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Surface sampling of reservoir fluids
Surface sampling of reservoir fluids primarily involves sampling individual gas and liquid streams from a production separator or similar installation, and it is by far the most common method of sampling pressurized hydrocarbon fluids. Usually, the objective of separator sampling is to obtain a fluid representative of the production of one well that enters the separator in its entirety, but the method also can be used to obtain a fluid representing commingled production from a number of wells into a single gas/oil separation plant. In either case, the objective is to collect separate samples of the gas and liquid exiting the separator and to measure the separate flow rates of the two phases and obtain the gas-oil ratio (GOR). Although the two phases are never in perfect equilibrium, providing that the two samples are representative of the separate flows, it is possible to mix the two samples together in the same proportion in which they are produced t obtain a recombined sample that represents the fluid entering the separator.
Some of the biggest errors affecting fluid samples are related to the measurement of separator-gas and -liquid flow rates, which are crucial for the recombination process in the laboratory. Good accuracy is often considered to be in the region of 5%, but the figure can be much worse, for example, if there is carry-over of liquid in the gas exit stream (or carry-under of gas in an oil with foaming tendencies). Problems are especially common for gas-well production tests, where very high flow rates can be used, and special techniques are available for trying to measure liquid carry-over in such situations. However, the best approach involves proper sizing and adjustment of the separator for the production rate. Another important source of error in this domain involves confusion over whether liquid flow rates are reported at separator conditions or at tank conditions; this has serious implications for gas/condensate fluids in which the separator-liquid shrinkage is typically much larger than in the case of an oil.
Although broad guidelines exist concerning the volumes of samples that should be collected, special attention should be given when collecting gas samples from separators operating at low pressures because the lower density may result in the collection of insufficient weight of gas. Fig. 1 enables the required volume of gas to be estimated simply as a function of separator pressure, GOR, and the volume of liquid that is required. This chart is reproduced with the kind permission of Saudi Aramco.
Separator sampling methods
In line with current trends in the oil industry, current recommendations are to use evacuated bottles for gases and piston bottles for liquids and avoid any use of mercury in sampling operations. At extremely low temperatures, piston bottles have been reported to leak past the piston seal, so sampling under such conditions should be avoided if possible. If these methods cannot be used, then repeated purging (a minimum of five times) should be used for gas samples, and the displacement of brine should be used for liquid samples unless high H2S or CO2 levels are present, in which case it is preferable to use separator water saturated with gas if it is available. The principal guidelines to be followed for surface sampling are given in Table 1.
The two most common separator sampling methods are described in the American Petroleum Institute's RP 44:
- Filling an evacuated container (method 1)
- Filling a piston-type container (method 3)
Filling an evacuated container
This method is especially simple and accurate. The principal undesirable feature of the method is the requirement that the vessel be evacuated before its transport to the sampling point (with possible loss of vacuum during transport), or that a vacuum pump be provided at the wellsite. Testing pre-evacuated vessels for adequate vacuum at the time of sampling should be done only by personnel well trained in vacuum-testing procedures because improper testing often leads to loss of vacuum or introduction of air into the sample vessel. (Collecting an additional sample may be preferable to vacuum testing.) A clean, evacuated container should never be purged with separator gas and re-evacuated in the field because any liquid that condenses in the container during the purge may not totally re-evaporate during evacuation in the field.
Sample collection is accomplished by the following steps:
- Locate an appropriate sample source valve A (see Fig. 2 ) on the separator from which the desired sample can be collected. Clean any debris from valve A; open the valve briefly to blow it out, and then close it.
- Connect the fitting on the flexible tubing of the sampling rig securely to valve A on the separator. Open the line valve B, and open the purge valve C.
- If a vacuum pump is available and personnel are qualified in vacuum techniques, connect the sample inlet valve D on the sample container to the fitting on the sampling rig, as shown in Fig. 2. Connect the vacuum pump to valve C, open valve C and valve B to evacuate the sampling rig, then close valve C and disconnect the pump. Slowly reopen valve A completely to establish full separator pressure on the entire sampling rig from valve A to valve D, and proceed to Step 6.
- If a vacuum pump is not available, open valves B and C, then open and close valve A in one quick burst to purge air from the sampling rig, and quickly close valve B. Slowly reopen valve A completely to establish full separator pressure on the entire sampling rig from valve A to valve B.
- Connect the sample inlet valve D on the sample container to the fitting on the sampling rig, as shown in Fig. 2. Open valve C, then open and close valve B in one quick burst to purge air from the line connecting valves B and D, and close valve C promptly. Note: Use a long vent line on valve C if H2S is present. Reopen valve B to establish full separator pressure on the entire sampling rig from valve A to valve D.
- Cautiously crack open valve D, while carefully monitoring the pressure gauge, and fill the container slowly. Continuously adjust valve D as needed to keep full pressure on the pressure gauge. Filling a large container can take as long as 20 minutes. The progress of the filling process can be monitored by listening for a hissing sound at valve D (and in the container) and by monitoring the pressure gauge. When you think that the container is full, open valve D further while listening to the container and monitoring the pressure gauge.
- When the container is full, close valve D, and then close valve B.
- Slightly open valve C to bleed the connections between valves B and D to atmospheric pressure. Note: The line from valve A to valve B, including the pressure gauge, is still under full pressure. Use a long vent line on valve C if H2S is present.
- Disconnect the sample container. This is the last step in collecting the first sample. The apparatus is now ready for collecting additional samples by repeating Steps 5–8.
- Following collection of the last sample, close valve A securely, then open valve B (and valve C, if it is not already open) to bleed pressure from all parts of the line and sampling rig before disconnecting the line from valve A. Note: Use a long vent line on valve C if H2S is present.
- Insert sealing plugs into the valves on each sample container; then check the valves for leaks by immersing them in water or painting them with soap solution. Before inserting the sealing plugs, the threads should be lubricated by stretching Teflon® tape into the threads or by applying pipe dope. After a container is determined to be leak-free, it should be tagged and otherwise prepared for storage or transit.
Filling a piston type container
It refers to the same sampling rig as that used for the gas-sampling method above, though the sample cylinder will contain a piston, and valve E will represent the hydraulic-fluid connection, as indicated in Fig. 3. Some steps in this procedure may need modification depending on exact equipment design; this is notable for sample cylinders, which have an additional purge valve at the sample inlet end of the cylinder.
This is a preferred method for nonmercury liquid-sample collection. It has the advantage that the liquid sample can be kept at the saturation pressure throughout the collection process, which avoids gas breakout from the sample. In addition, the sample does not come into contact with any other fluids during sampling or during transfer in the laboratory. The undesirable feature of the method is that with sample containers, the potential for contamination with hydraulic fluid exists if the seal on the piston leaks. (Water can be used as the hydraulic fluid to minimize the possibility of contamination, but the operator should first check with the manufacturer to ensure that water will not damage the container.)
If a piston-type container is being used, hydraulic fluid must be preloaded behind the piston so that the piston position is fully toward the sampling end. A danger is that inexperienced personnel may not know this and may attempt to use this type of container without a proper fill of hydraulic fluid and without proper hydraulic-pressure support on the piston seal. In such a case, full pressure will not be maintained on the separator oil during sampling, and the process essentially will be the same as filling an empty container, except that the seal on the piston might leak. The manufacturer’s instructions should be consulted to ensure that the operation of the piston-type container is completely understood before commencing the sampling operation.
The procedure is as follows:
- Locate an appropriate sample source valve A on the separator (see Fig. 3) from which the desired oil sample can be collected. Clean any debris from valve A, hold a rag over the valve (or attach a temporary purge line connected to a suitable container), open valve A slowly, purge sufficient oil through the valve, and then close valve A. Remove the rag or temporary purge line. Note: Use a long vent line if H2S is present.
- Connect the fitting on the flexible tubing of the sampling rig (see Fig. 3) securely to valve A on the separator. Open the line valve B, and open the purge valve C.
- If a vacuum pump is available and personnel are qualified in vacuum techniques, connect the sample inlet valve D on the sample container to the fitting on the sampling rig, as shown in Fig. 3. Connect the vacuum pump to valve C, open valve C and valve B to evacuate the sampling rig, and then close valve B. Slowly reopen valve A completely to establish full separator pressure on the entire sampling rig from valve A to valve B. Open valve D to evacuate the connection and the small dead volume in the container (the internal volume between valve D and the face of the piston when the piston position is at the sampling end), then close valve C and disconnect the pump. Slowly reopen valve B completely to establish full separator pressure on the entire system from valve A through valve D to the face of the piston in the container, and proceed to Step 6. Be sure that valve D is completely open.
- If a vacuum pump is not available, close valve B and open valve A slowly (the pressure on the gauge should rise to the separator pressure). Close valve A, attach a purge line at the end of the rig below valve C, close valve C, and open valve B to let the pressure deplete to atmospheric. Close valve B, then slowly reopen valve A completely. Slightly open valve B, and slowly purge a volume of oil equivalent to several times the volume of the sampling rig, collecting the purged oil in a suitable container (maintain full separator pressure on the pressure gauge during this purge). Close valve B, and remove the purge line. Full separator pressure should now be on the entire sampling rig from valve A to valve B.
- Connect the sample inlet valve D on the sample container to the fitting on the sampling rig, as shown in Fig. 3, and attach a purge line at the end of valve C. Open valve D, close valve C, and open valve B slowly to pressure up the connection with the container and any dead volume in the sample container. Close valve B, and open valve C to let the pressure deplete to atmospheric. Close valve C, then slowly reopen valve B completely. Slightly open valve C, and slowly purge a volume of oil equivalent to several times the volume of the connection, collecting the purged oil in a suitable container (maintain full separator pressure on the pressure gauge during this purge). Close valve C, and remove the purge line. Full separator pressure should now be on the entire sampling rig from valve A through valve D to the face of the piston in the sample container. Be sure that valve D is completely open. Note: This method is not perfect because the oil in the dead volume in the sample container has not been purged under pressure. However, if the piston position is fully toward the sampling end of the container, the amount of oil in the dead volume will be negligible.
- Cautiously crack open sample outlet valve E while carefully monitoring the pressure gauge, and allow the sample fluid to slowly displace the preload hydraulic oil into a suitable collection vessel. Continuously adjust valve E as needed to be sure that the rate of sample collection is sufficiently slow so that full separator pressure is maintained on the sample side of the piston (as indicated by the pressure gauge). The sampling operation can be ended when a desired volume of sample is collected (as indicated by a given volume of hydraulic fluid being displaced to the collection vessel). The operation must be stopped with at least enough preload liquid left in the container to provide the "outage" required in Step 7. Close valves E, D, and B, in that order. (If the container has a magnetic indicator to show the position of the piston, then nitrogen gas can be used as the hydraulic fluid behind the piston, and Step 7 can be eliminated so long as approximately 10% volume of nitrogen remains on the hydraulic side of the piston.)
- Open valve E slightly (with valve D closed), and drain into the collection vessel a volume of hydraulic oil equal to approximately 10% of the container volume. This will create the necessary vapor space in the container without altering the overall composition of the oil sample. (Be sure to leave at least some hydraulic oil behind the piston so that there is pressure support on the seal and very little pressure drop across the seal). Close valve E securely.
- Slightly open valve C to bleed the connections between valves B and D to atmospheric pressure. Note: The line from valve A to B, including the pressure gauge, is still under pressure. Use a long vent line if H2S is present.
- Disconnect the sample container. This is the last step for the first sample and leaves the apparatus ready for collection of additional samples by repeating Steps 5–8.
- Following collection of the last sample, close valve A securely, then open valve B (and valve C, if it is not already open) to bleed pressure from all parts of the line and sampling rig before disconnecting the line from source valve A.
- Wipe the valves on the sample container clean and inspect for any signs of leakage. After a container is determined to be leak-free, insert plugs in the valves, then tag the container and otherwise prepare it for storage or transit. Before inserting the sealing plugs, the threads should be lubricated by stretching Teflon ® tape into the threads or by applying pipe dope.
Wellhead sampling, more commonly known as flowline sampling, involves the collection of a fluid sample at the surface from the wellhead itself or from the flowline or upstream side of the choke manifold, provided that the fluid is still in one-phase condition. This option is restricted to wells producing dry gas, very-low-GOR oils, and some high-pressure/high-temperature reservoir fluids. Dry-gas wellhead samples can be collected as for gas sampling from a separator, whereas wellhead sampling of other or unknown fluids should be performed as for separator liquids. However, all equipment must be compatible with maximum wellhead pressure, and as the state of the fluid is not usually known with certainty, separator sampling also should be performed if possible, as a backup.
Isokinetic sampling, also known as split-stream sampling, involves collecting samples from well production in two-phase flow, using a small side stream to allow the two-phase fluid to be collected and measured in laboratory scale equipment at the wellsite. There are two principal challenges in this approach: controlling the side stream so that it is removed from the main flow at identical velocity (hence the term isokinetic) to avoid disproportionate sampling of the two phases, and ensuring that the flow is turbulent upstream of the sampling probe so that the minor phase is finely distributed in the major phase. Although this special type of sampling has been used for more than 60 years, mainly for sampling gas/condensate production, many still consider it to be at the development stage, and it has never achieved wide acceptance. A more recent development of isokinetic sampling involves sampling of the exit gas stream from a separator and calculation of a figure for separator efficiency. This efficiency is then used to modify the GOR used for recombining separator samples, but it should be compared to the separator efficiencies.
- API RP 44, Sampling Petroleum Reservoir Fluids, second edition. 2003. Washington, DC: API.
- Williams, J.M. 1998. Fluid Sampling under Adverse Conditions. Oil & Gas Science and Technology - Rev. IFP 53 (3): 355-365. http://dx.doi.org/10.2516/ogst:1998031
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