Field system design needs crude knowledge

Heavy oil characterization identifies flow assurance risks that drive system designs

The necessity for accurate characterization of heavy oil separation properties and emulsion formation tendency is critical for any design team, as it has a major impact on the field development concept selection.

This includes, for example, the fluid lifting mechanism for production wells, on the system detailed design and, finally, on the selection of fit-for-purpose operational solutions, especially for offshore new field developments.

Heavy and extra heavy oil are crude oils which are so viscous that they will not flow easily. In this article, heavy oil is referred to as oil with an API gravity of less than 23o, noting that classification schemes exist that organize such oils into medium heavy, heavy, extra heavy and bitumen oils, based on density and dynamic viscosity.

As oil and gas resources in current basins of the world continue to decline, heavy oil provides a significant solution to the world’s thirst for energy.

There are enormous reserves of heavy oil, potentially up to 1Gbbl (1x10^9) bbl, depending on the recovery factor. However, with current technologies the recovery rate is currently low and so far less than 1% has been produced or is under development.

If the recovery rate could be increased to 50% of the available deposits, 50% of North American demand would be met for more than 50 years, according to Francois Cupcic, heavy oil research leader at Total.

There are numerous flow assurance challenges associated with the production and processing of heavy and extra heavy oils in both onshore and offshore environments that directly impact the ability of any process system design to successfully handle such fluids.

Many of these challenges are centered on the production separation system and are generally addressed in the early stage of the field development. These challenges include:

• Artificial lift options (gas lift, electrical submersible pumps, hydraulic submersible pumps, etc.)
• Space and weight restrictions on platforms and floating production systems that govern separation vessel sizes and residence times:
• Accuracy of the fluid characterization data
• Heating and power requirements
• Separation vessels’ Internal design
• Process vessels’ configuration
• Oil, water, emulsion, foam, microbiology, and solids handling
• Optimal treatment chemicals application.

A critical aspect of the heavy oil production separation system design is the accuracy of the fluid characterization data upon which the design is based. During the appraisal phase of a new heavy oil field development, when appraisal well tests occur and fluid samples are being collected for other issues such as PVT (pressure, volume, and temperature) studies, it is paramount that good, quality, representative samples of oil, and indeed formation water, are collected.

This is so some samples can be allocated for the accurate determination of fluid physical properties and production chemistry characteristics, as this is crucial information for the basis of design of any production and separation system. The degree of drilling mud contamination, especially if it is oil-based mud, is very important to know.

Both live downhole and stabilized oil samples need to be collected and analyzed. The dynamic viscosity of the oil and its water-in-oil emulsion is one of the more important parameters required. It defines the factor between dry oil and emulsion viscosity for a system and is a key flow assurance parameter input for any system design.

It crucial that representative fluid samples are collected, and that the laboratory test methods selected to define the rheological properties and to characterize the production chemistry issues are fit for purpose for each system.

An example of this would be the measurement of fluid dynamic viscosity, where the test shear conditions must directly relate to the passage of live fluids through the points of significant shearing, i.e. shearing due to passage through electric submersible pumps, hydraulic submersible pumps, gas lift, and wellhead choke valves.

Calculation of representative flow regime and shear conditions for an emulsion creation in the laboratory is recommended. Otherwise, the measured properties used for design purposes will be misleading, resulting in unnecessary production equipment redesign and upgrades at a later date.

Following laboratory measurement of rheological properties, it is then possible to model live fluid viscosities using PVT software and viscosity correlations, including algorithms, to calculate relative viscosity and the ratio of emulsion viscosity to stabilized crude oil viscosity. Heavy and extra heavy oils can exhibit unusual rheological properties and variable flow assurance characteristics during production and processing, which must be properly defined. These include, but are not limited to, the following, all of which must be considered in any new facility design:

• Variable, multiphase flow and slugging characteristics in surface and subsea pipelines (need to be modelled)
• High, dynamic, oil viscosities causing lifting issues in production wells, which influences lifting-mechanism selection, especially in remote, offshore locations with subsea wells
• Subsea systems with long, tieback flowlines, because of problems with high viscosity fluids that require relatively high restart pressures, following lengthy unplanned shutdowns
• Variable tendencies of oil types to form stable emulsions, including differing amounts of emulsifying agents
• High emulsion viscosities, especially up to a critical maximum prior to the emulsion inversion point, Fig.1.
• Onerously slow oil-water separation rates linked to typically low oil-water density contrasts and high, continuous, oil-phase viscosities, unless the fluids are heated to 120°C or more to lower the fluid viscosities
• Swelling of the continuous oil phase by flashed gas, causing gas undercarry downstream
• Stable foam formation in separators and slug catchers that causes liquid carryover to the gas treatment system
• Heavy oils with associated formation- derived sand and fines solids production can further stabilize emulsions and drop out subsea and in topsides vessels to plug and erode internals and to reduce vessel residence times.
• Varying propensity to form precipitated solids from the production fluids during production and depressurization while flowing to the surface, which can generate various unwanted solids that form flow restrictions and even plug lines.

Case studies

Offshore Western Australia

The facility involved offshore production of heavy oils of between 17°API and 20°API gravity to a floating production, storage and offloading vessel with common production facilities in an environmentally sensitive area. This facility required good performance of the primary separation vessels as being vital in ensuring standards were met for produced water discharge quality in addition to export oil quality.

A series of studies were carried out to review the post-FEED (front end engineering and design) proposed designs of production vessels (separators and an electrostatic coalescer) in the system, covering all aspects of vessel designs that would affect separation performance, and later a review study took place after operation of the system to help optimize performance further.

Discoveries were made at the design stage and later after operation of the field on-site with recommendations implemented by the operator that led to an optimized separation system design including:

• Key emulsion stabilisers in the production fluids were found to be carboxylate soaps, so demulsifiers could be reformulated to directly handle such components and improve separation vessel performance leading to in specification separated water and export oil.
• Separator vessel internals (including inlet pipes, inlet device tubes, vane packs, and gas outlet demisting devices) were critiqued and redesigned to improve performance.
• An optimal emulsion treating temperature range of 55-60°C was identified and set in the design.
• Hot water recycling to the first-stage separator inlet from the second-stage separator and coalescer was incorporated in the design to aid separation performance and help keep the fluid from being too near its emulsioninversion- point zone of peak emulsion viscosity, which can hinder good separation performance.
• A revised subsea and topsides chemical application strategy was implemented.
• Later trials of a subsea-injected demulsifier indicated that it could decrease the fluid viscosity significantly, while achieving some moderate increases in oil flow rate and reduction in flowline pressures. The effect was very dose-rate sensitive and care must be taken to not add too much chemical, due to the risk of water dropping out too rapidly, which would result in water accumulation promoting slug formation and subsequent pressure fluctuations in the flow lines.
• New operational practices were put in place to handle fluids from newly drilled flowback to host production wells, including optimized use of the slops tank.

UK North Sea

A new oilfield development in the UK North Sea comprised of a 14°API gravity heavy oil produced from subsea wells to a FPSO required a full review of the critical fluid-sampling and production-chemistry characterisation data, to ensure that potential issues were identified and incorporated into the production facilities design and development planning process, so as to generate a fit for purpose design. One area of focus was on forward emulsions rheological properties.

A variety of laboratory-based production fluid rheology studies with stabilized crude oil were undertaken on dry oil and its forward emulsions. It was found that the measured dry oil and forward emulsion viscosity was very sensitive to the mixing method used (including the type of blender or stirrer used, plus shearing duration) as indeed was the measured emulsion inversion point and the emulsion stability of the samples.

It was also necessary to control the shear rate for viscosity measurement to closely mimic the conditions on fluid passage through production equipment, like submersible pumps and wellhead choke valves, to get representative data for design purposes.

In this case and for analog fields, it was found that the emulsion viscosity multiplier (to dry oil) was between 3 and 5, though if too excessive shearing was employed, unrepresentative results of >6 could be achieved and, if too gentle mixing was employed, the multiplier would be below 2, and again unrepresentative.

This was the most representative result for this particular oil (and close analogs) that was used for design purposes. For other heavy-oil fields, it is recommended that such detailed test work be repeated for each case, or the design premise will be incorrect.

Laboratory experimental data was then compared to theoretical relative emulsion viscosity data versus water cut using various published algorithms to determine the validity of the data, noting that the algorithms do tend to give a spread of predicted emulsion viscosities. Correct interpretation of the data comparison gives confidence in the data used for design purposes, Figure 2.

• In this particular heavy oil case, the generated fluid rheological data was used in making a number of decisions including the following:
• Help decide upon the lifting-mechanism concept for the subsea wells.
• Determine the risk of flow assurance issues, like the restart pressures required of subsea wells with cold fluids.
• Help decide upon the use of recycled, produced water to aid surface production separation characteristics. Decide on the requirement for downhole chemical injection, e.g. demulsifier, wrt protection of submersible pumps, and maintenance of production rates.
• Decide upon the number of subsea chemical umbilicals required. OE

Colin Smith is a process chemistry consultant at Maxoil Solutions with 25 years experience in the oil industry. He initially worked in the wax, hydrates, and asphaltene laboratories of the BP Research Centre, Sunbury, followed by global on-site troubleshooting work at various oilfield consultancies. Smith earned a graduate chemistry degree from University College Dublin.