The Drilling FluidLecture 4

PET

Q 1 2015-2016

Abu Dhabi Polytechnic

Drilling fluid technology has become increasingly sophisticated and deserves treatment as a separate element of the drilling system

Drilling Fluid

• The first drilling fluids consisted of water, later "mud”

• Use of salt water, various types of natural and synthetic oils, air, nitrogen, natural gas, mist and foam

• Selection of drilling fluid to use for a particular well, and the selection of materials to add is determined by: well objectives,

expected formation types,

subsurface temperatures and pressures,

anticipated drilling problems, and

material cost and availability

The variability of drilling parameters necessarily gives rise to a wide range of mud types, but we can broadly classify them into three categories:

Drilling Fluid Types

• Water-base systems, fresh or salt water, commercial additives designed to optimize mud properties

• Oil-base systems, diesel oil or a synthetic oil. These have less tendency to alter formation properties and cause damage than water-base muds, and are more suitable for high-temperatureenvironments.

• Pneumatic systems, air, gas, or foams, which are for use in hard rock or severely depleted formations

Drilling Fluid Types

A properly designed and well-maintained mud system is instrumental in:

• Controlling subsurface pressure

• Removing and transporting drilled cuttings, and holding cuttings in suspension when circulation is halted

• Sealing off permeable formations and stabilizing the wellbore

Drilling Fluid Functions

Drilling Fluid Functions

• Preventing formation damage

• Cooling and lubricating the bit and drill string

• Transmitting hydraulic horsepower to the bit

• Facilitating the collection of formation data

At any point in a wellbore, a column of drilling fluid exerts a hydrostatic pressure (P) that is equal to the density of the fluid (ϱ) times the height of the fluid column (h):

Control of Subsurface Pressures

Control of Subsurface Pressures

controlling the pressure of the fluid column by adjusting the drilling fluid density, or mud weight, such that the resulting wellbore pressure counteracts the formation pressure-TVD (h) being controlled by drilling

Control of Subsurface Pressures

The goal is to keep the mud weight high enough so that formation fluids cannot enter the wellbore, while at the same time keeping it low enough to optimize drilling performance and avoid fracturing the subsurface formation

Cuttings Removal, Transport and Suspension

• The drilling fluid must have sufficient viscosity to lift rock cuttings from underneath the bit as they are drilled, and carry them up the annulus to the surface

Cuttings Removal, Transport and Suspension

• a drilling fluid must exhibit sufficient gel strength to hold cuttings in suspension when circulation stops,

• the fluid velocity is also important to proper hole-cleaning

• Annular velocities between 100 and 200 ft/min [0.5-1.0 m/s] are usual

• Annular velocities can be used to calculate the time at which a given sample of cuttings was removed from the bottom of the hole, which can be correlated to depth

Sealing off Permeable Formations, Stabilizing the Wellbore and Preventing Formation Damage

• Drilling fluids should be designed to deposit a thin, low-permeability "filter cake" of solid mud particles against the borehole wall opposite permeable formations

This improves wellbore stability and prevents a number of drilling and production problems, such as "tight" hole conditions, poor log quality, increased torque and drag on the drill string, stuck pipe, lost circulation and formation damage

Hard rock formations that do not have a tendency to cave in can be drilled with air, foam, or water as a drilling fluid

Sealing off Permeable Formations, Stabilizing the Wellbore and Preventing Formation Damage

Wellbore stability is controlled partly by the hydrostatic pressure of the mud, and partly by chemical interactions that may occur between the mud and the minerals within a given formation

Chemical additives may promote a mud's ability to preventing formation instability, but even more important is the placement of a quality filter cake on the walls to keep fluid invasion to a minimum

Cooling and Lubricating the Bit and Drill String

This function is primarily performed at the bottom of the drill string, where the bit is forced against the bottom of the hole and rotated

Force applied to the bit may range from 10,000 to 100,000 lb [45 to 445 kN], and rotating speed may range from 50 to 200 rpm

Cooling and Lubricating the Bit and Drill String

This combination of weight and speed generates tremendous frictional heat within the bit that must be removed by the circulating fluid to prevent rapid wear. Lubricants added to the mud system can help reduce friction at the bit and between the drillstring and hole

Transmitting Hydraulic Horsepower to the Bit

During circulation, the rate of fluid flowshould be regulated so that the mud pumps deliver the optimal amount of hydraulic energy to clean the hole ahead of the bit

Hydraulic energy also provides power for mud motors to rotate the bit and for Measurement While Drilling (MWD) and Logging While Drilling (LWD) tools

Transmitting Hydraulic Horsepower to the Bit

Hydraulics programs are based on sizing the bit nozzles to maximize the hydraulic horsepower or impact force imparted to the bottom of the well

Facilitating the Collection of Formation Data

The mud system is critical to a successful formation evaluation program. During drilling, for example, mud loggers monitor mud returns and drilled cuttings for mineral composition and indications of hydrocarbons

Facilitating the Collection of Formation Data

They record this information on a mud log that shows lithology, penetration rate, gas detection and oil-stained cuttings

Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD) procedures are likewise influenced by the mud program, as is the planning and execution of coring programs and wireline logging programs

Drilling Fluid Formulation

A wide variety of mud components and chemical additives are available to help control drilling fluid properties and optimize system performance. Some common examples are:

• Viscosifiers, such as bentonite, which improve a drilling fluid’s ability to remove cuttings from the wellbore and to suspend cuttings and weight materials during periods of noncirculation

• Weighting materials, such as barite ( barium sulfate), which are used to increase drilling fluid density;

• Rheology control additives, such as lignosulfonatesand polymers, which are used as thinners to imporve flow properties

• Lost circulation materials: any of a variety of items that act to plug porous, permeable zones, including walnut hulls, shredded cellophane, mica flakes, and vegetable fibers

• Many other additives are designed for functions ranging from pH control to shale stabilization to corrosion inhibition