8802 Scobee Street, Suite 100 • Bakersfield, CA 93311 • 661.617.6000 • Fax: 661.617.6013 • www.process-instruments.com • pwade@process-instruments.comU.S.A. | OMAN | BAHRAIN

Portable Well Testing & Validation PrincipleProcess Instrument’s well test and validation design philosophy started with two simple questions.

Q. How do you measure the accuracy of an oil/water cut probe?

A. Use a second calibrated 0-100 percent cut probe and compare the two.

Q. How do you check the accuracy of the 0-100 percent cut probe?

A. Grab a sample and spin a cut.

While there is general agreement that a sample needs to be taken, few understand what taking an accurate sample entails. Most people think you can simply take a sample from the flow-line, put it in a container, shake it up, pour part of it into a centrifuge tube, spin it and read the results.

Process Instruments, Inc. implemented the first well test validation system in January of 2000. The first system was a Coriolis mass flow meter, a static mixer and isokinetic sampler, and a PLC. This system was connected on the discharge of a fixed automated well test system. The validation system was in series with the AWT and the results of the validation system and AWT were compared. Next, a gross measurement and net oil error were compared to see how well the AWT was performing.

This simple but effective validation system allowed the customer to improve the accuracy of their AWT’s, while also having a proven system to compare against. Using a sampling system which follows API Chapter 8 (Section 2: Standard Practice for Auto Sampling of Liquid Petroleum), the decision was made to add the patented Accuflow dual separator design upstream of the Coriolis meter and sampling system to create a state of the art portable well test system. The same basic principals applied to the original design are in use today. They consist of the following:

1. Use an effective separator design (Accuflow design with a vertical and horizontal separator).

2. Measure the flow with an accurate repeatable none mechanical meter (Coriolis mass meter).

3. Condition the flow stream prior to sampling (static mixer).4. Take a sample proportional to the flow-rate (time-weighted

average).5. Take an isokinetic sample.

6. Handle and analyze the sample properly (cut by weight).API Chapter 8 discusses many of the methods followed by Process Instruments.

Basic Principal #1Measure 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 free gas is not present. We chose the Endress+Hauser mass meter because of its technology, compact design and reputation for having a stable zero.

Basic Principal #2Condition 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.

Basic Principal #3Take a sample proportional to the flow-rateIn order to get a representative sample, it is critical the sample is taken proportional to the flow-rate. In order to accomplish this, a meter is required to measure the flow-rate. The PLC monitors the flow-rate and for every portion of a barrel measured, it sends a signal to the sampler, actuating it and causing it to grab a sample. The faster the flow-rate, the faster the sample rate. If there is no flow-rate, then there is no sample.

Basic Principal #4Take an Isokinetic sample.An isokinetic positive-displacement sampler is used for grabbing the sample. The sample is taken 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 feet/second, the sample is grabbed at that velocity. Then, 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 of it escaping.

8802 Scobee Street, Suite 100 • Bakersfield, CA 93311 • 661.617.6000 • Fax: 661.617.6013 • www.process-instruments.com • pwade@process-instruments.comU.S.A. | OMAN | BAHRAIN

Basic Principal #5Handle 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 to 2 percent error in the absolute water cut. Based on our experience and history of validation work, there is no question that the only way to determine an accurate cut is by weighing the sample. Determining the net amount of water and dividing it by the total sample weight provides a very accurate water cut result.

Using the volume method of filling separatory funnels, taking a visual reading, draining the free water, taking another reading, filling centrifuge tubes with solvent heated to one temperature and emulsion which was yet another temperature, allows for too many variables in determining the water cut.

Free Gas Removal - Accuflow DesignThe patented Accuflow Multiphase Metering System can handle large surges of liquid and gas. By utilizing the Accuflow and incorporating the five basic principles described above, the Process Instruments Well Test & Validation System is a complete design that is used for well testing and validation of well test systems. The Accuflow design allows for complete removal of free gas. Fluid enters the Accuflow and passes by the cyclonic entry point of the vertical separator section. Any remaining entrained gas travels into the horizontal separator section where a thin gas bearing emulsion layer exists. This layer is the key to allowing the 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 the small retained volume of fluid which allows a minimum purge volume. Each time a system is built, the vertical system is sized for the maximum anticipated gas flow and the horizontal section is sized based on the maximum anticipated liquid flow rate.

Self ValidationIn light oil applications, real time self validation is possible by knowing the oil and water densities. With enough differential between the two, the water cut is calculated using the Coriolis mass meter. This real time feature is only used as a self validation of the water cut. The actual results of the isokinetic sample are compared to the mass meter results. Having two methods of determining the water cut sets Process Instruments apart from the competition. However, the key to this method is establishing an accurate oil and water density.

Gas Measurement Ultrasonic flow meters are used for gas measurement. Using this method allows an 80:1 turndown ratio. In addition, the ultrasonic meter provides online diagnostics to determine if there is any liquid carryover. This feature allows a test to be validated if there is any question regarding the gas measurement. Pressure and temperature are measured so the gas volume can be corrected and reported in a Scf/day volume.

Conclusion Well testing measurements are difficult to make. Due to the well dynamics there are large variations in flow-rates, both in gas and liquid. The water cut ranges from 0-100 percent. High volume wells with high water cuts and high GVF wells are the most difficult to measure. Utilizing any type of electronic device requires frequent calibration and a physical method of checking to see if the calibration is correct. A 1 percent absolute error on a well that produces 1000 bbls/day at a 90 percent water cut can cause a net oil measurement error of 10 bbls. The only accurate method to determine the water cut is to take an accurate physical sample proportional to flow rate.

According to API standards, before a sample is taken• Thesamplemustbeproperlyconditioned(mixed).• Thesampleshouldbetakenusinganisokineticsampler.• Thesampleshouldnotbedividedonceithasbeentaken.• Thesamplemustbeanalyzedproperly.

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

The well test/validation system described here is simple to understand and easy to operate. Due to its simplicity, each test features accurate and repeatable results. 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 flow volume is also accurate and repeatable.