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Model M Manual : Appendix
Rate of Fill-up Graph
The accompanying graph is very useful in liquid level work. The graph indicates the initial rate of fill-up between casing and tubing in a well for a certain producing rate after the well is shut-in. The chart is read as follows: If the well produces less than 100 BPD, find the point on the bottom scales corresponding to the producing rate, read straight up until intersecting the line indicating the pipe sizes, then read to the left to find fill-up rate. For over 100 BPD, find the point on the bottom scale corresponding to 1/10 of the producing rate, read up until intersecting the line indicating the pipe sizes, then read to the left to find fill-up rate and multiply by 10. For example, if a well which has 2” tubing and 51/2” casing is producing 60 BPD, the initial fill-up rate is 2.2 feet per minute. A well with 21/2” tubing and 41/2” casing, producing at 700 BPD will have an initial fill-up rate of 59 feet per minute.
Use of Rate of Fill-up Information
If a well is shut down for ten minutes for surface connection work prior to a liquid level shot and the initial fill-up rate is only 1.3 feet per minute, then the maximum error would be only 13 feet due to the shut-in time. However, if the fill-up rate is 35 feet per minute, the maximum error could be as high as 350 feet.
The fill-up rate found in the chart is the initial rate at which liquid will fill the casing annulus. This rate decreases as the pressure in the well bore approaches the static reservoir pressure. For example, if the static reservoir pressure is 1000 PSI, the reservoir pressure will support approximately 2500 feet of liquid (assuming low casing pressure), so the fill-up rate found on the chart would be within 10o/o for the first 600 feet or so. However, if the static reservoir pressure would support only 900 feet of liquid, then the fill-up rate would be reduced to one-half by the time the well had filled with 600 feet of liquid.
The rate of fill-up graph can be used to estimate the production rate of a well. A first fluid level depth measurement is performed while the well is at normal producing conditions. Then, the well is shut down and the liquid level in the casing annulus is allowed to rise. The height that the liquid rose and the amount of shut-in time is recorded. This data is used in conjunction with the casing and tubing sizes to determine the production rate of a well. This procedure is more accurate in wells that produce small amounts of gas up the casing annulus.
If the well is producing gas up the casing annulus, the rate of fill-up is not as predictable. Also, using the rate of fill-up to estimate the production rate is not as accurate. A casing pressure build-up rate in excess of 1 PSI in three minutes indicates that the casing annulus liquid contains a substantial amount of free gas and the fill-up data should be used with caution.
