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Drilling Fluid: Lifeblood of the Well

In 1900, while drilling an oil well in Spindletop, Texas, workers ran a herd of cattle through a pit filled with water. The mud that resulted, a viscous, muddy slurry of water and clay, was pumped into the borehole. Drilling fluids are still called mud, but engineers no longer rely only on water and clay. Instead, they carefully design compounds and mixtures to meet specific needs under various drilling conditions. Modern drilling fluids are truly the lifeblood of the well. Today's deep wells could not exist without them.
         
Drilling fluids are referred to as mud.
You can see why.

History
Long ago, people were generally drilling for water, not for oil. In fact, they were annoyed when they accidentally found oil since it contaminated the water!
Early wells were drilled for water to be used for drinking, washing, irrigation and for brine, which is used as a source of salt. It was only in the 19th century that drilling for oil became widespread as industrialization increased the need for petroleum products.
The earliest records of well drilling date back to the third century BC in China. The technique - Cable Tool Drilling - involved dropping a heavy metal drilling tool and removing the pulverized rock with a tubular container. The Chinese were relatively advanced in this art and are credited with the first intentional use of fluids in the process of drilling. In this case the fluid was water. It softened the rock making penetration easier and aided in the removal of the pieces of pulverized rock known as cuttings. (It is important to remove the cuttings from the borehole so the drill bit is free to dig further.)


Rotary drilling from an offshore rig

In 1833 a French engineer named Flauville was watching a cable tool drilling operation in which the drilling apparatus struck water. He realized that the gushing water was very effective in lifting the cuttings out of the well. The principle of using moving fluid to remove cuttings from the well bore was established. He conceived of an assembly in which water would be pumped down the inside of a drilling rod and carry cuttings with it as it returned to the surface in the space between the drilling rod and the wall of the well bore. This procedure remains standard today.

Rotary Drilling has largely replaced Cable Tool Drilling. With this technique the drill bit is at the tip of a rotating pipe. The process is similar to that used with a hand-held electric drill or auger that you might use to drill into a piece of wood. But instead of drilling a few inches or centimeters into wood, modern oil wells may reach thousands of feet or meters into the ground. When drilling wood the cuttings are cleared out of the hole via spiral grooves along the shaft. This works for a shallow hole, but not a deep well. Instead, the cuttings are carried up to the surface along with the circulating mud.
As wells get deeper, drilling fluids have taken on increased importance, serving a number of purposes and solving a variety of problems that vary greatly from place to place.                    
The Many Roles of Drilling Fluids
The harsh environment in underground drilling operations encouraged the research and development of drilling fluids that can fill several crucial roles in the drilling process: suspension, pressure control, stabilization of formations, buoyancy, lubrication and cooling.
Suspension
The flow of drilling fluid down the drill pipe and up the borehole sometimes stops, either because of a problem, or in order to raise the drill pipe up and out of the hole to allow the bit to be changed. When drilling stops the cuttings suspended in the fluid can sink to the bottom of the hole jamming the drill. Drilling fluids are designed to have a very interesting property that takes care of this problem. The thickness, or viscosity, of the fluid increases as movement of the fluid slows. When the fluid stops moving it forms a thick gel that suspends the rock cuttings and keeps them from sinking to the bottom of the borehole. When the fluid starts moving again it becomes thinner and reverts to its previous thin, liquid form. It is called the Thixotropic Property  of drilling mud.

Pieces of rock are scraped free by the rotating drill bit. Drilling fluid is pumped down the drill pipe, picks up these cuttings and brings them to the surface.

Pressure Control
There is a popular image of oil gushing from a rig, high into the sky, while workers rejoice at having found oil. Actually, such blowouts are rare and no cause for celebration since the goal is to extract the oil in a controlled manner. Mud is designed to prevent such accidents by counteracting the natural pressure of fluids in rock formation. A proper balance must be achieved in which the pressure of the drilling fluid against the walls of the borehole is enough to counter the pressure exerted by both rock formations and by oil or gas, but not so much that it damages the well. If the weight of the drilling fluid is too great it could cause the rock to fracture and the drilling fluid would be lost into the ground.
The pressure of a liquid depends on its density. Weighting agents may be added to the drilling fluid to increase its density, and thus, the pressure it exerts on the walls of the borehole. The density of the liquid may be adjusted to meet the conditions in the well.
Stabilization of the Exposed Rock Formation
There are two phases of the drilling process. At first drilling proceeds through rock that does not contain oil. The goal is to move as quickly as possible and get to the oil-bearing rock - the reservoir. The priority is on keeping the exposed rock formation in the borehole stable while avoiding the loss of drilling fluid. By maintaining drilling fluid pressure above rock formation pore fluid pressure, there is a natural tendency for the drilling fluid to enter permeable rock in the formation. With special additives in the drilling fluids this can be prevented.

The drilling fluid may interact with the surrounding rock in other ways. For example, if the rock is laden with salt, water will dissolve the salt and tend to make the walls of the borehole unstable. An oil-based fluid would be better in this situation. Rock formations with a high clay content also may tend to be washed away by water. Such formations require an inhibitive fluid to maintain a stable wellbore and prevent enlargement, or wash outs. As drilling progresses, the wellbore is lined with a steel casing which is cemented in place to provide both wellbore stability and a route to the surface for oil when the reservoir is reached. After reaching the reservoir the composition of the drilling fluid may have to be changed to avoid clogging the pores of the rock. Keeping the pores open will allow oil to flow more freely into the borehole and up to the surface.
Buoyancy
A well may be many thousands of feet or meters deep. A steel drill pipe of such great length weighs many tons. Immersing the drill pipe in fluid produces a buoyancy effect, reducing its weight and putting less stress on the drilling mechanism.
Lubrication and Cooling
When metal moves against rock there is friction and heat. Drilling fluids provide lubrication and cooling to keep the process moving along smoothly, and to extend the life of the drill bit. Lubrication may be especially important on extended reach or horizontal wells where the friction between the drill pipe, drill bit and rock surfaces must be kept to a minimum.


Suspension of Cuttings in static borehole: Thixotropic Property
(special Rheology property)

It is an interesting and unusual property of the oilwell drilling fluid (mud). It can change form - from thick to thin. This allows the drilling fluid to suspend cuttings when drilling stops

At rest, it is a gel that supports the weight of the small rock shown here (Left).

When stirred it becomes more liquid and the rock sinks (Right).

Mud Cycle At A Wellsite

Most of the mud used in a drilling operation is recirculated in a continuous cycle:

1. Mud is mixed and kept in the mud pit.

2. A pump draws it out of the mud pit and sends it, through the hollow center of the drill pipe, down into the borehole.

3. Mud emerges from the drill pipe at the bottom of the borehole where the drill bit is grinding away at the rock formation.

4. Now the mud begins the return trip to the surface carrying the pieces of rock, called cuttings, that have been scraped off the formation by the bit.

5. The mud rises in the annulus, the space between the drill pipe and the walls of the borehole. The typical diameter of a drill pipe is about 4 inches (10 centimeters). At the bottom of a deep well, the borehole might be 8 inches (20 centimeters) in diameter.

6. At the surface the mud travels through the mud return line, a pipe that leads to the shale shaker.

7. The shale shakers consist of a series of vibrating metal screens which are used to separate the mud from the cuttings. The mud drips through the screens and is returned to the mud pit.

8. The rock cuttings slip down the shale slide to be disposed of. Depending upon environmental and other considerations, they may be washed before disposal. Some of the cuttings are taken to be examined by geologists for clues about what is going on deep down inside the well.


Environmental Challenges

Today the major challenge in formulating drilling fluids is to meet the increasingly demanding conditions of high temperature and pressure found in some deep wells and extended reach and horizontal wells while avoiding harm to the environment. The components of drilling fluids should be selected so that any discharge of mud or cuttings has the minimum possible environmental impact. Environmental concerns are a major driving force behind current drilling fluids research and development. Health of rig workers also is an important influence in the use of drilling fluids, and products are selected to minimize health risks.
Although fluids are essential for the successful drilling of an oil well, they can also be one of the messier aspects of a drilling operation. Cuttings that are brought up out of the borehole have to be disposed of, as does any drilling fluid that remains attached to them. And while the environmental footprint at a wellsite is relatively small, being confined to the vicinity of the drilling operation, the environmental impact near the rig can be significant. The degree of impact drilling fluids have on the environment depends on the type of mud used and the prevailing environmental conditions. Offshore, water-based muds are generally the least damaging compared to oil-based. (In contrast, discharges of drilling waste on land have different types of impact and the salt content of mud may pose more of a problem than the hydrocarbon content.)
With many pollutants the impact on the environment is influenced by the way the pollutant is discharged and subsequently dispersed throughout the environment. Oily cuttings, when discharged under water, do not disperse as much as water-based muds and may form cuttings piles which blanket parts of the sea bed. High concentrations of organic material such as oil can have a profound effect on plants and animals living on the seabed. As the organic matter decomposes oxygen is used up and toxic sulfides may be produced. Such conditions can result in the almost total elimination of bottom-dwelling organisms very close to the rig.
Surrounding the immediate area of the rig there is a recovery zone where there are plants and animals that are able to tolerate some degree of pollution. The less tolerant organisms, which live further from the source of pollution, gradually reappear closer to the rig as the site recovers. Most of the disruption occurs within 500 meters (about 1600 feet) of the rig site, but some biological effects have been reported as much as 10 km (over 6 miles) away. When drilling offshore in regions where there are strong water currents, the discarded cuttings tend to spread out leaving a thinner covering of the seabed near the discharge site. This makes them more susceptible to the action of microorganisms that act to degrade the entrained drilling fluid, speeding up recovery of the sea bed.
Why use synthetic-based drilling fluids?
The environmental impact of cuttings contaminated with oil-based muds has resulted in severe restrictions of their use in many parts of the world and has led to the development of more environmentally friendly, synthetic-based drilling fluids that not only perform well but are less toxic and, in most cases, more biodegradable.
How are drilling fluids tested and regulated?
Regulation of drilling fluids varies according to geographic location and local legislation. Testing is performed to determine the toxicity of various chemicals. Additional tests are performed to gather data about biodegradation and bioaccumulation.
Toxicity tests
Toxicity tests are also used to predict the impact of a pollutant on the receiving environment. The results of these experiments are used to estimate the maximum amount of material that can be discharged without having a direct, toxic effect on the environment. The exact type of test performed depends upon local legislation and the likely fate of the contaminant. For example, in some areas oil-based muds are tested on bottom-dwelling creatures known as sediment re-workers. These animals obtain nutrition by eating sediment and are likely to be affected by oily cuttings that accumulate on the seabed. Water-based muds, on the other hand, are tested on fish who are more likely to be exposed to water- soluble substances.
Reducing Environmental Impact
Biodegradation is a key factor in reducing the long-term environmental impact of drilling fluids. Another consideration in drilling fluid design is reducing toxicity to fish, sediment re-workers, algae and zooplankton. But it is equally important to reduce the amount of waste generated in the first place. This is achieved by recycling drilling fluids as much as possible and by designing them in such a way as to make this easier to achieve. For example, on shale shaker screens low-viscosity fluids separate more readily from the cuttings. This improves the recovery of drilling fluid and reduces the amount of organic material discharged to the sea.
Drilling fluids began as mud - just clay and water. Now little is the same but the name. Modern muds are designed for a wide range of drilling conditions. Many factors must be carefully weighed and balanced, not the least of which is environmental safety.

Biodegradation
Biodegradation is the breakdown of an organic substance, such as oil, by the action of living organisms, usually microorganisms and especially bacteria. Some substances biodegrade more rapidly and more completely than others. Ultimate biodegradation  results in a compound being converted to water and carbon dioxide. Some substances may degrade to smaller, intermediate molecules. This is called primary degradation. These molecules are usually intermediates in the process of ultimate biodegradation, but can in some instances be more persistent or more toxic than the original pollutant.
Biodegradation may occur under both aerobic (with oxygen) and anaerobic (without oxygen) conditions. If the contaminant is well dispersed in water there is usually more oxygen available for aerobic biodegradation. Water-based mud is more readily dispersed because it is water-soluble. Oily cuttings do not disperse as well and tend to settle in a small area of the seabed near the rig. This high concentration of organic material can result in the generation of anaerobic conditions when rapid bacterial activity uses up the available oxygen in an area
Bioaccumulation
The accumulation of chemicals in an organism's cells is called bioaccumulation. This amount of bioaccumulation depends on the balance between the rate at which the substance enters the organism's cells and how quickly it is broken down or excreted. If an organism ingests a small amount of a pollutant it may be able to eliminate it without significant accumulation, however if the organism is unable to eliminate the contaminant from its body then bioaccumulation will result. Alternatively, when an environment is heavily contaminated an organism may absorb more of the substance than it can excrete in the same amount of time. Bioaccumulation will result unless the concentration of the contaminant is reduced

Environmental Case Study

The Hibernia Project
The Dowell Drilling Fluids Team on the Hibernia project, off the coast of Newfoundland, faces some interesting challenges. As the platform is located in an environmentally sensitive area, discharges to the sea must be carefully monitored.


There are three major types of drilling fluids: water-based mud (WBM), oil-based mud (OBM) and synthetic-based mud (SBM). The drilled cuttings from water-based muds are much less damaging to the environment and are normally discharged o the sea. Water base systems are not always as effective as oil- or synthetic-based fluids, especially for solving the problems associated with these particular wells.

In the most recent wells a combination of water-based and oil-based muds have been used. Water-based mud is used early in the drilling process. Then oil-based mud is substituted as the well gets deeper and reaches the limit of the water-based mud, in terms of lubricity and well bore stabilization.
Since oil-based muds, while effective as a drilling fluid, can be toxic to marine plants and animals so their use is normally tightly controlled. For the Hibernia project , the cuttings from the mud have to be washed (on the platform), in order to remove the oil, prior to discharge. This washing process takes time, and has proved to be a factor limiting the rate of drilling.

In the future, government legislation may allow for discharge of cuttings using synthetic-based mud, which exhibits similar performance to the oil-based mud, but has a better environmental profile. Although the initial cost of the fluid will be approximately double that of oil-based mud, substantial time and dollar savings will be achieved due to the fact that these cuttings do not have to be washed.
THIXOTROPY EXPERIMENT: GEL STRENGTH

In Drilling Fluid: The Lifeblood of the Well you have read about an interesting property of drilling fluid: Thixotropy. The drilling fluid is a liquid while in motion, but when at rest it turns into a thick gel. This makes it useful because when the circulation of the drilling fluid stops, the gel suspends the rock cuttings and prevents them from sinking to the bottom of the borehole. Fluids like this that become more viscous when standing still are called thixotropic.
Another thixotropic liquid is ketchup. Have you ever had trouble getting ketchup to come out of the bottle? You shake and shake but it's stuck. Then, once it gets going it flows quite freely. Ketchup that has been sitting still becomes thick. When it's shaken or stirred it becomes thinner. Here's a way to explore the thixotropic property of ketchup by seeing how rapidly weights fall through it.
Tools & Materials
You will need:

  • Beaker full of ketchup (Let it stand at room temperature for an hour or so)
  • Ring stand
  • Mirror
  • Several weights or steel balls. We used 10 gram brass weights that came with our balance scale.
  • Spoon
  • Stopwatch.

The Experiment
Here's what to do:

  • Put the beaker on the ring stand with the mirror underneath as pictured to the right. The mirror is there so you can see when the weights reach the bottom without twisting your neck.
  •  

Trial

Still Ketchup

Stirred Ketchup

1

5.34

4.38

2

7.84

4.00

3

6.72

4.75

4

8.53

4.19

5

10.75

5.81

average

7.44

4.65

  • Drop a weight into the ketchup from just above the surface. Time how long it takes to reach the bottom.
  • Drop and time four more weights the same way. Be careful not to drop a weight in the same spot where you previously put one. You want each weight to fall through undisturbed ketchup.
  • Retrieve the weights with the spoon.
  • Stir the ketchup thoroughly.
  • Repeat steps 1 through 3. Use the chart below to record your results.

Look at your results. Which is more viscous (thicker), still ketchup or stirred ketchup?
What are some good strategies for getting ketchup to flow easily out of the bottle?

Results:
This chart shows the time it took, in seconds for 10 gram brass weights to drop through to the bottom of a beaker of ketchup. We used 10g weights because 20g weights dropped through the ketchup very quickly and we had trouble timing them with the stopwatch. The 5g weight wouldn't sink at all.
The results in the first column were with ketchup that had been sitting still for about an hour. Then we stirred the ketchup vigorously for a minute or so and tested it again. The results of this test are shown in the second column.

 



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