International audienceThe poromechanical behaviour of hardened cement paste under isotropic loading is studied on the basis of an experimental testing program of drained, undrained and unjacketed compression tests. The macroscopic behaviour of the material is described in the framework of the mechanics of porous media. The poroelastic parameters of the material are determined and the effect of stress and pore pressure on them is evaluated. Appropriate effective stress laws which control the evolution of total volume, pore volume, solid volume, porosity and drained bulk modulus are discussed. A phenomenon of degradation of elastic properties is observed in the test results. The microscopic observations showed that this degradation is caused by the microcracking of the material under isotropic loading. The good compatibility and the consistency of the obtained poromechanical parameters demonstrate that the behaviour of the hardened cement paste can be indeed described within the framework of the theory of porous media

A method is presented for the evaluation of the permeability-porosity relationship in a lowpermeability porous material using the results of a single transient test. This method accounts for both elastic and non-elastic deformations of the sample during the test and is applied to a hardened class G oil well cement paste. An initial hydrostatic undrained loading is applied to the sample. The generated excess pore pressure is then released at one end of the sample while monitoring the pore pressure at the other end and the radial strain in the middle of the sample during the dissipation of the pore pressure. These measurements are back analysed to evaluate the permeability and its evolution with porosity change. The effect of creep of the sample during the test on the measured pore pressure and volume change is taken into account in the analysis. This approach permits to calibrate a power law permeability-porosity relationship for the tested hardened cement paste. The porosity sensitivity exponent of the power-law is evaluated equal to 11 and is shown to be mostly independent of the stress level and of the creep strains.

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Ghabezloo et al. (2008): The effect of undrained heating on a fluid-saturated hardened cement paste 150The effect of undrained heating on a fluid-saturated hardened cement paste AbstractThe effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests.The anomalous thermal behaviour of cement paste pore fluid is back analysed from the results of the undrained heating test.

The reactivity of a crushed well cement in contact with (1) a brine with dissolved H2S-CO2; (2) a dry H2S-CO2 supercritical phase; (3) a two-phase fluid associating a brine with dissolved H2S-CO2 and a H2S-CO2 supercritical phase was investigated in batch experiments at 500 bar and 120, 200 degrees C. All of the experiments showed that following 15-60 days cement carbonation occurred. The H2S reactivity with cement is limited since it only transformed the ferrites (minor phases) by sulfidation. It appeared that the primary parameter controlling the degree of carbonation (i.e., the rate of calcium carbonates precipitation and CSH (Calcium Silicate Hydrates) decalcification) is the physical state of the fluid phase contacting the minerals. The carbonation degree is complete when the minerals contact at least the dry H2S-CO2 supercritical phase and partial when they contactthe brine with dissolved H2S-CO2. Aragonite (calcium carbonate polymorph) precipitated specifically within the dry H2S-CO2 supercritical phase. CSH cristallinity is improved by partial carbonation while CSH are amorphized by complete carbonation. However, the features evidenced in this study cannot be directly related to effective features of cement as a monolith. Further studies involving cement as a monolith are necessary to ascertain textural, petrophysical, and mechanical evolution of cement.

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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

Cement sheath design has been undergoing very deep changes for the last five years, mainly thanks to Carbon Capture and Storage (CCS). Indeed, for this application, Operators will have to demonstrate to national regulators that cement sheath can provide zonal isolation not only during the operational phases, but also during another phase: the long term storage. Stakes are as big as the market is, covering CCS plus traditional Oil and Gas market, where harmless operations to the environment know-how has become decisive to open new business opportunities. To ensure zonal isolation, cement system should be properly designed. It should be correctly placed to ensure it is hardening as per designed. Finally, its mechanical properties should be in adequacy with the successive operations the well is submitted to. Many years dominated by rules of thumb to evaluate cement sheath damage have progressively been replaced by engineered methodologies implemented in software. They can be numerical or analytical based, dealing with mechanics of solid or porous media. What should matter are not their pros or cons, but the correct assessment of their limitations to appreciate simulation results. To evaluate stresses a material can withstand before failure, constitutive laws and failure criteria are necessary but not sufficient: Initial state of stress is mandatory as it sets the initial distance from failure. After presenting the state of art and its weaknesses to evaluate cement sheath initial state of stress, this paper presents Total's methodology based on porous mechanics where initial state of effective stress results from cement hydration evolution with time under downhole conditions. The final state of stress is the consequence of the initial state plus additional stresses from operations. This methodology has been implemented in Total's dedicated software and has been successfully applied to design cement sheath on various operations worldwide: One detailed in this paper. Introduction After a deep look at well design best practices, there is a significant gap between all the achievements done for casing design and so little for cement sheath design, whereas both components are mandatory to achieve perennial primary and secondary barriers. If primary models and methodologies for cement sheath design were issued in the 90s, it is only recently that the petroleum industry's consideration towards this essential well component has considerably increased. An explanation may be the dramatic increase in deep water wells and their associated costs since the late 90s, which have pushed companies to find solutions to mitigate associated risks. One of the main risk is sustain casing pressure (SCP), possibly leading to casing collapse and heavy workover or worse, to the loss of 10 to 50 million dollars wells. Another explanation may be that new wells are more and more technical and expensive, which suggest better understanding when designing and greater practices to mitigate these risks. However, it seems that CCS, considered as one of the solution to mitigate climate change1, and presenting a potential market of 220–2,200 Gtons of CO2 cumulatively, has the most largely contributed to this cement sheath design renewal. Indeed, wells are considered as the main potential leakage pathway for storage and companies should prove that no well will leak over time, especially once abandoned, to safely deploy CCS technology. This has kicked off the interest in tools and methodologies to design cement sheaths that remain tight over the life of the well.