I am encouraged that we, as an industy, continue to refine and tweak our practices to solve zonal-isolation and cementing challenges in every well environment in which we work. As cementing techniques are improved, so, too, are the cement-evaluation methods and work flows. This paper demonstrates a new way to create gas-tight seals during well abandonment, overcoming the limitations of traditional methods and reducing the operator’s liability and potential environmental impact after decommissioning has been completed. This paper discusses shale creep and other shale-deformation mechanisms and how an understanding of these can be used to activate shale that has not contacted the casing yet to form a well barrier. Well RXY is located in Cairn’s Ravva offshore field in the Krishna-Godavari Basin in India.
Green fields today mostly can be regarded as marginal fields and successfully developed. It covers the complete assessment of the oil and gas recovery potential from reservoir structure and formation evaluation, oil and gas reserve mapping, their uncertainties and risks management, feasible reservoir fluid depletion approaches, and to the construction of integrated production systems for cost effective development of the green fields. Depth conversion of time interpretations is a basic skill set for interpreters. There is no single methodology that is optimal for all cases. Next, appropriate depth methods will be presented. Depth imaging should be considered an integral component of interpretation. If the results derived from depth imaging are intended to mitigate risk, the interpreter must actively guide the process.
Pulsed-neutron logging has evolved over the last 50 years, but the intrinsic physical measurements have remained unchanged, which means that operators cannot obtain a complete picture of the rock and fluids behind casing with conventional tools. However, advances in tool design and a new fast-neutron cross-section (FNXS) measurement provide for an alternative gas-identification technique. Gas in open holes is typically identified from neutron porosity and gamma-gamma density crossover. In casedhole environments, gamma-gamma density measurements are challenging because of the large casing and cement corrections needed. Previous gas identification in casedhole environments has relied on the formation hydrogen index (HI) or neutron porosity (TPHI) log and sigma. In openhole environments, density and neutron porosity crossover is a typical gas identifier, but, in many instances, shale can mask the identification of gas. This is a common problem in some gas reservoirs in Alaska, and it leads to ambiguous interpretations about the gas saturation and potential producibility of different zones. Gas identification in casedhole environments is even more complicated because the density measurement is not commonly available. The FNXS measurement responds primarily to formation atom density, for which most rocks, clays, and liquids have similar values.
Pulsed-neutron logging has evolved over the last 50 years, but the intrinsic physical measurements have remained unchanged, which means that operators cannot obtain a complete picture of the rock and fluids behind casing with conventional tools. However, advances in tool design and a new fast-neutron cross-section (FNXS) measurement provide for an alternative gas-identification technique. Gas in open holes is typically identified from neutron porosity and gamma-gamma density crossover. In casedhole environments, gamma-gamma density measurements are challenging because of the large casing and cement corrections needed. Previous gas identification in casedhole environments has relied on the formation hydrogen index (HI) or neutron porosity (TPHI) log and sigma.
This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Houston, Texas, USA, 23-25 July 2018. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC.
ABSTRACT: The integrity of the cement sheath is the key part to maintain zonal isolation and prevent the inter-zonal communication. Loads arising from multiple stages of wellbore life span may induce various modes of cement failure within the cement (disking and radial cracks) and at the cement-casing and the cement-formation interfaces (debonding fractures). Micro-annuli (MA) are the systematic and inter-connected debonding fractures which is the most hazardous mode of cement failure and can cause serious wellbore leakage problems. This paper utilizes the extended finite element method (XFEM) to study the cement failure under various loading conditions (i.e. cement volume change during hardening, mechanical loads due to borehole pressure change) and investigate the influence of the cement failure to the MA generation during the latter operation stages. A staged 3D finite element analysis approach including loads from various operation procedures during the life cycle of a composite wellbore system is used to establish an in-situ downhole condition and study the conditions of MA generation and evolution. Modeling results indicate that radial cracks are likely to occur during the cement volume shrinkage during the cement hardening and MA (debonding fractures) tend to occur under the periodically cooling during the injection stage. The results also show that the initial stress state in the cement for each procedure is a key factor determining the initiation of different cement failure types. In summary, the more compressive the cement state of stress, the lower the likelihood for radial cracks to initiate and the more likely debonding occurs during thermal cycling. The results with respect to varying cement Young’s modulus show that a high Young’s modulus promotes the initiation of radial cracks. The initiation of interface debonding is independent of cement Young’s modulus. The results presented indicate that a cement system with a low Young’s modulus and high tensile strength provides favorable conditions to promote the cement sheath integrity.
Inter-zonal communication is the unwanted fluid migration along the wellbore through fractures and cracks within the formation rock and cement, and along the composite system interfaces (Parcevaux and Sault, 1984; Dusseault et al., 2000; Nelson and Gilliot, 2004). Zonal isolation is maintained by the mechanical integrity of the wellbore system, especially the integrity of the cement sheath. Various factors during the life cycle of the wellbore (i.e. drilling, completion, injection/production, and well abandonment) may damage the cement sheath and compromise wellbore integrity. Therefore, understanding the occurrence of cement failure under different loading conditions during the life cycle of the wellbore is important in order to predict potential hazards and optimize operation procedures.
ABSTRACT: Throughout the life of a well, its cement sheath may fail through the development of micro-annuli along the boundaries of its structure. The occurrence and attributes of these annuli may be influenced by the processes of drilling and completions, in addition to the in-situ conditions. This paper presents a staged finite element modeling approach to determine the effects that variations in the elastic properties between cement grades and rock lithologies, as well as encountered stress regimes, have upon the properties of micro-annuli formation within water injection wells. The model set-up is achieved through an initiation of the chosen stress field, implementation of borehole fluid pressures, and temperature decrease for the water injection phase. The effects of the difference in elastic parameters between the cement grade and formation rock on micro-annulus formation are achieved by evaluating multiple model combinations of both components. Similarly, the influences of the stress regime on the micro-annuli are determined by implementing differing intensities of stress contrast into the model, as well as considering depth variations. The research results indicate that the aperture magnitude of the developed micro-annuli, as well as its timing of initiation, depend upon both the Young’s moduli contrast between cement and formation materials and the depth considered. Additionally, the orientation of the aperture appears to rely on the stress regime in which the wellbore is enveloped. These observations could be utilized to better prevent wellbore leakage by selecting cement-formation pairs that possess a lower chance of micro-annuli formation and by being aware of the asymmetric and aperture widening effects of different stress regimes on this pervasive mode of failure.
Gas migration (a form of wellbore leakage) has been identified as a major problem in the oil and gas industry for decades (Oyarhossein and Dusseault, 2015). This failure of wellbore integrity may lead to extreme operational difficulties and detrimental environmental damage, both of which require costly remedial activities (Dusseault et al. 2000; Bois et al. 2011). Moreover, the problem has only been exacerbated in recent times by the exploitation of more hostile environments, such as High Temperature - High Pressure (HTHP) and ultra-deep-water fields, as well as the increased use of more complicated development operations, including CO2 sequestration, water injection, and gas production wells (Feng et al. 2016).
ABSTRACT: Micro-annuli existing on the interface of cement-sheath might lead to the failure of wellbore integrity. Based on the solutions of the elastic and elastic-plastic deformations of a thick-walled cylinder with the internal and external pressures; meanwhile combining with the displacement continuity, a nonlinear equation set about the interfacial pressures and displacements of a casing-cement-formation system in the loading and unloading processes is established. Via solving the nonlinear equation set, the interfacial pressures and the displacements can be obtained. If the obtained interfacial pressure is larger than the bonding strength of rock, micro-annuli appear on the interface, and its size is equal to the difference of the interfacial displacement in the loading process and that in the unloading process. Numerical examples show that the micro-annuli are at an order of tens of micrometers. Moreover, as the increase of the elastic modulus of cement, the micro-annuli become broader. As a comparison, if the uniaxial compressive strength of cement increases, the micro-annuli become narrower. Therefore, to avoid the generation and development of micro-annuli, it is suggested that the cement with high uniaxial compressive strength and low elastic modulus should be used for the field practices.
Wellbore integrity needs to be investigated in the whole life cycle of a well, and the sealing of the cement sheath is the key. In the completion and production process, a variety of reasons can lead to the sealing failure of the cement sheath. For instance, both the wellbore uncleanness and the residual mud cake can result in the reduction of the cementing quality. During the hardening process of cement slurry, gas channel formed by weight loss can trigger the sealing failure. Due to the rapid change of the internal casing pressure, or the variation of the casing temperature, the cement interface can also separate from formation.
Bois et al  mentioned that the loading and unloading process of the internal casing pressure is one of the key factors affecting the cement sealing. During the oil test, pressure test, or hydraulic fracturing, etc., the loading and unloading process of the internal casing pressure is inevitable. Under these situations, the cement will experience the similar process. Once the load applied on the cement sheath overcomes its elastic limitation, the cement sheath will appear plastic deformation, which will be remained after unloading, thus microannuli form on the interfaces, including the first and the second interfaces.
Dada, Akindolu (Heriot-Watt University) | Muradov, Khafiz (Heriot-Watt University) | Wang, Hong (Heriot-Watt University) | Nikjoo, Ehsan (Heriot-Watt University) | Villarreal, Edsson (Repsol Ecuador S.A. formerly Heriot-Watt University) | Davies, David (Heriot-Watt University)
The continuous stream of data from wells completed with multiple permanent, downhole sensors has created new monitoring possibilities. One new workflow, TTA (Temperature Transient Analysis) has proven to be particularly valuable for allocation of flow rates and phase cuts as well as for analysis of the properties of the near-wellbore formation. However, the measured temperature signal suffers considerable attenuation when the gauge is installed distant from the producing layer. This loss in signal quality has to be accounted for before carrying out TTA.
This paper investigates the effect of heat transmission in the wellbore on TTA, evaluates existing transient thermal wellbore models and develops models to reconstruct the sandface temperature from the temperature measured by a gauge located at some distance from the sandface. The possibility of estimating the thermal properties of the wellbore and surrounding formation using these models is also studied.
Two approaches are proposed for mitigating the remote gauge problem. The first approach reconstructs the sandface temperature from the degraded gauge temperature measurement data. Temperature reconstruction was found to be possible providing an accurate model of the wellbore is available. Both numerical, transient, thermal wellbore simulators and analytical thermal wellbore models may be used. Numerical inversion of the analytical transient thermal wellbore model is necessary since analytical inversion is impractical due to the result being very sensitivity to errors in measured gauge temperature.
The second approach requires producing the well under conditions that minimize the attenuation. Empirical methods may be used to quantify the heat transfer effect, when conditions are such that the transient temperature signal is "good enough" for TTA. This approach also also allows estimating the degree of uncertainty (due to wellbore heat transfer) on the TTA.
This work would extend the application of TTA to wells where analysis was previously impossible because the gauge was installed distant from the producing layer. This extension of TTA further increases the value-added by installing permanent downhole gauges.