Summary Microannuli at the well cement-sheath interfaces may result in loss of zonal isolation, which is the source of many problems, such as sustainable annular pressures, crossflows between reservoirs, and undesirable flow behind the casing. The microannuli are commonly explained by variations in cement volume during hydration (chemical shrinkage/expansion) or by contraction of the casing because of a decrease in mud density/temperature because these could create a gap if the cement is unable to follow the induced deformations. However, these two modes are not sufficient to predict all possible types of microannuli encountered in oil and gas wells, meaning that other modes have been missed. This paper presents a comprehensive mechanistic analysis of microannulus formation to highlight and explain other modes and to detail the conditions under which they can appear. It is grounded in both theoretical and experimental evidence and takes into account most of the features that characterize cement after it has been placed, including cement volume variations and heat production during hydration, mud-density and temperature variations, cement thermo-poro-elasto-plastic behavior during and after hydration, thermo-poro-elasto-plastic behavior of the formation, and initial state of stress in the formation.

fr -joelle.hy-billiot@total.com -virgile.rouchon@ifpen.fr -gerard.mouronval@total.com -marc.lescanne@total.com veronique.lachet@ifpen.fr -nicolas.aimard@total.com * Corresponding authorRésumé -Une méthode géochimique pour la surveillance d'un site pilote de stockage de CO 2 : Rousse, France. Approche combinant les gaz majeurs, l'isotopie du carbone du CO 2 et les gaz rares -Ce papier présente la caractérisation géochimique des différents gaz, naturels et anthropogéniques, impliqués dans un pilote de stockage de CO 2 en champ de gaz naturel appauvri (Rousse, France). Dans ce pilote, le CO 2 est produit par oxycombustion d'un gaz naturel transformé en gaz domestique à l'usine de Lacq. Ce CO 2 est transporté dans un pipeline de 30 km de longueur jusqu'au réservoir de gaz appauvri de Rousse. Les gaz produits à Rousse avant injection de CO 2 , le gaz commercial de Lacq et le CO 2 résultant de l'oxycombustion ont été échantillonnés, ainsi que les gaz situés dans un puits de surveillance (à une profondeur de 45 m) et les gaz du sol situés au voisinage de Rousse. Pour tous ces échantillons, la composition en gaz majeurs, la signature isotopique du carbone ainsi que l'abondance et signature isotopique des gaz rares ont été déterminées. Les compositions gazeuses du gaz naturel de Rousse sont comparables à celle du gaz domestique de Lacq avec le méthane comme composé principal et la fraction C 2 -C 5 et CO 2 comme gaz résiduels. Les gaz des sols reflètent typiquement des mélanges entre l'air (pôle pur) et le CO 2 d'origine biogénique (avec des teneurs maximales de l'ordre de 9-10 %), tandis que les gaz présents dans le puits de monitoring reflètent typiquement la composition de l'air sans excès de CO 2 . Le gaz de Rousse et le gaz domestique du site de Lacq ont une composition isotopique δ 13 C CH 4 égale à -41,0 ‰ et -43,0 ‰ respectivement. Le CO 2 injecté sur Rousse a une composition isotopique δ 13 C CO 2 égale à -40,0 ‰ à la sortie de la chambre d'oxycombustion, tandis que la composition isotopique δ 13 C CO 2 des gaz des sols est comprise entre -15 et -25 ‰. Le gaz naturel de Rousse et le gaz domestique du site de Lacq sont tous les deux enrichis en hélium, appauvris en néon, argon et krypton par rapport aux valeurs de l'air (standard naturel). Le procédé de combustion produit un CO 2 enrichi en hélium, hérité du gaz domestique de Lacq, et une composition en néon, argon et krypton reflétant celle de l'oxygène produit par l'unité de séparation d'air. En effet, le néon est appauvri relativement à l'air, tandis que le krypton est enrichi de 10 fois, résultant de la séparation cryogénique des gaz rares au sein de l'unité de séparation d'air. Les gaz rares des échantillons de sols ont une composition équivalente à celle de l'air. À partir de ces résultats, les compositions des pôles purs impliqués dans le site pilote de stockage de CO 2 montrent que les compositions en gaz rares produits par le procédé d'oxycombustion sont suffisamment

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Micro-annuli at the well cement sheath's interfaces may result in loss of zonal isolation, which is the source of many problems, such as sustainable casing pressures, cross flows between reservoirs or any undesirable flow behind casing. They are commonly explained by cement volume variations during hydration (chemical shrinkage/expansion) or by contraction of the casing due to a decrease in mud density/temperature as these could create a gap if cement cannot follow induced deformations. However, these modes are not sufficient to predict all possible types of micro-annuli encountered in oil and gas wells, meaning that other modes have been missed. This paper presents a comprehensive mechanistic analysis of micro-annulus formation to highlight the forgotten modes, to explain them, to detail under which conditions they can appear, and to present counter measures to prevent them. It is based on both theoretical and experimental evidences and takes into account most features that characterize cement after it has been placed, including cement volume variations during hydration, cement heat production during hydration, mud density and temperature variations, cement thermo-poro-elasto-plastic behavior during and after hydration, formation thermo-poroelasto- plastic behavior, and formation initial state of stress. Introduction Cement sheath is a key element for maintaining well integrity. The loss of zonal isolation due to the cement sheath can result not only in severe operational difficulties possibly leading to the loss of the well but also to dramatic environmental damages and ultimately, to fatal injuries. This defect of isolation can be due to improper cement placement resulting from difficult well inclination, from poor hole calibration, poor centralization, poor selection of chemical agents for mud removal or from poor adequation between volume/rheologies/displacement rate of cement train. It can be the result of pollution of cement slurry by the invasion of fluid from the surrounding formations to the cement matrix while cement sets, especially if free water develops or cement settles. All of these mentioned above can generate poor mud removal on wellbore/casing or contaminated cement, inducing risk of mud channels. Loss of zonal isolation can also be the result of inappropriate set-cement properties; pressurizing the casing in excess or applying thermal loads, like for Steam Assisted Gravity Drainage (SAGD) applications, can create mechanical stresses strong enough to badly damage the cement sheath and give ways for fluids to pass the cement barrier. The purpose of this paper is to describe micro-annulus formation through an exhaustive analysis, as it is important to identify the possible causes responsible for the leakage mechanisms, so a competent seal can be provided with cement sheath during the life of the well. Focusing one's attention to the cement slurry design or set-cement properties is not sufficient to provide and maintain zonal isolation. All the parameters linked to the well have to be taken into account as previously described. The cement slurry has to be mixed and pumped as per design to expect a satisfactory result but the various parameters like displacement parameters are of importance. Assuming the centralization of the casing/liner is optimized to guarantee a successful placement of the slurry, the quality of the mud combined with selection of the BHA and good drilling practices are also of primary concern to prevent the formation of wellbore wash-out and to provide an in-gauge wellbore for the next operations. One can take additional precautions to validate the expected volumes and somehow to ensure a correct coverage of the zones of interest like pumping special pills or using a caliper

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