Validation of Automated Well Test Systems

Validation & Portable Well Testing

Process Instruments, Inc implemented the first well test validation system in 1998. The first system did not have the ability to separate the gas and liquid phases. It was used downstream of an automatic well system.  The system was very useful for improving well test system accuracies. In order to use this system for portable well testing and validation, we added the patented Accuflow separator. The system consist of the following:

We apply API Chapter 8, Sec. 2 – Standard Practice for Automatic Sampling of Liquid Petroleum for our sampling and handling. 

Principal #1 – Measure the flow with an accurate repeatable none mechanical meter.

The flow meter Process Instruments uses is a Coriolis mass flow meter manufactured by Endress+Hauser in Basil, Switzerland. Coriolis meters have no wearing parts and are extremely accurate if there is no presence of free gas. We chose the Endress+Hauser mass meter because of its technology, compact design and its reputation for having a very stable zero. 

Principal #2 – Condition the flow stream prior to sampling.

A multi-vane static mixer is used upstream of the isokinetic sampler to ensure that oil and water is reduced to small micron droplets and dispersed evenly across the cross section of the flow-stream.  This method has proven to be very effective in conditioning the flow-stream for sampling.

Principal #3 – Take a sample proportional to the flow-rate

In order to get a representative sample, it is critical the sample is taken proportional to the flowrate. In order to accomplish this; a meter is required to measure the flowrate. The PLC monitors the flowrate and for every portion of a barrel measured it sends a signal to the sampler, actuating it and causing the sampler to grab a sample. The faster the flowrate the faster the sample is taken. If there is no flowrate then there is no sample. 

Principal #4 – Take an Isokinetic sample.

An isokinetic positive-displacement sampler is used for grabbing the sample. The sampler grabs the sample from the center of the flow-stream at the fluid velocity. In other words, if the velocity of the fluid is 2 feet/second or 10 f/s the sample is grabbed at that velocity, it is isolated and discharged by a positive displacement piston.  The positive displacement assures that all of the sample is discharged without the possibility of any sample escaping. 

Principal #5 – Handle the sample properly.  

Water cut is determined by weight instead of the conventional method which is by volume. Implementing the Cut by Weight method, has removed variables that could cause a 1-2% error in the absolute water cut. Based on our experience and the history of validation work, there is no question the only way to determine an accurate cut is by weighing the sample. By determining the net amount of water and dividing it by the total sample weight it provides to be a very accurate water cut result. 

Free Gas Removal – Accuflow Design

The patented Accuflow Multiphase Metering System handles large surges of liquid and gas. By utilizing the Accuflow and incorporating the 5 Basic Principals described above, the Process Instruments Validation System is a complete design that is used for validation of Automatic Well Testers and Wet LACT Metering systems. The Accuflow design allows for removal of free gas. Fluid enters the Accuflow and passes by the cyclonic entry point entering the vertical separator section. The remaining entrained gas travels into the horizontal separator section where a thin gas bearing emulsion layer exists. The thin layer is the key to allowing gas the time required to reach the surface where it breaks free and travels into the gas phase.  An additional benefit to the Accuflow is a small retained volume of fluid which allows for a minimum purge volume. 

Gas Measurement 

A vortex gas flow meter is used for our gas measurement. Vortex meters have a large turndown ratio of 30:1 allowing the system to measure broad flow ranges.  The pressure and temperature are measured so the gas volume can be corrected to a standard condition (Scf/day).  

Conclusion 

Well testing is a difficult measurement to make.  Due to the well dynamics, there are large variations in flow-rates both in gas and liquid. The water cuts range from 0-100%. High volume wells with high water cut and are the most difficult wells to measure. A 1% absolute error on a well that produces 1000 bbls/day at a 90% water cut can cause a net oil measurement error of 10 bbls. The only method of knowing the water cut is to take an accurate physical sample. 

To recap and according to API standards, before a sample is taken;

• The sample must be properly conditioned. 
• The sample should be taken using an isokinetic sampler.
• The sample should not be divided once it has been taken.
• The sample must be analyzed properly. 

The Accuflow LT series separates and conditions each flow stream for proper measurement. It has the ability to utilize dual liquid and gas meters in order to cover the full range of measurement. 

The validation system described here is simple to understand and easy to operate. Due to the simplicity we can obtain accurate and repeatable results for each test or validation. Special attention has been given where it matters most. The method of Cut by Weight is critical for high water cut ranges. By utilizing the coriolis mass meter, the volume flow is also accurate and repeatable. 

Validation of New Technology

Any new technology for well testing should always be validated with an accurate and proven system. Process Instruments has the experience and knowledge required for your validation needs.