Nonlinear Stress-Strain Model for Confined Well Cement

Abstract

The cement sheath is the key for providing the zonal isolation and integrity of the wellbore. Oil well cement works under confining pressure, so it exhibits strong nonlinear and ductile behavior which is very different from that without confining pressure. Therefore, for the accuracy of the simulation and the reliability of well construction design, a reliable compression stress–strain model is essential for confined well cement. In this paper, a new axial stress–strain model for confined well cement is developed based on uniaxial and triaxial test data, examinations of failure mechanisms, and the results of numerical analysis. A parametric study was conducted to evaluate and calibrate the model. The model is simple and suitable for direct use in simulation studies and well design. Results from this study show the nonlinear compressive behavior of confined well cement can be predicted using the traditional uniaxial compressive strength test measurements.

Chemo-Physico-Mechanical Behavior of Class G Oil Well Cement Pastes

“…This range of values for cement paste Young's modulus agrees with previous measurements, e.g. [140,[206][207][208]. A reduction in the Young's modulus with increasing porosity is also expected based on previous observations, see e.g.…”

“…In the case of samples tested under confining pressure, the stress-strain response exhibits significant ductile behavior. This observation is in line with previous studies [173,208,210]. Compared to the unconfined samples that failed at an axial strain of approximately 0.55%, the confined reference samples were able to achieve 1% axial strain without losing load-carrying capacity.…”

“…The relatively small deformation load capacity in the absence of confinement is in agreement with previous research on cement pastes, e.g. [115,140,[208][209][210].…”

“…This value is close to that found by e.g. Jimenez et al [207] and Li et al [208]. Deviations from the initial linear behavior is observed starting from approximately 0.2% axial strain, or some 30 MPa axial stress.…”

“…This soil mechanics approach showed that the strength increase of cement paste can be linked to a compaction mechanism that occurs under increasing mean effective stress. MPa and 20 MPa, the ultimate strength increases upward to 80-100 MPa [208].…”