It may, in some cases, be impossible to entirely exclude all water from an oil well. Tinder such conditions some reasonable limit to the permissible amount of water should be established. This amount will depend upon the average daily rate of oil pro duction of other wells in the locality, and also upon how greatly water interferes with normal production. For instance, the limit considered reasonable in California is about 5 bbls. of water per 24 hours. This figure is probably excessive for some fields and too low for others. However, in any field or locality, conditions should be carefully studied for the purpose of establish ing a limit, and therefore all tests should be reduced to a common basis of barrels per 24 hours.
The amount of water entering a well during a test is determined by the difference of level noted at the time of two separate observations. A diagram similar to Fig. 15 will be found convenient for computing the volume of water rising in various sizes of casing during various intervals of time.
Perforating Casing to Test Formations.—Where several strata of oil or water bearing formations are penetrated by a well it has sometimes happened that the upper strata have not been thoroughly tested. Such procedure leaves doubt as to whether or not some of these strata are susceptible of furnishing com mercial quantities of oil, also as to whether water or gas have been prevented from passing from one stratum to another. The upper formations may be tested either by drilling another well into the shallow formations, or, in the case of deep wells, by perforating the casing which has shut off the formations. When the test by perforating the casing is adopted, it is assumed, of course, that some steps were taken, when the well was drilled, to permanently seal or separate the several strata. This sealing may have been accomplished by injecting cement or mud back of the casing, or the formations may have caved and settled around the casing.
At first glance it may appear extremely hazardous to deliber tely perforate a string of casing which has been inserted in a well for the specific purpose of excluding water. However, this test has been successfully carried out in a number of wells, and in some instances has definitely proved that productive formations were cased This process, in the hands of skilled workmen, is undoubtedly susceptible of considerable further development and use. The test should be preceded by a thorough study of the geology of the immediate neighborhood in order that it may be intelligently guided. Some method must be devised to prove that the perforating machine has actually cut through the casing and cement. One method is to paint the knife of the perforator before perforating, and to examine it after the operation for evidence of degree of penetration.
For a detailed presentation of the subject of perforated casing, the reader is referred to the work of E. W. Testing Casing For Leaks.—Where casing is inserted in a well or the purpose of excluding water the first requirement is, of course, that the casing shall itself be water-tight. Although his requirement would seem to be obvious it is frequently gnored. An excellent method of conducting this test is des ribed by R. E. Collom.' "Causes of casing leaks are usually one or more of the following: Insufficient tightening of casing, collapse, defective welds, corrosion, wear of drilling-line, or shifting formations.
" In a drilling well considerable time and expense can be saved by testing a water string, if cemented, before the plug is drilled out for test of shut-off.
"If this program is not followed, the test for water shut-off may be inconclusive and it may be necessary to again plug the shoe of the casing with cement or run a casing tester.
"By plugging in the shoe of the casing and making bailing tests, the existence of a casing leak and the rate of flow can be detected. By the use of a casing tester, the position of the leak, as well as the rate of flow, can be determined.
"A casing-tester, or swab-bailer as it is sometimes called, can be used to locate definitely any one of a number of leaks. There are a number of variations in the so-called swab-bailer or casing-tester. Figures 16 and 17 illustrate three of the varieties in common use. All testers have a closed bottom. The casing must be bailed free of fluid below the depth to be tested. The tester is run to a predetermined depth and allowed to stand for a given length of time. When the tester is removed, the amount of fluid therein can be measured and the rate of leakage reduced to terms of flow during 12 or 24 hour periods.
"When the position of the leak has been determined, the next thing to do is to repair it. The method of repair depends to a great extent upon the nature of the leak." A detailed description of various steps to be taken in deter mining the source of water in oil wells, by H. W. includes the following information: Packers Used in Locating Source of Water.—" Packers run into wells on tubing or casing and set in casing or formation are used to good advan tage for testing source of water. The packer and casing act in the same manner as a cement bridge, and stop or retard the passage of fluid past the point at which the packer is set. The subsequent testing by bailing or pumping is done below the packer, whereas in the case of a bridge the reverse is true. In other words, the packer eliminates the producible fluid from above and the plug or bridge eliminates the producible fluid from below.