Salinity sensing under the sea
New technology helps subsea wet gas meters optimize oil and gas operations
By Svein Eirik Monge
The growth in offshore brownfield projects and operators looking to develop new fields in remote areas means it is more important than ever to adopt cost-effective, flexible, and innovative technologies that can both negate production threats and optimize subsea oil and gas production.
To this end, operators are looking for sophisticated technologies that provide real-time information on flow performance, protect well integrity, ensure more effective production operations, and preempt production threats. New salinity sensor technologies exist today that can meet these subsea oil and gas exploration challenges.
Subsea oil and gas operators face a variety of challenges when it comes to identifying fluid composition and securing flow assurance. These include a wide range of operating conditions, particularly with the growth of wet gas fields. In wet gas fields, the dangers of water breakthrough and condensate increase, bringing with them greater chances of saline formation water entering the flow.
Formation water and salinity lead to scaling and the formation of hydrates, corrosion, and overall threats to the integrity and availability of subsea wells and infrastructure. In worst-case scenarios, formation water and water coning-a problem in which bottom water infiltrates the perforation zone in the near-wellbore area and reduces gas production-in the reservoir can jeopardize hydrocarbon production and lead to well shutdowns.
Another challenge is subsea tiebacks. This is one of the more economical means of developing deep water fields, as it connects new discoveries to existing facilities in order to extend the life of the production infrastructure. Industry analyst Douglas-Westwood predicts expenditures on subsea tiebacks will be around $94.3 billion from 2016 through 2020.
In a tieback setup, however, it can take hours or even days before any onset of formation water is detected by the topside or onshore measurement system. The delay adds risk to the accompanying potential dangers. It is in this context that detecting changing fluid composition and water salinity in real time is vital.
Salinity measurement, for example, tells the reservoir engineer if formation water is entering the flow and helps the process engineer adjust the rates of scale and corrosion inhibitors. Once measured, fluctuating operating conditions can be accommodated, risk reduced, and remedial action taken against hydrates, scaling, and corrosion. Other remedial action might include adjusting the choke setting or instigating zonal isolation.
Any one of these measurements must operate quickly and in real time. Given the right conditions, hydrate formation can accelerate at an alarming pace, with a critical time window of as little as 20 minutes needed for preventative action.
Lastly, it should be noted that other key drivers in subsea operations are safety, flexibility, and the ability for subsea measurement instruments to fit seamlessly within existing infrastructure. The latter is an especially crucial criteria for engineering, procurement, and construction companies. The size, weight, and compactness of instruments are essential, as manifolds are often already crowded with instrumentation and have little room to spare.
Traditional vs. new techniques
Traditionally, operators have used several techniques to address these challenges. Chief among them are laboratory analysis of a water sample and conductivity measurement.
Laboratory analysis of a water sample is used to determine the salinity of the water. However, this is a time consuming and costly process that does not capture rapid variations in the water salinity. Formation water detection has historically been based on an indication value that remains stable if there is no salinity but increases when more saline water enters the flow. The downside of this form of measurement is that it is only a qualitative indication, not a quantitative measurement essential to flow assurance and salinity detection in today's operations.
There is a perception that subsea multiphase and wet gas meters are unwieldy and expensive, with operators put off by the perceived scale and potential expense of such deployments. However, there are alternatives on the market now. One is a cost-effective and compact design including salinity measurement.
Although traditional formation water detection was based on an indication value that increased as more saline water entered the flow, new technologies provide quantitative measurements in many types of field conditions. One of these technologies (created by Emerson's Roxar unit) consists of a salinity sensor mounted flush against the wall of a subsea wet gas meter, which provides individual flow rates of gas, condensate, or oil and water. This salinity sensor design is ceramic and based on microwave (MW) resonance technology (figure 1).
Electromagnetic resonance field lines go out of the ceramic sensor into the metal surrounding it, including the two small antennas below the ceramic cylinder. From a microwave sensor perspective, the sensor is categorized as a waveguide cavity resonator, shortened on one end and open on the other, the side that is facing the flow. As part of the MW measurements, electromagnetic waves bounce off reflective surfaces, such as the metallic surface used in the ceramic sensor. A resonance typically occurs when a reflection bounces back and forth in such a way that a constructive interference between the incident and reflected wave occurs. This is similar to how a pressure wave creates resonances at certain frequencies in a flute or a brass horn.
Within this system, the higher the Q-factor-a dimensionless parameter that describes the resonator's bandwidth relative to its center frequency-the "stronger" the resonance.
That means the energy dissipated per cycle is small, with the Q-factor decreasing if more energy is dissipated per cycle. Higher water conductivity from increased salinity causes the resonator to leak more of its energy to the surroundings, which again results in a decreasing Q-factor. The Q-factor shift versus the frequency shift determines the conductivity of the water on the probe.
The result is an immediate response to salinity changes, and the ability to measure water conductivity with a high level of accuracy. Small pockets of formation water leaking into the flow can be detected instantaneously-something that no other technology has achieved to date.
Figure 1. Conceptual drawings of the ceramic salinity sensor based on microwave resonance technology.
Extensive testing of the salinity sensor took place at the Colorado Experiment Engineering Station (CEESI) based on a qualification process prescribed by a leading operator. Three separate flow tests took place at CEESI with close to 700 individual test points in total. Testing was conducted along the full range of the subsea wet gas meter from 85 percent to 100 percent gas volume fraction (GVF), 0-100 water liquid ratio, and a wide range of conductivities. The sweet spot of the salinity measurement technology (figure 2) is conductivity of < 2 Siemens per meter and high GVF. In the sweet spot, the probe shows better performance than ±0.5 S/m and uncertainty is kept at up to 99.98 percent GVF at low conductivities.
The testing results demonstrated that the new salinity probe can perform effectively in a wide range of conductivities, detecting changes in water conductivity as low as ±0.004 S/m.
With oil and gas wells being produced over a broader range of process conditions, and water salinity and conductivity being a key operational parameter for reservoir management and flow assurance, new salinity and subsea multiphase measurement devices are enabling oil and gas companies to expand their operations into areas that were not previously feasible.
The new measurement tool described in this article helps to meet the need for decision support based on accurate process information, which is crucial for operators in identifying production, safety, and environmental threats. Being able to respond quickly and judiciously to these situations goes a long way toward minimizing downtime, reducing risk, and increasing operational efficiency, which in turn translates into greater profitability.
Figure 2. Overview of performance test points in high gas volume fraction (GVF)