Drilling fluid is used anytime a hole is drilled deep into the Earth. This includes oil drilling, gas drilling, water drilling, and exploration drilling. It is used because it helps provide hydrostatic pressure which keeps unwanted fluid from entering into the hole. Two additional functions of drilling fluid are to keep the bit clean and carry drilled cuttings out of the hole. The ingredients and chemical additives that are found in drilling fluid can vary significantly according to the type of formation being drilled. In all, drilling fluid is a critical part of the drilling process that needs to be managed by Mud Engineers.
The drilling fluid must be properly engineered so that it moves the drill cuttings out of the bore hole. For example, it must be able to carry the cuttings up the hole but also keep them suspended while the drill is not in motion. To do this it must be able to become a gel-like substance when it is not circulated by the drill's movement. This allows the cuttings to "float" in the drilling fluid while the drill pipe is lengthened or while maintenance is carried out. If any of the drill cuttings fall to the bottom of the bore hole, it can result in stuck pipe and dramately slow down the drilling process. Also, the longer it takes for the mud to remove the cuttings from the hole the more they break up and become more difficult to remove.
The formation's pressure is another thing that the drilling fluid controls. For example, if the pressure in the formation increases too much, barite is typically added to the drilling fluid to increase its weight. However, if the mud weight becomes too high it can cause instability and create fractures in the formation. Blowouts are caused by unbalanced pressures in the well. In addition to maintaining formation pressure, correctly managed hydraulic pressures help to manage instability caused by shifts in the Earth's tectonic plates.
When the drilling formation is porous, the mud is used to create a seal. This seal is called a filter cake and is made when the pressure inside the well is greater than the formation pressure. A thin layer of drilling fluid then sees into the formation and creates this thin filter cake. If the filter cake becomes too thick, it can cause a tight bore hole, stuck pipe, damage to the formation, or loss of circulation. When the formation is so porous that the mud actually flows into it instead of forming a thin deposit, other steps must be taken. This means using "bridging agents" which are often calcium carbonate or ground cellulose to block the pore spaces before the mud can form a filter cake. Certain substances can also help with the quality of the filter cake. These include bentonite, polymer, asphalt, and gilsonite.
Along with sealing the bore hole, the mud also helps to ensure well stability. It does this by offsetting the mechanical forces at work with its weight. If the well becomes unstable it can cause unusual hole conditions. When the hole gets too enlarged, it can become weak and unresponsive to attempts at stabilization. This can result in low annular velocities and inefficient removal of drill cuttings. When drilling in porous formations such as sand or sandstone, a well-formed filter cake can help to prevent hole enlargement. The hole's enlargement can also be kept in check through moderate usage of hydraulics and nozzle velocities. While drilling in shale formations, the bore hole is usually much more stable and the mud weight is all that is needed to keep the hole's diameter constant. However, when dealing with water based muds there can be a reaction between the shale and the mud which can cause the shale to become softer. Fractured or brittle shale formations can cause extensive mechanical problems while drilling. Potassium, calcium, glycols, polymers, asphalt, salt, and oil can be used to control the reactions between water based muds and shale formations. Since water based mud and shale can have such complicated reactions, most drillers will use an oil based or synthetic mud when drilling in shale.
In addition to its other purposes, drilling fluid also lubricates and cools the drill bit. The heat in question is caused when the bit rubs against the formation and when the pipe rotates against the casing. Mud also reduces the amount of drag that affects the drilling assembly by improving the coefficient of friction. Oil based and synthetic muds are better at this than water based muds which is why they are used more in areas where the drilling is found to generate higher friction levels due to the type of formation being drilled into.
The heavy weight of the drill string can also be partially supported by mud. The buoyancy created by the density of the mud helps to hold up the drilling equipment which reduces the "hook load" on the derrick. This can be very valuable when the depth of a bore hole requires more pipe than the derrick can hold up. Part of the drill string's weight is supported by the drilling fluid which allows deeper drilling to take place since the weight capacity of the derrick.
Mud is also instrumental in the analysis of the drilling situation. The cuttings brought up by the mud's circulation can be examined to further improve drilling technique and determine the depth from which the cuttings are coming from. However, continuous friction between the drill string and mud can increase corrosion. Corrosion can also be sped up by oxygen in the mud which results in aeration or foaming. Drilling fluid is an essential and unavoidable part of drilling but it is not without its complications.