One of the objects in cementing is to have all the cement pos sible between the wall of the hole and the casing. To insure this several methods have been employed. Two of the principal methods are described briefly.
Perkins the Perkins method two plugs are used. (see B, Fig. 81). A wooden plug is dropped into the casing. Cement is pumped above this plug, which is driven to the bottom of the casing. The bottom of the casing is a few inches off bot tom. A special shoe is used on the bottom of the casing.
When all the cement is in the casing, a second plug is dropped inside, and water is pumped in above this plug. The pressure of the water drives the second plug downward upon plug 1. When plug 2 reaches plug 1, all the cement is out of the hole. Also the back-pressure on the pump shows that the second plug has reached bottom, so the pump is stopped and the casing is lowered to bottom. Later the soft wood plugs are readily drilled up by the bit when drilling is commenced.
Tubing second method of cementing commonly used is the tubing method. In this case tubing is put in the hole inside the casing. This string of tubing extends to within 5 or 6 ft. of bottom. A top packer is placed between the tubing and casing, so that when water or cement is pumped into the hole, circulation of water takes place outside the casing.
Cement is pumped through the tubing (see Fig. 82). When all the cement has been put in the hole, water is pumped in above the cement. A sufficient time for all the cement to reach bottom and collect outside the casing is allowed for.
The casing is next lowered to bottom, the tubing withdrawn, and the cement allowed to set.
Cementing methods •consistently followed will give good results and insure safety from water. Cement not only acts as a plug to shut off water, but also protects the casing somewhat from direct contact with acid waters that eat it up.
In some California oil fields the casing is badly eaten by the waters in the wells. In some fields casing will not last over 3 years, due to the corroding action of the water. This means either expensive replacement of casing or the flooding of a well by infiltrating water.
"Mudding Off" Oil methods are used in many places where cement might be used. It consists of pump ing a mixture of mud-fluid into the hole behind the casing, or putting it in with a bailer. This mud-fluid has different prop erties than water or mud.' When properly made the mud particles will not settle in the hole and "freeze" the casing. The fluid will effectually seal off water, as water cannot penetrate it. Mud-laden fluid has a specific gravity of from 1.15 to 1.3, and weighs from 72 to 81 lb. as against 62.5 lb. per cubic foot of water. It exerts a pressure of 0.499 to 0.564 lb. per square
inch as against 0.434 for pure water. Some oil-field waters have specific gravities of 0.4 to 0.50, so the specific gravity of the mud-fluid must vary accordingly.
Mud-fluid may even take the place of casing and has the advan tage in that when properly made it does not settle and bind the casing. As a consequence, casing may readily be pulled from the well 5 or 6 years later. Another advantage is the fact that the mixture protects casing from the corrosive action of oil-field waters. Also drilling may be carried on through the fluid. Mud fluid is made by mixing pure clay or shale with water until a creamy consistency is obtained. Tests can be made to determine the proper specific gravity.
Mud-laden fluid may be pumped into a well just like cement, using either the tubing method, or it may be placed with a bailer. Figure 83 illustrates the use of mud-fluid in shutting off sands.
In rotary or in circulator drilling it is simpler to handle the mud-fluid than with the cable-tool system, as the pipe mix ing facilities are already in use. Instead of tubing the drill pipe is the medium used for introducing the fluid.
Another use for the mud-fluid is in overcoming a flow of gas during drilling. In numerous cases high-gas flows check drilling as the tools will not go through the gas which exerts sufficient pressure to force the tools out of the well. In most cases this can be remedied by the following methods: Lubricating. — If a control head is on the well the gas may be shut in or greatly reduced. If a control head is not used, put a gate valve on the casing. This may call for several days' work with a heavy gasser but by leaving the gate valve open so the gas may go through it, and by careful manipulation, the well may be "capped." To enable further drilling, the gas flow must be "killed." This is done by lubricating. A joint of casing is screwed on top the gate valve (Fig. 84, page 192). This joint of casing has a top valve to which are connected fittings so that mud-fluid may be pumped into the joint of casing. The joint of casing is filled with mud-fluid and the top valve closed. The bottom gate valve on the casing i then opened slowly. The gas pressure on the mud is quickly equalized, and the mud falls to the bottom of the hole. When the top joint of casing is empty, the bottom valve is closed, the top opened, and another batch of mud-fluid is pumped hi. This procedure is continued until the gas is killed, which is not until the pressure is overcome by the mud. In some cases filling the hole with water will kill the gas, but at times the water pressure is insufficient to offset the gas pressure and mud-fluid must be used.