To determine the closed pressure of a well it is best to allow the well to blow down to its natural pressure also, that all the water in it may be blown off, which otherwise would retard the gas; then the steam or spring gauge is to be screwed in place and the well shut in, then the pressure noted. It is customary to note the number of pounds pressure as indicated by the gauge at the end of each minute for several minutes, also noting the final pressure, which in some wells may be reached in a few minutes, while in others it may take twenty-four hours or more before the total pressure is obtained. This will be the closed or rock pressure of the well.
The pressure of a well at the end of the first minute may also be used to estimate the open-flow volume of a well and a close approximation may be obtained by the following method: add 15 pounds to the gauge reading, then multiply by the depth of the well, times one-half the square of the diameter of the tubing in the well; the resulting figure will be the volume of the well. This approximation will in all cases be less than the actual volume of the well. If the diameter of the tubing is not constant the entire depth of the well, separate calculations must be made for each different diameter to the entire depth of the well, and the separate results are to be added together.
In order to realize the value of a gas well one must have a thorough understanding of the difference between open-flow volume and closed pressure. It is a common occurrence to hear people say that a well is a big one, as the pressure is Soo pounds to the square inch; and this is the most common information one hears about gas wells, and such data are of no value unless the open flow volume is also obtained. It has been stated in another chapter that the rock pressure has a certain relation to depth, and it will be noticed that in a new field wells in the same sand will have approximately the same rock pressure; and of two wells offsetting each other, one may make ioo,000 cubic feet of gas per day, and the other 5,oco,000 cubic feet during the same time; both will have the same rock pressure, yet one is a big well and the other very light. Of two wells having the same open-flow volume but different pressures, the one with the greater pressure will feed more gas onto a gas line than will the other, and therefore will be more valuable. Pressure is required to put
gas on a line which also has a pressure, and high-pressure gas lines may carry from too to 30o pounds of line pressure, and in order that a well may put gas on that line it will have to have a greater pressure than that of the gathering line. It must also be remembered that the volume of a gas well as determined by the Pitot-tube will not be equal to the volume that it will put on a line against pressure; such a volume depends upon the size of the well, its rock pressure and the line pressure against which the well is working.
Where large operators use gas compressors, the volume of a well is the main consideration, as the compressors enable the keeping of the line pressure down, allowing wells with small rock pressure to deliver their gas onto the line.
Wells are in the care of well blowers, who visit each well frequently, and note the pressure as well as blow off accumulated water and watch the actions of the well.
A word of caution against the ordinary information gained regarding the size of gas wells, as the smallest volume well will make considerable noise or whistle if opened up after being shut in for any length of time, and such action will convey the impression to the uninitiated that it is a large well, but that may not be the case and all such information should be verified wherever possible by allowing the pressure of the well to blow down and then properly gauging it. Small and shallow wells when left open for one-half or one hour will blow down sufficiently for proper gauging. Deeper and larger wells should be allowed to blow anywhere from six to twenty-four hours, as it may be possible that it will take all that time for it to blow down; some times gas may find lodgment in an upper sand and until such a sand is exhausted the true volume of the producing sand cannot be determined.
Never gauge a well through an opening in a gate:as such open ings are not as large as the size of the gate would indicate. Thus a 2-inch gate's opening will not be circular, but in the neighborhood of one inch by two inches long; try to keep con siderable distance from any fittings, and it is best to carry a 2-inch nipple or better still a 2-inch pipe and gauging through that.