Mechanical Expansion of Steel Tubing as a Solution to Leaky Wellbores

The reported experimental procedure has two main components that are critical: composite cylinders that simulate wellbores and the expansion fixture that is used to carry out mechanical manipulation of the cement.

Wellbores are the main gateway for production of subsurface fluids (water, oil, gas, or steam) as well as injection of various fluids. Regardless of its function, the wellbore is required to provide a controlled flow of produced/injected fluids. Wellbore construction has two distinct operations: drilling and completion. Wellbore cement, part of the completions procedure, primarily provides zonal isolation, mechanical support of the metal pipe (casing), and protection of metal components from corrosive fluids. These are essential elements of uncompromised, fully functioning wellbores. The integrity of the wellbore cement sheath is a function of the chemical and physical properties of the hydrated cement, the geometry of the cased well, and the properties of the surrounding formation/formation fluids^2,3. Incomplete removal of drilling fluid will result in poor zonal isolation since it prevents formation of strong bonds at interfaces with rock and/or metal. Cement sheaths can be subjected to many types of failure during the life of a well. Pressure and temperature oscillations caused by completion and production operations contribute to the development of fractures within the cement matrix; debonding is caused by pressure and/or temperature changes and cement hydration shrinkage^4,5,6. The result is almost always presence of microannular fluid flow, although its occurrence can be detected early or after years of service life.

Heathman and Beck (2006) created a model of cemented casing subjected to over 100 pressure and temperature cyclic loads, which showed visible debonding, initiation of cement cracks which can pose preferential pathways for migrating fluid⁷. In the field, the expansion and contraction of metal components of a wellbore will not coincide with those of cement and rock, causing interfacial debonding and formation of a microannulus, leading to an increase in permeability of the cement sheath. An additional casing loading can cause the propagation of radial cracks in the cement matrix once the tensile stresses exceed the tensile strength of the material⁸. All of the aforementioned cement failures can result in micro-channeling, which leads to gas migration, the occurrence of SCP, and long-term environmental risks.

A considerable number of producing and abandoned wells with SCP constitute a potentially new source of continuous natural gas emission⁹. The analysis conducted by Watson and Bachu (2009) of 315,000 oil, gas, and injection wells in Alberta, Canada also showed that wellbore deviation, well type, abandonment method, and the quality of cement are key factors contributing to potential well leakage in the shallower part of the well¹⁰. The existing remedial operations are costly and unsuccessful; the squeeze cementing, one of the most commonly used remedial techniques, has a success rate of just 50%¹¹.

In this paper we report on the evaluation of the Expandable Casing Technology (ECT) as a new remediation technique for leaky wellbores^12,13. ECT can be applied in new or existing wells¹⁴. The first commercial installation of this technology was performed by Chevron on a well in shallow waters of the Gulf of Mexico in November 1999 ¹⁵. The current operating envelope for expandable tubulars encapsulates an inclination of 100° from vertical, temperature up to 205 °C, mud weight to 2.37 g/cm³, a depth of 8,763 m, hydrostatic pressure of 160.6 GPa and a tubular length 2,092 m ¹⁶. A typical expansion rate for solid expandable tubulars is approximately 2.4 m/min ¹⁷.

This study offers a unique approach to the adaptation of ECT technology as a new remediation operation for SCP. The expansion of the steel pipe compresses the cement which would result in closure of the gas flow at the interface and seal the gas leak. It is important to mention that the focus of this study is the sealing of an existing microannular gas flow, therefore we only focused on that as a possible cause of leaky wellbores. In order to test the effectiveness of newly adapted technology for this purpose, we designed a wellbore model with an existing microannular flow. This is obtained by rotating the inner pipe during cement hydration. This is not to simulate any field operations, but simply to fast-forward what would happen after decades of thermal and pressure loading in a wellbore.