Collecting and Preserving Water Samples
In the previous unit you learned how a water analysis can reveal telling information about the water in a production system. It is important for you to realize, however, that the information reported on a water analysis is based on a sample of water that is supposed to represent the system as a whole.
In this unit, you will learn how to properly collect and preserve a sample of water from a production system so that you can have confidence that the analysis of that water is accurate.
Contents
What is the process for collecting a water sample?
Water sample collection procedures may vary from area to area, but there are a few basic rules that apply wherever you work. They include the following:
- Purge the collection point, which means to let the water flow for a bit in order to clean away dirt and other debris that might be encrusted around the valve. Remember: you want the water sample to reflect what’s in the system, not what’s caked on the outside.
- Do not use “dead legs”. In other words, don’t collect water from a part of the system where the water is not flowing or is otherwise stagnant. Samples like those do not represent the system overall.
- Filter your samples. (Paper coffee filters are good for this.) Make a note of any foreign substances, such as a rust flake, etc., for the techs back at the lab. Although it is desirable to filter the sample so that field tests can be performed with clean water, there may be times when you may not want to filter the samples. For example, if you are interested in capturing data on all dissolved and suspended solids in the water, such as the total iron content, you would collect a non-filtered sample.
- Label with date and location. It’s a good practice to label both the bottle AND the cap.
- Preserve your samples.
- Use appropriate sample containers. Plastic are best, but glass with non-metal caps work fine too. Avoid metal containers or metal lids, since they usually corrode with resulting contamination or loss of the sample.
(Note: There are bottles actually meant for collecting water samples and you can get some by contacting your district lab tech. If they don’t have any tell them to get some because you need them for good water analyses. Having the right bottles on hand will keep you from having to use coke bottles, Styrofoam cups, etc.)
An accepted water sample collection process is included in the appendices of this manual. Regardless of what process you decide to use, however, always remember your primary reason for collecting a water sample: to get water that represents the water in the system.
How much water do I collect?
That depends. The Scale Group asks for a quart (liter), but ask some technicians in the field and they’ll tell you that there’s no way to get a quart of water out of some wells.
Try to collect enough that will allow the laboratory to conduct a thorough analysis for you.
Two six-ounce bottles will usually suffice for laboratory analyses. One sample un acidized and the other sample acidized to a pH of 2 or less. You may want to add a scale inhibitor if bad scaling is a problem, but make a note of it and inform the lab.
Keep in mind that any unused sample (oil, water or solids) is hazardous waste and must be disposed of properly.
What is a field analysis?
Water has certain characteristics that begin to change the instant the water leaves the system. Even the most precise and accurate water analysis will have little significance if the sample has been incorrectly taken and does not represent the water as it is in the system. Since your goal is to get a sample that most represents the water in the system, there are certain analyses that must be performed in the field immediately after drawing the sample.
The following properties of the sample should be field tested immediately:
Carbon Dioxide (CO2)
When you open a soft drink, the dissolved CO2 begins immediately to escape the solution. The same holds true for water once it’s free from the pressure exerted by the system. Although the analyses methods discussed here are adequate in most cases, the most accurate predictions of the system’s water chemistry are accomplished with software modeling programs. These tools calculate the dissolved acid gas content and pH based on the partial pressure of the acid gases.
You will learn more about these modeling programs in the next level of you technical training. In the meantime, however, use the following guidelines to determine which method should be used:
1 Pressure < 50 psig or downstream of the low pressure separator - FIELD TITRATION AND DIRECT pH MEASUREMENT IS OK
2 Pressure > 50 psig or upstream of the low pressure separator – USE CALCULATION METHOD
Procedures for performing a CO2 field analysis are included in the appendices.
Hydrogen Sulfide (H2S)
measure the water for H2S as soon as possible after the sample is drawn. Remember, however, the results will still not reflect actual H2S concentrations as they are in the system. A procedure for performing a H2S field test is included in the appendices.
Bicarbonate (Alkalinity)
Theoretically, bicarbonate alkalinity will not degrade between the time a sample is taken and the time it reaches the laboratory. Nevertheless, it is still a good practice to take the reading in the field because there are chances that the sample could be degraded. For example, some sample preservation techniques that use acid will destroy the bicarbonate alkalinity. Plus, other preservatives can interfere with the alkalinity titration, so it’s best to do the titration before preserving the sample. The field analysis procedures for HCO3 are also included in the appendices.
pH
Performing a field analysis for pH is a much debated topic. There are those who say that you should test the sample with pH paper as soon as it is drawn. Others say that unless you have a pH meter you cannot get an accurate reading. Some believe that testing for pH is fruitless because pH is closely related to the concentration of CO2 in the water. Since CO2 begins to offgas as soon as the system is opened, those with this opinion say that it’s impossible to get an accurate pH measurement anyway. You should find out what the practice is in your district and use it.
Regardless of the method used, however, it is important to take the measurement immediately due to the time critical nature of pH.
A word about titration
The titration process involves judging when the color of liquid changes. In order to be consistent in your assessments, it is recommended that you compare your wet titration end points with readings taken of the same sample with a pH meter. This will enable you to equate a specific hue of color with a measured pH value.
Chlorides
Chloride counts are a key indicator of water’s corrosivity and, therefore, important data in the water analysis. Although there are procedures that enable water samples to be preserved for accurate chloride determination in the lab, some techs and sales reps also like to test for them in the field.
A procedure for field testing with silver nitrate is included in the field testing procedures found in the Appendix of this course. Chloride field test should not take the place of lab tests.
Suspended Solids
Another analysis that can be performed in the field is the Millipore Filter Test. This analysis consists of flowing water through a membrane filter to determine:
1. Amount of non-dissolved solids in the water; 2. Types of non-dissolved materials.
These are important considerations in cases where the water will be re-injected into the formation. A procedure for performing a Millipore test in the field is included in the Appendix.
Oil Carryover
Typically, an oil-in-water extraction is performed in the field and the extracted oil is then sent to the lab for an IR analysis. Some offshore platforms are equipped with IR equipment on-site. A procedure for conducting oil-in-water extraction is included in the appendices.
Why and how are samples preserved?
The chief reason for preserving samples is to keep certain constituents in the water from precipitating out during transfer to the laboratory. For example, the concentration of chlorides is an important indicator of water’s potential corrosivity. For example, iron oxide and calcium carbonate can form solids between the time the samples are collected and when they get to the lab.
Acid will stabilize iron (Fe) and calcium carbonate (CaCO3).
HCl is the most common acid used for preserving samples, but keep in mind that you are adding chloride to your sample. (This could affect your chloride levels, so keep track of how much HCl is added to the water.)
You should preserve any sample from which constituents important to your analysis could precipitate. You should always include with your preserved sample the volume of the preservatives added to the sample and the volume of the sample itself.
Procedures for properly preserving samples are included in the appendix of this manual.
References
Ostroff, A.G. Introduction to Oilfield Water Technology. NACE, (1979).
