High Temperature Multiphase Flowmeters in Heavy-Oil Thermal Production
P. Mehdizadeh, SPE, Consultant, Production Technology Inc.
The accurate measurement of Oil, Water and Gas/Steam in heavy oil thermal production (SAGO and other Steam Flood Processes) is a very diffcult task faced by the heavy oil industry. The accuracy of these measurements is critical for reservoir management and production diagnostics. Mulitphase flow meter technology has been used successfully around the world for over 10 years and in heavy oil "cold" production in Venezuela and other countries. But multiphase technology has never been used in Extra Heavy Oil Thermal Production. The Canadian heavy oil thermal producers regularly see production temperatures exceeding 200 C (392 F) and some wells are approaching 232 C (450 F). New technology is required to accurately measure wells producing at these elevated temperatures. The first field tests using a multi phase flow meter in a heavy oil thermal project was conducted by one of the major Canadian producers in the fall of 2004. Additional tests were completed during the summer of 2005 with another heavy oil producer. This paper will review the unique problems encountered with testing heavy oil in high temperature applications. The test results from multiple well tests and the accuracy of the multi phase flow meters when compared to the field reference will be presented.
An estimated six trillion barrels of heavy oil and bitumen is available worldwide. A majority of these reserves are located in United States, Canada and Venezuela (Ir Thermal-based recovery methods have been used since 1950's to recover oil from these reservoirs. Significant changes related to reservoir management and production facilities have been made (2). In the past decade new thermal recovery techniques such as the steam assisted gravity drainage (SAGO), two cyclic steam stimulation (CSS), steam and gas push (SAP) and vapor extraction (Vapex) processes have been developed and used to enhance the recovery of very heavy oil (i ,3,4). Among these processes, the steam assisted gravity drainage has emerged as an effective technology for recovering oil from sand deposits that are too deep to be recoverable by surface mining(3,4). In the SAGO process, steam is injected continuously down one well while the mobilized bitumen and condensate steam are produced continuously up a second welL. The injector and producer are drilled approximately 5 m apart. In some developments, horizontal wells are used to enhance reservoir access and well productivity (4). In the injector well, steam injection creates a chamber that grows as the steam condenses on the chamber walls and releases heat. Heated bitumen and condensed steam drain by gravity into the lower producing well and are pumped out.
The oil-in-water emulsion produced by the SAGO process is very stable and requires chemical treatment and processing to separate the oil and water (5). Conventional gravity based test separators used in measuring well rates are not able to deal with this stable emulsion, in the presence of steam condensate and produced gas, as well as the high temperatures. Reference 5 has reported on a SAGO plant that processes a reverse emulsion of about 360 m3/day of oil and 1200 m3/day of water at 195-200 C and 1800 kPa (260 psig). At the operating temperature of the separator (about 200 C) the produced water is less dense than bitumen. Wet oil exits from the bottom of the high temperature separator and enters a flash treater operated at temperatures above 145 C to flash out the steam. These conditions make it diffcult for conventional gravitybased test separators, with limited retention time, to produce accurate measurements of the produced fluids.
Multiphase metering technology has been used successfully for well testing in the production of heavy oil (stable emulsions) in cold processes (6). Multiphase measurement technology has been an enabling technology for the owners of the Petrozuata heavy oil operation (6). The installation of 37 multiphase meters in conjunction with the multi phase pumps in place of separators, liquid pumps, and gas compressors paid significant dividends in CAPEX and continue to pay year on year in aPEX. Multiphase metering techniques have also been used successfully in steam flood operations (7, 9) to improve field allocation factors. The field tests described in this paper were undertaken to assess the capability and advantages of using multiphase metering technology to overcome the shortcomings of the conventional gravity based test separators used in a number of high temperature thermal recovery operations such as CSS and SAGO.
Fluid Processing and Facilities
Figure 8(page 7) shows the comparison of the well rate data from multi phase meter with the data from the emulsion meter. There is close agreement with the emulsion meter. Figure 9 (page8) shows the comparison with the HTS #2 separator output. As noted in Figure 9, the multiphase meter test data is consistent in trending the fluid outputs from the separator to better than 50 M"3/0.
Some amount of over reporting of oil and under reporting of water was noted - especially after the shut down on May 23 for workover operations. This could be partly due to the uncertainty introduced by the reference water cut device. This device had previously shown discrepancies with water cut from the field production data. Additionally, the water cut device was observed to flat line at 10% water cut regardless of the water cut in the oil/bitumen stream that was supposed to run at 3-5% water cut. A second source of error may have been the deposition of heavy oil components on the probes in the multi phase meter, due to equipment cooling down, during the shut down of May 27-28 for workover. We noted that the discrepancy in the water cut, which would be the cause of over reporting oil and under reporting water, was high immediately after the restart but diminished substantially as the test progressed. These issues will be the subject of further investigations.
As in the tests conducted at Site A, the ability to assign absolute accuracy level to the multiphase meter data, obtained in a field test, is limited by the quality of the reference data. In practical terms, the quality and the consistency of the measurements that was provided by this multi phase meter under very challenging field conditions is remarkable and the accuracy of the measurements are felt to be very adequate for well testing.
The field tests described in this paper were undertaken to assess the capability and advantages of using multiphase metering technology to overcome the shortcomings of the conventional gravity based test separators used in a number of high temperature thermal recovery operations such as CSS and SAGO. The SAGO thermal recovery process produces measurement conditions, shown in Table I, that challenge multiphase measurement technology to the limit. The hardware must be able to operate at very high temperatures. The measurement strategy and techniques must be able to handle the tight emulsion - made up of water and bitumen that have very close densities - as well as a gas phase that is made up of produced hydrocarbons and superheated steam. The superheated steam can condense to water with reduction in temperature. The superheated water can flash to steam with reduction in pressure. The performance of a multi phase meter made by Agar Corporation was evaluated under these challenging conditions and is reported in this paper. Field tests were conducted at two sites (A and B) operated by different companies. Both operators employed SAGO or a modification thereof to produce the oil bitumen. Consequently, Sites A and B had different fluid conditions as described in Table I.
At Site A, a number of different "tank gauging" procedures were tried to obtain reference flow rate to check the performance of the multiphase meter. These methods are summarized in Table 2. The method that proved accurate enough as a reference measurement for the multiphase meter performance verification is shown schematically in Figure 2. Subsequent performance tests of the multiphase meter against the tank data indicated that the accuracy of the multiphase meter is as good as the "best" tank test procedures and water cut sampling that can be attained under practical field conditions as shown in Figures 3 and 4.
A 12 well field test campaign was conducted after the initial qualification tests at Site A. The results are shown in Figure 5. The multiphase meter data is in good agreement with tank test, which are the best reference measurements that can be practically obtained in the field. The results form historical well tests on these wells are also plotted in Figure 5. The historical data is obtained from conventional test separator and production measurements. For this site, the allocation factor obtained by comparing the total production from the reference tank tests vs. multi phase meter is 0.97, where as the allocation factor for the historical field data vs. tank test is 1.40. This improvement in allocation factor, through the use of multiphase metering was noted in other applications (6, 7, and 9) and is an important measurement parameter for improving the monitoring of production processes.
A 30 day well testing campaign was conducted at Site B on 2 pairs of SAGO wells. Figure 7(page 7) shows the test layout at Site B. Figure 8 shows the comparison of the well rate data from multi phase meter with the data from the emulsion meter. There is close agreement with the emulsion meter. Figure 9 shows the comparison with the HTS #2 separator output as shown in Figure 7.
As in the tests conducted at Site A, as well as other field tests (6, 7, 8, 10, and 13), the ability to assign absolute accuracy level to the multi phase meter data, obtained in the field, is limited by the quality of the reference field data. Figure 9 shows that the multi phase meter test data is consistent and trends the fluid outputs from the separator to better than 50 M"3/D. Some amount of over reporting of oil and under reporting of water was noted - especially after the shut down on May 23 for workover operations. This could be partly due to the uncertainty introduced by the reference water cut device. This device had been known previously to have discrepancies with water cut from the field production data. A second source of error may have been the deposition of heavy oil components on the probes in the multiphase meter during the shut down of May 27-28 for workover. Having conducted the qualification tests at Site A (Figures 3 and 4), one can reasonably assume that multiphase meter is reporting well rate data that is close to the actual values at Site B.
Field verification data obtained at Sites A and B indicated that the multi phase meter used in these field tests can perform well under the challenging fluid and high temperature operating conditions shown in Table i and provided reliable flow rate and water cut data. The multi phase meter provided consistent measurements. The accuracy of the measurements matched the level of accuracy that can be attained with the very rigorous tank measurements, shown in Figure i, and was felt to be very suitable for well testing. In addition, the multiphase meter provided online data as shown in Figure 10 (page 8) that can be used for production and well diagnostics (6). The use of multi phase meters should therefore be considered as a viable and improved measurement alternative to conventional test separators for well testing under SAGO process conditions.
It should be also noted that to obtain the quality of flow rate and water cut data from conventional measurement methods in the above field tests, the operators had to go through considerable amounts of effort and equipment modifications as well as revisions to measurement practices as shown in Table 2 and Figure i. Normally this level of attention can not be afforded to measurement and well testing activities in the field. Using the multiphase meter, the same, or perhaps better, quality of information was made available to the field on-line without the need for all the equipment modifications and extra personnel interventions, to perform the tank testing and manual/automatic water cut sampling. The justification for using the multi phase meter in a project should consider these factors as well as the accuracy of the data and capital cost and maintenance (6) of the equipment.
The author would like to acknowledge the input and technical discussions provided by Michael Olugan, Long Lake Project, NEXEN, in preparing this paper. The author also wishes to acknowledge the technical support provided by personnel from Zirco and Agar Corporation during the field tests.
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