TESTING THE CONDITION OF OIL WELLS At various stages in the progress of drilling an oil well, it is necessary, if good work is desired, to test the mechanical condi tion of the well and casing.
Bailing Test.—The exclusion of water from an oil well is of prime importance. Tests designed to prove that water has been excluded will go far toward proving that the well has been properly drilled. Obviously the best test is to remove all water from the well and see if any more comes in. Sometimes condi tions are so complex that such a simple procedure is not feasible. The following rules have been evolved from several thousand tests.
1. Measurements to the bottom of the hole and to the bottom of the casing shoe must be carefully checked before the casing is landed or cemented. A steel tape should be used in determining the distance that the sand line or drilling line "measures over." 2. Casing must be tested by bailing the well to a safe depth (see collapsing strength of casing, page 43) before drilling below the shoe. Old casing may collapse with less pressure than that indicated for new casing. Testing by applying pump pressure inside the casing will not always reveal leaks.
3. The removal of cement or other material in the casing by drilling must be carefully done so as to avoid damage to the shut off. The drill must be run merely far enough to go through the cement and below the shoe. A distance of from five to ten feet below the shoe would be ample. By drilling too far below the shoe complications may arise which will render a positive test impossible.' 4. Bailing should continue until all fluid is removed from the hole, unless there is danger of collapsing the casing. It is advisable to run the bailer until it brings up nothing but mud, and then after waiting an hour or more, to give the water which was accu mulated on the inside of the casing time to run down, again apply the bailer. When a well is in such a condition that it can not be safely bailed dry, the fluid should be lowered to a certain depth, (depending upon pressure and strength of casing), by continu ously running the bailer to that depth until no more fluid is brought up. A permanent target should be placed on the line
to mark this bailing point. If both oil and water are present in a well which can not be bailed dry, it may be necessary to remove only the water by operating the bailer until it fails to bring up water. The bailing should then be discontinued for several hours, and again resumed to increasing depths until the point where it again picks up water is determined. At this point the well is ready for inspection, and the water may after wards be removed by bailing from the bottom of the well, a record being kept of the amount bailed out. It may be necessary to repeat this process several times in order to determine whether the water is being exhausted. In the case of a high-pressure flow of gas, or in the case of heaving formation, it will probably be possible to test the well by pumping only.
5. When a tight or closed bailer is used in a deep well, some sort of valve or other outlet should be provided to relieve the gas pressure which may otherwise burst the bailer and endanger the lives of persons in the derrick.
A leaky bailer should never be used in testing.
A proper test to determine whether or not a sand stratum carries oil or water can not be made unless water from all other possible sources is excluded.
When gas is flowing from a well in large quantities and under great pressure, it is sometimes difficult to determine whether or not water has been properly shut off. It has frequently been assumed that a constant flow of dry gas proved that the water had been shut off. There is, however, a possibility that the gas pressure may be great enough to hold back water which would otherwise enter the well. When the flow of gas is confined to a comparatively small outlet it will sometimes be entirely free from water while if the gas flow is entirely unobstructed very large quantities of water may accompany it. In such a case a final test must be postponed until the well is completed and its general condition noted for a considerable period of time.