Numerous casing and production-liner deformation/failure problems have been reported in high-porosity chalk formations in both the overburden and the reservoir sections, causing costly operation problems that prevent workovers and recompletions. This paper presents the results of studies performed to investigate stability of an openhole, cemented liner and uncemented-liner completions in a highly compacting chalk formation. The effects of critical cavity dimensions caused by various acid-stimulation techniques were also investigated.
On the basis of the review of historical caliper-survey data, we ascertain that axial-compression collapse is a major liner-deformation mechanism in the reservoir zones. Axial-compression collapse has been found in both low-angle wells (also in buildup sections of horizontal wells) and horizontal laterals. The casing deformation in low-angle sections is a result of reservoir compaction (i.e., change in the vertical formation strain), while the deformation in horizontal sections is primarily induced by increased axial loading because of cavity deformation. The current completion practice using cluster perforations and high-volume acid treatments causes vertically enlarged cavities, resulting in poor radial constraint.
A series of laboratory triaxial tests was performed on selected reservoir chalk samples to measure the stress/strain and failure behavior of the chalk formation considering a wide range of porosity and water saturation and different levels of confining pressures. Using the chalk-failure criteria and constitutive relations developed from the analysis of laboratory triaxial-compression-test data, a 3D nonlinear poroelastic/plastic finite-element-method (FEM) model was developed for the openhole stability analysis. The simulation results show that the abnormally high ductility of chalks after pore collapse around a borehole could actually enhance borehole stability, with a magnitude beyond expectation.
In this study, analytical and numerical models are also developed for evaluating cavity-induced axial-compression collapse of production liners. Model results indicate that the risk of the cavity-induced axial-compression collapse substantially increases for short perforated intervals stimulated with large acid treatments. However, increasing the perforation-interval lengths along the entire liner axis results in more-uniform acid distribution and will greatly reduce the chance of axial-compression collapse caused by localized cavity deformation. On the basis of these analysis results, key completion design criteria and stimulation strategies were developed for wells completed in highly compacting chalk reservoirs to reduce risk of casing and liner mechanical problems.