New testing, clean-up techniques meet demanding requirements
Mario Nava
Cristina Ribeiro
Gilbert Conort
Schlumberger
Multiple new technologies have been integrated into a well testing services platform that increases the operating envelope while redu- cing the environmental impact.
The well testing service acquires accurate dynamic reservoir data while ensuring the processing of well effluents in exploration, appraisal, and development wells, both onshore and offshore. This approach combines a multiphase flow metering system, a separation process designed around an adaptable water weir, and a computerized process and control program. The system enables dynamic reservoir testing under a wide range of conditions, including handling solids-producing environments, low API oils, high flow rate oil-brine separation, strict environmental controls on water and hydrocarbon disposal, integration with water treatment processes, and rig space restrictions.
The new testing and clean-up approach has been demonstrated in some of Petrobras' deepwater wells offshore Brazil, for Pemex in the southern producing region of Mexico, and offshore Angola for TotalFinaElf E&P.
Process improvement
Well testing is a procedure applicable during all stages of a well's life, from exploration and appraisal through development. Increasing cost pressure and rising environmental concern about the proper handling of effluents has led to a reduction in the frequency and duration of exploration and appraisal well testing, despite the loss of data critical to field assessment and reservoir development. However, the need to minimize production uncertainty in development wells magnified the importance of more efficient methods of clean up and well effluent disposal. Furthermore, it also made sense to find ways to acquire critical reservoir data during the development well clean up period.
Historically, conventional well testing and clean-up systems have suffered from a number of notable shortcomings, particularly with regard to cumbersome operations and logistics, as well as environmental impact. For example, the limited introduction of innovative technologies has caused the well testing/clean-up process to remain labor and equipment intensive. Bulky equipment, taking up precious rig space and requiring massive rig-up/rig-down procedures, has been the norm, although much of the equipment is only required for measurement corrections.
The need of past systems for in-situ flow measurement calibration and operator interaction has necessitated a large number of people to conduct the process. From the environmental perspective, there has been no established means of handling clean-up effluents, addressing unprocessed water, high water-cut oil disposal, or potential liquid dropout from open-ended gas flares.
Taking these limitations into consideration, Schlumberger designed the CleanTest services platform as a series of integrated, plug-and-play components to be configured to meet the needs of a wide variety of testing and clean-up situations. Safety, data quality, minimized environmental impact, and cost efficiency are the goals of a well-engineered operation.
Using this new system, a well is flowed through a controlled process throughout the testing period, from the start of clean-up to the last flow or sampling period. As a result, there is no need to dispose of unseparated fluid phases. Flow rates through the test equipment are known, and no solids remain in the low pressure/high velocity pipe system. Less equipment and fewer people are needed to clean up and appraise a well's performance with minimal environmental impact. Using this system for clean-up has saved one operator two days of rig time compared to similar operations conducted with conventional equipment.
System components
The system includes multiphase flow meter and oil-brine separation technologies. Optional cyclonic solids removal, water treatment, and gas flaring/oil burning components can be added to form a comprehensive well testing and clean-up solution suited to handling specific situations.
The tester technology provides continuous flow rate measurement of each phase in a flowing well. The device allows multiphase testing and monitoring of all well types, from initial flow and clean-up in new wells to production optimization of mature wells. When incorporated into the services platform, phase and flow rate information from the PhaseTester could be used to provide the early intelligence that contributes to the efficiency in the rest of the process.
A new well test separator uses proprietary Smart Weir technology and forms the second key component in the services platform. The separator is positioned in the process line just after the tester unit. It is in-line at all times to provide control of the separation process and improved oil/gas flaring efficiencies during well clean-up. The seamless process separates the well effluent into individual oil, gas, and water phases. It also can handle mixtures of hydrocarbon and non-hydrocarbon effluents, like those produced during well clean-up, including limited amounts of solids.
The weir, key to the new separator design, can be adjusted remotely so that the oil collection point inside the vessel is just above the midpoint (or 55% of vessel height) or as low as 15% of vessel height. This allows the system to automatically handle a wide range of water flow rate volumes. No phases go unmeasured or unseparated, eliminating the traditional problem of having to estimate flow rates during clean-up or the need to store and then dispose of unseparated effluents during the early well clean-up stage. Surge tanks and gauge tanks are eliminated, saving deck space offshore.
Improved data accuracy and quicker well clean-up are among the benefits of this innovative separating system. It also increases safety and environmental compliance, while reducing onsite equipment needs, rig-up time, and crew size.
When needed, a wellhead desander, or cyclonic solids removal unit that is located upstream of the choke, can be included in the services platform. It is suited for high-pressure applications such as post-frac well clean-up, well test clean-up, and high-rate tests of unconsolidated reservoirs. It has a large flow rate envelope while providing safe, efficient solids removal and discharge.
A continuous water treatment package is placed downstream of the separator water outlet and completes the process of separating any remaining oil from the water using the patented TORR (Total Oil Recovery and Remediation) technology, licensed from Earth (Canada) Corp. It reduces remaining oil content from a few percents to a few tens of ppm, typically from 2% (20,000 ppm) to 10 ppm and it can process 5,000 bwpd at temperatures ranging from 32° F to 212° F. The de-oiled water, down to 15 mg/l, is released overboard in offshore situations. Recovered oil is returned to the separator.
The oil burning system is placed at the end of the services process line. It was the first such system to meet environmental burning requirements by eliminating liquid fallout, visible smoke emissions, and oil dumping at the end of a burn sequence. The system operates effectively with water cut up to 25%. Remote monitoring of the system provides a web-based data delivery with secure, real-time two-way communication to the wellsite.
Key components of the CleanTest services platform include the PhaseTester Vx multiphase flowmeter and Smart Weir oil-brine separation technologies. The cyclonic solids removal, water treatment, and gas flaring/oil burning components are optional. Together, all components form a complete testing and clean-up system.
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Field applications
The system recently was used on a dynamically positioned rig operating for Petrobras in deep waters offshore Brazil to handle the clean-up period of a heavy oil (±17°API oil) well while providing precise measurements and environmentally safe effluent disposal. It was the first job worldwide to combine the three systems. The well clean-up phase was particularly critical, because about 2,500 bbl of fluid had been lost into the formation during completion. The operator wanted both well lift optimization and basic sediment and water real-time measurements to help define the end of the clean-up period.
The new separator technology allowed the well to be opened on a large 1-in. choke, which provided a shortened but safe clean-up period. The system saved several days over the use of conventional clean-up technologies. Real-time measurements of water cut in the oil outlet line were helpful for monitoring burn quality while reducing the amount of water to be de-oiled. Equipped with a low flowrate kit, the burner handled 1,856 b/d (295 cu m/d) fluid with a water cut of up to 13%. The GOR was 346 bbl/scf (55 cu m/cu m).
The system achieved all objectives set by Petrobras. The desired real-time quality data were delivered, which helped with well lift optimization. All environmental requirements were met thanks to improved burn quality.
At another location, in the southern producing area of Mexico near Villahermosa, particularly in the Comalcalco region, operator Pemex Exploracion y Produccion has tested over 56 wells to date using this service. On two occasions, the system was used for well clean-up in areas of stringent environmental regulations. At the other wellsites, the system was used for closed-loop testing. Specifically, Pemex tested the accuracy of the Coriolis meters in the Smart Weir separator in comparison to conventional atmospheric gauge tanks, as well as the environmental impact of the new system. Reduced vapor emissions and gas burning are desired in the region.
The Pemex wells tested had a wide range of characteristics as far as fluid type, well behavior, pressures and temperatures. Both low-pressure, low-temperature wells and HPHT situations were encountered. A range of operation has been established for the system and Pemex has approved its use. Already, the Comalcalco region is almost exclusively requesting the system for its operations.
Offshore Angola, drillstem tests were conducted on five deep exploration wells using the system approach, with particular emphasis on the value of using a multiphase flowmeter for fluid control, measurement and disposal as compared to conventional well testing procedures. In all five of these wells, the uncertainties inherent in flowing exploration wells were handled effectively, while safely and efficiently obtaining the continuous flow measurements desired.
In the end, the multiphase flowmeter data were found to be preferable to discontinuous, undetailed separator measurements alone. Both the continuous nature of the multiphase flow rate records achieved with the new equipment, as well as the instantaneous flow variations revealed added value especially for nitrogen lift and clean-up optimization in these wells. By controlling and adjusting the nitrogen injection pressure and pump rate, and monitoring the returns at surface, a continuous and sustainable lift of the liquid column was achieved, reducing nitrogen consumption and clean-up time.
Further, a full account of recovered fluids added confidence about when the wells were effectively cleaned up, eliminating the need for extended, high drawdown clean-up periods. The multiphase testing equipment also had no problems handling the low surface flowing pressures or challenging fluid conditions present in some of the wells.
