As a subsea well is drilled, successively smaller casings are introduced creating a number of fluid-filled annular spaces. Only the inner-most annulus (A) can bleed off through the subsea tree. The outer annuli (B and C) may be open to fluid movement through open shoes but, in practice, these annuli frequently get plugged off during cementing by formation collapse or solids settlement.
Due to the combination of extreme water depths and hole depth in many modern offshore wells, HP/HT reservoir fluids can reach temperatures as high as 350ºF while flowing. When a well is brought on line these hot fluids rise up the casing string, elevating the temperature of the casings and annular fluids during flow.
If thermal expansion of the annular fluids is restricted by plugging, a major pressure increase, known as annular pressure build up (APB), occurs. Pressure increases of 80-150psi/ºF are typical, meaning that even a relatively modest temperature increase of 100ºF can result in pressures approaching 15,000psi.
It is not generally cost effective and, in many cases, impractical to design the casing string to tolerate the potentially massive APB’s in modern wells. Engineered APB mitigation systems are therefore required to accommodate such expansion.
APB is known to have caused the rupture of intermediate and production casings on several wells over the last 10 years. The financial impact and recovery implications of such failures are enormous.
The theory of APB mitigation
Recognised APB mitigation systems include precision-engineered bursting discs and nitrogen fluid spacers. Potentially, however, the most simple mitigation system is that of syntactic foam elements with designed collapse performance located within the casing annuli.
The annular volume increase resulting from this engineered foam collapse accommodates the thermal expansion of the trapped fluid and significantly reduces APB.
Balmoral is acknowledged as an innovative and proactive developer in the fields of advanced syntactic foam systems for subsea buoyancy and insulation use. The company has now developed a family of unique syntactic foam systems that provide protection against excessive APB in casings during the start-up of subsea wells.
Known as Balmoral Casing Guard (BCG), in-house technologists developed the materials by applying behaviour patterns for syntactic foams, originally established for more traditional buoyancy and insulation systems, to the unique requirements of APB mitigation.
Pure syntactic foams comprise hollow glass microspheres (HGMS) suspended in a thermoset resin matrix. HGMS grades are available with collapse pressure ratings ranging from 250-28,000psi; these collapse pressures are not temperature sensitive. Discrete steps in collapse pressure mean that, in the absence of other factors, protection against APB could only be provided at those specific pressures determined by the glass microsphere pressure ratings.
However, the collapse pressure of each microsphere grade can be enhanced by the selection of a matrix resin system with appropriate mechanical performance. By careful selection, syntactic foams with a continuum of collapse pressures from 250-28,000 psi can therefore be created.
As the mechanical performance of the matrix resin system is temperature dependant so too is the collapse pressure of the syntactic foam even though the collapse pressure of the glass microspheres is not temperature dependant in isolation. It is therefore possible to formulate a syntactic foam which will suffer hydrostatic collapse (HCP), and thus APB mitigation, at effectively any combination of pressure and temperature.
The effects on HCP of microsphere grades of temperature and resin matrix Tg are illustrated in Figures 1 and 2.
The amount of foam volume contraction upon hydrostatic collapse - known as the ‘collapse percentage’ - is a function of the volume fill of the foam by microspheres. Contractions of 30-40% are routinely available.
The volume contraction behaviour of a typical medium Tg grade of Balmoral Casing Guard is given in Figure 3.