Liquid loading phenomenon is known as the inability of the produced gas to carry all the co-produced liquid to the surface. Under such condition, the non-removed liquid accumulates at the wellbore resulting in reduction of the production and sometimes cause the death of the well. Several studies were carried out and correlation were developed based on field and experimental data with the aim to predict the onset of liquid loading in a gas well. However, each model provides different indication on the critical gas velocity at which the liquid loading exists. Thus, to have a clear understanding on the difference between most used models, experiments were performed in an upward inclinable pipe section. The 60-mm diameter test pipe was positioned at angles of 30°, 45° and 60° from horizontal. The fluids used were air and light oil. Measurements include fluid velocities and fluid reversal point. High-speed video cameras were used to record the flow conditions in which the onset of liquid loading initiated. Experimental results were compared with existing models by
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.
More-accurate estimates of the fatigue damage on subsea wellhead might prolong the service lifetime of the equipment. Nevertheless, there might come a point during the life of a well on which the fatigue capacity is nearly depleted, without the possibility of further interventions being carried out, and thus imposing the abandonment of the well. This paper studies how employing a BOP tethering system may reduce the bending moment load transferred to the wellhead.
A BOP tethering system may be described as an assembly of anchors disposed around the subsea wellhead, which are connected to the BOP by mooring lines. The goal of the system is to reduce dynamic loads transferred to critical wellhead fatigue components and minimize the damage rates by decreasing the bending moment that is transferred from the riser to the wellhead.
The scope of a wellhead fatigue assessment comprises a riser response analysis. This paper presents the expected reduction on the calculated fluctuating bending moment load transferred to the wellhead for a series of possible configurations of the tethering system.
The results of the study conducted have shown that tethering the BOP system during drilling or re-entry operations has potential to decrease accumulated fatigue damage in the wellhead and can be regarded as an alternative for mitigating wellhead fatigue. The gains in petroleum production because of the increased operational life of the well have the potential to surpass the costs inherent to installing the tethering system. The results of simulations for different design options have shown the potential of this approach to increase the remaining service life of a wellhead, potentially doubling it. Even if installation restrictions prevent the optimum design to be used, this approach could still be advantageous.
Mitigating wellhead fatigue may prevent early well abandonment. Approaches considered to mitigate wellhead fatigue by actively reducing the load transferred to the wellhead, such as a reactive flex-joint, have been presented before. The BOP tethering system is an alternative to these previous systems and provides operators an additional solution for consideration to their specific needs.
De Andrade, Jesus (Norwegian University of Science and Technology) | Fagerås, Sondre (Norwegian University of Science and Technology) | Sangesland, Sigbjørn (Norwegian University of Science and Technology)
The annular casing cement is an important part of the well barrier throughout the life cycle of a well. With the increasing number of subsea plug and abandonment (P&A) operations, increased attention is now given to annular cement evaluation and the ability to prove adequate zonal isolation. Today, cement evaluation by logging is performed almost exclusively using acoustic logging tools. One of the concerns when it comes to cement integrity is the frequently occurring micro-annulus at the casing-cement interface. Yet, a typical cement evaluation tool may lack of accuracy on its evaluation. Hence, a novel concept for the evaluation of casing-cement micro-annulus has been proposed.
This paper describes a mechanical-based approach for cement evaluation – the Annulus Verification Tool (AVT). The AVT applies a radial force on the casing inner wall that yields an ovalization of the cross-section, while recording the radial displacement of the casing. A prototype of the AVT has been constructed along with an experimental setup to allow for initial testing of the tool. This comprises the construction of full-scale diameter samples representing a typical 9 5/8-in. production casing cement job, with the possibility to generate a uniform micro-annulus of a known size at the casing-cement interface.
The tests performed have shown that the AVT is able to differentiate a casing supported by an annular cement sheath from a free pipe, due to the stiffness contrast. By measuring the casing radial displacement with high resolution, the results have shown that a microannulus can be detected and its size quantified with good accuracy. Experimental tests performed with tool eccentricity and tilting has shown that the AVT should be kept centralized to achieve accurate quantification of the microannulus size.
The AVT module is meant to complement the acoustic tool sting used today, and to improve evaluation of the cement sheath's sealing capability, especially in cases where a micro-annulus is detected or suspected. If an existing microannulus is suspected, the AVT logging response may confirm its occurrence, quantify its size and aid the planning of remedial operations to restore the annular barrier.
Silva, Thiago Lima (Federal University of Santa Catarina and Norwegian University of Science and Technology) | Codas, Andrés (IBM Research) | Stanko, Milan (Norwegian University of Science and Technology) | Camponogara, Eduardo (Federal University of Santa Catarina) | Foss, Bjarne (Norwegian University of Science and Technology)
A methodology is proposed for the production optimization of oil reservoirs constrained by gathering systems. Because of differences in scale and simulation tools, production optimization involving oil reservoirs and gathering networks typically adopts standalone models for each domain. Although some reservoir simulators allow the modeling of inflow-control devices (ICDs) and deviated wells, the handling of gathering-network constraints is still limited. The disregard of such constraints might render unfeasible operational plans with respect to the gathering facilities, precluding their application in real-world fields. We propose using multiple shooting (MS) to handle the output constraints from the gathering network in a scalable way. MS allowed the handling of multiple output constraints because it splits the prediction horizon into several smaller intervals, enabling the use of decomposition and parallelization techniques. The novelty of this work lies in the coupling of reservoir and network models, and in the exploitation of the problem structure to cope with multiple network constraints. An explicit coupling of reservoir and network models is used to avoid the extra burden of converging the equations of the integrated system at every timestep. Instead, the inconsistencies between reservoir and network flows and pressures are modeled as constraints in the optimization formulation. Hence, all constraints regarding both reservoir and network equations are consistent at the convergence of the algorithm. The integrated-production-optimization problem is solved with a reduced sequential quadratic programming (SQP) (RSQP) algorithm, which is an efficient gradient-based optimization method. The MS ability to handle such constraints is assessed by a simulation analysis performed in a two-phase black-oil reservoir producing to a gathering network equipped with electrical submersible pumps (ESPs). The results showed that the method is suitable to handle complex and numerous network constraints. Because of the nonconvex nature of the control-optimization problem, a heuristic procedure was developed to obtain a feasible initial solution for the integrated-production system. Further, a case study compared long-term optimization with short-term practices, where the latter yielded a lower net present value (NPV), arguably because it could not anticipate early water-front arrivals.
The objective of this paper is to describe experiments conducted to investigate osmosis as a mechanism for low-salinity enhanced oil recovery (EOR). For this purpose, an experiment was designed to facilitate enhanced oil recovery by osmosis-induced connate water expansion, while at the same time reducing the contributions of other proposed low-salinity mechanisms. Considerations were also made to achieve osmotic water transport rates comparable to what is expected at reservoir temperature.
The correlation between enhanced oil recovery and the surface-to-volume ratio was of particular interest. Because the osmotic pressure gradients occur over distances comparable to the pore size, it is plausible that fluid redistribution due to osmosis would lead to a fairly local redistribution of oil, and thereby have a small impact on overall enhanced recovery in the field. However, near exposed surfaces, this local redistribution may result in oil production.
Previous investigations of osmosis as an underlying low-salinity mechanism have consisted of visualization experiments, where water transport and oil movement under influence of osmotic gradients have been observed. Our experiments are intended to increase the understanding of the relative importance of osmosis in both small-scale low-salinity experiment results, and for field-scale low-salinity flooding.
In the experiments, oil-wet sandstone samples with different surface-to-volume ratios were saturated with high-salinity water and oil to irreducible water saturation. The samples were first left to spontaneous imbibe in high-salinity water and afterward in low-salinity water. Additional oil production from spontaneous imbibition of low-salinity was recorded and compared with the surface-to-volume ratio. The experiment was performed twice, at both ambient and elevated temperatures.
The experiments at ambient temperature resulted in increased oil production values of 8-22% of pore volume by low-salinity spontaneous imbibition. No clear correlation was found between increased oil recovery and the surface-to-volume ratio. A correlation was, however, seen between increased oil production and the pore volume. Thus, increased oil production by low-salinity imbibition seems to be proportionate to the pore volume.
The experiments at elevated temperature resulted in low values of increased oil production by low-salinity spontaneous imbibition, and the values do not seem to correlate with either surface area or pore volume. The low response is believed to be caused by thermal effects from repeated heating and cooling of the samples during the preparations.
Our results cannot dismiss osmosis as an important mechanism for low-salinity EOR. Possible explanations for the correlation between increased oil production and pore volume are hysteresis and simultaneous connate water expansion throughout the core.
The main design principle of unlined pressure tunnels/shafts in a hydropower scheme is to provide sufficient confinement (minor principal stress) to withstand the internal hydrostatic water pressure. Rock support is only provided in areas where very weak rockmass is encountered. The principle uses maximum hydrostatic water head as a design parameter and thus implies that occasional pressure transients will not have significant impact on the stability of a pressure tunnel. Minor rock falls and resulting headloss are accepted as a compromise against higher cost of supporting the whole tunnel. However, because of changing operational regime of power plants in recent years, start and stop sequences have become more frequent in Norway. This has resulted in more frequent water hammer and mass oscillations in pressure tunnels compared to the time when these tunnels were designed/constructed. Consequently, collapses have occurred in some unlined pressure tunnels/shafts, which have been in long-term operation.
This article aims to assess the influence of pressure fluctuation in the long-term stability of unlined inclined pressure shaft of Svandalsflona Hydropower Project located in southern Norway. The main issue is to review and analyze the collapse that was identified in 2008 at the inclined shaft of the headrace system of the project. The article reviews the engineering geological condition of the collapse area, evaluates mechanical properties of the rockmass at the failure location, tunnel hydraulics over the period of its operation and hydraulic impact on the rockmass/installed rock support. As the outcome, the mechanics of failure resulting due to pressure variations and conditions that may have led to the instability in this inclined shaft subjected to frequent start and stop sequences during operation are discussed. The manuscript is a part of large research initiative in Norway in the field of renewable energy called HydroCen (Norwegian Research Centre for Hydropower Technology).
The total length of hydropower tunnels in Norway is more than 4000 km, most of which were constructed during the period between 1950 and 2010. Almost 100 unlined pressure shafts or tunnels with static water head above 150 m are successfully operating in Norway with the highest being 1047 m (Broch, 2016). The design and construction of unlined pressure tunnels has been one of the most successful developments in Norway. This concept has gained worldwide reputation for its cost effectiveness, time saving and simplicity in construction of water tunnels in hard rock mass with favorable in-situ stress conditions (Panthi and Basnet, 2016).
The transition zone, i.e. where the pressurized water enters the steel lined section of the waterway, is a key component of the entire tunnel system in any underground hydropower project utilizing unlined pressure tunnels. Not only is it important for conveying the pressurized water into the turbine, its positioning also defines the location, length and layout of several other tunnels. The key for deciding on positioning of the transition zone is to identify a place with sufficient in-situ rock mass stress to withstand the internal water pressure generated from the hydraulic head of the pressure tunnel. To prepare complete tender documents, preliminary positioning of the transition zone must be defined before sufficient stress data for final positioning are available. In Norway, such early assessments have often been made on empirical basis with little or no testing. Even though this approach normally is necessary in the preliminary phase of a project development, the final decision on transition zone positioning, must be based on in-situ testing. Recognizing that stress testing is required for safe design of the transition zone, flexibility in both layout and construction schedule should be incorporated in the project. To ensure this flexibility, the tender documents must include a specific plan that describes what ground investigations should be done, where they should be done, what the acceptable criteria for such tests are, and how the results should be adopted in the design. In this paper, a proposal on the outline of such a plan will be presented based on Norwegian experience and practice in investigation, design and construction of the transition zone area for selected projects. The work presented is part of the hydropower research being performed at HydroCen, based at NTNU in Trondheim, Norway.
As water conduits for energy production, rock tunnels constitute an integral part of underground hydropower projects. In Norway, the use of unlined pressure tunnels has become the norm for any underground hydropower project, and the concept has achieved worldwide recognition (Rancourt, 2010). The main motivation for adopting the unlined concept is the potential for very large cost reductions when replacing a steel or concrete lined tunnel with an unlined tunnel. There are, however, several reports of unlined pressure tunnels not performing as expected due to hydraulic failure of the tunnel caused by the internal water pressure, as documented by Broch (1982), Merritt (1999) and Palmstrom and Broch (2017) amongst others. To reduce the risk associated with hydraulic failure of unlined pressure tunnels, knowledge about rock stresses in the area of interest is required. Obtaining such information in due time for preparation of tender documents can be difficult due to the inherent limitations of the stress measuring methods, combined with the often difficult physical access to the investigation point. Therefore, project owners and engineers often have to make their tender stage tunnel design without direct stress measurements, and postpone rock stress measurements to the construction phase. This implies that changes to the tender design must be allowed for, in case insufficient stress levels are identified during construction stage stress measurements (Halvorsen and Roti, 2013).
A specific plan describing the testing- and design methodology for the siting of the transition zone is a useful common basis for both Contractor and Client when deciding the final design. The outline of such a plan is presented herein, based on Norwegian experience and practice in investigation, design and construction of the transition zone area in hard rock conditions. The plan is specially emphasizing the rock stress measurements, though it is recognized that knowledge about the overall rock mass conditions is essential in any underground design.
A key requirement for using unlined pressure tunnels is that, at any point along the pressurized waterway, the rock stress surrounding the tunnel must be larger than the internal water pressure. If not, the water pressure can lift, or jack, the rock mass so that hydraulic failure and associated large leakages occur. Excessive leakages from the pressure tunnel can cause catastrophic events such as flooding of nearby underground structures and even landslides as reported by Brekke and Ripley (1987), Benson (1989) and Palmstrom and Broch (2017). To avoid such incidents it is absolutely required to ensure that any section of unlined pressure tunnel has sufficiently high rock stresses to accommodate the internal water pressure.
Technical soundness, cost effectiveness and long-term sustainability are the major issues that one should achieve while planning, designing, constructing and operating the hydropower schemes. With the changing world of competitiveness with other sources of renewable energies such as wind and solar, it is important that the hydro-energy still is able to compete in the renewable energy market. One of the possible solutions that helps to achieve the cost effectiveness, technically soundness and timely completion of the hydropower scheme is the use of unlined / shotcrete lined pressure tunnels. Such approach may help replace the traditionally used concrete and steel lined pressure tunnel systems, which are both cost intensive and long construction time taking. Nevertheless, there are several technical, geological and geo-tectonic challenges associated to this solution, which limits the use of this innovative concept. This manuscript will review the history of the use of unlined pressure tunnels / shafts in the world and presents prevailing design approaches in the use with the aid of the use of some very important cases of failures and successes where unlined / shotcrete lined pressure tunnels and shafts were used in the past. The detail evaluation and analysis on the geological and geo-tectonic environment are carried out. As an outcome, the state-of-the-art upgraded design criteria as well as guidelines for the use of unlined / shotcrete lined pressure tunnels are suggested with the belief that the vast hydropower resources still untapped in the Asian region will be even more cost effective, technically sound and long-term sustainable solution.
Unlined pressure tunnels in hydropower schemes are becoming popular worldwide due to the cost effectiveness compared to the tunnels that are lined with concrete and/or steel pipes (Basnet and Panthi, 2018a &; 2018b). The unlined tunnels are relatively easy in construction and take much shorter construction time in condition that the topography, geology and geo-tectonics favor. Regarding the history of application, Norwegian unlined high pressure tunnels and shafts are by far the most valuable examples in the world than the one constructed in other countries. The earliest attempts to use unlined pressure tunnels in hydropower projects with the surface powerhouse in Norway was already in 1920s, which is almost 100 years back from now. However, emphasis was given to keep all waterway system and powerhouses inside the mountain after the completion of World War II (Broch, 2013). Today, Norway has over 230 underground powerhouses and over 4300 km unlined tunnels and shafts. Such tunnels and shafts were considered to be possible due to the favorable engineering geological and geo-tectonic conditions that persist in the Scandinavia (Johansen, 1984; Panthi, 2014). Experiences gained in design, construction and operation of such waterway systems has led to the development of different design criteria for unlined tunnels (Broch, 1982).
As oil fields mature, the produced water content of the production stream will often increase over time, and produced water management will eventually become a bottleneck in production. Subsea separation of produced water enables prolonged lifetime of brown field installations, increased recovery rates and more energy efficient production. In addition, implementation of subsea water separation will also enable future tie-ins to existing facilities, and reduce the need for new and expensive transport lines. Existing installed subsea produced water bulk separator technologies are limited to gravity and compact gravity vessels, such as Troll and Tordis, and the Marlim pipe separator. These are large installations, which are costly to manufacture, transport and install. In addition, the gravity and compact gravity vessels are not suited for deep-water installations, and there is a need for novel solutions to both reduce the weight and size of bulk water separators, making the technology more attractive for new business cases.
In order to investigate improved subsea bulk water separation technologies, a multiphase oil-water test loop has been developed at the Norwegian University of Science and Technology (NTNU). Facility test fluids are ExxsolD60 and distilled water with wt%3.4 NaCl. In this paper, a new separator design, utilizing multiple parallel pipes will be presented. The design allows reduction of required wall thickness at large water depths, shorter residence times and hence a shorter separator length compared to traditional gravity based technologies. Initial performance data of a constructed medium scale prototype will be reported, including separation efficiency estimations over a range of flow rates, water cuts (WC) and water extraction rates (ER). Tested flow rates vary from 250L/min to 750L/min at 30%, 50% and 70% WC. Water extraction rates are varied from 50% to 100% of the inlet water rate.
Based on this initial test campaign, the concept proves promising, displaying good separation efficiencies (>98%) for both water continuous and oil continuous inlet flows at moderate flow velocities. At higher flow rates, performance decreases, and water extraction rates must be limited in order to maintain high efficiencies. Photos of flow conditions at the water outlet are included, providing a visualization of the occurring two-phase flow phenomena inside the separator.
The presented concept adds to an expanding portfolio of proposed subsea separation solutions, and displays a new way of utilizing parallel pipes to achieve oil-water bulk separation.