Casing Design for Trapped Annular Pressure Buildup
- A.S. Halal (Enertech Engineering & Research Co.) | R.F. Mitchell (Enertech Engineering & Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1994
- Document Type
- Journal Paper
- 107 - 114
- 1994. Society of Petroleum Engineers
- 1.3.2 Subsea Wellheads, 1.14 Casing and Cementing, 5.6.4 Drillstem/Well Testing, 5.1.5 Geologic Modeling, 5.2.2 Fluid Modeling, Equations of State, 1.14.1 Casing Design, 1.10 Drilling Equipment, 4.1.5 Processing Equipment, 1.2 Wellbore Design, 4.1.2 Separation and Treating, 1.6 Drilling Operations
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Conventional single-string analysis for casing design with annular-fluid expansion can underpredict or overpredict pressures between strings because multistring effects are neglected. Multiple-string systems with multiple sealed annuli behave as composite interactive systems. This paper presents a constitutive-based multistring analysis method for composite string effects and complex fluid behavior. The composite stiffness of cemented casings is determined from elastic stress/strain relationships, and the nonlinear fluid behavior is modeled by direct use of fluid PVT relations in the formulation and solution. The method is incorporated in a computer model linking comprehensive stress calculations to accurate temperature and pressure predictions. Sensitivity studies of the system response to various key parameters and operating conditions are presented, and comparisons are made with single-string analyses to demonstrate the strong interaction between casing strings.
The readily accessible wellheads of onshore and platform wells allow the operator to monitor and bleed off all annuli. However, this is not the case with subsea wells, where limited access to the wellhead requires careful consideration of annular heatup pressures during casing design. Subsea high-pressure, high-temperature (HPHT) wells can experience significant casing heatup, not only during production, but also during testing and drilling. Tubulars elongate and trapped annular fluids expand, causing severe loads that must be considered during casing design. Burst and collapse are not the only concern. Casing axial loads from constrained thermal elongation together with "reverse ballooning" from high annular pressure can generate sufficient compression to relieve all hanging weight and cause upward forces at the mudline hanger.
Attempts to assess heatup pressures and casing stresses using single-string analysis methods yield results that are in most cases too high because of the usual assumption that the wellbore around the casing of interest behaves as a fully rigid structure. In this case, the presumed rigidity of the outer casings or formations more than offsets the relief caused by the assumed absence of heatup pressure on the inside. This results in significant overprediction of annular pressures and possible overdesign of the casing.
On the other hand, if only the uncemented portions of the neighboring casings are assumed to be flexible, then it would be extremely difficult to determine whether the results are overpredicted: this becomes a function of the location of top of cement (TOC), among many other factors. Therefore, one must be exceedingly prudent when using this approach because casing underdesign is possible as a result of underprediction of burst or collapse loads.
In reality, the elevated annular pressures caused by fluid heatup and the corresponding radial, axial, and hoop stresses must attain collective equilibrium across the whole wellbore. Furthermore, the formation and cemented portions of the casings behave as flexible composite systems, and their elastic response should not be ignored. Hence, conventional single-string analysis methods, may be inadequate, highlighting the need for a multistring approach. Adams presented a multistring analysis method that takes into account axial forces and casing interdependence. He used two separate analysis models: a "classic" model for hydrostatic and heatup pressures and a 1D finite-element model for axial forces. The two models were used in an iterative manner to arrive at a converged solution.
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