Annular Pressure Build-up (APB)
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.
Next: The theory of APB mitigation
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