Casing Failure in Salt Rock: Numerical Investigation of its Causes

Abstract

Numerous cases of casing failure have been reported worldwide. Depletion-induced compaction is one of the main factors that may cause casing failure. Stress perturbation in salt layers may accommodate rock flow which endangers the stability of cased wells. Besides, poor cementing jobs are recognized as one of the wellbore instability causes. These factors need to be considered to determine the mechanism behind the casing failure. In this study, the creep behavior of the caprock salt layer in the southwest Iranian oil fields is experimentally studied through a number of creep tests under conditions of elevated temperature and pressure. Then, a three-dimensional finite element analysis is utilized to examine the validity of various casing failure scenarios after calibrating the model with well logs and experimental data. The results confirmed that the combined effects of creep behavior of rock salt and cementing imperfection can be the main cause of failure in the wellbore under study, because the failure time and depth predicted by the numerical simulation were found to be in accordance with the real field data. By history matching, the geomechanical parameter of the salt layer has been modified for study of the casing failure in other wells in the same formation.

Appendix: Definition of Equations

The vertical effective stress at each depth is (Terzaghi 1943):

$$S_{{\text{V}}}^{^{\prime}} = S_{{\text{V}}}^{{}} - P_{{\text{p}}} ,$$

(5)

where, $S_{{\text{V}}}^{^{\prime}}$ is the vertical effective stress, $S_{{\text{V}}}^{{}}$ is the vertical stress integrated over the distance from the surface down to the desired depth according to density of the overlying layers (Table 1), and P_p is the pore pressure.

The horizontal effective stress at each depth can be estimated as follows (Fjaer et al. 2008):

$$S_{{\text{H}}}^{^{\prime}} = S_{{\text{h}}}^{^{\prime}} = \frac{\upsilon }{1 - \upsilon }S_{{\text{V}}}^{^{\prime}} ,$$

(6)

where, $S_{{\text{H}}}^{^{\prime}}$: Maximum horizontal effective stress, $S_{{\text{h}}}^{^{\prime}}$: Minimum horizontal effective stress, $\nu$: Poisson's ratio, and ${\text{S}}_{V}^{^{\prime}}$: Vertical effective stress.

The relationship between the minimum time increment and the element size in the numerical simulation is (Dassault Systèmes Simulia Corp. 2012):

$$\Delta t > \frac{{\gamma_{{\text{w}}} }}{6Ek} \left( {\Delta l} \right)^{2} ,$$

(7)

where Δt is the time increment; γ_w is the specific weight of the pore fluid, E is the elastic modulus of the rock, k is the permeability of the rock, and Δl is the typical element size near the boundary condition.

Von Mises stress is stated as follows (Bellarby 2009):

$$\sigma_{{{\text{VM}}}} = { }\frac{1}{\sqrt 2 }\sqrt {\left( {\sigma_{{\text{z}}} - \sigma_{\theta } } \right)^{2} + \left( {\sigma_{\theta } - \sigma_{{\text{r}}} } \right)^{2} + \left( {\sigma_{{\text{r}}} - \sigma_{{\text{z}}} } \right)^{2} } ,$$

(8)

where σ_VM is von Mises stress; σ_z is the overburden stress; σ_θ is hoop stress; and σ_r is the radial stress.