Exclusion of Water from Oil Wells.—The damage caused by the entrance of water into oil wells is so well recognized that it is deemed unnecessary to dwell upon that feature to the extent of citing examples. However, it appears that many experienced oil operators do not yet realize that the damage can be avoided, or even remedied; therefore consideration of precautionary and remedial measures is clearly worth while.
Damage to oil fields by water arises from the fact that when water enters a porous stratum saturated with oil, it tends to displace the oil. If the entrance of water is at or near an oil well it frequently drives the oil away from the well, which then produces only water. When this condition continues along the strata, neighboring wells are affected in a like manner.
The entrance of water into an oil-bearing stratum in the drill ing of wells is due to two primary causes; first, incomplete knowledge of geological conditions surrounding the well; and second, faulty mechanical conditions existing in its drilling or maintenance. Many combinations of these two causes exist, and a general idea of them can most easily he gained by the examination and consideration of the sketches shown in Figs. 4 to 13. These sketches merely illustrate a few typical cases, and do not cover all the various complications due to geological and mechanical conditions.
Damaging water conditions are frequently accounted for by the statement that all the oil has been exhausted from a stratum, and therefore water naturally replaces it. Another frequent explanation is that the oil and water have both collected naturally in the same underground reservoir, and therefore the production of clean oil is impossible. Neither of these explana tions should be accepted until a careful and exhaustive examina tion has eliminated all other possible explanations.
Definitions of the various terms used herein in the control of water in oil wells can be most readily understood by reference to the sketches mentioned above.
Formation Shut-off.—The landing of casing in the walls of the well in such a manner as to prevent the passage of water without use of cement, as illustrated in Fig. 4.
Water String.—The casing placed in the well primarily for the purpose of preventing water from flowing from its original position to other strata below the shoe of the water string, as illustrated by the casing of larger diameter in Fig. 5.
"Top Water."—The water found in strata lying above the productive oil formation penetrated by a certain well, as illustrated in Figs. 4 to 10, inclusive. The term is indefinite and should be avoided by using a statement specifically mentioning the depth at which the water occurs.
"Bottom Water."—The water found in strata lying below the productive oil formation penetrated by a certain well, as illus trated in Fig. 11. The term is indefinite and should be avoided as mentioned in the foregoing paragraph.
Intermediate Water.—The water found in strata between productive oil formations penetrated by a certain well, as illustrated in Fig. 12.
Edge Water.—The water found in a sand which contains oil at a higher elevation or, in other words, farther up the dip. Illus trated in Fig. 13.
Water can in most cases be excluded from an oil well while it is being drilled, if sufficient care and attention are given to the matter. The old adage, "an ounce of prevention is worth a pound of cure," is particularly applicable to this problem. The foregoing pages have outlined methods for gathering pertinent. facts, and the next chapter shows how they can be assembled in useful and intelligible form. The mechanical methods involved in actually excluding water from a well must always be governed by the existing underground conditions many of which can only be surmised from the facts observed.
Methods of Excluding Water.—In most oil fields some water is found in the formations overlying the oil sands. There are a number of methods in use for excluding this water from the wells, and of course local conditions must be taken into account in the choice of the method used in any particular case. Water in shallow wells is often successfully handled by landing the casing in impervious strata, but in deep drilling in California hydraulic cement is used, in the majority of cases.
Formation shut-off is the term applied to a shut-off where cas ing is landed in a bed of shale or clay without using cement or other device to plug the space between the casing and the wall of the hble. In this operation much depends upon the kind of cas ing-shoe which is used. It should be a plain shoe, as distin guished from the notched Baker shoe and from the types used in rotary drilling. A shoe 14 to 20 in. in length is usually used, although in a few cases in deep work shoes from 6 to 20 ft. in length have been successfully used. It is important not to have the largest diameter of the shoe at the bottom in order that it can be driven tightly into the formation. When a suitable shale or clay is reached where it is desired to land the water string, the hole is drilled ahead with a smaller-sized bit and the casing is driven until the shoe is tight in the formation. This will prevent the water from following down the outside of the casing and entering the well below.