MODIFIED VISCOMETER REMOTELY MEASURES DILUTED HEAVY OIL

J.S. Pasini-Taylor
AEC Pipelines
Edmonton, Alta.

AEC Pipelines, Edmonton, has successfully modified a conventional viscometer for remote, on-line measurement of diluted heavy-oil.

These modifications included additional instrumentation to monitor and control the temperature of the product sample within the viscometer and instrumentation and controls remotely to operate the viscometer and monitor its status.

MEASURING PRODUCT QUALITY

A pipeline company transporting diluted heavy oil must monitor product quality. Viscosity, one of the properties of diluted heavy oil, is measured on-line and continuously. Natural gas condensate is used to dilute heavy oil to meet pipeline-transportation viscosity specifications.

The heavy-oil producer is responsible for ensuring that the diluted oil meets specification, while the pipeline company uses the on-line viscometer to monitor the product entering the pipeline to ensure the specification is met.

The viscosity signal is continuously transmitted to the producer to reduce, if necessary, the addition of condensate to the oil, an expensive commodity in limited supply.

With the increasing development of heavy-oil resources, transporting diluted heavy oil will become more prevalent. The need for continuous on-line measurement of viscosity at specified temperatures will be required to meet pipeline specifications while keeping tight control of condensate costs.

COLD LAKE PIPELINE

AEC Pipelines, a division of Alberta Energy Co. Ltd., owns and operates the Cold Lake heavy-oil pipeline.

This transportation network (Fig. 1) moves diluted heavy oil from in situ heavy-oil projects in the Cold Lake region in northeastern Alberta through a 610-mm (24-in.) pipeline to Edmonton, a distance of 240 km (149 miles). A parallel 323.9-mm (12-in.) pipeline flowing in the reverse direction supplies natural-gas condensate diluent (C5+) to the producers for dilution of the heavy oil.

The pipeline is remotely operated from the control center near Edmonton and the quality of the diluted heavy oil is monitored on a 24-hr basis.

AEC Pipeline's modification of an off-the-shelf viscometer was to suit it to a remotely operated pipeline application and to meet the unique temperature requirements of the viscosity specification.

The goal is simultaneously to hold viscosity at a maximum and to meet pipeline-transportation specifications. This requires reducing costs through minimum condensate addition while transporting the maximum percentage of heavy oil. (Increased volumes of condensate in diluted heavy oil result in higher overall transportation costs.)

HIGH TEMPERATURES

There are several unique features of this application for viscometers.

One is the high inlet temperature of the diluted heavy oil (50 C.) in comparison to the specification temperature (2-18 C.). The viscosity specification requires that the diluted heavy oil not exceed 250 cSt (cSt = cp/p) at specification temperature as established by the interprovincial pipeline carriers.

The specification temperature is typically changed monthly and is the lower of either:

The actual temperature of the diluted heavy oil at the Edmonton delivery point (where the product is transferred from AEC Pipelines to the interprovincial pipeline carriers); or,

The average pipeline temperature of the interprovincial pipeline carriers.

The heavy-oil producer delivers the diluted heavy oil to AEC Pipelines at an elevated temperature because the heavy oil is produced with the thermal extraction method of cyclic steam stimulation.

After extraction, the heavy oil is treated to remove water, and then condensate is added by the producer at a ratio of approximately 30% condensate to 70% heavy oil.

The exact ratio is controlled by the on-line viscometer to maximize viscosity through minimum condensate addition.

Table 1 shows a comparison of the viscosity and density of in situ heavy oil, produced heavy oil, and onspecification diluted heavy oil.

Because AEC Pipelines receives the diluted heavy oil at an elevated temperature, the sample must be chilled to a 50 C. for the viscosity of the oil to be measured at the specification temperature.

In the past it has not been possible to measure viscosity at an elevated temperature (less than 20 C.) and accurately correlate that to a viscosity at specification temperature.

The reason is the diluted heavy oil has two distinct thermal ranges over which its viscosity behavior shows different patterns.

At less than approximately 20 C., the blend is relatively insensitive to changes in shear rate and is almost Newtonian in its flow behavior.

In this thermal range, however, it does have a marked sensitivity to changes in temperature.

At greater than 20 C., the viscosity variation with temperature is much less rapid but it becomes sensitive to shear rate.

Fig. 2 illustrates the problem.

At shear rates greater than 10 sec-1 (the shear rate of the pipeline being approximately 17 sec-1), the oil is insensitive to changes in shear of < 20 C. When the viscosity of the oil is measured at a shear rate of greater than 10 sec 1 and >20 C., the shear rate changes markedly in all samples tested.

This change in the fluid's behavior from Newtonian to non-Newtonian makes it difficult directly to predict diluted heavy-oil viscosity at specification temperatures of 2-18 C. (based on measured viscosity at pipeline inlet temperature of 50 C. or any other arbitrary temperature).

It has proven more accurate to chill the product sample to the specification temperature and directly measure the viscosity of the diluted heavy oil.

A second unique aspect of the application is that the Cold Lake heavy-oil pipeline is operated and monitored remotely through a sophisticated supervisory control and data acquisition (scada) system.

An off-the-shelf viscometer is not usually designed for a remote application. Rather, it is typically used in a refinery installation where technicians are able to check the status of the unit and calibrate or adjust the temperature of the unit on a regular basis.

A viscometer was needed which could be remotely operated and monitored so that a technician need only check the calibration once a week. A standard viscometer required modifications to meet this criterion.

MEANS OF SOLUTION

The viscometer chosen for this application employs the Hagen-Poiseuille theory which states that for laminar flow (Re < 2100) through a circular tube of known length and diameter with steady, incompressible fluid flow and constant back pressure, the viscosity varies directly with differential pressure across the capillary tube.

The pressure drop across the tube results entirely from the viscous stress at the walls of the tube and is therefore proportional to viscosity for Newtonian fluids. 12 The viscometer measures absolute viscosity.

For a horizontal capillary the defining equation is the following:

m = p Ap R4

------- (1)

8LQ

Following are the concerns that were addressed to meet the application criteria particular to this installation:

TEMPERATURE

Modifications were required remotely to monitor and control the product sample to the specification temperature approximately 50 C. lower than the inlet temperature of the product.

The following changes and additions were made to a standard viscometer:

A 100-ohm, platinum, tipsensitive, band 5 accuracy resistance temperature detector (RTD) was inserted into the process line downstream, but as close as possible to the measuring capillary tube.

The RTD was connected to a 4-20 ma temperature transmitter remotely to transmit the signal. This feature of monitoring actual product temperature is not typical in a standard capillary viscometer.

Typically, a standard viscometer would have its capillary tube submerged in a bath of mineral oil. The oil bath would be temperature controlled, and presumably the product flowing through the tubing would be the same temperature as the bath.

In this application of high AT, however, it was critical to get an accurate sample temperature measurement. The RTD should in no way interfere with the sample flow through the tubing and should therefore be tip sensitive.

In order to meet the high AT requirement, it was necessary to replace the standard primary shell and tube heat exchanger with a surface area of 0.11 sq m with an exchanger with a surface area of 0.68 sq m.

Replacing the single-pass exchanger (28 tubes x 1 pass) with a four-pass exchanger(13 tubes x 4 pass) allowed the velocity through the unit to be increased by a factor of 2.2. The efficiency of heat transfer was improved with this replacement and the high AT requirement was satisfied.

Liquid-crystal display (LCD) indicators for viscosity (cp) and temperature (C.) were locally mounted so that a technician could readily see the analog output while servicing the unit. These indicators were looped with the 420 ma signals being transmitted to an on-site computer and further transmitted to the control center.

Further to improve the heat-transfer efficiency, the chiller refrigerant was upgraded from R12 (chlorodiflouromethane) to R22 (dichlorodiflouromethane), a refrigerant that operates at a cooler temperature. A problem resulted when the amperage draw on the refrigerant compressor motor exceeded the motor rating.

A crankcase pressure regulator was installed on the compressor to keep the load within limits. A low-temperature thermostat was also added to match the cooler temperatures the chiller was capable of attaining.

REMOTE OPERATION

Modifications were required to upgrade the viscometer to be remotely operated and monitored from the control center.

The viscometer signal is transmitted to the local onsite computer, a programmable logic controller (PLC). If the diluted heavy oil exceeds specification, a valve closes to divert the product back to the producer and the viscometer remains on-line to detect when the product has returned to specification (Fig. 3).

The viscosity signal is continuously transmitted to the heavy-oil producer so that the condensate-injection rate can be adjusted to meet pipeline specifications while viscosity is held to maximum levels to reduce condensate costs.

The scada system is used to control and monitor the unit status from the control center. The following changes to the viscometer were required to meet the remote-operation considerations:

For effective remote operation of the pump stations, the viscometer was required to respond to the several situations: station fire alarm, station emergency-shutdown alarm, pipeline booster pump shutdown, pipeline booster pump start-up, viscosity specification exceeded, and viscosity specification return to normal.

Contractors were installed in the circuitry of the viscometer's internal pump motor. The PLC signals the contactors to switch the viscometer pump on or off and maintains the temperature control system within the unit.

This becomes important during an extended shutdown situation. The low product-specification temperature must be maintained within the unit or the return to normal will be lengthy while the chiller cools the diluted heavy-oil sample stream. The viscometer's internal pump is only shut down when the pipeline booster pumps are shut down.

Programming of the onsite computer (PLC) was required to set the logic for shutdown and start-up of the viscometer and associated equipment under the conditions previously stated and to ensure only product that meets the viscosity specification is transported.

FILTRATION CONSIDERATIONS

Because the diluted heavy oil is asphaltic in nature and has high particulate loading, filtration is a problem at the low specification temperatures. The viscometer had 6mm nominal size tubing with a capillary tube inner diameter of 4.45 mm (0.07 mm).

Asphaltenes tend to precipitate at the lower specification temperatures. That tendency caused a high level of particulate precipitation on the tubing internals until proper filtration was employed.

Particulate dropout is encouraged by the laminar flow regime that is inherent to the unit's principle of operation. Particulate drop out in the capillary tube increased the differential pressure across the tube and caused drifting in the viscosity readout, and frequent calibrations became necessary.

Some of the filtration problems were addressed as follows:

An oversized 10-m filter was installed on the inlet of the viscometer but downstream of the primary heat exchanger so that it could capture any temperature-dependent precipitates. The filter has disposable cartridges that are changed weekly.

The viscometer was initially equipped with a 100-[L strainer which was inadequate for products with a high particulate loading. During sizing of the filtration system, care was taken to ensure that retention of the diluted heavy-oil sample in the filter housing did not create excessive time delays in the viscosity readings.

During an upset, the producer's plant can allow a small volume of water to enter the diluted heavy oil. At lower specification temperatures, the water would then freeze inside the filter or the heat exchanger.

A shutdown switch was installed that would stop the viscometer's internal pump if freezing occurred. This switch was required to stop sample flow through the unit so that the viscometer's internal pump could not be starved or otherwise adversely affected.

Before installation of the 10-m filter, a high-velocity flush was performed to clean the viscometer's tubing internals.

This was required because the unit had run for some time with only the 100-m filter installed and there had been a buildup of debris.

The viscometer tubing can be properly flushed of particulate only if the internal metering pump is bypassed or removed and a flush in the turbulent regime is performed with a solvent such as toluene.

The internal pump was an obstacle to attaining turbulent flow and the bypass worked well leaving the tubing clean and free of internal coating.

Currently, the viscometer is flushed with solvent on a weekly basis without removal of the internal pump. This routine flush is performed before a technician recalibrates the unit.

If the viscosity signal is out of calibration as compared to a standard sample, a routine flush may bring the readings back within acceptable limits. A high-velocity flush may still be used if need arises in the future.

RESULTS

Remote monitoring and control of the diluted heavyoil sample to the specification temperature with the described modifications have had the following results:

With a AT requirement of up to 50 C., the viscometer successfully maintained the diluted heavy-oil product sample to the specification temperature within 0.20 C.

In addition, the refrigerant compressor on the chiller operates 30-40% of the time to reduce wear on the mechanical components, thereby increasing the life of the equipment.

The modified viscometer has successfully met the requirements for remote operation and monitoring.

Specifically, the logic of the on-site computer (PLC) has been successful in automatically stopping the station's main pumps and diverting off-specification diluted heavy oil back to the producer's tankage. Upon the oil's return to specification, the pipeline pump station is available for the producer automatically to resume shipment.

Also, the viscosity signal is transmitted to the producer who adjusts the condensate-injection rate accordingly through a control loop (Fig. 3) to meet the viscosity specification.

At a measured viscosity of 250 cSt, Esso Resources Canada, one heavy-oil producer, estimates a savings of approximately $400,000/year for every 10 cSt of accurately measured diluted heavy-oil viscosity. This savings is based on production of 20,000 cu m/day of diluted heavy oil.

Finally, the results of the filtration modifications are as follows:

With the 10-m filter installed, periodic removal and inspection of the capillary tube shows it to be clean and free of coating or buildup of debris.

The viscometer has operated continuously and remained accurate to within 1 % of calibrated samples with maintenance consisting of only routine solvent flushes and filter changes.

Additionally, disposable filters are easily replaced. The viscometer is more reliable because the filter is being regularly changed.

And maintenance of the viscometer internal pump has been reduced in two ways.

First, the pump is automatically switched off in the event of freezing in the filter or exchanger and therefore the pump cannot be starved.

Second, accurately metered flow of product through the unit is critical to match the Hagen-Poiseuille theory. Metering pumps with very fine tolerances are used in this viscometer and the slightest accumulation of debris can damage and alter the flow through the pump.

Because the diluted heavy oil is well filtered, there has been less wear and less maintenance on the internal pump and viscosity readings are accurate.