Drilling Mud Performs Many Different Jobs

One purpose of drilling fluid, or drilling mud, is to provide hydrostatic pressure.  

This pressure prevents formation fluids from entering the well bore. 

The deeper the well is drilled, the higher the formation pressure.

As a well bore gets deeper, more fluid from the formation is wanting to seep into the well bore.  

A well bore that is 10 feet deep does not have much formation pressure.  However, a well bore that is 10,000 feet deep has multiple times the formation pressure.

Think of it like this:  The deeper in the water you dive, the more pressure from the water all around you.  This analogy is not 100% concise, but it's an easy way to think of formation pressure. 

Depending on the formation, the drilling crew will have to "mud up".  Failure to "mud up" at the right intervals can have serious consequences to the operation.

Various Purposes of Drilling Fluid

1. Create wallcake.
2. Clean hole and cool bit.
3. Maintain wellbore stability.
4. Provide hydraulic energy to bit and drilling tools

Mud that is too thick will inhibit the drill bit from drilling at a high rate of penetration.  Too thin can result in a "kick" or "blowout".  Kicks and blowout can damage equipment, not to mention kill personnel.  Therefore, mud weight has to be just right.

Drilling fluid also keeps the bit cool.  Without drilling mud, the drill bit would overheat and burn up long before it should.

It's also used to clean the hole.  As rock formations are crushed and broken by the pdc or tricone bit, these cuttings need to be cleaned out of the way.  If they're not, the rock drill bit will just continue to crush and grind the same cuttings over and over.  

Additionally, as the bit does make headway in drilling the hole, the cuttings will become compacted around the bit and drill string, eventually bringing a halt to the entire operation.

Drilling mud also transmits hydraulic energy to the drill bit and drilling tools.  This energy provides energy to the mud motor, which in turn rotates the bit.

When drilling operations have stopped, such as when coming out of the hole or adding a new section of drill pipe, cuttings need to be suspended and held in place rather than allowed to fall back to the bottom of the hole.  Drilling fluid accomplishes this feat because it gels when static.

Drilling mud maintains wellbore stability.  The wellbore is the inside diameter of the drilled hole.  

When drilling in permeable formations, wall cake will form on the well bore.  Wall cake is drilling fluid that has permeated the porous formation, leaving behind a residue.  Wall cake is needed, in just the right amount, to maintain well bore stability.  

Types of Drilling Mud

Air - Air is pressured either down the annulus or drill string.

Air and water - Water is mixed with air to control dust, increase viscosity, or to cool the bit.

Water - Sometimes plain water without chemicals is used as the drilling fluid.

Water based mud - Special chemicals are added to water to create a slurry thicker than water but not as thick as syrup. Water based mud is a liquid but will gel when static.

Oil based mud - A product that is based on a petroleum product, such as diesel fuel. This type of mud provides greater lubricants and doesn't break down as easily. However, it's more costly and creates more environmental concerns.

Synthetic based fluid - Based on synthetic oil. This type of drilling fluid is most often used in offshore drilling rigs.

Related Pages:

Bakken Shale:  The Bakken Shale was at one time considered to be a marginal find because the oil was trapped in such low permeable shale.

Deepwater Horizon:  Transocean's Deepwater Horizon burned for two days and then sank - an equipment loss of over $200 million.

Directional Drilling:  Directional drilling is common practice in shale gas plays such as the Bakken and Barnett Shale, and the selection of drilling tools is critical.

Drilling Mud:  Drilling mud can be oil, air, or water based.  It cools and cleans the bit, stabilizes the well bore, and creates wallcake.

Exxon Valdez:  The effects of the Exxon Valdez oil spill are still being felt almost 30 years later.

Natural Gas Drilling:  Natural gas drilling looks to continue because the natural gas reserves of the United States are large enough to fuel our needs for many years to come.

Oil Well Drilling:  Oil well drilling requires many pre-drilling decisions, such as whether to reverse circulate and which type of drilling mud to use.

Shale Oil:  Shell Oil Company's in-situ conversion process requires a "freeze wall" around the outside perimeter of the shale oil extraction zone to prevent groundwater contamination. 

Spindletop:  The oil revolution began when Spindletop gushed black gold 150 feet in the air for nine days in 1901, and Howard Hughes invented the roller cone bit.

Tar Sands:  Tar sands are a black, tar-like combination of clay, sand, water, and bitumen.  They are sometimes called "oil sands" to make them seem more appealing.

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