DRILLING FLUIDS

Drilling fluid -mud – is usually a mixture of water, clay, weighing material and a few chemicals. Sometimes oil may be used instead of water, or oil added to the water to give the mud certain desirable properties. Drilling fluid is used to raise the cuttings made by the bit and lift them to the surface for disposal.

But equally important, it also provides a means of keeping underground pressures in check. The heavier or denser the mud, is the more pressure it exerts. So weighing materials -barite – are added to the mud to make it exert as much pressure as needed to contain formation pressures. The equipment in the circulating system consists of a large number of items. The mud pump takes in mud from the mud pits and sends it out a discharge line to a standpipe. The standpipe is a steel pipe mounted vertically on one leg of the mast or derrick. The mud is pumped up the standpipe and into a flexible, very strong, reinforced rubber hose called the rotary hose or kelly hose.
The rotary hose is connected to the swivel . The mud enters the swivel the swivel:goes down the kelly, drill pipe and drill collars and exist at the bit. It then does a sharp U-turn and heads back up the hole in the annulus. The annulus is the space between the outside of the drill string and wall of the hole. Finally the mud leaves the hole through a steel pipe called the mud return line and falls over a vibrating, screen like device called the shale shaker. Agitators installed on the mud pits help maintain a uniform mixture of liquids and solids in the mud. If any fine silt or sand is being drilled, then devices called desilters or desanders may be added. Another
auxiliary in the mud system is a device called degasser.

DRILLING FLUID PRODUCTS

Many of the components in drilling fluids can affect the efficiency of solids-control devices. As discussed in the previous section, fluid rheology, shale inhibition potential, wetting characteristics, lubricity, and corrosivity can all affect both the properties of cuttings and the performance of solids-control equipment. Key components that affect those properties include colloidal materials, macropolymers, conventional polymers, and surface-active materials.

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Types of drilling fluids

Drilling fluids are classified according to the type of fluids and other primary ingredients

1.Gaseous:Air, nitrogen
2.Aqueous: Gasified–foam, energized(including aphrons)clay, polymer, emulsion
3.Nonaqueous:Oil or synthetic–all oil, invert emulsion

True foams contain at least 70% gas(usually N2,CO2,or air)at surface of the hole,while energized fluids, including aphrons, contain lesser amounts of gas.Aphrons are specially stabilized bubbles that function as a bridging or lost circulation material(LCM)to reduce mud losses to permeable and microfractured formation.Aqueous drilling fluids are generally dubbed water-based muds (WBMs), while nonaqueous drilling fluids (NAFs) are often referred to as oil-based muds (OBMs) or synthetic-based muds (SBMs). OBMs are based on NAFs that are distilled from crude oil; they include diesel, mineral oils, and refined linear paraffins (LPs). SBMs, which are also known as pseudo– oil-based muds, are based on chemical reaction products of common feedstock materials like ethylene; they include olefins, esters, and synthetic LPs.

Detailed classification schemes for liquid drilling fluids are employed that describe the composition of the fluids more precisely. One such classification scheme is shown in Figures 2.1 and 2.2. An even more precise classification scheme is described in Table 2.1, which includesthe mud systems most commonly used today, along with their principal components and general characteristics.

Functions of Drilling Fluids

A drilling fluid, or mud, is any fluid that is used in a drilling operation in which that fluid is circulated or pumped from the surface, down the drill string, through the bit, and back to the surface via the annulus.
Drilling fluids satisfy many needs in their capacity to do the following
[M-I llc]:
. Suspend cuttings (drilled solids), remove them from the bottom of the    hole and the well bore, and release them at the surface
. Control formation pressure and maintain well-bore stability
. Seal permeable formations
. Cool, lubricate, and support the drilling assembly
. Transmit hydraulic energy to tools and bit
. Minimize reservoir damage
. Permit adequate formation evaluation
. Control corrosion
. Facilitate cementing and completion
. Minimize impact on the environment
. Inhibit gas hydrate formation
The most critical function that a drilling fluid performs is to minimizethe concentration of cuttings around the drill bit and throughout the well bore. Of course, in so doing, the fluid itself assumes this cuttings burden, and if the cuttings are not removed from the fluid, it very quickly loses its ability to clean the hole and creates thick filter cakes. To enable on-site recycling and reuse of the drilling fluid, cuttings must be continually and efficiently removed.