Oilfield waters are often referred to as brines, especially when they contain significant quantities of dissolved salts. They also frequently contain dissolved gases and may contain small quantities of the heavier hydrocarbons found in oils. Water can be present in a surface separator during production, either from liquid water in the zone being tested or by condensation from water vapor in the produced gas, or possibly from both. Water from aquifers or seawater may also need to be analyzed in connection with water-injection activities. >> OIl & Gas Engineering Books <<
Need for water analysis
The analysis of oilfield waters has a wide range of applications, including identifying the origin of produced water, characterizing aquifer properties, interpreting wireline-log measurements, predicting formation damage from water incompatibility, investigating scaling tendencies in surface and downhole equipment, monitoring fluid movement in reservoirs, identifying the presence of bacteria, evaluating disposal options and environmental compliance, and predicting and monitoring corrosion. Water analyses also can be useful in diagnosing and correcting numerous oilfield operating problems.
The American Petroleum Institute (API) publishes Recommended Practice 45, which contains information on the applications of oilfield-water analyses and gives recommendations for the proper collection, preservation, and labeling of oilfield-water samples. RP 45 also gives a description of numerous analytical methods and recommends appropriate reporting formats for analytical results. This publication should be consulted for more information about specific analytical methods and any special sampling or storage requirements linked to such methods. Numerous analytical methods are also available as American Society for Testing and Materials (ASTM) standards.
Measurements to be carried out on water sample
When sampling and analysis are part of a long-term monitoring program, such as those required by government authorities or those forming part of custody-transfer contracts, the methods defined in the appropriate documentation or contracts must be followed as closely as possible, even if this constitutes differences with the procedures or recommendations in this text or with the industry standards cited here. However, the guidelines provided here should be taken into consideration before contracts are drafted or when existing contracts are renewed.
If samples are to be collected for the measurement of trace components, biological species, or reactive chemicals that are likely to be affected by storage, container material, or ambient conditions, on-site analyses should be considered. API RP45 lists the following measurements that should be carried out immediately in the field after sampling and filtering oilfield waters:
- Dissolved oxygen
- Total and soluble iron
- Turbidity on an unfiltered sample
- Total suspended solids with at least primary filtration and washing performed in the field
- Bacteria with filtering and/or culturing in the field and incubation and counting performed in the laboratory
Biological determinations are outside the scope of this document but are covered in detail elsewhere.
For many other analyses, special preparation and preservation measures are required to be performed in the field. This can involve acidification with various acids, addition of other chemicals, refrigeration [ideally 39°F (4°C)], and storage in the dark. If there is any uncertainty concerning sample storage conditions, the laboratory that will perform the analyses should be consulted for advice. If no information is available, it is advisable to keep samples cool and out of the sunlight.
Water sampling during drillstem test
For a drillstem test (DST) that does not flow to the surface, great care must be taken to determine if the test has flowed sufficient fluid to allow representative reservoir brine into the tool. The best practice is to sample the water after each stand of pipe is removed. Normally, the total-dissolved-solids content will increase downward and become constant when pure formation water is obtained. A test that flows water will give even higher assurance of an uncontaminated sample. If only one DST water sample is taken for analysis, it should be taken just above the tool because this is the last water to enter the tool and is least likely to show contamination.
Surface sampling is commonly used to obtain a sample of formation water from a sampling valve at the wellhead or another sampling point. A plastic or rubber tube can be used to transfer the sample from the sample valve into the container. Fig. 1 shows a simple method of excluding air when sampling water in this way. After purging the sample valve and line to remove any foreign material, water is delivered to the bottom of the sample bottle, which is placed in a large, much taller beaker until the water fills the beaker and overflows. Then, the cap is immersed in the beaker and inverted to eliminate air bubbles before removing the delivery tube and closing the sample bottle under water. This technique cannot be used when acid or other preservatives must be added to the sample.
Fig. 1 – Water-sampling method to avoid air.
An alternative sampling technique for use when a clean source of water is available is shown in Fig. 2. Here, once the sample point and line have been purged, the sample is collected in the sample cylinder by closing the two valves. This system should not be used to collect pressurized water samples.
Fig. 2 – An alternative method for collecting air-free water samples.
Fig. 3 – Oil/water separator and water-sample filtering system.
For some measurements, it is necessary to obtain a field-filtered sample. The filtering system shown in Fig. 3 is simple and economical and can be used for various applications. It consists of a 50-mL disposable syringe, two check valves, and an inline disk-filter holder. The filter holder takes size 47-mm-diameter, 0.45-μm pore-size filters, with the option of including various prefilters. The syringe fill line should be connected to a source of brine free from oil or gas, either directly to a suitable sample point or to the brine outlet from a suitable separation vessel, as shown in Fig. 3. The brine is drawn through the inlet line into the syringe and then forced through the filter into the collection bottle. The check valves allow the syringe to be used as a pump for filling the collection bottle without needing to open and close valves. If the filter becomes clogged, it can be replaced in a few minutes. Approximately 2 minutes are required to collect 250 mL of sample. Usually two samples are taken, with the one being acidified to pH 3 or less with concentrated hydrochloric or nitric acid. The system can be cleaned easily or flushed with brine to prevent contamination.
If pressurized water samples are required, most of the procedures described previously for pressurized hydrocarbon fluids can be used, including downhole sampling. Piston sample bottles are essential because the sample cannot be collected by the displacement of water or brine.