Drilling Fluids Have Many Purposes. Utilize a Good Drilling Fluids Software Program to Keep Track of Them All.

Drilling fluid might look unassuming to the outside observer, usually, a brown mud that travels through a series of pits and pipes. It goes down into the ground and comes back out. Perhaps the outsider has seen drilling fluids explode onto a rig floor in a Hollywood movie, but usually, that’s where the knowledge stops. Drilling fluid provides many crucial tasks on a drilling project. It is a complex mixture with chemical and rheological aspects that can affect the success or failure of a project.

Drilling fluid accounts for 10% of the total cost on the average well and usually is the component that can balloon a budget on a drilling, workover, or completion project in the oil and gas industry. A drilling fluids engineer typically uses a series of spreadsheets or, better yet, a cloud-based software system. These programs account for all of the tasks, metrics, chemistry, hydraulics, and cost that go along with maintaining a proper drilling fluid system on a well.

What are the purposes of drilling fluids?

Controlling Fluids Below the Surface

Traditionally, drilling fluids were embraced to keep formation oil, gas, water, and steam in place in their geologic formations. Many rock zones are porous and hold these fluids like a sponge holds water. Until the operator determines that it is time to transport those fluids to the surface for testing or production, it is imperative that they stay out of the hole as it is being drilled.

Fluids that weren’t appropriately controlled (or at all) resulted in the blowouts seen in the early days of the oil and gas industry. In these first failures or recent ones such as the Macondo blowout, oil was collected in oak barrels or cleaned up in environmentally catastrophic national disasters – not ideal.

Failing to control subsurface fluids may only be a minor problem if a flow of fresh water is allowed into an unweighted water-based mud system. However, the outcome can be much worse. Saltwater that flows into a freshwater drilling fluids system from the formation will cause the mud to become too viscous to be effective, and needs to be fixed. In an oil-based drilling fluids system, a water invasion into the annulus can destroy the entire mud system. An unwanted steam flow can heat up the mud to hazardous temperatures for personnel and equipment. All of these scenarios pale in comparison to the worst potential disasters. In 2010, the entire globe witnessed what is possible when subsurface fluids and pressures are not controlled. In the Gulf of Mexico, the Macondo blowout saw the loss of eleven lives, the state-of-the-art rig was wholly destroyed, and an unprecedented environmental cleanup followed suit. The total cost of this disaster went above $40 billion.

Other drilling fluids related problems such as lost circulation and hole cleaning come secondary to proper well control for this reason. Well control issues are always the number one priority to protect lives and equipment.

Transporting Cuttings to the Surface

Anyone who has ever done basic carpentry knows the feeling of drilling a hole into a piece of wood. They will eventually realize that if the cuttings are not displaced from the hole, the bit will become lodged in place or hard to remove. Wells are the same. They are drilled with a string of pipe that connects to a drill bit at the end. The series becomes longer as the well gets deeper; joints of pipe (usually 30’ to 45’) are added to the drill string as the drilling progresses by the rig hands.

The pipe is hollow in the middle and drilling fluid is pumped through the middle of the pipe from the rig’s circulatory system. The clean fluid travels down the pipe and around the larger diameter bit where it contacts the fresh hole cuttings. The fluid travels up the annulus of the hole and back up to the surface. The cuttings are carried with the liquid and disposed of with equipment or solidifying treatments on the rig site location.

If the drilling fluid is not functioning correctly, the cuttings will remain in the hole. A sound drilling fluids software system like MudWorks, ensures the drilling fluids engineers keep the fluid in good condition. The software tracks the daily checks of the mud chemistry and provides exacting details on the rheology of the fluid (how effectively it is moving throughout the hole). Viscosity is generally the property responsible for carrying the cuttings out, so it is paramount to maintain viscosities that will accomplish this goal. All wells are different, so effective viscosities may vary. For instance, some wells are drilled with air. The air acts as the drilling fluid and carries the cuttings via the massive amount of volume being pumped by the surface equipment.

There are many drilling fluid types, such as the following: saltwater, freshwater, oil-based, mist, foam, and air. Each of these fluids has hole cleaning properties that must correlate to the mud pumps and the diameter of the drilled hole. Other factors to be considered are the penetration rate and drill pipe size.

If the hole is efficiently cleaned out, formation cuttings will accumulate in the wellbore. When the bit and drill string are pulled out, the pipe and become stuck, sometimes permanently. It is not uncommon for entire wells to be lost in this fashion.

Drilling Fluids Prevent Hydration of Shales, Clays, Salt and Other Sensitive Formations

Not all rock formations are created equally; some provide specific challenges. A sound mud system should be designed for any problems that may arise. Drilling a hole in the earth requires a drilling fluid intended for the type of rock encountered. Reducing the amount of water that filter from the drilling fluid is an effective way to help prevent mud-related well instability. Another method is to use salt in the water. Other special mud additives are commercially available that protect certain formations from becoming insecure.

Clay is a particularly daunting formation to deal with on a drilling project. Specialized surface equipment is even used just to deal with the problems clay creates. Other geologically young rocks like the formations found in California, also have very problematic clays. If allowed to swell, and the properties that keep swelling down are not adequately maintained, the clay will close off the wellbore, and the pipe will become stuck. Anyone who has ever had a blood pressure test will know the feeling of when the bag swells around their arm; there would be problems pulling their arm out of the bag. Swollen clays behave like this around the pipe.

Shale is a sedimentary rock formation mainly composed of clay. Many of the problematic clays that drilling engineers and other rig personnel encounter are shales. The youngest shales are still very saturated with water, and the clays hydrate quickly creating problems with viscosity. Sometimes these rocks are called mudstone or claystone. The harder shales usually have microfracture planes that run throughout the formation like the grains on wood. Some shales are stable, but others will shift like a deck of playing cards if the bedding planes are lubricated with H2O.

Failure to stop these unstable rock formations from hydrating can result in the loss of the entire uncased section of the well. These first three functions of drilling fluids are the most important. The rest can have serious consequences, but failure to address well control, cuttings management, and formation instability can cause loss of life, equipment, and the well itself.

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Bit Cooling and Lubrication

Earlier drill bits had open bearings that were lubricated by the drilling fluid. Currently, most bits used in commercial drilling for oil and gas are tri-cone bits, which have moving parts that are lubricated with sealed bearings. The other common type of bit is a PDC bit that has fixed diamond cutters with no moving parts. Tri-cones crush and pulverize rock while PDC bits tear and shear formations. The steel teeth and cutters of these bits are rotating against rock at 40 to 100 rpm with many thousands of pounds of pressure. The mud decreases the heat produced by this enormous amount of friction.

The consequences of insufficiently cooling the bit will result in a total bit failure, which can cost thousands to tens of thousands of dollars. Not only this, but the entire string of drill pipe has to be tripped out of the hole so that the bit can be replaced. Tripping a drill string out of the hole can take up to a day or more. As one can imagine, an entire day lost on a drilling project is incredibly expensive since these projects can run into hundreds of thousands and millions of dollars.

Prevent Filtrate Invasion Into Producing Formations

Mud filtrate and fine particles in the drilling fluid can penetrate the producing formations. Why is this a problem? These hydrocarbon-rich zones are the primary reason most wells are drilled in the first place. Fluid loss to the formation can introduce damaging emulsifiers and polymers in such quantities that it can ruin the productivity of a well when the operators decide to produce it. Fine particles create a vital wall cake that creates a barrier between the rock formations and the annulus of the well.

Drilling fluid is a non-Newtonian fluid, which means that it can act strangely under different stresses. Water is a Newtonian fluid. If it is poured into a cup, it will immediately take the shape of that cup. Non-Newtonian fluids such as paint, ketchup, or drilling fluid don’t behave in this way. Ketchup will become less viscous when introduced to stress caused by a hand slapping the bottom of a near-empty bottle to get the remaining liquid out. Some non-Newtonian fluids like cornstarch mixed in water become more viscous when force is applied, such as being stirred rapidly. When it comes to drilling fluids, experience, and testing their fluid under various stresses will dictate the filtrate value needed to protect the formations on the way to the producing formations.

Mud engineers need to understand the behavior of their fluids under different rheological stresses in the hole. The particles that are suspended in drilling fluid don’t always stay suspended under constant conditions. Too much wall cake can cause the pipe to get stuck and too little can allow the damaging elements to get through to the valuable formations.

Provide a Medium for Analyzing the Well Bore after the Well Has Been Drilled

When a well drilled to a completion depth, specialized tools are run up and down the hole on a wire-line to ascertain the location of desired areas within the production zones. These tools have to send or receive information from the rocks within these zones. The devices are suspended in the drilling fluid. If there is too much salt within the mud, then it can interfere with the analysis.

The drilling fluid must also maintain a stable environment for these tools which have little weight and no circulation to make it to the bottom of the hole. If these tools don’t make it to the bottom of the hole, then the wrong data can be acted upon. The incorrect depths can be produced in this case, resulting in the loss of an otherwise productive well.

Provide a Method to Evaluate the Formations that are Being Drilled

Cuttings are collected and analyzed from the surface equipment that separates this waste from the fluid at the surface. Rock samples are gathered and labeled by the depth they were drilled up. The oil and gas content of these samples is measured, creating a narrative in the rock correlated to the depth.

A consistent drilling fluid column needs to be maintained so that these samples are relayed up to the Mud Loggers without contamination. Oil and gas content that comes from the formation and up through the drilling fluid medium are also analyzed.

Power Hydraulic Tools and Act as a Communication Method with Down Hole Tools

The drill string doesn’t just include the drill pipe and a bit. There are components directly above the bit that make up a “bottom hole assembly.” Some of these multi-million dollars, pipe-shaped tools are highly technical. These tools, typically a hydraulically powered mud motor and a variety of “measurement while drilling” tools, provide the ability to deviate the well path to different angles and azimuthal directions.

These pricey tools can also provide logging while drilling data in real-time. Formation resistivity, porosity, pressure while drilling, gamma radiation, and vibration data is pumped up the mud column. One popular method of telemetry is called mud pulse technology. These tools send binary code sequences through the mud column that are picked up by sensors on the surface that decode this information into useable data.

Most wells in the current era aren’t drilled straight down but at varying degrees of angle and direction. A mud motor’s unique design allows this to angle building and turning to happen by stopping the rotation of the drill string and only rotating the bit. A small angle upset in the 30’ mud motor allows a desired well path to be established overextended depths and distances. It looks like a bent drinking straw. As the drilling fluid travels down the inside diameter of the pipe during drilling operations, it powers a series of rotors and stators within the mud motor. This hydraulic power produced by the drilling fluid spins the drill bit independently of the rotating action from the rig on the surface.

Allow Easy Separation of Cuttings and Drilling Fluid at the Surface Cleaning Equipment

Drilling fluids must be designed, monitored, and maintained to separate cuttings from the liquid when the slurry reaches the surface. When cuttings reach the shakers and other solids control equipment, most of the mud should return back to the circulation system, and most of the cuttings should be disposed of.

Drilling fluids must be both thick and thin. To transport cuttings up the hole efficiently, it must be thick. It must also be thin so that the cuttings can separate easily from it at the surface. To accomplish this feat, drilling fluids need to be extremely shear thinning. The faster that the mud is moving up at the surface, the thinner it is. Due to pressure loss further down the hole, the fluid moves slower and has a higher viscosity in the annulus. The non-Newtonian aspects of drilling fluids allow this possibility.

If these requirements aren’t met, a creeping problem comes into plays. If finer solids from the cuttings aren’t removed from the fluid, the entire system becomes too viscous for use. Unnecessary cost is added to the project by failing to address this problem.

Support Weight Materials in the Fluid

Weighting agents such as barite, calcium carbonate, or hematite are added to the fluid to provide weight. Water weighs 8.34 lbs per gallon. Drilling fluids need to weigh more than water most of the time. The drilling fluids need to keep the lighter water, oil, and gas from the formation from overpowering it. However, the fine particles in these weighting agents tend to fall out of drilling fluids and settle if the drilling fluid is not continuously agitated. Unless low shear rate viscosities are provided, suspension of these particles will collapse, and particles will sift out of the fluid and settle on the bottom of the surface pits or the hole itself.

If these particles settle to the bottom of the mud pits, then fine silt can build up and restrict flow. If the weighting particles fall out of the fluid in the hole (especially horizontal holes, it can create a condition called barite sag. This condition creates a silt bed that makes it challenging to move pipe in and out. Pipe that is in a 90-degree horizontal hole is subject to gravity that makes it sit on the bottom of the hole. Pipe that needs to slide in and out of sticky situations like this can get stuck.

Preventing Loss of Whole Mud

If whole mud is lost to the formation, then hole cleaning becomes ineffective. The drilling fluid that should be carrying the cuttings up to the surface no longer does its job, and the cuttings accumulate in the hole. Packing off the well with cuttings causes stuck pipe. These situations are called “lost circulation.”

While drilling, the bit is drilling through different rock formations. Some formations have different fracture gradients than others. If the drilling fluid is too heavy for some formations, it will surpass the fracture gradient, and the fluid will break into the formation. Lost circulation to the formation can cause another problem.

The column of fluid maintains a hydrostatic pressure that keeps oil, gas, and water from invading the hole. If too much whole mud is lost to the formation, then the drilling fluid column can’t support the pressure forces from the formation. If the hydrostatic pressure provided by the weighted fluid column is compromised, formation fluids will overpower and travel to the surface.

Drilling fluid is then used to carry lost circulation materials down to the formation to patch it up. Materials like walnut hulls or cottonseed fibers are pumped down through the mud to create a barrier and fix the leaks.

How do we Maintain and Manage a Good Drilling Fluid?

Drilling Fluids Engineers must maintain a good working relationship with the rig crew, especially the derrick hand who adds the necessary materials to the pits in hoppers. The engineers must be vigilant in performing mud checks that inform all personnel how the mud is performing and what additives are needed to maintain a healthy system.

A good software system is also necessary to track the progression of drilling fluids. The chemical aspects, rheology, and hydraulics are different for each project. Drilling fluids software like MudWorks performs the calculations and allows the mud engineers to generate the daily reports and metrics that the rest of personnel need to see on the project. Cost is a vital component, and the easier a software platform makes it on the engineers, the smoother a drilling project will go.