Influence of Casing Centralization on Cement Sheath Integrity During Thermal Cycling

Abstract

The objective of primary cementing is to protect the casing and to ensure zonal isolation. It can be difficult to obtain a good cement job along the full length of a well, and casing centralization is one of the main factors that influence this. Even if the dependence of cement placement on casing centralization is well-known, little information is available on how the degree of casing centralization affects the well during its production phase. Well temperatures cycle up and down as a part of normal production operations – and well barrier materials, in particular steel, cement and rock, will consequently repeatedly expand and contract their volumes. Over time, this is likely to induce debonding and radial cracking of the cement sheath which threatens well integrity. This paper reports the results of an experimental study mapping how, where and when the annular cement loses its sealing ability upon temperature variations, and how this is dependent on casing centralization. The studied samples consisted of rock, cement and casing, and the temperature was cycled in a controlled and programmable manner. In-situ monitoring by Acoustic Emission (AE) sensors detected the development of cracking and debonding in the samples during thermal cycling. Initial and post-experiment computed tomography (CT) scans provided complementary three-dimensional (3D) information on the geometry and location of the induced cracks and debonding. Our study compared the thermal cycling resistance of two samples, one with centralized casing and one with a 50% casing stand-off. The AE monitoring results indicated that most of the cracking/debonding occurred during the actual heating and cooling, and not in between cycles when the temperature was held constant. The CT analyses showed that the thermal cycling caused considerable enlargement of cracks and voids initially present in the cement sheath, and this enlargement was significantly more severe when the casing was not centralized. The paper presents, for the first time, a 3D visualization of cracks and debonded volumes in the cement sheath, and it underlines the importance of obtaining a good initial cement job. Also, it is shown that it is important to obtain a good casing centralization during well construction – not only for optimal cement placement, but also for maintaining well integrity during production.

Study of Thermal Variations in Wells During Carbon Dioxide Injection

“…Similar trends, displaying that both pressure and temperature variations aggravate the casing/cement bond, were observed by Boukhelifa et al (2004) and Bois et al (2012). Detailed experimental studies of annular-cement debonding and cracking were also recently performed by Albawi et al (2014) andDe Andrade et al (2014). Both these studies show that only a few thermal cycles in the range of -20 C to 120 C cause cement debonding and cracking that are visible in computed-tomography images.…”

Integrity of Downscaled Well Models Subject to Cooling

“…Several recent studies report on application of thermal loads to downscaled wellbore section models (Albawi et al 2014;De Andrade et al 2014). Albawi et al (2014) developed an experimental setup for thermal cycling with monitoring of acoustic emission and temperature, and tested it in a set temperature interval of Ϫ20 to 120°C to simulate conditions in an Arctic well.…”

“…Some increase in void/crack volumes was observed for both specimens after the thermal cycling, especially for the stand-off specimen. The temperature ranges used in these studies (Albawi et al 2014;De Andrade et al 2014) were limited to temperatures above 0°C. These studies revealed the need for cold thermal cycling (below 0°C).…”

“…;Kosinowski and Teodoriu 2012;Shadravan et al 2015) and thermal cycling of casing/cement/rock assemblies(Albawi et al 2014;De Andrade et al 2014), suggest failure of the cement sheath including debonding and/or radial cracks. Although thermal expansion/contraction of the casing during our thermalcycling experiments must have imparted some stresses on the cement sheath, no destructive changes in the cement structure were observed.…”

Study of Thermal Variations in Wells During CO2 Injection

“…Similar trends, displaying that both pressure and temperature variations aggravate the casing-cement bond, have been observed by Boukhelifa et al (2005) and Bois et al (2012). Detailed experimental studies of annular cement debonding and cracking have also recently been performed by Albawi et al (2014) andDe Andrade et al (2014). Both these studies show that only a few thermal cycles in the range Ϫ20°C to 120°C cause cement debonding and cracking that is visible in computed tomography (CT) images.…”