Cement holds the most critical role for providing long-term zonal isolation for permanent abandonment phase. The loss of cement integrity is undesirable as it may threaten the surrounding environment and safety on the surface. The quality of cured cement is commonly associated with the properties of cement material and cement placement in the wellbore. However, there are still limited investigations that link these factors specifically to the sealing ability of cement plug, especially with the lack of proper equipment in the past.
In the present work, a small-scale laboratory setup has been constructed to test the sealing performance of a cement plug. The cement plug is contained inside a test cell, connected to a pressurizing system and placed inside a heating cabinet. Consequently, the test can be simulated at downhole conditions in a controlled manner. By using this setup, it is possible to monitor the minimum pressure required for the plug to fail and the gas leak rate.
Two different cement systems, neat- and silica-cement, were prepared as plugging materials. Both cement systems are placed inside pipes with three different levels of surface roughness and then tested. Results show that the inner surface roughness of the pipes affects cement plug sealing significantly, and the effect is independent of the type of cement systems. Plugs placed inside a very-rough pipe significantly reduce the gas leak rate. Our results also show that an immediate gas leak occurs in all samples from leak paths formed at the cement/steel interface.
Cement sheaths are among the most important barrier elements in petroleum wells. However, the cement may lose its integrity due to repeated pressure variations in the wellbore, such as during pressure tests and fluid injections. Typical cement sheaths failure mechanisms are formation of radial cracks and microannuli, and such potential leak paths may lead to loss of zonal isolation and pressure build-up in the annulus. To prevent such barrier failures, it is important to study and understand cement sheath failure mechanisms.
This paper describes a series of experiments where we have used a tailor-built laboratory set-up to study cement sheath integrity during pressure cycling, where the set-up consists of down-scaled samples of rock, cement and casing. Cement integrity before and during casing pressurization is characterized by X-ray computed tomography (CT), which provides 3D visualization of radial cracks formed inside the cement and rock. We have studied how contextual well conditions, such as rock stiffness, casing stand-off and presence of mudfilm, influence cement sheath integrity.
The results confirm that the rock stiffness and casing stand-off determine how much casing pressure the cement can withstand before radial cracks are formed in the cement sheath, where the rock stiffness is significantly more important than casing stand-off. Furthermore, it is seen that the radial cracks in the cement sheath continue into the rock as well. However, when a thin mudfilm is present at the rock surface, the cracks stop at the cement-rock interface, and the cement sheath withstands less pressure before failure. The bonding towards the rock is thus of importance.
Plug and abandonment (P&A) of subsea wells is very costly and usually requires semi-submersible drilling rigs (SSR). To reduce total costs of the subsea P&A campaigns, it is beneficial to perform P&A operations with riserless light well intervention (RLWI) vessels instead of rigs. Currently, a drilling rig is required for performing P&A operations in the reservoir section and overburden, whereas intervention vessels can be used for preparatory work and wellhead removal.
This paper discusses how it can be technologically feasible to perform full P&A of subsea wells with RLWI vessels. It is shown that, for wells of simple and medium complexity, innovate approaches with use of existing technologies can enable full P&A of the entire well with RLWI vessels. This is demonstrated by thorough analyses of operational procedures using available technologies, where RLWI operations are compared to rig operations for different well scenarios. Furthermore, to quantify the cost benefits of the innovative approaches, a cost-optimization tool has been used to estimate the resulting cost and time durations of the different approaches and scenarios.
Cement-sheath integrity is important for maintaining zonal isolation in the well. The annular-cement sheath is considered to be one of the most-important well-barrier elements, both during production and after well abandonment. It is well-known, however, that cement sheaths degrade over time (e.g., from repeated temperature and pressure variations during production), but the link between leak rate and the cause of cement-sheath degradation has not yet been established.
In this paper, we have studied fluid flow through degraded cement sheaths. The degree of degradation of the cement sheaths varied from systematically connected cracks to real microannuli. The leak paths, created by thermal-cycling experiments, were imported into a computational-fluid-dynamics (CFD) simulation software. The pressure drop over the cement sheath was used as a boundary condition, and the resulting pressure-driven flow was studied using methane gas as the model fluid. The Forchheimer equation was used to estimate the effective permeability of the cement sheaths with defects.
Our results show that the pressure-driven flow is complex and greatly affected by the geometry of the flow paths. A nonlinear pressure-buildup curve was observed for all experimental cases, indicating that Darcy’s law was not validated. For homogeneous microannuli, the pressure-buildup curve was linear. The estimated effective permeability for all cases was observed to be orders of magnitude larger than that of a good cement sheath.
De Andrade, Jesus (Norwegian University of Science and Technology) | Sangesland, Sigbjorn (Norwegian University of Science and Technology) | Skorpa, Ragnhild (SINTEF Petroleum Research) | Todorovic, Jelena (SINTEF Petroleum Research) | Vrålstad, Torbjørn (SINTEF Petroleum Research)
J. De Andrade and S. Sangesland, Norwegian University of Science and Technology; R. Skorpa, J. Todorovic, and T. Vrålstad, SINTEF Petroleum Research Summary The annular cement sheath is one of the most-important well-barrier elements, both during production and after well abandonment. It is, however, well-known that repeated pressure and temperature variations in the wellbore during production and injection can have a detrimental effect on the integrity of the cement sheath. A unique laboratory setup with downscaled samples of rock, cement, and pipe has been designed to study cement-sheath-failure mechanisms during thermal cycling, such as debonding and crack formation. With this setup, it is possible to set the cement under pressure and subsequently expose the cement to temperature cycling under pressure as well. Cement integrity before and after thermal cycling is visualized in three-dimensional by X-ray computed tomography (CT), which enables quantification of and differentiation between debonding toward the casing, debonding toward the formation, and cracks formed inside the cement sheath itself. This paper describes in detail the development and functionality of this laboratory setup along with the experimental procedure. Several examples to demonstrate the applicability of the setup, such as tests with different types of casing surfaces and different rocks, are also shown. Introduction Zonal isolation is one of the most-important objectives of primary cementing. The cement sheath needs to retain its integrity throughout the lifetime of the well, including after well abandonment. Long-term sealing of the annular cement sheath can, however, be difficult to maintain, and cement-sheath failure is one of the reasons that many wells develop well-integrity problems such as sustained casing pressure as they age (Bourgoyne et al. 1999; Vignes and Aadnoy 2008). This is particularly critical for subsea wells, where the wellhead arrangements do not provide access to all annuli.
Cement sheath integrity is important to maintain zonal isolation, as the annular cement sheath is considered to be one of the most important well barrier elements in the well, both during production and after well abandonment. It is however well known that cement sheaths degrad over time, e.g. from repeated temperature variations during production, but the link between the actual leakage rate and the degradation of cement sheaths have not yet been established. In this paper, we have mapped actual leak paths of degraded cement sheaths, created by thermal cycling experiments, by X-Ray Computer Tomography (CT). The resulting leak path was imported into a Computational Fluid Dynamics (CFD) simulation software, making it possible to determine flow rates of various fluids through actual degraded cement sheaths. The pressure drop over the cement sheath was used as boundary condition of the system.
The objective of plug and abandonment of wells can be described as "restoring the cap rock". In that respect, the long-term integrity of the plugging material is crucial. I.e. it is important that the plugging material can resist downhole chemicals and otherwise withstand downhole conditions. In this paper, we have performed ageing tests with cement samples at relevant downhole conditions to determine the long-term integrity of well cement as plugging material. Portland cement samples with and without silica flour as additive have been separately exposed to crude oil, brine and H2S (in brine) at 100 °C and 500 bar for 1, 3, 6 and 12 months. The long-term integrity of the samples was determined by measuring changes in weight, volume, mechanical strength and permeability, as well as physical appearance.
It is seen from the results that the addition of a pozzolan such as silica has a significant impact on the long-term integrity of Portland cement, especially in a corrosive environment such as H2S. All the samples were affected by most of the different chemical environments, but the cement samples without silica were considerably more affected than the samples with silica as additive. Furthermore, the exposure to H2S in brine resulted in the formation of an unexpected white deposit, which precipitated both inside and outside the samples.
Aas, Bjarne (IRIS/DrillWell) | Sørbø, Jostein (IRIS/DrillWell) | Stokka, Sigmund (IRIS/DrillWell) | Saasen, Arild (Det Norske Oljeselskap) | Statoil, Rune Godøy (SINTEF Petroleum Research/DrillWell) | Lunde, Øyvind (SINTEF Petroleum Research/DrillWell) | Phillips, Conoco (SINTEF Petroleum Research/DrillWell) | Vrålstad, Torbjørn (SINTEF Petroleum Research/DrillWell)
Well abandonment operations can be very time-consuming and costly, and thousands of wells need to be permanently plugged and abandoned offshore Norway during the upcoming years. One possible way to reduce costs during P&A operations is to leave most of the production tubing in the well, as this would save significant rig time. A major concern with such an approach is, however, whether the cement will properly displace the original fluid, due to lack of tubing centralization and possible unfavorable flow dynamics in the annulus. In this paper, we demonstrate by full-scale experimental tests that it is possible to obtain good cement placement when the tubing is left in the hole, with and without control lines. Full-scale tests have been performed with both conventional and expandable cement to determine the sealing ability of annulus cement when tubing is left in hole. The quality of the cement placement was evaluated by pressure tests with water; where leakage rates and pressure drops over the test sections were recorded, and by visual inspection after cutting the test assemblies at different places. It is seen from the experiments that cement is well placed in the annulus when tubing is left in hole, but some microannuli are detected.
Vrålstad, Torbjørn (SINTEF Petroleum Research/DrillWell) | Skorpa, Ragnhild (SINTEF Petroleum Research/DrillWell) | Opedal, Nils (SINTEF Petroleum Research/DrillWell) | De Andrade, Jesus (Norwegian University of Science and Technology (NTNU)/DrillWell)
The cement sheath is one of the most important well barrier elements in the well, both during production and after abandonment. However, normal production operations which involve temperature variations in the well, such as steam injection, stimulations and shut-down periods, may damage the integrity of the cement sheath. Temperature increase and decrease, i.e. thermal cycling, cause the casing to expand and contract, which creates debonding and cracking of the cement sheath and thereby loss of zonal isolation.
This paper presents novel results from an experimental study of cement sheath integrity during thermal cycling. The temperature was cycled repeatedly from 5 °C to 125 °C in a controlled manner from inside the casing, and Portland cement with silica additive was tested with both sandstone and shale as surrounding rock. Debonding and cracking of cement were quantified and visualized by X-ray computed tomography (CT), and it was found that cracking and debonding occurred for the sandstone sample, whereas the shale sample remained almost unaffected. There were some initial defects in the cement sheath in the sandstone sample, and these small and scattered defects grew together during thermal cycling into a continuous leak path; i.e. resulting in a loss of zonal isolation.
The digitalized 3D geometry of this leak path was imported into Computational Fluid Dynamics (CFD) software, thereby enabling a unique visualization of fluid flow through an actual leak path in degraded cement and an estimation of leak rates for different pressure differences. It is seen that microannuli are not homogeneous or uniform, and that fluid flow through microannuli and cracks is complex and not easily predictable.
Moeinikia, Fatemeh (University of Stavanger) | Fjelde, Kjell Kåre (University of Stavanger) | Saasen, Arild (Det norske oljeselskap ASA and University of Stavanger) | Vrålstad, Torbjørn (SINTEF) | Arild, Oystein (International Research Institute in Stavanger)
F. Moeinikia and K.K. Fjelde, University of Stavanger; A. Saasen, Det norske oljeselskap ASA and University of Stavanger; T. Vrålstad, SINTEF; and Ø. Arild, International Research Institute of Stavanger Summary In a few years, there will be a need for performing a considerable number of subsea plug-and-abandonment (P&A) operations on the Norwegian continental shelf. There are certain challenges associated with this. These include more-difficult access to subsea wells compared with ordinary platform wells, high daily rates of semisubmersible rigs, and the fact that these rigs will be allocated for drilling new exploration-and-production wells to sustain the hydrocarbon production. At the moment, there is large focus on finding rigless technologies to reduce P&A cost and use of rig time. A probabilistic approach should be used to assess the costand duration-saving potential of such technologies relative to semisubmersible rigs. In this paper, we will consider rigless technologies for performing parts of the P&A operation. For the first time, a probabilistic approach, including learning curves, correlations, and possible risk events, is used to evaluate subsea batch-operated P&A. A realistic example that includes the use of a semisubmersible rig for batch-operated P&A of subsea production wells is used to show how a probabilistic methodology can be implemented for obtaining probabilistic estimates of P&A cost and duration. Inclusion of learning curves and correlations makes the application of a probabilistic approach for multiwell campaigns challenging. It is demonstrated how to incorporate learning curves, correlations, and possible risk events to capture a realistic range of cost and duration for multiwell P&A operations. In a second operational example, a light-well-intervention vessel is used to accomplish preparatory work and wellhead cutting and removal for P&A of the subsea production wells in batch campaigns. These two scenarios will be compared from a cost and duration point of view. Moreover, the advantages associated with riserless technologies for multiwell abandonment are discussed. Introduction According to Oil and Gas UK (OP061 2011), plug and abandonment (P&A) is considered complete when the following three phases are accomplished.