Drilling muds and fluids are two well known concepts in petroleum engineering industry. This post provides some fundamentals of them briefly in short summarised points with no explanation.
DRILLING MUD AND FLUID: Overview
Drilling muds and fluids are used in drilling operations and have the same functional meaning: a fluid that is being circulated in the borehole to do some fucntions inside the hole and they will be discussed later. But, the difference between muds and fluids is the concept. Muds are a mix of two or more components, which must be solids materials mixed with liquids. Fluids are a wider concept and dont have to be in liquid phase, we can say that muds are fluids, but fluids are not muds.
In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells.
The three main categories of drilling fluids are water-based muds (which can be dispersed and non-dispersed), non-aqueous muds, usually called oil-based mud, and gaseous drilling fluid, in which a wide range of gases can be used.
Drilling fluid Functions
1- Minimize impact on environment
2- Remove cuttings from well
3- Suspend and release cuttings
4- Control formation pressures
5- Seal permeable formations
6- Maintain wellbore stability
7- Minimizing formation damage
8- Cool, lubricate, and support the bit and drilling assembly
9- Transmit hydraulic energy to tools and bit.
10- Ensure adequate formation evaluation
11- Control corrosion (in acceptable level)
12- Facilitate cementing and completion
Types of drilling fluids
Many types of drilling fluids are used on a day-to-day basis. Some wells require that different types be used at different parts in the hole, or that some types be used in combination with others. The various types of fluid generally fall into a few broad categories:
Air: Compressed air is pumped either down the bore hole's annular space or down the drill string itself.
Air/water: The same as above, with water added to increase viscosity, flush the hole, provide more cooling, and/or to control dust.
Air/polymer: A specially formulated chemical, most often referred to as a type of polymer, is added to the water & air mixture to create specific conditions. A foaming agent is a good example of a polymer .
Water: Water by itself is sometimes used.
Water-based mud (WBM): A most basic waterbased mud system begins with water, then clays and other chemicals are incorporated into the water to create a homogenous blend resembling something between chocolate milk and a malt depending on viscosity). The clay (called "shale in its rock form) is usually a combination of native clays that are suspended in the fluid while drilling, or specific types of clay that are processed and sold as additives for the WBM system.
The most common of these is bentonite , frequently referred to in the oilfield as "gel". Gel likely makes reference to the fact that while the fluid is being pumped, it can be very thin and free-flowing (like chocolate milk), though when pumping is stopped, the static fluid builds a "gel" structure that resists flow. When an adequate pumping force is applied to "break the gel", flow resumes and the fluid returns to its previously free-flowing state. Many other chemicals (e.g. potassium formate ) are added to a WBM system to achieve various effects, including: viscosity control, shale stability, enhance drilling rate of penetration, cooling and lubricating of equipment.
Oil-based mud (OBM):
Oil-based mud can be a mud where the base fluid is a petroleum product such as diesel fuel. Oil-based muds are used for many reasons, some being increased lubricity, enhanced shale inhibition, and greater cleaning abilities with less viscosity. Oil-based muds also withstand greater heat without breaking down. The use of oil-based muds has special considerations. These include cost and environmental considerations.
Synthetic-based fluid (SBM): Synthetic-based fluid is a mud where the base fluid is a synthetic oil. This is most often used on offshore rigs because it has the properties of an oil-based mud, but the toxicity of the fluid fumes are much less than an oil-based fluid. This is important when men work with the fluid in an enclosed space such as an offshore drilling rig.
The Overall Drilling Mud Circulation Procees
On a drilling rig, mud is pumped from the mud pits through the drill string where it sprays out of nozzles on the drill bit, cleaning and cooling the drill bit in the process. The mud then carries the crushed or cut rock ("cuttings") up the annular space ("annulus") between the drill string and the sides of the hole being drilled, up through the surface casing, where it emerges back at the surface. Cuttings are then filtered out with either a [shale shaker], or the newer shale conveyor technology, and the mud returns to the mud pits. The mud pits let the drilled "fines" settle; the pits are also where the fluid is treated by adding chemicals and other substances.
The returning mud can contain natural gases or other flammable materials which will collect in and around the shale shaker / conveyor area or in other work areas. Because of the risk of a fire or an explosion if they ignite, special monitoring sensors and explosion-proof certified equipment is commonly installed, and workers are advised to take safety precautions. The mud is then pumped back down the hole and further re-circulated. After testing, the mud is treated periodically in the mud pits to ensure properties which optimize and improve drilling efficiency, borehole stability, and other requirements listed below.
Drilling fluids depends on geological factors such as rocks that are going to be drilled and anticipated down hole temperatures and pressures, where the level of inhibition is one of the key parameters. The selection of fluid type can be influence by technical requirements, cost, availability and environmental concerns.