Development of a Hydrate Inhibition Monitoring System
Figure 2: Possible Shifts in Methane Hydrate Phase Boundaries Owing to the Maximum Measurement Errors of the Prototype C-V Device
250
metal wall of pipelines and is not easily interfered with by unexpected sound waves, such as common sounds and low frequency vibration. Therefore, measuring ultrasound velocity can be an ideal technique for online applications.
200 Artificial Neural Network 150 100 50
Here, an artificial neural network (ANN) technique was used to determine the inhibitor and salt concentrations simultaneously using the measured electrical conductivity, acoustic velocity and temperature. An ANN is initially trained using a set of carefully measured data and then validated using another set of independent data. It does not require an analytical formula or understanding of the underlying physical relationships. Therefore, an ANN is especially useful for applications in which multi-parameter correlations are needed but where the interaction and relations between the measured parameters are not well known.24,25
0 -10 -5
10% MeOH 10% MEG
0 Temperature (ºC)
10.5% MeOH 10.5% MEG
510
25% MeOH 40% MEG
25.5% MeOH 40.5% MEG
Error bars = ±0.4ºC for the curves with 25 mass% MeOH and 40 mass% MEG; ±0.3ºC for the curves with 10 mass% MeOH and 10 mass% MEG, in the presence of 5 mass% NaCl. MEG = monoethylene glycol; MeOH = methanol.
Figure 3: Evaluation of the Prototype C-V Device Using Two Real Produced Water Samples Containing Unknown Concentrations of Methanol, a Corrosion Inhibitor and Multi-component Salts in Addition to the Presence of Condensate
100 120 140 160 180 200
60 80
40
012345 Temperature (ºC)
PW-1 (FPD) 6789 10 PW-1 (C-V device) PW-2 (C-V device) PW-2 (measured)
property must be independent of the electrical conductivity in that it is not correlated with the electrical conductivity in any way. Furthermore, it should be sensitive enough to identify changes in hydrate inhibitor and salt concentrations. Ideally, the technique should be robust for potential online application. Acoustic velocity (acoustic property) is such a property. It is well known that the sound propagation can be directly related to salt concentration.18
In recent
years, acoustic velocity has been successfully applied to investigate a variety of situations.19–23
In addition, ultrasound can penetrate the 38 15
Determination of Hydrate Phase Boundary (the Hydrate Safety Margin) An in-house thermodynamic model, HWHYD,26,27
was used to determine
the hydrate phase boundary (the hydrate phase safety margin). To do so, the salt and inhibitor concentrations (as determined by the ANN) and the composition of the hydrocarbon fluids (oil or gas) are fed into the thermodynamic model.
Figure 1 shows the methodological chart of the hydrate inhibition monitoring system. The acoustic velocity and electrical conductivity are functions of salt concentration, inhibitor concentration and temperature. The velocity, conductivity and temperature of the aqueous sample under examination are measured and fed into a pre-trained/validated ANN correlation. The salt and inhibitor concentrations are determined by the ANN. By loading the composition of the hydrocarbon fluids, the hydrate phase boundary can be determined by the thermodynamic model HWHYD. The hydrate safety margin is determined by superimposing the pipeline temperature and pressure.
Prototype Device Instrumentation
The C-V prototype device consists of four units: a sample cell, an electrical conductivity measurement unit, an ultrasonic test unit and a PC. The sample cell is a stainless steel container in which the aqueous sample under examination is added. The conductivity measurement unit includes a four-pole conductivity probe and a conductivity meter. One advantage of the four-pole probe is to minimise the effect of polarisation on electrical conductivity measurements, especially for high salinity solutions. The electrical conductivity and the sample temperature are measured by the conductivity meter and fed into the PC. The ultrasonic test unit is composed of two ultrasonic transducers, a pulser/receiver (P/R) and a high-speed data acquisition card (DAC). The two transducers are high-damp transducers with a central frequency of 1MHz. One acts as a transmitter, whereas the other acts as a receiver. The P/R sends a pulse that excites the transmitting transducer, generating an ultrasonic signal. The ultrasonic wave passes through the sample and is received by the receiving transducer. The received ultrasonic signal is sent back to the P/R. The P/R then amplifies the received signal and sends it to the DAC, where the analogue signal is converted into a digital signal and sent to the PC. Specially developed software is installed in the PC. The software integrates three programmes under
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 1
Pressure/bar
Pressure/bar
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