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 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 with roller-cone bits.
- 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 from entering the well or hold the oil/gas inside, prevent a blowout and to physically stabilize the geologic formation. The exception to this is when drilling an underbalanced well.
- Build a filter cake on the hole wall, preventing fluid loss by 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 (345 to 517 bar).
[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 more properly Drilling Fluids Engineers, is arduous.
In offshore drilling, with new technology and high total day costs, wells are being drilled extremely fast. Having two mud engineers makes economic sense to prevent down time due to drilling fluid difficulties. Two mud engineers also reduce insurance costs to oil companies for environmental damage that oil companies are responsible for during drilling and production.
The cost of the drilling fluid is typically about 10% (may vary greatly) of the total cost of well construction, and demands competent mud engineers. Large cost savings result when the mud engineer performs adequately.
The mud engineer is not to be confused with mudlogger, service personnel who monitor gas from the mud and collect 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 the same as water based mud and could be disposed of in offshore waters due to low toxicity to marine mammals. New regulations restrict the amount of synthetic oil that can be discharged. These new regulations created a significant burden 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.
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
