If strata are known to exist which should be protected against infiltrating water or escaping gas, a well must be abandoned in such manner as to practically replace the impervious strata which have been penetrated. In other words some sort of impervious plugs must be placed in the well at points between the porous strata which are to be protected.
The easiest method of plugging a well is to fill it with mud fluid which may fill and seal all porous formations. Evidence is not plentiful to prove that this method is thoroughly effective, although some state laws approve it.
There are two methods of plugging a well with clay or mud; first, by mixing the clay with water in the well, and second, by mixing the clay and water at the surface and introducing the mixture into the well.
In describing the two methods G. McGregor says:' "In the first method cable tools are used, being allowed to swing at the particular spot where it is desired to mud. The clay is then shoveled dry, or slightly moistened, into the top of the casing along with a con stant stream of water. The water serves the double purpose of prevent ing the clay from "bridging" when it strikes the fluid, and also to a certain extent increasing the head of water in the casing, thus helping to carry the clay outside the casing and into the formation. The action of the tools swinging in the open hole is similar to the action of rotary drill-pipe, and as the clay settles down around the tools it is pounded back into the formation. Care must be taken that the tools do not
become "mudded up" or that circulation does not stop while clay is being put in." With the second method, various mechanical devices are used, such as a concrete mixer, to mix the clay and water. Some kinds of clay may be mixed sufficiently by merely throwing a strong stream of water against the clay bank.
Where old records are not complete and it is desired to plug wells in order to prevent water from entering them, mud has been found useful. Its use makes it possible for the tools to enter caving sands which could not otherwise be penetrated, and fur thermore, the flow of water into the well is at least checked until some sort of solid plug can be put in place. It is necessary to reach the original bottom of the well and to split or destroy the casing so that the mud will have free access to the walls of the well.
Mud is sometimes forced into a well under a pump pressure of several hundred pounds per square inch, either through tubing or directly into the casing, fittings being provided at the top of the casing so that pressure can be applied to the mud. The mud is forced down until no more can be put in. In some cases circulation will be established to the surface, outside the easing, while in other cases no such surface evidence appears.
The following rules have been applied to many abandonments with a view to affording protection which could be tested to some extent.