will usually force out the mud, and the oil, gas, or water will enter the hole again; but as long as a sufficient column of fluid remains in the hole the contents of the sealed formation can not enter the hole. It is believed that the efficiency in sealing off the porous formations in a well depends more upon the mud forced into the pores of the formation and retained by the weight of the column of fluid than upon the mud plastered on the walls of the hole, although the mud coating probably aids in protecting the walls from caving.

"When a well has been treated with mud fluid the contents of each formation is confined to its original stratum, so that there can be no movement of oil, water, or gas either from the sands into the well, from the well into the sands, or from one sand into another. Thus waste and intermingling are prevented, corrosive waters can not reach and attack the casing, and the strata are entirely sealed off from each other as they were before the well was drilled.

“Mud fluid, besides preventing caving, as stated above, is also an aid in keeping loose sands from entering the hole. The fluid clogs up all pores or crevices, and makes a solid wall which the weight of the fluid in the hole will hold up. Furthermore, the mud which has entered the formations, or is plastered on the walls, protects them from contact with air and water, which would cause slaking and caving. The fluid is especially helpful in drilling through a loose sand that otherwise would run into the hole and make drilling difficult.

“The mud-laden fluid may be prepared from clay obtained from surface deposits or from material derived from drillings. Ordinarily there will be enough clayey or shaley material in the formations encountered in the well to provide all the fluid necessary. This has been found true both in drilling with rotary tools and with cable tools. Drillings from sandstones and limestones should not be allowed to enter the slush pit. The mud fluid can be mixed and prepared in a few hours by ordinary unskilled labor whenever it is desired.

"Settling out sand, limestone cuttings, etc., in order to avoid freezing of casing and of tools, is important."

FIELDS

CHAPTER VIII

CASING METHODS-CASING USED IN VARIOUS COLLAPSING PRESSURES SAFE LENGTHS OF STRING-CASING EQUIPMENT.

Casing (steel or iron pipe, usually with finer, or more, threads per inch than those used on ordinary pipe) is used in nearly all oil and gas wells for the following purposes: Shutting off water.

Casing off running sand and caving formations.

Passing through caverns and workable coal measures and mines.

Shutting off intermediate oil or gas bearing strata when it is desired to drill deeper.

"Oil string" for casing through caving oil sands.

The shutting off of water is the chief and the most important purpose for which casing is used. The process consists of setting a string of casing in an impervious formation, preferably shale, at a point in the well below the lowest water bearing formation and above the oil or gas bearing sand, the object being to exclude the water from the productive sands.

The water in the stratified rocks presents many problems to the oil and gas operator, is the cause of much expense in drilling, and when careless or unintelligent methods of shutting it off are employed, may be a menace not only to his own property, but to the properties of his neighbors. Refer to Fig. 159.

In the early days of oil and gas development of the Eastern fields, scant attention was given to the casing of the well, perhaps largely for the reason that the problem in those fields was comparatively simple. The rock formations stood up and usually there were thick beds of hard impervious shale below, water bearing formations to provide a tight seat for the casing. The casing was simply lowered in the reduced

hole provided for it, perhaps driven a few inches to set it, and a few shovelsful of sand pumpings poured down outside to pack it.

Casing shoes and packers were not used and collapsing pressures were considered only in a rule of thumb way. If the casing collapsed the operator put in another string, of heavier weight if he could secure it. Unquestionably many strings of casing have been put in wells in the fields of Eastern United States where the collapsing safety factor was much less than two and in some cases it was little more than one-that is practically nil.

The lighter weights of casing that served the purpose in the Appalachian fields were found totally inadequate for the long strings needed in the fields of California. Pipe manufacturers, to meet these requirements, began making better and heavier casing, until today we have 61⁄4-inch casing, for example, in varying weights of from twelve pounds to twentyeight pounds per foot, the excess weight all being added to the inside, thus reducing the inside diameter of the heavier weights. This adds somewhat to the driller's problems when putting in smaller strings of casing within a next size larger or in running fishing tools.

The heavy California weights of casing have become standard in the fields of Wyoming, North Texas and in foreign fields.

Perhaps the first attempt to pack casing that could not be made tight was by means of the seed bag, a cotton bag of flaxseed or small grains which, when saturated with water, would expand and seal the bottom of the casing.

The several types of packer next were developed and they have come into general use in the fields where the rock formations are sufficiently hard to provide a firm support for the packer and where caving will not defeat its purpose. Now it is the custom in the fields of Kansas and Oklahoma to use a heavy casing shoe on the bottom of each of the outside strings of casing and a bottom hole packer or anchor packer