|
|
|
Preparation & Validation of One fundamental purpose of sampling is to verify online electronic instruments are providing accurate indications of water content. The method to determine water content must be carefully considered by observing the types of crude oil present, the installation of the equipment (both sampling and electronic measurement installations) and the temperature, pressure, and level of water present. EMULSION TYPE: An oil and water emulsion can be one or a mix of two emulsion phases which are called oil continuous (oil surrounding water) and water continuous (water surrounding oil). The type of emulsion will create different challenges in the attempt to extract a representative sample. A. Oil continuous emulsions: These can range from 0% water up to 90% water in the oil continuous phase. The light oil and NGL and condensate do not have the emulsifiers in the liquid and therefore the water is typically in a free state and will separate out very rapidly. In low flow conditions, this causes severe problems in sampling. The stream must be very well mixed with a good sample probe to extract the sample. With heavy oils the water can exist in streamers (water continuous phase) inside of the oil continuous emulsion. This causes extreme sampling problems due to the non- homogeneous mixture which can also flow at a different rate than the imbedded water phase. B. Water continuous emulsions: In very light oils (West Texas 40-50 API oil), NGL’s and condensate, the range of this emulsion can be from 30% water to 100% water. Opening a valve on the main line to pull a sample tends to favor the water phase and the sample will be biased. The higher the water cut the more difficult it is to pull a representative sample without mixers and pitot tube sample ports. Typical water continuous emulsion water cut range is 65-100%. FLOW RATES: Flow rates will impact the sample quality. Low flow allows the more dense liquid to favor the bottom of the process pipe. Higher flow rates begin to mix the liquids as the turbulent flow regime is entered. If the sample port is too close to an elbow in the pipe, the centrifugal force will tend to separate the heavier product and send it to the outside of the elbow. Mixing due to a static mixer or a centrifugal pump will aid in the dispersion of one phase in the other. These must be considered carefully since in many cases they still do not provide good distribution and dispersion of the water. SAMPLE PORT DESIGN: The key points are to have a design that will remove the sample in a representative cross section of the main line at the same velocity as the main line. The best will sample across the area of the main line and place the sample into a closed chamber. Most well testing systems open directly to atmospheric pressure when taking the sample into an open jar. Higher velocities such as opening a port to atmospheric pressures from the main line which is at a higher pressure can bias the sample due to the change in velocity at the point where the sample is being pulled. If the port is open to the atmosphere when pulling a sample the bias may come in due to the water slipping around the exit favoring the oil. If the sample was pulled at the same velocity as the flowing line, the "bow wave" effect is eliminated and water cannot slip past. The port design should take a cross section of the main line without creating a bias for oil or water. If the slipstream is pulled at the center of the main line, the liquids need to be well conditioned just upstream. A large sample port size will create several problems. A large sample port will force the operator to pinch down on the valve so that the oil will not spray. If the stub between the liquids in the main line is large, the pipe between the liquids and the valve becomes a "dead leg" where water may start to accumulate when pulling the sample. A recommended sample port will be a 3/8" or a maximum of 1/ 2" tubing with a through swage fitting into a weldolet in the main pipe that will be sampled. Figure 1 gives an example of a 1/ 2" type of system using a 45 degree cut on the tubing in the middle of the stream. A mark is made on the tubing on the front side of the 45 degree cut so that after it is inserted the technician can make certain that it is facing upstream. A static mixer is shown just upstream to mix the fluids.
1. _3000# Threadolet Figure 1. Sample Port Configuration
METHODS OF HANDLING THE SAMPLE AFTER IT IS PULLED: Emulsions will "age" as they cool down and time has passed from its removal from the line. This "aging" makes it difficult to process in the lab. If the sample is pulled using a gate type of valve or a needle valve, these operate slowly with the opening of the valve, the velocity will change depending on the valve opening. This will change the apparent water cut as different operators open the valve to different port sizes. The amount of sample pulled should be just enough to process using the analysis method of choice. In the case of centrifuge, this should not be more than about 100 to 200 ml. The entire sample should be centrifuged using the ASTM API method called out in IP359/82. Karl Fischer (KF) should only be used for 0-5% water cuts and the sample pulled must be processed soon after obtaining the sample. Pulling a sample for KF is difficult due to the very small size that is processed – typically only 1ml. If the sample is left to "age" the results will not be representative of the true water cut. Mercaptan and some sulfurs will affect KF depending upon the chemicals used. LABORATORY METHODS: Most preferred is the distillation procedure although it is the most difficult and time consuming. Centrifuge is the typical method for well testing applications (0-100% water cuts) but, it is often used for 0-5% water in oil. ASTM/ API Chapter 10.4 (Field Method) and IP359/82 (Laboratory Method) are standard test methods for centrifuge which need to be followed to obtain reasonable results. One overlooked issue is that of the use of water saturated toluene as a solvent. This is of particular importance when the water cut is lower than 0.5%. Kerosene and Stoddard solvent do not suffer from absorption of water. Several of the centrifuge vendors sell tubes that do not have any graduations between major marks, these are very hard to use. The procedure does not mention using the complete sample but, many instances this is the only way to achieve reasonable results. In this case multiple tubes are filled and all of the numbers are added together to obtain the average results. Notice the repeatability and reproducibility numbers at the end of the laboratory method standard. For 0-3% water cut the repeatability is 0.12% and the reproducibility is 0.28%. This is not between separate samples pulled at the same time and run separately or for higher water cuts. The ASTM standards do not address the error above 1% water cut for centrifuge or Dean Starke. In addition, many cases of laboratory bias have been seen. One way to quantify the reproducibility is to obtain a 200 to 500 ml sample from a well mixed stream where an on line analyzer is available to make certain that the water cut is not changing during the sample pulling. Immediately after pulling the sample shake well and separate it into three or four samples. Mark each with a different well name and submit to the laboratory. The results can be very interesting but, help to shed light on the problem. HANDLING OF THE DATA: Data should be plotted with the lab results on the x-axis and the water cut analyzer on the y-axis. Be certain that the data from the meter is read at the same time when the sample is being pulled and that it is a fairly steady reading. If it is varying the sample becomes the average of the analyzer readings and this is difficult to track. Differences in DCS or PLC timing of data collection usually means that writing down the time of day and then looking up the analyzer reading at that time does not give useful information. A scatter plot of the data for a given pipeline or well vs time will help in observing the probable error in sampling and method. Fitting a best fit line through the data will show the offset from the analyzer to the laboratory methods. After obtaining 10-30 points the pattern should be uniformly distributed about the 45 degree line and have correlation between the data. During the data taking, no adjustments should be performed on the analyzer or these should be noted on the record so that the plots will be meaningful. The scatter of the points will give an indication of the deviation between the two sets of data. If a constant offset in either direction can be seen then it is time to place this offset into the analyzer. Constantly changing the offset in the analyzer is not productive because this is most likely due to the reproducibility of pulling and processing the sample. Once the graphed data shows a clear offset this should be entered once and more data collected. If records are kept and each change is noted the performance of the analyzer vs time will be seen.
EXAMPLE OF DATA: Figure 2 shows real data from a well test where the data was obtained and compared to a centrifuge. The middle line is the best fit line and the two outer lines are the +/- 5% lines. This data set has a few "outliers" which are data points that have a problem in the data collection. Even with the data limits set at +/-5%, the samples do not always fall within these boundries. This graph was selected because all of the data came from one test separator with 20 wells where the sampling was typical. Sampling was not done from the center of the 3" pipe but instead from the side of the wall. There was no static mixer upstream and upstream of the sample port was a 6 foot straight run. The sample port was too large causing the operator to pinch back on the valve to avoid splashing out of the sample jar. The water continuous emulsion phase shows more scatter than the oil continuous which also states that it is harder to obtain a good sample in the water continuous phase. SUMMARY: Sampling is always a very difficult problem. Flow dynamics, physical properties of the fluids, emulsion properties, temperature, pressure, physical pipe layout, sample method and analysis procedures all make dramatic changes in the resultant data. Data plots vs time comparing the instrument value vs the chosen validation method usually show trend and provide information that cannot be obtained in any other manner. If only a random correlation exists between the two data sets there is something very wrong with one or both systems of measurement.
Figure 3. Correct Installation of Sampler, Courtesy Welker Engineering Co., Sugar Land, Texas Figure 3 shows some of the issues in the sampling of crude oils. Notice the oil at the bottom of the pipe, after the mixer, after the sample loop, and before the sampler. These problems exist in many installations with the best of intentions to eliminate them foiled by the product’s characteristics as mentioned earlier. At best sampling is an art and the water cut analyzers are making a measurement which will reduce the difficulties in obtaining good data. The problem is in the validation of the water cut analyzer due to the sample and laboratory issues. Performance Acceptance Test Procedure The purpose of this procedure is to qualify a water cut analyzer for use in a well test application. It is assumed that proper sampling, laboratory and handling procedures have been implemented. 1. Obtain a Sample: 1.1 Prepare clean sample receivers not larger than 200 ml. Assure that the selected laboratory method is available and appropriate for the water cut to be seen. 1.2 Upon arrival at site of sampling, verify that the water cut analyzer is seeing a relatively constant water cut so that sampling will be over very small changes in water. If the analyzer display is not close to the sample point, have another technician observe the reading while the sample is being pulled. 1.3 Using a waste bucket, purge the sample port line of existing liquids and then grab a sample into the container without shutting off the flow in the sample line until an appropriate amount of liquid has been accumulated. 1.4 Write down the information in the attached form for the conditions at sampling. This is data for the well under test including Stream Number, water cut, temperature, Oil Adjust, Emulsion Phase (Oil or Wat) and salinity displayed for the current Stream number. Oil Adjust is available by pressing MENU four times from Home at the Phase Dynamics, Inc. analyzer. Water Adjust should always be equal to zero. 1.5 Pull two or three samples and label them accordingly. It would be preferred that the samples have a different water cut. Be careful that the water cut is not changing quickly and therefore the sample will not represent the analyzer reading.
2. Process the Samples: 2.1 Using the ASTM recommended procedures for centrifuge or distillation process the sample noting the results on the line with respect to the sample number. Using Karl Fischer for water cuts higher than 5% is not recommended. Process the samples taken within _ hour of the collection. Longer waits may create a more stable emulsion which will bias centrifuge results.
3. Process the Data: 3.1 Calculate the Error column on the data sheet by taking the Laboratory water cut and subtracting the observed water cut on the analyzer. 3.2 Determine the Average Error. At the bottom of the Error column calculate the Average Error by summing all of the numbers in the Error column and dividing by the number of samples. This should only be done for samples of the same crude type and density. 3.3 Plot the data on X-Y in Excel or by hand with the Laboratory water cut on the X-Axis and the Analyzer water cut on the Y-Axis. 3.4 Calculate the best fit line though the data points using Excel or another program. 3.5 Place Error Limits around the best fit line. The ASTM standards do not cover accuracy for water cuts greater than 1%. Typical limits seen across many field conditions are +/-3% in the oil continuous phase and up to +/-5% in the water continuous phase. These errors are made up of: analyzer, sample port, sample handling, and sample processing errors. These error bounds are to establish a reasonable definition to look at the sampling and analyzer issues.
4. Analyzer Calibration: 4.1 Oil continuous emulsions require an offset to be added or subtracted to correct for the differences between the sample values and the PDI values. This is accomplished using the Oil Adjust. Be certain to press Enter after selecting the appropriate offset. 4.2 Water continuous emulsions differences between samples and PDI values are compensated for by a change in the apparent salinity entered into the analyzer. Water Adjust should always be equal to zero. Increasing the salinity lowers the water cut. If the analyzer is showing "Wat" phase and the sample is consistently showing a lower water cut than the Phase Dynamics, Inc. analyzer, change the salinity to reflect the difference in water cut. The salinity ONLY makes a difference in the water continuous emulsion ("Wat" ) phase. Be certain that the analyzer is in this phase before changing the salinity to adjust for the difference between the sample value and the PDI value when the sample was taken. Another method to compensate salinity for water continuous emulsions would be to obtain a good 100% water sample under process conditions and then performing a Standard Salinity calibration. The resultant salinity should be the most accurate water cut obtainable. 4.3 STANDARD Salinity Calibration Routine: 4.3.1 Make certain that the Home Menu is showing WAT for the water continuous emulsion phase. If this changes to OIL at any time during this procedure, your calibration will be terminated. 4.3.2 Step through the Menu Items until Salinity Calibration Appears on the Screen, press ENTER, the next Menu item will be Standard Salinity Calibration, press ENTER once again. If you pressed Select and it says Detailed Salinity Calibration do not use this method, instead press Select once again so that Standard Salinity calibration is displayed then press ENTER. 4.3.3 The screen will now show the Stream XX and Water = xx.x% and ENTER Starts Sample. Press Enter as you either start to pull a sample or know that the flowing stream is 100% water. 4.3.4 Wait at least 4 seconds before pressing the ENTER again to End Sample. If you press Enter to soon it would not have enough time to sample the frequency. 4.3.5 The next screen is Stream xx, Enter Water Content If you obtained a 100% water dump during this test then use Value and Select to have 99.9% on the screen and press ENTER. If instead of obtaining a water dump of 100% water you pulled a sample and determined the water cut, enter this value instead of the 99.9%. 4.3.6 The system will calculate the apparent salinity and display the New salinity and the Old salinity and request you to press ENTER if you accept the new salt. 4.3.7 If you pressed ENTER in the above step, the system will place the calculated salinity into the Menu item for this selected Stream.
This procedure should give you the best obtainable accuracy for the water phase since the sample was taken at pressure and temperature.
Sample Data Sheet
Mean Error (%)_______
|
