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Cement-sheath integrity is fundamentally important for well integrity and helps to ensure that the asset is operated safely and economically. If the cement sheath is damaged during well operations, it could lead to interzonal communication, annular pressure, and casing damage. Remedial jobs are then necessary to fix the problem and continue well operation. In some cases, it might not be economical to fix the problem, leading to well abandonment.The challenge to cement-sheath integrity is caused by the stresses resulting from change in pressure and temperature during well operation. Examples of wells that could be subjected to changes in pressure and temperature are deepwater, high-pressure high-temperature (HPHT), gas storage, steam injection, and geothermal. Gas storage wells in Italy are addressed in this work. An integrated, intelligent, and interventionless solution has been applied to solve the zonal-isolation challenges in such wells. The well events during the life cycle were integrated into the analysis and a cement sheath was designed and tested so that it was not damaged. Safety considerations were built into the design procedures based on the teachings of designs of other materials. This design procedure helped the cement sheath withstand cyclic loads, which could be significant in gas storage wells.During well operations, if the stresses on the cement sheath exceed the design limits, the cement sheath could be damaged. The initial damage could be in the form of small cracks and micro-annuli. If these damages can be fixed immediately and without intervention, then the formation fluid can be prevented from entering the annulus. Intelligent features were built into the cement sheath so that small cracks and micro-annuli could be automatically sealed if formation fluid enters the annulus.The integrated, intelligent, and interventionless zonal-isolation features of the solution offer a unique opportunity to prevent the annular pressure buildup if the cement sheath inadvertently fails. This solution has been implemented in a number of wells and is described in this work. The job implementation and subsequent results during the life of the wells have been successful. The unique features of the solution discussed and presented in this work is a game-changer and should help the industry solve a pressing problem and operate wells safely and economically.

One of the main objectives of a primary cement job is to prevent formation fluids from migrating into the annulus. To achieve this objective, the cement sheath should withstand the stresses induced by the various well operations and maintain integrity during the life of the well. However, the majority of the cement design programs in the industry today consider only the slurry properties and do not assess the effect of the mechanical properties of the cement sheath on the final well design. A design procedure has been developed to estimate the risk of cement failure as a function of cement sheath and formation characteristics and well loading. A few examples of well loading are pressure testing, well completion operations, hydraulic fracturing, and hydrocarbon production. The design procedure is based on a finite element analysis and simulates the sequence of events from drilling through cement hydration, well completion, and production operations. The cement failure modes simulated are debonding, cracking, and plastic deformation. The cement is assumed to behave linearly as long as its tensile strength or compressive shear strength are not exceeded. The material modeling adopted for the undamaged cement is a Hookean model bounded by smear cracking in tension and Mohr-Coulomb in the compressive shear. Shrinkage and expansion of the cement are included in the material model. The need to design a fit-for-purpose cement sheath is accentuated by the sustained casing pressure observed on a number of wells after they were put on production and on some HPHT wells after the displacement fluid was changed over to a well-completion fluid. The pressure in the annulus side sometimes results in an inability to continue further operation. Applications are discussed and examples are provided. From the processes reviewed in this paper, one can estimate the risk of failure of various cement systems and select a fit-for-purpose system that will minimize the overall cost. This process should improve the economics of constructing and producing oil and gas wells (cost effective life cycle design) and also improve safety because zonal isolation failures may be reduced.

Recent experience in the field has demonstrated that the mechanical properties of the annular sealant are a critical factor in the success of a well. A demanding operational regime such as High Pressure/High Temperature (HP/HT) and well interventions, e.g. pressure testing stimulation, place high demands on the mechanical design of a candidate sealant. The following steps should be followed to determine the required mechanical properties of such well sealant candidates: The stability of the bore hole should be evaluated prior to placing the sealant in the annulus. The stability is affected by the far field stresses, the weight (s.g.) of the drilling fluid and the mechanical properties of the formation. The integrity of the sealant should be investigated for the operational envelope of the well. Finite Element models were used to determine the influence of wellbore stability aspects and the effects of loading on the integrity of the sealant. The mechanical properties of the sealant considered were elasticity, tensile strength, shear strength and bonding strength. In this way it was possible to examine aspects of non-linear material behavior of the sealant. The following factors were investigated in this study: What are the well parameters and its operating envelope? What are the total stresses on the formation and on the sealant under various HP/HT conditions? Which mechanical properties does the sealant have to possess in order to be able to withstand different types of stress? The study indicated that there is a strong interaction between on the one hand the behavior of the sealant and on the other hand the formation in-situ stresses, the formation properties, the well completion parameters and its operating envelope. Depending on these input conditions, the sealant was observed to behave in one of the following manners. The sealant remained intact without any damage. A micro annulus occurred between sealant and casing/formation. The sealant failed because of either tensile or shear deformation. Calculations were performed for various wellbore stability scenarios, well completion characteristics, well loading envelopes and properties of the sealant. Typical oil well cement slurries, latex slurries, foam cements and cement hybrids were investigated. Typical results from the study are discussed and procedures for field application are given.

SPE Members Abstract This paper presents case histories that demonstrate the utility and economy of monitoring circulatable hole (CH) immediately before a primary cement job, and then taking real-time action to correct any deficiencies detected. Although it is well established that effective hole-cleaning is critical to the success of a cementing job, a reliable method has only recently been made available for continuously monitoring the volume of the casing-to-wellbore annulus. Accurately determining the annular volume involved in the circulation process is the key to verifying that the hole is clean. Traditionally, operators have circulated the annulus for an arbitrary length of time; CH monitoring, however, has allowed for accurate assessment of hole condition with as much as 75% less circulation time before cementing. The case histories presented describe methods used to correct CH deficiencies once they are established. These methods include–Pumping at higher rates–Rotating and/or reciprocating pipe–Adding spacers/flushes to the mud system Based on the case histories presented, the following benefits can be derived from the new measurement method:–Circulation times originally averaging 2 hours may be reduced to as little as 30 minutes.–Pipe movement can contribute greatly to the amount of CH.–Operators can save the expense of certain casing equipment traditionally used to increase CH since they know 100% of the annulus is in circulation. Introduction After several years of test studies, the industry has, to some extent, solved the problem of drilling fluid displacement from the annulus between the casing and borehole, by cement and other fluids. Although studies have shown how important nearly full displacement of the drilling fluid is to provide a long-term seal of the annulus, different theories exist about the most effective way to accomplish near-100% displacement. The main difference in these theories is the means by which to increase the shear stress on the static annular fluid by the dynamic annular fluid. P. 271