It is necessary to keep track of the gas, oil, and water production from each well to be able to manage the reserves properly, evaluate where further reserve potential may be found, and diagnose well problems as quickly as possible. Proper allocation of income also requires knowledge of daily production rates as the royalty or working interest ownership may be different for each well.
In simple facilities that contain only a few wells, it is attractive to route each well to its own separator and/or treater and measure its gas, oil, and water production on a continuous basis. In facilities that handleproduction from many wells, it is sometimes more convenient to enable each well to flow through the manifold to one or more test subsystems on a periodic basis. Total production from the facility is then allocated back to the individual wells on the basis of these well tests.
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The frequency with which wells must be tested and the length of the test depends upon well properties, legal requirements, requirements for special studies, etc. Most oil wells should be tested at least twice a month for four to twelve hours. Gas wells should be tested at least once amonth. Due to the need to put troublesome wells on long-term tests, the need to repeat tests whose results might be suspect, and the need to test several wells whenever there is an unexpected change in total production, one test system can handle approximately 20 oil wells.
In order to obtain a valid test, the test system should operate at the same pressure as the system to which the well normally flows. That is, if a well normally flows to a high-pressure separator the first vessel in the test system should operate at that pressure. If other Wells normally flow to an intermediate- or low-pressure separator, the first vessel in the test system must be able to operate at that pressure as well. Thus, in our example facility, Figure 2-1, we must either install separate high-, intermediate-and low-pressure test systems, or we must arrange the gas backpressure valves on the first vessel in the test system so that the vessel can operate at any of the three pressures by just switching a valve.
A test system can be made up of any of the components we have discussed (e.g., separators, FWKOs, treaters) arranged in any combination that makes sense to obtain the required data. A three-phase separator could be used where oil/water emulsions are not considered severe. The amount of oil in the water outlet is insignificant and can be neglected. The water in the oil outlet can be determined from a net oil computer, which automatically corrects for the water, or by taking a sample and measuring its oil content. This would be particularly well suited for gas wells.
A vertical treater could be used where it was considered necessary to heat the emulsion in order to measure its water content. Standard treaters are low-pressure vessels with limited gas and free water capacity. For this reason they would tend to be used on low-pressure oil wells. If it is desirable to use the treater on a higher pressure oil well, this could be done by including a separator upstream of and in series with the treater. If a great deal of free water is expected, the treater could be designed with a large FWKO section or a three-phase separator could be installed upstream.
Some facilities use a high-pressure three-phase separator for the highand intermediate-pressure wells that do not make much water and a treater for the low-pressure wells. Figure 2-18 shows an enlargement of the well test system for the example.
