DRILLING FLUIDS

OVERVIEW

Drilling fluid or drilling mud as many people call it is a vitality in a rotary drilling process. The term “drilling fluid” includes air, gas, water and mud. “Mud” refers to the liquid that contains solids and water or oil. The mud is made up with clay and other additives that give it desirable properties.

MUD TYPES

Water Based Mud

Often, water is the base of drilling mud. Water makes up the liquid part or phase of a water-based mud. Crew members put clay and special additives into the water to make a mud with the properties needed to do its job well. For example, clays give it thickness or viscosity. The water in the mud may be fresh water, sea water or concentrated brine (salt water). The one used depends on its availability and whether it gives the mud the needed properties to drill the hole efficiently.

Oil Mud

At times, down hole drilling conditions require the crew to add oil to the mud, or in some cases, crew members use oil instead of water as the base of the mud. This is called oil-based mud. Oil based mud has many advantages. It can stabilize the formation and reduce downhole drilling problems. However, it is harder for the crew to work with because it can create slippery conditions and environmental precautions must be used. From an environmental standpoint, mud with oil is more difficult to handle because the oil clings to the drill cuttings. The oil must be cleaned off the cuttings before they’re disposed of.

Drilling with Air

Sometimes drilling fluid is dry air or natural gas. Here, dry air is coming out of the rig’s Blooey Line, carrying very fine drilled cuttings. Air drilling uses very large air compressors instead of mud pumps. Drilling with air or gas can prevent formation damage and can overcome severe lost circulation problems. And it allows the bit to drill very fast. Down hole conditions have to be just right for air or gas to be usable. For example, the bit cannot drill through formations containing large amounts of water. The water mixes with the cuttings and the air or gas and clogs up the hole.

Foam Drilling

If small amounts of water are present in the formations being drilled, special equipment can inject a foam agent into the air stream. The foam helps separate the cuttings and remove water from a hole.

Aerated Drilling

In some cases, the rig operator may use aerated mud, which like foam drilling, helps prevent clogging of the well bore. Aerated drilling uses both mud and air pumped into the standpipe at the same time.

DRILLING FLUID FUNCTION

Overview

When circulated down the drill string and up the hole, drilling mud serves many functions. For example, mud cleans the hole, cools and lubricates the bit & the drill string, lifts cuttings to the surface, carries information about formations being drilled, stabilizes the well bore, controls formation pressure and suspends cuttings when pumping stops.

Cleaning the Hole

One function of mud is to clean the hole. A clean hole allows the bit to drill into uncut formation rock. Here is an example of what can happen when cuttings are not moved off bottom. Mud jets out of the bit and moves cuttings away from the bottom of the hole. The mud then carries the cuttings up the annulus and to the surface for disposal.

Cooling / Lubrication

Heat is encountered down hole. Deep formations can be very hot and friction from rotating drilling components generates a lot of heat. High temperatures increase drill string and bit wear. Drilling fluid helps to reduce the temperature in the drill string down hole while drilling. In addition, drilling fluid provides lubrication to the drill string and bit that helps prevent wear.

Protecting Wellbore Walls

Mud stabilizes the hole, keeps it from caving in. As mud moves up the hole, it usually flows by permeable formations. Permeable formations although allow the fluid to flow, when the mud is next to a permeable formation, pressure forces the liquid part of the mud, the filtrate, into tiny openings or pore spaces in the formation. This leaves behind a thin sheet of solid particles, known as mud cake. These solids plaster the side of the hole, much like the plaster on the wall of a building. The wall cake helps keep the well from caving in.

Controlling Formation Pressure

The column of mud in the well creates pressure down hole, called hydrostatic pressure. The hydrostatic pressure of the mud column offsets formation pressure. Mud is the first line of defence in well control. As long as the hole is full of mud, that is the right weight, the well cannot kick and perhaps blowout. A kick is the entry of formation fluids into the well bore. The kick forces drilling mud out of the hole. If crew members fail to control a kick, a blowout can occur. A blowout is the uncontrolled flow of drilling mud and formation fluids out of the hole.

Obtaining Downhole Information

Mud is also used to obtain information about formations down hole. Mud loggers, by examining cuttings at the surface, can gather important information about the formation being drilled and the conditions down hole.

MUD PROPERTIES & ADDITIVES

Bentonite

In water, or oil based drilling mud, crew members usually add clay, called bentonite, or similar mineral. Bentonite swells in water, therefore thickens the mud, gives viscosity, to help clean the cuttings from the hole and provide other desirable properties.

[TOOL BOX]: Viscous fluids are more resistant to flow. Honey is a good example of a viscous fluid, pure water is not viscous.

Barite

Barite is a heavy mineral. The crew adds barite to mud to make it heavy or dense. Barite is over four times heavier than water. Dense mud exerts more pressure than light mud. Weighted mud controls formation pressure. This is called “Primary Well Control”.

[TOOL BOX]: Primary well control is using the density or weight of the drill fluid to provide sufficient pressure to prevent the influx of formation fluid into the well bore. If sufficient mud pressure is not used while drilling, the pressurized formation fluid forces the mud up the well bore where it blows out of control. When the hole is full of mud that weighs the right amount, the pressure of the mud equalizes the pressure of the formation, so formation fluids can’t enter the well bore.

PH

The control of many mud properties depends on its PH. The PH of mud is a measure of its acidity or alkalinity. The PH scale runs from 0 to 14. If the mud is neutral, neither acidic nor alkaline, it has a PH of 7. Mud with a PH below 7 is acidic, a PH above 7 shows that the mud is alkaline. Most drilling muds require a high PH, at least 9, or higher.

Prompt quiz: We’ve said that most drilling mud should have a PH of 9 or greater. Will the mud be called acidic or alkaline?

Caustic Soda

Because mud needs to have a high PH, another common mud additive is Caustic Soda or Sodium Hydroxide. Caustic soda is often called “caustic”. Crew members add caustic soda to the mud to control PH. Caustic soda increases the PH value, it makes the mud more alkaline. In general, caustics are the most dangerous chemicals that you’ll handle on the rig. High strength solutions can seriously burn your skin. Be very careful when handling it to avoid injury, wear the proper personal protective equipment, also remember to always add caustic soda to water, never add water to caustic soda. If you do, the caustic soda will boil up, splatter and cover you with a burning chemical.

Gelled Mud

When drilling stops, say let the crew make a connection (add a joint of drill pipe to the string), the driller normally stops pumping mud. When pumping stops, the mud stops moving. At rest, mud gels, that is it becomes a semi-solid like gelatin. Gelled mud suspends the cuttings. Gelling keeps the cuttings from falling down hole and piling up around the bit. The ability of a gel to keep the cuttings suspended is measured by its gel strength. When the driller starts the pump and resumes mud’s circulation, the mud’s gel strength reduces, which allows the drilling fluid to flow easier.

MUD TESTS

Overview

We’ve just covered a few key points about mud additives and the properties that mud should have to allow a successful drilling. On the rig, it is important for crew members to constantly monitor and maintain these properties. An important member of the drilling team is the mud engineer. The mud engineer runs tests on the drilling fluid. The mud engineer’s job is to monitor and maintain the mud’s properties to the specifications of the well operator. He may also recommend changes to improve drilling, such as adding more caustic soda to increase the mud’s PH. In this section, we will learn about tools that’re used to monitor mud properties.

Mud Balance

The density, or weight per unit volume of the drilling mud determines how much hydrostatic pressure the mud column exerts on the formation. It is therefore important to know the mud’s density at all times. To determine mud density, the mud engineer or helper uses a mud balance. The person weighing the mud puts a small amount of mud in the mud container at left on the balance. He then slides the adjustable counterweight to the right or left until the arm balances on the fork room. The person then reads the mud density at the point on the arm next to the counterweight. In many areas, mud density is read in pounds per gallon but can also be reported in pounds per cubic foot, milligrams per liter, and other units. Mud density is usually called mud weight by the rig crew.

[TOOL BOX]: Calculate the density of mud by adjusting the counterweight on the mud balance. Click on the correct density when you’ve finished.

Marsh Funnel

The viscosity of the mud is thickness or resistance to flow, is also an important factor. The mud’s viscosity determines how well it can carry cuttings up the hole. One measure of a mud’s viscosity is its funnel viscosity. That is how many seconds does it take exactly one quart of mud to flow out of a special funnel called a Marsh Funnel. A Marsh Funnel has a hole in the bottom that’s the standard size. The mud engineer or helper pours one quart of mud into the funnel and records the time that it takes to run out into a pitcher or beaker. In this example, one quart of mud flows out of the funnel and into the beaker in 35 sec, so this mud has a funnel viscosity of 35 sec. A less viscous or thinner mud would flow through the funnel faster; a more viscous or thicker mud would flow through the funnel slower.

Rotational Viscometer

This device also measures mud’s viscosity. It is a more scientific viscosity measure than the Marsh Funnel. A Fann V-G Meter measures the mud’s viscosity in centipoises. A centipoise is a unit of measure for viscosity, just as an inch is a unit of measure for length. The Fann V-G Meter works by spinning a rotor or bob in a sample of mud at two different speeds. In addition, a Fann V-G Meter is used to determine a mud’s yield point, which is a measure of the mud’s resistance to flow. Combined with a timer, the Meter also measures the mud’s gel strength. Gel strength is the mud’s ability to temporarily solidify or gel when it’s not flowing.

[TOOL BOX]: Here’s a mud with high gel strength. Click the button labeled “lower gel strength” to see what would happen if the gel strength wasn’t this high.

Filter Press

This is a Filter Press. Inside the white container is a piece of porous paper called filter paper. Also inside the container is a mud sample. The mud engineer puts the mud sample under 100 pounds per square inch of pressure for 30 minutes. The pressure forces the liquid part of the mud, the filtrate, through the filter paper and into the graduated cylinder. By measuring the amount of the filtrate, the mud engineer can get an indication of the amount of filtrate that will be lost to down hole formations and the amount of solids or wall cake build up on the wall of the hole.

Chloride Test

Mud engineers may run other drilling mud tests. One common test is for salt or chloride in the mud filtrate. By adding Potassium Chromate and other chemicals, the engineer can determine if the hole has penetrated a salt formation. It can also determine whether salt water has entered the well bore, which may be a sign of a kick.

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