metodo de caracterizacao do comportamento mecânico e de cimentos. a presente invencao se refere a um processo para a determinacao de parâmetros mecânicos de um sistema cimentario, em funcao do tempo, e em funcao da finura do sistema comentario, da pressao e/ou da temperatura, representativas das condicoes in situ encontradas nos pocos de perfuracao. a composicao inicial do sistema cimentario, sua finura (fi) e a velocidade das ondas de compressao e em funcao do tempo vp(t) sao os unicos dados de entrada do processo. este compreende: - uma etapa a de estimativa do grau de hidratacao do sistema cimentario, em funcao do tempo (alfa)(1), a partir de vp(t), a uma pressao p1 e uma temperatura ti; - uma etapa b, na qual (alfa)(1) e determinada em funcao de valores desejados de finura (fi)n do sistema cimentario, de pressao pn, e/ou de temperatura tn; - uma etapa c, na qual a composicao do sistema cimentario em funcao do tempo c(t) e em funcao de valores desejados de finura (fi)n do sistema cimentario, de pressao pn e/ou de temperatura tn, a partir de (alfa)(t) determinado na etapa b; e uma etapa d de determinacao de pelo menos um parâmetro mecânico do sistema cimentario, em funcao do tempo e em funcao dos valores desejados de finura (fi)n do sistema cimentario, de pressao pn, e/ou de temperatura tn a partir de c(t) determinada na etapa c. de acordo com o processo da invencao, esses parâmetros podem ser determinados desde o mais novo tempo do sistema cimentario. em particular, os parâmetros de deformabilidade estatica e parâmetros de acoplamento hidromecânicos sao determinados pelo processo, de acordo com a invencao.

Cement sheath integrity under high-pressure and high-temperature (HPHT) conditions in the Maharaja Lela Jamalulalam (MLJ) field in Brunei is challenging to achieve because of the harsh downhole conditions. Pressures reach 15,000 psi [103 MPa], downhole temperatures rise to 165°C [330°F], and the narrow margin between the pore pressure and the fracture gradient makes mud removal and cement placement even more challenging. In addition, pressure and temperature cycling during production means cement selection with right properties is essential to maintain long-term wellbore integrity. The cement system must deliver cement mechanical properties that can withstand the downhole stresses over time. Mechanical failure of the cement sheath would create a path for wellbore fluid migration that could result in sustained casing pressure (SCP) or interzonal communication. This paper will discuss the challenges of cementing in the tight pore-to-fracture window in the MLJ field, and the design and placement of one of the world's heaviest (2.48SG [20.7 lb/gal]) flexible and expandable resilient cement systems. Wells in the MLJ field reach into a deep reservoir (4500 to 5000 m true vertical depth, TVD) where the pressures can reach up to 15,000 psi with bottomhole static temperatures reaching 165°C. The well is drilled using 2.18 SG [18.2 lb/gal] mud weight, and the production string is pressure tested at 15,000 psi; during the drilling and completion, the temperature variation is on the order of 60°C [108°F]. Based on the above well conditions, advanced computer-based simulations were used to determine the cement mechanical properties required to withstand the pressure and temperature changes. To achieve the required mechanical properties, special blends were prepared with engineered particles to impart flexibility and expansion for a 2.48 SG cement system. To manage the cement placement, state-of-the-art cementing design software was used to accurately simulate the bottomhole circulating temperatures and pressures during placement as well as to simulate pipe centralization; a 3D model for fluids displacement also proved valuable.

The knowledge of the behaviour of oil-well cement paste from the early age to the hardened state is important in predicting the performances of the cement sheath in oil/gas wells, specially for prediction of the risk of micro annulus creation between the cement sheath and the rock formation or the casing. Characterization of the early-age mechanical behaviour is of particular importance, because during the well construction, the cement sheath is submitted to various mechanical loadings when the cement paste is not completely hydrated, as for example during a casing test. In this paper, the early-age mechanical behaviour of a class G cement paste is studied experimentally. A specially designed experimental device is used to investigate the mechanical behaviour of cement paste from the first hour of hydration, under stress states close to in-situ conditions. The macroscopic shrinkage of the cement paste as well as its stress-strain response under oedometric loading are studied for various pressures during hydration, from 0.3 to 45 MPa, and for hydration temperatures from 7 to 30 degrees C. These relatively low temperatures are used to slow down the hydration rate. The experimental results show that the macroscopic shrinkage increases significantly with the pressure during hydration. When submitted to a mechanical loading cycles at a given age, the cement paste hydrated under lower pressures, corresponding to shallower depth, exhibits higher deformability, showing a higher risk of creation of a micro-annulus during the mechanical loadings, as for example during a casing test. The experimental results clearly show the influence of the loading history on the mechanical behaviour of the cement paste at a given age. The oedometric experiments are associated with UCA (Ultrasonic Cement Analyser) and isothermal calorimetry experiments for a deeper insight into the behaviour of early age cement paste.

The purpose of this experimental study is to highlight the effects of leaching (by ammonium nitrate solution) on time-dependent behaviour of an oil-well cement paste under the temperature of 90 degrees C. Uncoupled creep tests on sound and leached samples and coupled chemo-mechanical creep tests on initially sound samples are performed under confining pressure of 3 and 10 MPa. Leaching significantly increases the porosity and the compaction of the material and weakens its mechanical properties. Compared to sound material, leached material has higher instantaneous and creep strains which are both amplified with confining pressure. Leaching very quickly generates tertiary creep and failure of the sample under confining pressure of 3 MPa due to how easily the ammonium nitrate solution permeates into the sample. However, the increase of confining pressure to 10 MPa slows down the penetration of the ammonium nitrate solution into the sample and the tertiary creep is not observed during the time of testing. Such coupling has to be taken into account in the evaluation of long-term behaviour.

It is well known that cement shrinks during hydration leading to a drop of stresses in the cement sheath below the hydrostatic pressure applied right after cement placement. This phenomenon might affect the integrity of the cement sheath under pressure and thermal loads taking place during the well lifecycle. A standard practice in the industry is to add to the cement expansion additives to balance the effects of shrinkage. When designing the cement recipe, a recurrent question is the percentage of additives by weight of cement (BWC) that needs to be added to fulfill technical requirements, yet at the lowest possible cost. It is believed for example that exaggerated expansion could be counterproductive because of the development of too high stresses that might fracture the set cement. Another important question is whether expansion can be activated without external water or pore pressure supply, which is the case if the cement is in contact with a shale formation or it is isolated from the reservoir by an impermeable mud cake or if the cement is placed between two casings. Cement permeability itself becomes an important parameter if the activation of expansion do require a source of water and/or pore pressure supply. The API RP 10B-5 (ISO 10426-5) recommends to use either the annular ring test or the membrane test to measure shrinkage/expansion of well cement formulations at atmospheric pressure. In the case of the ring, the cement specimen is in direct contact with water while in the membrane test it is not. Many companies modified the protocol of ring test by applying a water pressure to mimic the hydrostatic well pressure and to be able to increase the temperature. The ring test can be considered to simulate the case of a cement isolating a permeable reservoir and the membrane test the case of a cement placed either in front of an impermeable formation (shale for instance) or between two casings. In practice, most of the time, expansion is evaluated in the ring setup without paying attention to its validity outside the conditions of this test. In the recent years, Total has developed advanced cement testing devices that allow continuous measurement during hydration of volumetric strains, e.g. shrinkage/expansion, as well as water supply under realistic stress, drainage and temperature conditions. For the purpose of the work presented in this paper, three types of testing protocols were performed: Drained tests in which the pore pressure is kept constant and the resulting in water inflow/outflow is monitored.Undrained tests meaning zero water flow inducing changes of pore pressure that can be monitored by pressure sensors put at the two ends of the tested sample.Hybrid tests starting by an undrained followed by a drained phase with the aim to test the cement under various levels of effective pressure, defined as the difference between confining and pore pressures. In parallel, API annular ring tests, with and without pressure, were performed for the sake of comparison. Moreover, a theoretical model was modified on the light of the results of all these tests. This approach brought new understanding of the way the expansion is developing and most importantly its sensitivity to the effective stress and to water supply which vary significantly during cement hydration and possibly after and depends on the mechanical properties of the cemented formation. The results show clearly that API tests are insufficient to fully characterize shrinkage and expansion of a cement sheath. The purpose of this paper is to first describe the advanced experimental set up and to compare its results with the ones obtained from tests recommended by API. Then, a theoretical model which simulates the process of hydration and subsequent shrinkage as well as expansion will be presented. It will be shown that to reproduce the observed behavior during laboratory tests and the differences between various testing protocols, it is necessary to introduce the concept of expansion force and to account for pore pressure and for water supply. From there, the model would be able to predict the efficiency of expansion additives and to optimize the expansion additive percentage BWC, should the expansion is believed to be active under local downhole conditions.

In this paper, the Creation of Residual Strain during the hydration of cement paste is studied by performing oedometric experiments on class G cement pastes during the six first days of hydration. Various conditions of temperature (between 7 and 30 degrees C) and pressure (between 0.3 and 45 MPa) are explored. It is found that after a hydration degree of about 18%, mechanical loadings can induce residual strain. This state is reached at a time called critical time for Creation of Residual Strain (CRS). It corresponds to the maximum axial strain rate in an oedometric test or to the maximum rate of wave-velocity evolution versus time recorded in a Ultrasonic Cement Analyzer (UCA) test. The Boundary Nucleation and Growth (BNG) model is used to estimate the critical time for CRS. Within the range of studied temperatures and pressures, the predictive capacity of the BNG model for estimation of the critical time for CRS is demonstrated.

Total operated the Lacq CO 2 capture and storage demonstration pilot between January 2010 and March 2013. The injector well, RSE-1, was plugged and permanently abandoned in April and May 2015. As part of a risk assessment study, the hypothetical presence of micro-annuli in the outer cements of the well was studied, consisting of a 2.2 km-long pathway through two cement sheaths. A model was set up to couple the overlying Lasseube aquifer, the well cements and the Mano reservoir. The model concluded that no CO 2 can flow upwards into the Lasseube aquifer as the CO 2 storage reservoir is strongly depleted. The volumes of aquifer water that may flow downwards into the reservoir are very limited, with simulations suggesting rates of less than 0.01 m 3 /day. The cumulative volume of water that could flow down to the reservoir by the end of 2200 would be 0.0006% of the aquifer volume in the worse case. Both computed flow rates and volumetric hydrogeological consequences were therefore found to be of very small amplitude. This study concluded on the lack of risks related to a hypothetical loss of cement bonds with the shaly formations all along the cement sheaths of the 9⅝ and 7 inch casings.

Two types of mechanisms could lead to loss of cement-sheath integrity: mechanical and chemical degradations. However, chemical degradation by CO2 does not seem to be a real threat when the cement sheath is initially without default. Hence, it is important to understand the mechanical mechanisms that could lead to loss of cement-sheath integrity before and during CO2 sequestration. This is with this objective that Total has developed an integrated perspective whereby all events in the life of the well, are scrutinized. The description of this perspective is the objective of this paper.

The Rousse field, located in the Lacq basin in the southwest of France, is a site for a pilot carbon dioxide (CO2) storage project operated by Total. Since 2010, CO2 has been injected into a depleted gas field at a depth of 4.5 km through RSE-1, a 45-year old production well converted to an injection well.Older wells are commonly recognized as the most likely pathway for CO2 to migrate from the injection zone to other zones or to the surface. A number of factors could increase the potential risk for these wells: completions and production history may have created defects in the sealing elements (cement, steel and elastomers); the history itself may not be known in sufficient detail to estimate a reliable risk figure and finally the injected or produced fluids may have corroded the structureOld wells, whether they are converted to injectors or are in the path of the injection pressure field, could require renewed evaluation and preventive repairs as part of the field-level containment management plan, if their integrity assessment request so.As part of the due diligence process, advanced cement and steel evaluation logging tools were run in the RSE-1 well to investigate the bottom kilometer of the two kilometer-thick caprock. The 3D integrity map produced by the acoustic logging tools was analyzed for the presence of connected defects (pathways) that could lead to unwanted CO2 migration. The logs were also compared to the original 40 year old low-resolution sonic log to assess the extent of degradation, if any.A particular issue for wells drilled before the 1990's, when technological advance almost eliminated the problem, is that of mud channels left behind during the cement placement process. These defects present the highest risk because of their relatively large flow area. Since the well was completed in 1967, special attention was paid to the occurrence, connectivity and extent of channels. Even though imaging of the cemented annulus revealed an eccentered casing, almost lying on the rock face, and an oval borehole, the few mud pockets trapped between casing and rock were confirmed to be intermittent, with good cement providing hydraulic isolation between them. The comparison of the through-casing caliper to the formation sonic log enabled identification of borehole breakouts, caused by stress anisotropy, as the reason for the ovalization of the borehole section. Casing-cement bonding was confirmed to be excellent (it actually improved compared to the original log), and model-based inversion of imaging data suggests that cement-formation bond is also good.Defect-oriented analysis of the logs thus confirmed that RSE-1 provided containment across the caprock and was suitable for conversion to a CO2 injector without any repair or improvement work. (C) 2013 The Authors. Published by Elsevier Ltd.

Abstract Long-term cement sheath integrity is important to maintain wellbore stability and effective zonal isolation. Various factors influence cement sheath integrity, such as effective placement of cement, mechanical and thermal stresses, and interaction with corrosive gases. Corrosive gases are known to chemically attack Portland cement. However, the challenge is designing a cement system that can sustain CO2 and H2S attack. Acid-gas-resistant systems are useful in various applications, including producers and CO2 injector wells. To understand how the cement sheath is affected by prolonged exposure to corrosive gases, detailed studies should be performed at different temperatures and pressure conditions. This paper documents a resilient cement system that was exposed to CO2 and H2S gas environments for a period of three months. For comparison purposes, a neat cement system was used as a reference and tests were conducted at 194 (90°C) and 284°F (140°C). X-ray density profiles obtained using tomography techniques were used to analyze samples. The cement samples responded differently to CO2 and H2S environments. Formation of CaCO3 on exposure to CO2 was evident and was reflected in X-ray density profiles. Conversely, leaching of neat cement was observed on exposure to H2S gas. In both cases, the resilient cement system specially designed for the work described proved to be a better option when compared to neat cement because the destruction was less prominent in the former case.

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 139668, ’Use of a Mechanistic Model To Forecast Cement- Sheath Integrity for CO2 Storage,’ by A-P. Bois, SPE, CurisTec, and A. Garnier, SPE, G. Galdiolo, and J-B. Laudet, SPE, Total, originally prepared for the 2010 SPE International Conference on CO2 Capture, Storage, and Utilization, New Orleans, 10-12 November. The paper has not been peer reviewed. Implementation of CO2 storage in geological media requires a proper assessment of the risk of CO2 leakage from the storage sites. In particular, it is necessary to evaluate the risk that cement sheaths represent as leakage pathways because leakage could occur if cement becomes damaged or debonded at one of the interfaces of the cement sheaths. Therefore, it is of paramount importance to understand, before CO2 storage, the mechanisms that could lead to cement-sheath loss of integrity. Introduction The isolation defect may be the result of improper cement placement. It may be the result of cement slurry contaminated by fluid from the surrounding formations while the cement sets. It also may be the result of inappropriate set-cement properties. Tests performed in the laboratory have shown that there exist two types of mechanisms that could lead to cement-sheath loss of integrity: (1) mechanical degradation when cement is submitted to compressive or tensile loadings that are too high and (2) chemical degradation when cement comes in contact with CO2-enriched water. However field data and degradation-kinetics data show that chemical degradation may not be very fast unless leakage pathways already exist. The worst case is when both degradation mechanisms occur at the same time or one after the other. A mechanistic model was developed that accounts for the various modes of cement-sheath loss of integrity after cement has been placed was developed. The full-length paper presents the mechanistic model and uses this model to show that cement-sheath loss of integrity depends not only on cement properties but also on well architecture and history. Cement-Slurry Behavior Good rheological models are mathematical expressions for the shear stress or the viscosity that give accurate fit of experimental data over a wide range of shear rates and involve a minimum number of independent constants. Those for which the shear stress is independent of the history of deformation are time-independent models. They can be divided into linear, pseudoplastic, and viscoplastic models. Among these models, viscoplastic models, which are characterized by a viscosity that tends to infinity when shear rate tends to zero (plug flow), are at the core of cement-placement modeling. Unfortunately, they lead to experimental and mathematical problems.

The variation of permeability of typical petroleum cement paste is investigated as functions of mechanical loading and chemical degradation under the temperature of 90°C. In sound material, the permeability classically increases with deviatoric stress due to microcracks and volumetric dilatancy but decreases with confining pressure. Chemical leaching leads to significant increase of porosity of cement paste. However, the permeability of degraded material is lower than that of sound material during triaxial compression tests; this is due to compaction of pores under confining pressure. Further, the permeability variation in degraded cement is much more sensitive to confining pressure than that of sound material. During triaxial creep test, the permeability of degraded material decreases with time while that of sound material increases; this shows that the chemically leached material has a higher potential of volumetric compaction which is a key mechanism of plastic deformation. Coupled chemical degradation and triaxial compression tests are also performed. Under low confining pressure (3 MPa), the permeability increases with propagation of leaching front, and there is formation of preferential flow paths in the axial direction. However, with high confining pressure (10 MPa), there is no increase of permeability during chemical leaching and creation of successive degraded layers in the flow direction.

The storage of CO2 is an expected solution by the oil industry: using petroleum wells as geological reservoirs is a very important and new research field. The durability of such storage has an importance to be predicted. The carbonation of oil-well cement (Class G type) under temperature (90 degrees C) is evaluated in this paper. In the first part of the paper, the fabrication protocol for samples is explained, and the test conditions are described. The evolution of cement carbonation over time is then presented. In the second part of the paper, the results obtained by triaxial mechanical tests with permeability measurements at 90 degrees C are discussed. Mechanical evolutions obtained for different confinement pressures show a remarkable decrease in permeability and an important increase of multiaxial strengths, which may exceed 100%. Long-term tests of carbonation will be necessary to evaluate the effect of carbonic acid on the chemical stability of carbonated cement. DOI: 10.1061/(ASCE)MT.1943-5533.0000174. (C) 2011 American Society of Civil Engineers.

One of the major technological issues for CO2 injection is the long-term behavior of the cement-based materials used to ensure the overall sealing performance of the storage wells. When water is present, the CO2 injected can react chemically with the cement (i.e. carbonation). The objectives of our experimental program are to assess the kinetics and phenomenology of the changes that occur in class-G Portland cements exposed to CO2-enriched aqueous fluids at 8 MPa and two different temperatures (90 degrees C and 140 degrees C). The experimental program presented in this paper consists of two carbonation tests (static tests) and a coupled chemo-mechanical test (dynamic test) which were performed on similar class-G cement and similar CO2-rich water. The main preliminary results show a carbonation front progressing from the fluid-sample interface towards the sample centre. The front moves faster at 140 degrees C than 90 degrees C because of the different carbonation process involved at this higher temperature. The results of a coupled chemo-mechanical test with injection of CO2-saturated water show that the CO2 flow rate in the cement rapidly decreases, finally resulting in carbonation clogging of the cement sample. They seem consistent with reported field observations. (C) 2011 Published by Elsevier Ltd.

SummaryMicroannuli 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.

Cement paste used in the Oil Industry is generally subjected to chemical degradation due to flow of acid fluids in various situations. The present study focuses on the evolution of thermo-hydro-mechanical (THM) behavior with chemical degradation of petroleum cement paste. Triaxial compression tests with different confining pressures (0, 3, 10 and 20 MPa) are carried out on a standard oil cement paste in sound state and completely degraded state by ammonium nitrate solution under a temperature of 90 °C. The results obtained show that the material in its initial state exhibits a small elastic phase and a strong capacity of compaction. The mechanical behavior depends on the load induced pore water pressure. Because of the increase in porosity caused by chemical degradation, the mechanical strength (cohesion and friction angle) and Young's modulus decrease. The dependence of mechanical strength and Young's modulus on confining pressure is smaller in the chemically degraded cement paste than in the sound one. In fine, the mechanical behavior of the whole material becomes more ductile. As a result, such effects of chemical degradation should be taken into account when modeling such cement paste materials exposed to such chemical degradations.