Modern drilling is based on rotary rig system. One important aspect of the rotary rig system is the fluid circulation system. Drilling fluid is to the rotary drilling system what blood is to the human body and even more.
How the Circulation System Works
In the fluid circulating system (above), the drilling is pumped from the suction tank/mud pit through the standpipe to the Kelly. From the Kelly the fluid continues its journey into the drill pipe and exits the drill pipe through the drill bit. The fluid then has to go through the space between the drill string and the borehole wall called the annulus and flows to the surface through the mud return line. At the surface the fluid then goes through the separating facilities such as the shale shaker. This journey is then repeated once again over and over again throughout the drilling of the well.
It is important to note that in terms of physical and chemical properties, it is not the same fluid that was pumped into the hole that exits the hole. Changes occur. Do not forget that the drill string is constantly rotating during drilling and the drilling fluid undergoes a shearing force due to this rotation as it is being pumped down under pressure. As the fluid exits the drill bit at the bit nozzles, it is forced to go through a smaller channel and shears even more. The fluid then transfers energy in the form of hydraulic energy to the rock as it jets out of the bit nozzle. Furthermore, the fluid then has to carry rock cuttings produced by the bit and even becomes dehydrated as it loses liquid to the permeable formation. By the time the drilling fluid gets to the surface, it has expended energy, become contaminated, filled with solids and become dehydrated.
This is why the fluid has to go through separation facilities at the surface to remove the contaminants. For water-based fluid, more water has to be added to replace the water lost to the formations and even lost additives are replaced. The drilling fluid is then tested and when results are satisfactory, it is allowed to resume its journey down the hole all over again.
Functions of Drilling Fluid
The drilling fluid is required to perform a range of functions. They include:
- Suspending and transporting drilled cuttings from the hole
- Minimizing formation damage
- Preventing well control issues
- Minimizing drilling problems
- Cooling and lubricating the drill string
Suspending and Transporting Drilled Cuttings from the Hole
The drilling fluid is expected to suspend drilled cuttings during static conditions and transport them during flow conditions.
There are certain times drilling is paused, such as during tripping operations or when we have to carry out wireline services. (Related reading: The Truth about the Drilling Break.) When there is a pause such as this, there is the tendency for the drilled cuttings to tend to fall down under gravity back to the bottom of the hole for vertical wells or on the side of the hole for horizontal wells. The drilling fluid should be able to suspend the cuttings and minimize the rate at which cuttings fall. When flow is resumed, the drilling fluid should be able to carry cuttings from the bottom of the hole all the way to the surface. These are very important functions of the drilling fluid because proper hole cleaning ensures optimal drilling performance and prevents stuck pipe.
Minimizing Formation Damage
When drilling overbalanced, some of the drilling fluid is forced into the formation. The drilling fluid should be designed in such a way that minimal damage is done to the pay zone as a result of the drilling fluid being flushed into the reservoir. The formation can be damaged if the drilling fluid reacts negatively with the formation fluid or formation fines. This can become very costly to repair later in the form of stimulation. This is why at the onset the drilling fluid should be designed in such a way that it will cause minimal formation damage even if it is forced into the pay zone.
Preventing Well Control Issues
Well control issues include borehole instability, swelling shale, and borehole erosion. A properly conditioned drilling fluid will help prevent these issues.
To prevent borehole instability the density of the drilling fluid has to be looked into and the hydrostatic pressure exerted by the drilling fluid should be slightly greater than the formation pressure. This will prevent kick and mobile formations from closing-in on the drill string.
For swelling shale, inhibitive drilling fluid can be used. Inhibitive fluids are carefully designed to overcome the problem of shale swelling.
Borehole erosion can widen the hole and even fracture weak formations. The drilling fluid pump pressure, flow rate, and carrying capacity can be adjusted to ensure the drilling fluid meets up with this function.
Minimizing Drilling Problems
The drilling fluid also serves to minimize drilling problems. For example loss circulation and stuck pipe. The filter cake deposited on the wall of the hole helps to prevent excessive loss of drilling fluid to the formation after the initial one that led to the deposition of the filter cake. This minimizes the loss of fluid to permeable formations. The hole cleaning property of the drilling fluid also helps minimize the incidence of stuck pipe.
Cooling and Lubricating the Drill String
The constant revolution of the drill string and its interaction with underground formations causes heat. When the drilling fluid is circulating through the drill string and up the annulus, there is a heat exchange from the drill string to the fluid. This helps to cool the drill string. In some cases, the drill string could get so hot when it gets to the surface that a heat exchanger has to be installed on the surface to help cool the drilling fluid before it is sent down into the hole again. Also the drilling fluid helps to lubricate the drill string. This limits the friction between the drill string and tight spots in the hole. Some fluids such as oil-based mud and synthetic-based mud are more suited for the function of lubricity than normal water-based mud.
The drilling fluid faces a risk of contamination each time it embarks on its journey downhole. It can be contaminated by unwanted gases like CO2 or H2S, saltwater from the formation, rock cuttings, and even oil or natural gas from certain formations. All these tend to change the properties of the drilling fluid and if nothing is done could limit its performance or even lead to well control issues.
Yet, once the fluid gets to the surface and passes through separation facilities and gets recharged, it is ready to embark on its journey all over again. The drilling fluid is the lifeblood of the drilling process and the role it performs ensures that drilling thousands of feet (hundreds of meters) into the ground is even possible in the first place.