Nonpressurized hydrocarbon fluid sampling
Sampling of reservoir fluids is key to understanding a reservoir and planning surface and subsurface equipment and facilities. While many samples are collected downhole or at the surface while the fluids are still pressurized, useful information may also be gained from samples collected at atmospheric pressure (nonpressurized). This article discusses considerations for collecting such samples. It also includes information from an American Petroleum Institute (API) standard on sampling from oil tanks that is no longer available.
The sampling of nonpressurized or atmospheric-pressure hydrocarbon fluids from lines is relatively simple to perform, but attention must be paid to the need to purge sampling lines and pipework with at least three times their volume of fresh fluid before each sampling session. This is especially important in some installations and processing facilities, where the sampling point may be at the end of a "dead-leg" or trap in which fluid has collected or stagnated over a long period of time. In general, oil or condensate samples should be collected from a sample tap on the side of the line or the top of the line to avoid any water or sediment that may have accumulated at the bottom of the line. Atmospheric gas samples are rarely collected, but if they are required, they should be collected in evacuated chambers to minimize contamination by air.
Collecting samples for analysis
Atmospheric hydrocarbon samples also may be collected from pressurized lines or from samples collected in pressurized chambers, such as downhole samples. Usually, this will involve the release of gas and the collection of oil or condensate. Because the separation procedure that releases gas is dependent on the temperature and pressure (which may be above atmospheric if the liquid is collected in a closed trap), the properties of samples collected in this way may vary. Also, because the fluid in a sample chamber may already be in two-phase condition (or may have segregated), liquid from the entire sample should be collected to minimize uncertainty in the sample quality.
Sampling from oil tanks
Sampling from tanks is complicated by the need to collect samples from various depths to allow for any property changes or segregation that may exist. The procedure given next is a traditional method used for measuring and testing a field tank of crude oil, frequently referred to as "running" when related to custody-transfer transactions. It was published as API Standard 2500 but is no longer available. The method is reproduced here with the permission of the American Petroleum Inst. It is intended to support operations still using this method, or methods derived from it, and serve as a guideline to engineers setting up similar methods. Detailed descriptions of individual calibration and measurement methods are available in the API Manual of Petroleum Measurement Standards (MPMS), which represents all branches of the petroleum industry and is the recognized standard for downstream measurement methods.
- The tank is vertical, nonpressurized, and has a fixed roof with side outlets; it is to be gauged by the innage method (a process to determine the depth of liquid in a tank, which is measured from the surface of the liquid to the tank bottom or to a fixed datum plate).
- The oil viscosity is less than 100 Saybolt seconds at 100°F and is a liquid at atmospheric temperature and pressure.
- A cup-case thermometer is used to read the temperature of the oil in the tank.
- A thief is used to obtain fluid samples from the tank. (A "thief" is an industry term for a bottom-closure, core-type sampler used to secure samples from chosen depths in storage tanks.)
- The API gravity scale hydrometer test method is used to determine the API gravity of the oil; the temperature of the oil has to be near 60°F (±5°F).
- The water and sediment in the oil are to be determined by the centrifuge method with a 203-mm (8-in.) cone-shaped tube.
Hydrocarbon fluid sampling
The following outline gives the sequence of steps to be taken and the key points to be noted at each step.
- Isolate the tank to be checked.
- Use safety precautions and fresh air bottles if an H2S hazard exists.
- Ground yourself to a stair railing or tank shell before reaching the top. This prevents static-electrical discharge in a hazardous area.
- Stand to the side of the hatch when opening it to permit wind to blow gas away from you.
- Measure the temperature: suspend a thermometer in the oil tank. The thermometer should be 12 in. or more from the tank shell and must be immersed in oil for 5 minutes.
Use an American Society for Testing and Materials (ASTM)-approved, wood-back or corrosion-resistant metal cup case. If atmospheric temperature differs by more than 20°F from that of the liquid in the tank, the cup case should be given at least two preliminary immersions. Empty the cup case after each immersion.
Rapidly withdraw the thermometer and read and record the temperature to the nearest 1°F. Note: The number of temperature measurements varies with the depth of the liquid.
In a tank containing more than 15 ft of liquid, three measurements should be taken: (1) 3 ft below the top surface of the liquid, (2) in the middle of the liquid, and (3) 3 ft above the bottom of the liquid.
In a tank containing 10 to 15 ft of liquid, two measurements should be taken: (1) 3 ft below the top surface of the liquid, and (2) 3 ft above the bottom surface of the liquid.
In a tank containing less than 10 ft of liquid, one measurement should be taken in the middle of the liquid.
For tanks over 10 ft high with a capacity of less than 5,000 bbl, one measurement in the middle of the liquid should be taken.
- With a thief, take sample(s) for a basic sediment and water (BS&W) centrifuge test. Note: The number of samples to be taken for BS&W determination varies.
In tanks larger than 1,000-bbl capacity that contain more than 15 ft of liquid, equal-volume samples should be taken (1) 6 in. below the top of the liquid, (2) at the middle of the liquid, and (3) at the outlet connection of the merchantable oil, in the order named. This method also may be used in tanks up to and including a capacity of 1,000 bbl.
In a tank larger than 1,000-bbl capacity that contains more than 10 ft and up to 15 ft of liquid, equal-volume samples should be taken (1) 6 in. below the top surface of the liquid and (2) at the outlet connection of the merchantable oil, in the order named. This method may be used on tanks up to and including a capacity of 1,000 bbl.
In a tank larger than 1,000-bbl capacity that contains 10 ft or less of liquid, one sample may be taken in the middle of the column of liquid.
- Place the basic sediment and water (BS&W) composite sample in a sample container. The sample should be a blend of the upper, middle, and lower samples (if three samples were required), containing equal parts from the samples taken.
- Seal the sample container. In the lower 48 states, with the exception of California, the sample is ready to be tested for BS&W, as described in Step 17. In California, the container should be labeled and delivered to the laboratory for BS&W determination. (Note: These US state references were part of the original standard.)
- With a thief, take a sample for gravity determination. The sample should be taken midway between the oil surface and the pipeline connection. Hang the thief in the hatch. Remove bubbles, and place the hydrometer in the oil sample.
- Determine and record the sample temperature to the nearest 0.5°F. The hydrometer must float away from the wall of the cylinder; the temperature of the surrounding media should not change more than 5°F.
Depress the hydrometer two scale divisions and release. Read the hydrometer to the nearest 0.05°API on a scale at which surface or liquid cuts scale.
- Read and record the sample temperature to the nearest 0.5°F. Repeat the gravity reading if the temperature of the sample before and after the gravity reading has changed more than 1°F. Apply any relevant correction to the observed hydrometer reading (correction scale on bulb) to the nearest 0.1°API. Record the mean temperature reading observed before and after the final hydrometer reading to the nearest 1°F.
Note: Hydrometer scale readings at temperatures other than calibration temperatures (60°F) should not be considered more than scale readings because the hydrometer bulb changes with temperature.
- Convert the relevant corrected value to standard temperatures.
- Take the bottom thief sample for BS&W height. Lower the clean, dry thief slowly into the oil to the desired depth, trip the thief, and raise it slowly to avoid agitation. When the sample is taken, the top of the thief must be 2 in. above the bottom of the pipeline connections.
- Determine and record BS&W height in the tank. Pour the remaining thief sample over a test glass until contamination appears. Measure and record (as the BS&W height) the distance from the bottom of the thief to the top of the contamination in the thief. If BS&W height is less than 4 in. from the bottom of the pipeline connection, do not run the tank.
- Gauge the tank. Do not gauge a boiling or foaming tank. Use steel innage tape with an innage plumb bob. Always make contact between the gauge line and the hatch while running tape into the tank.
Gauge the tank only at the reference point on the gauging hatch. On tanks of 1,000-bbl capacity or less, read to the nearest 1/4 in. On tanks of 1,000 bbl or more, read to the nearest 1/8 in. Record the reading immediately; repeat until two identical gauges are obtained.
Saturate solvent with water. Toluene is approved solvent; it is flammable and toxic. Care should be taken when using toluene.
Fill a 1-qt or 1-L glass bottle with a screw top with 700 to 800 mL of toluene. Add 25 mL of either distilled or tap water. Screw the cap on; shake vigorously for 30 seconds. Loosen the cap; place the bottle in a bath for 30 minutes. Maintain the bath at a constant temperature of 140 ±5°F. Remove, tighten the cap, and shake vigorously for 30 seconds. Repeat three times.
Allow the bottle of water/toluene mixture to sit in the bath for 48 hours before using. Be sure that no free water is left in the bottle.
- Shake the sample container until the sample is well mixed. Fill two 203-mm (8-in.) cone-shaped centrifuge tubes with 50 mL of sample. Use a pipette to add 50 mL of toluene. Toluene should be water saturated at 140°F. Read the top of the meniscus at both the 50- and 100-mL marks. Add a 0.2-mL demulsifier if necessary for a clean break in the oil/water contact.
Stopper the tube tightly; invert the tube 10 times to ensure that oil and solvent are uniformly mixed.
- Loosen the stopper slightly. Immerse the tube to the 100-mL mark in a bath for 15 minutes. The bath must maintain a temperature of 140 ±5°F; by contract agreement, the bath temperature may be 120 ±5°F.
Remove the tube from the bath and tighten the stopper. Invert the tube 10 times to ensure that oil and solvent are uniformly mixed.
- Place the tubes in trunnion cups on opposite sides of the centrifuge. Spin for 10 minutes while maintaining minimum relative centrifuge force of 600.
Following the spinning, read and record the combined volume of water and sediment at the bottom of each tube. Read to the nearest 0.05 mL for oil from 0.1- to 1-mL graduation. Read to the nearest 0.1 mL above 1-mL graduation. Estimate to the nearest 0.025 mL below 0.1-mL graduation.
Return the tube to the centrifuge without agitation. Spin for 10 minutes at the same rate. Repeat this operation until the combined volume of water and sediment remains constant on two consecutive readings.
- Record the final volume of water and sediment in each tube. The sum of the two admissible readings is the vol% of water and sediment in the sample.
After the tank has been run, the following closing data should be obtained.
- Closing oil temperature: no closing temperature is necessary on tanks of 5,000 bbl or less; on tanks of 5,000 bbl or more, always read to the nearest 1°F.
- Obtain a closing gauge reading at the same point and in the same manner as the opening gauge reading.
- Obtain the bottom thief. If the BS&W level is lower than the opening gauge, report this to a supervisor.
More information concerning the specific measurement methods referred to here can be found in the API MPMS. The manual is being updated continually, and care should be taken that the current standard or chapter is used.
- API MPMS, Manual of Petroleum Measurement Standards. 1974. Washington, DC: API.
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