Low cycle cement fatigue experimental study and the effect on HPHT well integrity

摘要

Cement sheath is very important to maintain wellbore integrity in high pressure high temperature (HPHT) wells and steam injection wells. Due to the HPHT cycles experienced in the process of hydraulic fracturing, production and steam injection, the failure probability of low cycle cement fatigue is high in these wells and is likely to cause the failure of the zonal isolation and increase the casing failure probability. The experiment was set up to simulate conditions under which cement low cycle fatigue failure can occur. In the test, the casing was applied with zero based cyclic pressure to study the cement failure characteristics. The cement mechanical properties were measured at 14 days curing time under three different conditions: (1) room condition; (2) 167 °F, 14.7 psi; (3) 212 °F, 2610 psi; and used as the input data for finite element method analysis. The cement elastic strain and plastic strain developed in the experimental test was calculated by finite element method and thereby the cement cycles to failure can be predicted based on the strain-cycle relationship. As the confining pressure increase, the cement shows more plasticity and can hold more pressure cycles. At the temperature below 300 °F, the stress developed by thermal expansion has minor effect on the cement low cycle fatigue. The cement with higher Poisson's ratio and lower Young's modulus shows better low cycle fatigue behavior. The results of strain-cycle relationship were applied in HPHT gas wells in south Texas to predict the cement fatigue failure under different operations. This paper proposes the low cycle fatigue failure envelope that can help reduce the cement failure and improve the cement design in HPHT wells and steam injection wells.