical team would be pleased to discuss project-specific enquiries.