Coriolis meter solves two-phase flow issue with CO2 for enhanced oil recovery

Accurate flow measurement in changing conditions is now possible.

Using carbon dioxide (CO₂) for enhanced oil recovery (EOR) is a proven method to greatly increase yields in mature oil fields. However, accurate measurement of CO₂ at subcritical conditions in a nonmiscible flood is a proven challenge. One of the largest midstream energy companies in America has found the solution by applying advanced Coriolis metering technology.

The company began injecting CO₂ into one of its West Texas oil fields as an immiscible flood to maintain pressure in the oil reservoir. While the process went as planned, engineers felt that they could do even better if they could more accurately measure the CO₂ flows in each well. However CO₂ is an elusive element to measure under changing process conditions. When it is above the critical point it exists as a “liquid” and is easily measured with standard flow measuring devices such as an orifice meter. But, below the critical point it can coexist in two phases, liquid and gas. Traditionally, two-phase flow measurement has proven too difficult and plagues flow measurement in nearly all process operations.

In this application, the company transfers the CO₂ in pipelines to each of the injection wells throughout the field. During transport, variations in ambient temperature and pressure outside the pipeline cause the CO₂ to vary from a liquid to a gas. For instance, on a cool morning, the lines areprimarily liquid CO₂. But in the afternoon, with elevated outside temperatures there is primarily gas. This phase variation presents a flow measurement challenge for which there is no easy solution.

Possible options are orifice plates with multivariable DP transmitters, Vortex meters, and conventional Coriolis flow meters. While traditional Coriolis technology is highly accurate in single-phase flow, with a 0.02 percent plus or minus error level, two-phase flow boosts the error rate to 20 percent or higher. None of these options met the performance level the company needed, so at the time orifice flanges were used as an inexpensive, temporary option.

With the orifice plate arrangement, measurement data consisting of orifice differential pressure and flow temperature was relayed via pressure transmitters and thermocouple, respectively, to a PLC. The PLC was programmed with CO₂ density data as a function of temperature and pressure. This information was used in a standard gas equation to calculate flow.

However, the company realized poor measurement performance immediately. Measurements from the orifice plate arrangement were compared to the custody transfer field sales meter, which operated upstream of the injection wells in a single dense phase at higher pressure. The difference in measurement between the sales meter and well measurements was off by as much as 80 percent.

In search of a solution, the company tested the Foxboro CFT50 transmitter a few months after the startup of CO₂ injection. The initial results were encouraging. Once everything was set up and running it became apparent that the Foxboro CFT50 meter was tracking expected flow significantly better that the orifice meters. The success of the initial tests led to a full field test that also proved successful resulting in the company installing Foxboro CFT50 meters at each of the 10 injection wells throughout the field.

A typical wellhead installation system includes the Foxboro CFT50, a control valve, PLC and SCADA equipment. As oil is produced CO₂ volumes are increased or decreased to maintain constant reservoir pressure. The control valves open and close based on flow measurements from the CFT50 compared to set points established for each well.

The CFT50 provided the solution as an information tool to monitor precisely how much gas is being injected into each well. This information is used to create a better understanding of the impact of the CO₂ on the reservoir and its subsequent increase in production.

With the added ability to accurately measure two-phase CO₂, the company is optimistic about the long-term advantages of developing strategies to maximize production efficiency.

With the Foxboro CFT50 they have achieved a total combined accuracy of +5 percent over 10 meters. They can now better correlate production efficiency with the volume of CO₂ injected, which is critical for developing oil reservoir strategies.

Wade M. Mattar is Flow Specialist at Invensys Process Systems, Foxboro Measurements and Instruments Division. He can be reached at wade.mattar@ips.invensys.com