Wellhead flowmeter pilot test on the Bakken Formation
At the invitation of a large Canadian oil and gas producer, several Coriolis flowmeter manufacturing companies participated in a pilot project designed to investigate the feasibility of direct wellhead measurements using Coriolis meters for better reservoir and operations management. Although each Coriolis flowmeter and transmitter were different, they were all tested in one installation, against the same separator on the same well. The trials took place on shale wells in southeast Saskatchewan and heavy oil wells to the west of the territory.
Finding a measurement technology that could successfully perform in measuring produced liquids directly on the wellhead would reduce the reliance on infrequent well tests and site visits to optimize operations and, ultimately, production. Building a track record of successful field performance could also pave the way for changes to local oil and gas measurement regulations to allow for Coriolis meters on each well as a viable substitute for separator measurements.
Each oil field and even each well within a field can have different production characteristics, so in order for the test to be representative of future field performance, it is important to test on a diverse set of wells and conditions. Testing was done through summer and winter weather on shale wells, which have lighter oil and produce more gas, in the Bakken and heavy oil wells in western Saskatchewan. Heavier and lighter oil production each have their own set of challenges; therefore, successful results in both would indicate a robust measurement method.
The challenges to meet
For this trial, the challenges were divided into two groups: technical challenges and implementation challenges. Both offered specific problems that needed to be solved to meet measurement accuracy and production requirements.
The technical challenge was primarily based on ensuring that the test was set up properly and that the participants, as well as the producer, could evaluate the measurement device properly without complications or needing additional tools or training.
The biggest implementation issue was following best practices for installation for the challenging conditions the wells provided. In many cases, this meant exceeding the generic installation requirements established by the manufacturer, as noted in the installation manual — to know when to point out and guide the producer’s team to size and install the meters using best practices for two-phase or multiphase processes. For example, knowing to maintain back-pressure to ensure accurate flow. The on-site team — with experience in upstream oil and gas — were able to direct the producer’s technicians to find the pressure that maximized measurement performance as well as the efficiency of the beam pump, resulting in better — repeatable and reliable — results. It should be noted that during the winter, the cold weather significantly affected the viscosity of the oil, so the ideal pressure setting was different than in the summer months.
Coriolis meters have an advantage over single-variable flow measurement technologies (e.g., differential pressure) by virtue of the ability to measure density and the advanced diagnostics that are available in the latest electronics. Measuring the liquid density allows the watercut (percent water volume produced) to be calculated if the oil and water density are known, and the advanced diagnostics allow the meter to detect when gas is in the meter, interfering with the liquid density measurement. These advanced capabilities make it possible for one measurement device to calculate an accurate total volume flow rate as well as distinct oil and water rate measurements.
Another challenge in these applications is that beam pumps produce very unsteady flow rates. With each stroke of the pump, the flow drops to zero or may even have backflow, so the measurement technology must be able to handle unsteady or pulsating flow as well as track the direction of the flow. This set of requirements means that Coriolis technology is the most viable choice for these applications.
Solids and debris that are pumped up along with the oil or gas — such as sand, leftover debris from drilling incompletion, concrete, pump parts, rubber and basically any material that is part of the drilling setup and generally lumped under the label “blood and guts” — poses a challenge for Coriolis meters, since the flow path is somewhat restricted. However, the design of the installation included a filter that would catch any solid large enough to risk catching in the meter, allowing smaller particles to pass through the meter. For the wells tested, this filter prevented plugging and did not need to be emptied too frequently.
The customer understood how challenging the application truly was and came up with a well-thought-out skid design to install the test meters on, which facilitated testing. They followed best practices for installation in multiphase process conditions for the meters under test, and they had Coriolis meters on each leg of the separator. This allowed the customer to detect any separation issues, which, if undetected, would cause errors by produced fluids to the wrong phase (i.e., counting water as oil or oil as gas). Separators are common and well understood in the industry, but by no means perfect, so validating that the reference system is performing as expected is crucial to completing a valid trial.
Pilot test solution
After months of testing, the Coriolis flowmeter — selected by at least one of the companies to meet the test parameters — was from the compact model series, ideal for process control applications. These meters are known to run efficiently under some of the toughest productivity challenges. Included with this particular meter was the top-level transmitter equipped with advanced phase measurement software, which is recommended for applications where multiphase flow, water in oil, bubbles in oil, or liquid in gas can be found.
The advanced phase measurement software helped achieve consistent and reliable measurements over the other Coriolis meters that were part of this pilot test.
Initial results
The customer required a goal of 5% to 10% accuracy in total oil measurements; this was a set requirement that all participating companies had to meet with their offered solution.
The chosen Coriolis manufacturer’s total fluid measurement came within 1% of the customer’s reference. Once the manufacturer was chosen, multiple pilot installations were permanently installed to monitor the wells continuously. With this data continuously available from the meters installed on the wellhead, the customer was able to get real-time production data, enabling production optimization, as well as immediately detect issues such as a stopped beam pump or water breakthrough. They were also able to see that when the well was routed to the test separator each month, the additional back-pressure on the system had an effect on the production rate, which means that the data from the monthly well test may not be representative of the normal production.
Success factors
Several factors contributed to the Coriolis manufacturer selected for the pilot project. The customer’s excellent test design, which set effective parameters for the participants to meet, is one important factor, as it delineated expectations and provided a design suitable to the wells in the test.
The individual Coriolis flowmeter and transmitter elements combined to create a successful outcome — the selected Coriolis manufacturer opted to use low-frequency sensors, which help reduce errors in variable conditions, particularly in multiphase flow conditions. And, adding transmitter software that reads and corrects for multiphase flow ensured that readings were accurate and close to set tolerances. Advanced phase measurement proved to be an invaluable tool.
Justin Hollingsworth works in innovation at Emerson Automation Solutions as the application research engineering manager. He is focused on creating solutions for flow and density measurement customers that address the unmet needs in challenging applications. Hollingsworth has experience in developing complex measurement and control systems in the aerospace, automotive and process industries.
