Field Trial Results of Annular Pressure Behavior in a High-Pressure/High-Temperature Well
- Pieter Oudeman (Koninklijke/Shell E and P Laboratorium) | L.J. Bacarreza (Shell E and P)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1995
- Document Type
- Journal Paper
- 84 - 88
- 1995. Society of Petroleum Engineers
- 1.14.1 Casing Design, 1.14 Casing and Cementing, 2 Well Completion, 2.4.3 Sand/Solids Control, 1.6.5 Drilling Time Analysis, 4.1.2 Separation and Treating, 3 Production and Well Operations, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 5.6.4 Drillstem/Well Testing
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The problem of annular pressure build-up due to heating of the strings by the (hot) well effluent, will typically be of concern in the design of casing strings for HP/HT subsea wells. In such completions bleeding off the pressure through the wellhead, as done routinely with surface wells, is not possible.
Theoretical studies indicated that, potentially, extreme pressures could develop in confined sealed annuli of subsea completions, posing a severe threat to well integrity. To investigate the effect under realistic conditions, Shell Expro (UK) ran battery operated gauges in the 9 5/8" X 13 5/8" annulus of an offshore HP/HT well, to record pressure and temperature changes during drilling, cementing and production testing.
This paper presents the results of these tests and a general model for pressure build-up in unconfined sealed annuli to correlate the field data. Since only pressure and temperature measurements were available certain assumptions had to be made with respect to e.g. formation response to pressure build-up, in situ liquid properties etc. Although these assumptions could be debatable, it is hoped that by publishing and sharing ideas, a better understanding of annular pressure behaviour can be developed.
The problem of annular pressure build-up due to fluid heating during production, in the sealed annuli of (HP/HT) subsea completions, has been addressed by various authors (see e.g. , ). Most theoretical treatises consider the annular fluids to be perfectly confined between the casings. In view of the incompressible nature of these fluids small changes in temperature, annular volume etc. lead to considerable changes in pressure in completely tight annuli. Hence, most model calculations predicted that extreme pressures could, potentially, develop in confined sealed annuli. Obviously, when this is taken into account in casing design for deep HP/HT wells, thick walled casing has to be selected to withstand the anticipated burst and collapse loads. In turn this gives rise to considerably increased requirements with respect to offshore rig capacity, maximum deck load etc.
In order to avoid the problem, Shell Expro (UK) adapted the policy of not cementing a casing up to the shoe of the previous casing, so that pressure could, in principle, bleed off to surrounding formations. A theoretical basis for this policy was lacking however. Such a basis, which includes the effect of liquid leakoff on annulus pressure is presented in the next chapter.
To validate theoretical expressions a dedicated field test with an instrumented casing string was carried out in an offshore HP/HT exploration well. The results of this test and a comparison with model calculations are presented. Finally, in view of the implications of these results for casing design, in particular for subsea wells, recommendations for further research will be discussed.
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