Drilling mud
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Drilling mud, also called drilling fluid, is a lubricant used while drilling oil and natural gas wells and in exploration drilling rigs.
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[edit] Purpose
The five primary purposes of drilling mud or drilling fluids are to:
- Remove cuttings from the formation produced by the bit at the bottom of the hole and carry them to the surface. This is achieved by adjusting the rheology of the mud system.
- Lubricate and cool the drill bit during operation as friction causes high temperatures down-hole that can limit tool life and performance. This is particularly important if the bit is of the roller-cone type.
- Maintain hydrostatic equilibrium so that fluids and gas from the formation do not enter the well bore causing the well to flow, kick or blow out. This is achieved by adjusting the mud weight (density). High-density additives (barite, hematite) are used for preparation of kill-weight fluids, which create hydrostatic pressure that prevents water entering the well or hold the oil/gas inside, prevent a blowout and to physically stabilize the formation. The exception to this role is when drilling an underbalanced well.
- Build a filter cake on the hole wall, preventing fluid loss in mud invasion of penetrated formations.
- Support and prevent caving of the wall of the hole.
Other characteristics are considered important in modern drilling. Some of these include:
- Safe for the environment
- Prevent dispersion of reactive clays (gumbo)
- Ability to seal formation fractures/voids
- Non abrasive to tools and rig equipment
Offshore mud systems run in pressure from 5,000 to 7,500 psi.
[edit] Details of Use
On a drilling rig, mud pumps through the drill string where it sprays out of nozzles on the drill bit (cleaning the bit in the process), the mud then travels back up the annular space between the drill string and the sides of the hole being drilled, up through the surface casing, and emerges at the surface. Cuttings are then filtered out at the shale shaker and the mud enters the mud pits. The mud is then pumped back down and is continuously recirculated. The mud is treated periodically in the mud pits to give it properties that optimize and improve drilling efficiency.
[edit] Composition of drilling mud
Water-based drilling mud may consist of bentonite clay (gel) with additives such as barium sulfate (barite), calcium carbonate (chalk) or hematite. Various thickeners are used to influence the viscosity of the fluid, eg. Xanthan Gum, guar gum, glycol, carboxymethylcellulose, polyanionic cellulose (PAC), or starch. In turn, deflocculants are used to reduce viscosity of clay-based muds; anionic polyelectrolytes (eg. acrylates, polyphosphates, lignosulfonates (Lig) or tannic acid derivates such as Quebracho) are frequently used. Red mud was the name for a Quebracho-based mixture, named after the color of the red tannic acid salts; it was commonly used in 1940s to 1950s, then was obsoleted when lignosulfates became available. Many other chemicals are also used to maintain or create some of the properties listed in the section titled "Purpose".
One classification scheme for drilling fluids is based on their composition, and divides them to
- water-base,
- non-aqueous or oil/synthetic based (oil, olefin, or other synthetic fluid) base, and
- gaseous, or pneumatic.
Note:Oil Based and Synthetic Based muds are frequently classified separately due to the vast differences in regulations when using them.
[edit] Mud engineer
The slang name given to an oil field service company individual who is charged with maintaining a drilling fluid or completion fluid system on an oil and/or gas drilling rig. This individual typically works for the company selling the chemicals for the job and is specifically trained with those products, though independent mud engineers are still common. The work schedule of the mud engineer or Drilling Fluids Engineer, as he or she is more properly called these days, is usually fairly strenuous, as are most jobs in this industry. Until a few years ago, the "mud engineer" rarely worked a set schedule and, if resident on an offshore installation, may have been on call for 24 hours a day, with few (if any) days off each month. With the advent some 15 years ago in Northern Europe, of having two mud engineers offshore due to Health, Safety and Environmental regulations and working hours restrictions in more advanced countries, the offshore mud engineer rarely works more than the normal 12 hour shift. On land, however, there usually still is only one mud engineer assigned and most of the time, he is allocated to more than one drilling rig. The economics of land-drilling demands that the land-engineer spends a greater part of the day driving from rig to rig, testing the drilling or completion fluids, making recommendations for its maintenance and then repeating the process at another rig(s). A daily stop like this, usually for an hour or two, is typically called a "Drive-By". A 24 hour assignment to a single land rig is called a "sitting" job.
Offshore drilling, with new technology and high total day costs for the operation, have wells being drilled extremely fast and day rates for operations have increased. Any down time is frowned upon and having two mud engineers makes economical sense to sensible oil companies, to prevent down time due to drilling fluid difficulties. Two mud engineers also reduce insurance loading to oil companies for possible environmental damage that oil companies are responsible for during their license to drill and produce.
The cost of the drilling fluid is typically about 10% (may vary greatly) of the total cost of well construction. This large cost overhead places a demand on the competency of the mud engineer. Large cost savings can result when the mud engineer adequately performs his job.
The mud engineer is not to be confused with a mudlogger, a service personnel who monitors gas from the mud and collects wellbore samples.
[edit] Compliance Engineer
The compliance engineer is the most common name for a relatively new position in the oil field, emerging around 2002 due to new environmental regulations on Synthetic Mud. Previously synthetic mud was regulated in the same way as water based mud and could be disposed of into offshore waters as needed due to its low toxicity to marine mammals. New regulations restrict the amount of synthetic oil that can be discharded as a percentage by weight of synthetic fluid on cuttings that have been drilled and are being discharged overboard. For olefin based fluids the limit is 6.9%, for ester based fluids it is a bit higher. These new regulations created a significant amount of additional work in the form of tests needed to determine the "ROC" or retention on cuttings, sampling to determine the percentage of crude oil in the drilling mud, and extensive documention to substantiate all of this.
A new monthly toxicity test is also now performed to determine sediment toxicity. The species used is Leptocheirus plumulosus picture. Various concentrations of the drilling mud are added to the environment of the Leptochirus plumulosus to determine its effect on the animals. This is controversial for two reasons:
- These animals are not native to many of the areas regulated by them, including the Gulf of Mexico
- The test has a very large standard deviation and samples that fail horribly may pass easily upon retesting
