Structural Integrity of Casing and Cement Annulus in a Thermal Well Under Steam Stimulation
Authors: Wong R.C.K. , Yeung K.C. 
Published: March 2010

Abstract
Steam stimulation is one of the viable methods to extract heavy oil from oil sand reservoirs. In this thermal process, the injection well is subjected to high temperatures. Numerical simulations were used to investigate the structural integrity of the well in such a high temperature environment. In this study, stresses in the metal casing, cement annulus and surrounding shale formation were examined in detail to evaluate if the induced stresses exceed the material strengths under varying boundary conditions. These include the effect of heating rate, poor cement and shale formation properties.

Introduction
In Alberta, thousands of deviated wells are drilled through shale formation to extract heavy oil from underneath oil sand formation using thermal recovery processes. Thermal deviated wells in shale impose more technical challenges than conventional wells in consolidated rock because of their unique features. Thermal recovery processes, such as steam assisted gravity drainage and cyclic steam stimulation, involve high temperature operations up to 300 °C. Shales found in Alberta are compaction type without strong cementation, and have been known to swell and lose structural integrity upon exposure to water-based fluids. The occurrence of casing equipment impairment and failure after years of service in a shale formation are frequent.
A completed deviated well is made up of a series of annular rings of steel casing, oil well cement and shale of varying thicknesses. There are differences in mechanical, hydraulic and thermal properties among these materials. The overall response of a thermal well subject to heating is governed by not only each material, but also the properties of interfacial steel-cement and cement-shale interaction. Material properties of steel, oil well cement and shale have been well studied separately. However, the behaviour of steel-cement and cement-shale interfaces have not received much attention because the interaction behaviour is very complex, dependent on the imposed boundary conditions.
In a well completion, a borehole is drilled and advanced with drilling mud. Then, steel casings are inserted into the drilled hole filled with mud. The mud in the annular space between the steel casing and the drilled hole wall is removed by a clear wash or spacer fluid, followed by the cement slurry. In an idealized situation, all drilling mud should be removed and the annular space should be filled with cement slurry. However, the complete displacement of drilling mud by the spacer fluid and cement slurry may not be achieved. Often a residual mud layer adheres to the inside and outside surfaces of the borehole and casing, respectively. The thickness of this layer depends on the displacement processes (fluid velocity and annular space) and fluid rheology and density. Undesired mixing can be induced because there are differences in fluid rheology and density of drilling mud, washer fluid and cement slurry (non-Newtonian fluids). Local channel or fingering could occur in narrow spaces since the fluid displacement takes place in a narrow eccentric annular configuration bounded by the ircumferential surfaces of the drilled well and the steel casing(1).