User:Adilullo/Flow Assurance

Flow Assurance
Flow Assurance in Petroleum Industry is the discipline which guarantees the achievement of the life-time production targets of a lifting and transport system, from the reservoir sand face to the storage tanks, by predicting, preventing and solving the problems originated by the behavior of the transported substances, i.e. gases, liquids and solids either separated or in multiphase conditions.

Along the route from the reservoir to the receiving facilities, reservoir fluids usually undergo significant changes in pressure and temperature conditions. Beyond affecting the liquid/vapour ratio and compositions, these changes of conditions may induce several physico-chemical effects in the fluids, such as decreasing the solubility of some classes of compounds or creating new phases. Moreover, the presence of several phases in the pipes used for transportation (either in the well or on the surface) is the source of several complex rheological and fluid-dynamic phenomena. All these effects are not predicted and described by the basic thermodynamic and fluid dynamic models and affect, often negatively, the transportation process. Flow Assurance is the technical discipline addressing such phenomena and their effects on hydrocarbon production.

Typical Flow Assurance problems are:

•	build-up of wax, asphaltenes, inorganic scale deposits or sand

•	sudden formation of hydrates plugs

•	occurrence of unacceptable flow regimes, like severe slugging

•	lack of reservoir pressure required to move hydrocarbons to the receiving facilities

•	increased pressure drop caused by high viscosity emulsions

•	erosion phenomena due to high fluid velocity or to sand

•	build-up of sand deposits or accumulation of liquids in gas pipelines

•	restart difficulties due to high yield stress fluids, in particular at low temperatures

•	corrosion phenomena due to the settling of water or to the water-wetting of surfaces in emulsions.

All the Flow Assurance problems (like those listed above) have a negative effect on production rate or on lifting costs. In the worst case, some of them may even lead to complete production stops, in which case, they are called “show stoppers”. The latter, especially, justify the name Flow Assurance for the efforts aimed at “keeping the hydrocarbons flowing”.

Typical Flow Assurance solutions are, respectively:

•	regular injection of chemical products acting as deposition inhibitors, batch use of solvent to dissolve deposits or regular launch of pigs to displace them outside the pipeline

•	injection of hydrate inhibitors (either thermodynamic, kinetic or colloidal)

•	appropriate selection of pipeline diameter / route or active flow control (by valves or chemical products)

•	downhole or surface pumping technologies, multiphase pumping, gaslift

•	usage of emulsion breakers

•	pipeline diameter or appropriate material selection

•	sand control or regular pigging

•	yield stress reduction chemicals (e.g. pour point depressants or demulsifiers)

•	optimal thermal insulation or active heating

•	application of corrosion inhibitors, pigging or appropriate internal coating

Flow Assurance scope and relevance
Historically, the term is reported to have been originated in the early 90s with the expression “garantia de fluxo”, coined by Petrobras, and later translated in English as Flow Assurance and systematically used in the Deepstar JIP project (http://www.deepstar.org/). The term “Flow Assurance”, per se, is generic and does not lead to a unique technical interpretation, so that different Oil / Service Companies have internally adopted slightly different definitions, restricting or enlarging the lists of phenomena or solutions considered. Also, several Flow Assurance problems can be predicted and addressed with chemical methods, thus creating a significant overlap between Flow Assurance and the discipline called Oilfield Chemistry. Actually, to be appropriately managed, Flow Assurance issues and solutions deeply involve also engineering methods and computations, thermo-fluid dynamic simulation, material selection, pumping/boosting technologies, pigging and repair techniques.

During production, the solution of unexpected Flow Assurance problems tends to be quite expensive, often involving temporary complete production stops, usage of large amounts of chemical products or even mechanical interventions. This implies that all the efforts are made to predict and prevent Flow Assurance problems since the earliest design phases. Nevertheless, the inadequacy of data about fluids and future production conditions, together with the partial accuracy of current mathematical models, easily lead to conservative design approaches and increased CAPEX. Moreover, several Flow Assurance problems, even if predicted and dealt with in the design phase, cannot sometimes be fully prevented, i.e. their probability and impact cannot be reduced to zero, but just lowered below a level considered acceptable as a business risk. Finally, during production, residual problems, unexpected facts or pressure towards cost reduction may make some Flow Assurance phenomena emerge and remedial solutions must be eventually identified and applied.

All the above implies that, while the main Flow Assurance focus is prediction and prevention in the design phase, this discipline is also very active during operations, where early detection and problem solving become the main objectives.

Flow Assurance spans an extremely wide range of subjects, going from laboratory characterization of micro-liter samples to heavy weight, field mechanical operations. Thus, multidisciplinarity and transdisciplinarity are Flow Assurance key success factors. For this reason. Flow Assurance methodology, tools and technologies have been often addressed by the industry in the form of Joint Industry Projects (JIP), thus leveraging on the experience and point of view of several Oil and Service Companies, one notable example being the Deepstar project.

Flow Assurance in Design
The structure of a Flow Assurance study for the design phase is the follows.

1) Gather information about the transported fluids • Compositional and thermodynamic data produced for the PVT study and corresponding tuned EOS thermodynamic model.

For Flow Assurance studies, the commonly used models are the Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR), which are generally considered to provide comparable results.

It has to be noticed that thermodynamic models for PVT studies are normally tuned to reproduce accurately the fluid behavior at reservoir conditions. Such conditions are usually far from surface transportation conditions with respect to pressure and temperature. This may create large inaccuracies in the predictions of the tuned thermodynamic model. To avoid this, it is advisable that some PVT data are also obtained at the foreseen surface transportation conditions and tuning is performed with the data which are closer to the relevant P and T conditions.

• Laboratory data about wax components, especially the Wax Appearance Temperature (WAT), often divided into a “minor onset” and a “major onset”, and the Pour Point temperature.

• Laboratory data or model predictions about hydrate formation, in the form of the Hydrate Dissolution curve in the P-T plane.

• Laboratory data about asphaltene components, especially the asphaltene instability envelope, which indicates the region of the P-T plane where asphaltenes are unstable and may form particulate depositing on pipe walls and within process facilities.

• Laboratory data about emulsion stability, viscosity and yield-stress as a function of water-cut.

• Laboratory data about fluid compatibility, in case production involves mixing of hydrocarbons from different layers or reservoirs. Fluid incompatibility results in the formation of particulate and possible, consequent deposits.

• Water composition and scaling behavior at the P-T conditions encountered by the fluids from sand-face to the terminal facilities.

2) Gather information about the geographic localization and setting, with particular reference to the relative distances and the characteristics of the elevation/depth profiles of the pipelines and to the environmental thermal data (e.g. winter/summer temperatures).

3) Gather preliminary production forecasts of oil, gas and water. Such forecasts usually come from preliminary reservoir studies which assume the technical feasibility of given wellhead pressure conditions and create the economic basis for field development.

With the above data and tools, the main results of a design Flow Assurance study are the following:

1) Deliverability and Operability verifications: verify the expected production profiles with the overall pressure drops and thermal behavior and, if necessary, propose artificial boosting methods. Select pipelines diameters, thermal management strategy and slug-catcher volumes for steady-state operation, shut-down and start-up operations. This activity also includes evaluation of erosion potential, ramp-up and turn-down operations, emergency depressurization, chemicals requirements, warm and cold restarts.

2) Hydrate and Wax management strategy: specify primary and secondary (back-up/support) strategies for managing hydrate and wax formation during steady state production, planned and unplanned shut-downs and start-ups

3) Scale, Asphaltene, Emulsion and Compatibility strategies: identify strategies and technologies aimed at preventing scale and asphaltene deposition, the formation of high viscosity/yield stress emulsions and the possibility that mixing incompatible fluids may cause deposits or obstructions.

4) Chemicals selection: when chemical inhibitors are needed, objective of a Flow Assurance study is also to verify that the required inhibition performances are (or can be made) available in commercial products. This laboratory activity is usually performed with support from the many available suppliers, such as Nalco and Baker Petrolite.

5) Pigging strategy: collaborating with corrosion and material selection studies, define the optimal pigging strategy for cleaning purposes, to support corrosion inhibitors or other operations and for pipeline inspection (in-line inspection by “intelligent pigs”).

6) Monitoring strategy: when appropriate, specify metering points and instruments necessary to perform Flow Assurance monitoring during production and to carry out the planned strategies. This may include pressure and temperature sensors (either local or distributed), flow meters or water-cut meters, sampling points, test lines, on-line analysis devices, etc.

Flow Assurance Integrated Studies
Sometimes, Flow Assurance analyses show that the hypotheses used to obtain the field production forecasts (e.g. wellhead pressures) are not realistic or economic. In this case, interaction may be required with the reservoir studies in order to create a coherent development picture.

Such “integrated” studies may require the interplay between reservoir simulation and well/pipeline transportation simulations and several tools exist on the market to perform this. Two widely used tools are Integrated Production Modelling suite from Petroleum Experts and the Integrated Asset Modeling tools from LandMark (Halliburton). Such tools use a simplified description of thermal and fluid dynamic phenomena which is suitable for faster simulation. A more accurate description of thermo-fluid dynamics can be obtained with specialized software, among which the most widely recognized (within the petroleum industry) is SPTGroup’s OLGA which can also be used in integrated simulations, normally at the expense of a longer simulation time.

Flow Assurance in Operations
There are several contexts in which the Flow Assurance discipline emerges during production and operations:

- Production parameters may vary, along time, beyond the original design specifications, e.g. due to unexpected reservoir evolution or to changes in production set-up. The latter, for example, may occur when a single phase pipeline is converted to a multiphase one to fulfill zero-flaring policies or new fields are tied-in to existing facilities.

- Production parameters may differ from design parameters, e.g, in case of faster increase in Gas Oil Ratio (GOR) or water-cut or lower operating temperatures than expected)

- Pipeline integrity management issues may occur, e.g. in case of unexpected corrosion phenomena or brown-fields pipelines re-qualification for life extension, leading to Intelligent Pig inspection and preliminary need of deposits removal and cleaning of the inner pipeline wall

- Water/condensates/sand accumulations in pipelines may increase pressure drops or originate unexpected surges or slugging regime, e.g. for offshore gas lines located between production platforms and process facilities.

- Chemical additives can be less effective than expected, thus leading to partial or complete obstructions by distributed deposits or localized plugs, e.g., after change of supplier, or for insufficient dosage or after commingling with unexpectedly incompatible fluids.

All these facts may lead to Flow Assurance issues during production. In this case, the main Flow Assurance objectives are:

1) Early detection of deposits accumulation.

As a rule, the smaller amount of deposits, the cheaper and easier the remedial actions. Flow Assurance monitoring has the main target to detect as soon as possible the formation of localized obstructions or of distributed deposits so that the optimal solutions can be identified and applied during normal production. When normal production must be reduced or stopped, bigger and bigger costs or missing revenues may be incurred.

Three main monitoring strategies exist and may be applied in the field:

A. Steady-state monitoring, which implies the continuous measurement of the operational parameters (pressures, temperatures, flow rates), under regular production conditions, and their comparison with the expected values (often computed with the aid of simulators). This monitoring technology has been engineered with the aid of several commercial simulators and is proposed by several providers. A few notable examples may be Petroleum Experts, Halliburton, SPTGroup, MSi, FMC products. The drawback of steady state monitoring techniques lays in their limitations in detecting localized deposits and in identifying their position. Moreover, the limited accuracy of current multiphase meters often precludes the early detection of deposits accumulation hiding their effect in the uncertainty band around the measured parameters.

B. Transient monitoring, which implies the creation of flow rate transients and the analysis of resulting pressure values, in a way similar to sonar or water-hammer principles. The technology in this category with the biggest record of field applications is Eni’s PRIMEFLO, which allowed non-invasive internal inspection of oil and gas pipelines exceeding 500km length. The drawback of transient based monitoring techniques is their essential limitation to single phase fluids (either liquids or gases) and very limited applicability to multiphase conditions.

C. Noise monitoring, which is based on the passive recording of the fluid-dynamic pressure noise present during flow and its processing in order to infer pipeline conditions far from the measurement point. No records of regular application of this technology are known to the writer. The main drawback is the limited distance of inspection from the measurement point, which may be of one or two hundred meters.

2) Problem remediation

The solution of unexpected Flow Assurance problems involve the interplay of several competencies and turns out to be particularly demanding owing to: -	psychological pressure (especially for completely obstructed lines) -	frequent lack of sufficient data for unique diagnosis and solution selection, in particular for multiphase pipelines -	narrow portfolio of technologies quickly available on local markets -	frequent application of de-structured, trial-and-error approaches which often progressively limits the residual intervention possibilities

Flow Assurance Intervention technologies ...