Drilling Fluids

DRILLING FLUIDS

The key to making the rotary drilling system work is the ability to circulate a fluid continuously down through the drill pipe, out through the bit nozzles and back to the surface.

The drilling fluid can be air, foam (a combination of air and liquid or a liquid.

Liquid drilling fluids are commonly called drilling mud.

All drilling fluids, especially drilling mud, can have a wide range of chemical and physical properties. These properties are specifically designed for drilling conditions and the special problems that must be handled in drilling a well.

Purpose of Drilling Fluids

1. Cooling and lubrication. As the bit drills into the rock formation, the friction caused by the rotating bit against the rock generate heat. The heat is dissipated by the circulating drilling fluid. The fluid also lubricates the bit.

2. Cuttings removal. An important function of the drilling fluid is to carry rock cuttings removed by the bit to the surface. The drilling flows through treating equipment where the cuttings are removed and the clean fluid is again pumped down through the drill pipe string.

3. Suspend cuttings. There are times when circulation has to be stopped. The drilling fluid must have that gelling characteristics that will prevent drill cuttings from settling down at the bit. This may caused the drill pipe to be stuck.

4. Pressure control. The drilling mud can be the first line of defense against a blowout or loss of well control caused by formation pressures.

The hydrostatic head produced by the mud in psi is = 0.052 x G x H
where G = density of mud in ppg
H = depth of the hole in feet.

This hydrostatic head will counter the formation pressure in order to avoid a blowout while drilling.

For example, Lets say a well is being drilled in a salt-water basin (pressure gradient of 0.465 psi/ft), the pressure in the formation at 10,000 feet would be expected to be:

10,000 x 0.465 = 4,650 psi

The weight of mud required to counter this pressure is calculated as follows.
P = 0.052GH
4,650 = 0.052 x G x 10,000
G = 8.94 ppg

5. Data source. The cuttings that the drilling mud brings to the surface can tell the geologist the type of formation being drilled.

6. To wall the hole with impermeable filter cake. This will give a temporary support to the wall of the borehole from collapsing during drilling.


Drilling fluid can solve problems

Many drilling problems are due to conditions or situations that occur after drilling begins and for which the drilling fluid was not designed.

Some of these problems can be solved by adding materials to the drilling fluid to adjust its properties.

Other cases, it may be necessary to replace the drilling fluid being used with another fluid system.

The most common changes is the mud weight or density. Weighting material is added when high-pressure formations are expected.


Some of the problems are:

1. Lost circulation

Lost circulation can occur in several types of formations, including high permeable formations, fractured formations and cavernous zones.

Lost circulation materials can be added to the mud to bridge or deposit a mat where the drilling fluid being lost to the formation. These materials include cane and wood fibres, cellophane flakes and even padi husks were used in oil drilling in Sumatra.

2. Stuck pipe

Stuck pipe can occur after drilling has been halted for a rig breakdown, while running a directional survey or when conducting other nondrilling operation.

The drill pipe may stick to the wall of the hole due to the formation of filter cake or a layer of wet mud solids on the wall of the hole in the formation.

3. Heaving or sloughing hole

This occurs when shales enter the well bore after the section has been penetrated by the bit. To solve this problem, drilling is suspended the hole is conditioned (by letting the mud in circulation for a period of time)


Types of drilling fluids

1. Water-base mud

This fluid is the mud in which water is the continuous phase. This is the most common drilling mud used in oil drilling.


2. Oil-based mud

This drilling mud is made up of oil as the continuous phase. Diesel oil is widely used to provide the oil phase. This type of mud is commonly used in swelling shale formation.

With water-based mud the shale will absorb the water and it swells that may cause stuck pipe.

3. Air and foam

There are drilling conditions under which a liquid drilling fluid is not most desirable circulating medium. Air or foam is used in drilling some wells when these special conditions exist.
Mud Properties

1. Mud density or mud weight

Mud weight is measured by means of a mud balance. The weight of water is 8.33 ppg. The mud weight can be increased by adding barite (barium sulphate). Barite has a specific gravity of between 4.2 – 4.3.

Other materials can be used to increase mud weight such as ilmenite (S.G of 4.58)

2. Mud viscosity

Mud viscosity is difficult to measure but in the field the Marsh funnel and the Fann V-G meter is commonly used.

The Marsh Funnel is filled with mud, the operator then notes the time, removes his finger from the discharge and measures the time for one quart (946 cm3) to flow out. Marsh funnels are manufactured to precise dimensional standards and may be calibrated with water which has a funnel viscosity of 26  0.5 sec.

3. Gel strength

The gel strength of a mud is a measure of the shearing stress necessary to initiate a finite rate of shear.

With proper gel strength can help suspend solids in the hole and allow them to settle out on the surface, excessive gel strength can cause a number drilling problems.

4. Filtration

The filtration, water loss or wall building test is conducted with a filter press.

The rate at which filtrate will invade permeable zone and the thickness of the filter cake that will be deposited on the wall of the hole as filtration takes place are important keys to trouble-free drilling

Drilling Fluid treating and monitoring equipment

In addition to the main mud pumps, several items of mud treating equipment are found on most rigs. Much of this equipment is aimed at solids removal, including shale shakers, desanders, desilters and centrifuges.

Shale shakers remove larger particles from the mud stream as it returns from the bottom of the hole. Shakers are equipped with screens of various sizes, depending on the type of solids to be removed.

Finer particles in the mud stream are removed with desanders, desilters and centrifuges. Each of these items of solids-control equipment is applicable only over a certain range of particle sizes.

In addition to removing solids, mud handling equipment may also include a mud degasser to remove entrained gas from the mud stream. Degassing the drilling fluid is sometimes necessary when small volumes of gas flow into the well bore during drilling.

Additional equipment include mixers to agitate mud in the tanks, smaller pumps to various duties and equipment for adding chemicals and solid materials to the mud system.

Drilling hazards

The following are some of the most common hazards in drilling and can be overcome by proper control of the mud properties.

1. Salt section hole enlargement

Salt section can be eroded by the drilling fluid and causes hole enlargement. These enlargement will require larger mud volume to fill the system and in case of casing the hole, larger cement volume is required.
To avoid these problems a salt saturated mud system is prepared prior to drilling the salt bed.

2. Heaving shale problems

Areas with shale sections containing bentonite or other hydratable clays will continually absorb water, swell and slough into the hole.
Such beds are referred to as heaving shales and constitute a severe drilling hazard when encountered.

Pipe sticking, excessive solid buildup in the mud and hole bridging are typical problems.

Various treatments of the mud are sometimes successful, such as

• Changing mud system to high calcium content by adding lime, gypsum etc which reduces the tendency of the mud to hydrate water sensitive clays.

• Increasing circulation rate for more rapid removal of particles.

• Increasing mud density for greater wall support

• Decreasing water loss mud

• Changing to oil emulsion mud

• Changing to oil-based mud.


3. Blowouts

Blowout is the most spectacular, expensive and highly feared hazard of drilling.

This occurs when encountered formation pressure exceed the mud column pressure which allows the formation fluids to blow out of the hole.

Mud density or the mud weight is the principal factor in controlling this hazard.

In drilling a blow out preventer (BOP) stack is always attached at the top of the conductor pipe. In case of a gas kick (a sign that may lead to a blow out) the BOP stack can close the annular space between the drilling pipe and the conductor pipe or casing or shut the whole hole (with a blind ram of the BOP).


4. Lost Circulation

Lost circulation means the loss of substantial amount of drilling mud to an encountered formation.

Lost circulation materials are commonly circulated in the mud system both as a cure and a continuous preventive.

These materials are the fibrous materials such as the hay, sawdust or padi husk and lamellated (flat and platy) materials such as mica, cellophane.

Christmas tree (oil well)

In petroleum and natural gas extraction, a Christmas tree, or "tree", (not "wellhead" as sometimes incorrectly referred to) is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells. It was named for its crude resemblance to a decorated tree.

Overview

Note that a tree and wellhead are separate pieces of equipment not to be mistaken as the same piece. A wellhead must be present in order to utilize a Christmas tree and a wellhead is used without a Christmas tree during drilling operations, and also for riser tie-back situations which would then have a tree included at riser top. Producing surface wells that require pumps (pump jacks, nodding donkeys, and so on) frequently do not utilize any tree due to NO pressure containment requirement.

Tree complexity has increased over the last few decades. They are frequently manufactured from blocks of steel containing multiple valves rather than made from multiple flanged valves. This is especially true in subsea applications where the resemblance to Christmas trees no longer exists given the frame and support systems into which the main valve block is integrated.

It is common to identify the type of tree as either "subsea tree" or "surface tree". Each of these classifications has a number or varieties within them. Examples of subsea include conventional, dual bore, mono bore, TFL (through flow line), horizontal, mudline, mudline horizontal, side valve, and TBT (through bore tree) trees.

The primary function of a tree is to control the flow into or out of the well, usually oil or gas. A tree often provides numerous additional functions including chemical injection points, well intervention means, pressure relief means (such as annulus vent), tree and well monitoring points (such as pressure, temperature, corrosion, erosion, sand detection, flow rate, flow composition, valve and choke position feedback, connection points for devices such as down hole pressure and temperature transducer (DHPT).

When the operator, well, and facilities are ready to produce and receive oil or gas, valves are opened and the release of the formation fluids is allowed to flow into and through a pipeline. The pipeline then leads to a processing facility, storage depot and or other pipeline eventually leading to a refinery or distribution center (for gas). Subsea wells and thus trees usually flow through flowlines to a fixed or floating production platform or to a storage vessel (known as a floating storage offloading vessel (FSO), or floating processing unit (FPU), or floating production and offloading vessel or FPSO or other combination of structures).

A tree may also be used to control the injection of gas or water injection application on a producing or non-producing well in order to sustain economic "production" volumes of oil from other well(s) in the area (field).

On producing wells, injection of chemicals or alcohols or oil distillates to prevent and or solve production problems (such as blockages) may be used. Functionality may be extended further by using the control system on a subsea tree to monitor, measure, and react to sensor outputs on the tree or even down the well bore.

The control system attached to the tree controls the downhole safety valve (scssv, dhsv, sssv) while the tree acts as an attachment and conduit means of the control system to the downhole safety valve.

Christmas trees are used on both surface and subsea wells (current technical limits are up to around 3000 metres and working temperatures of -50°F to 350°F with a pressure of up to 15,000 psi). The deepest installed subsea tree is in the Gulf of Mexico at approximately 9000 feet.

Valves

Subsea and surface trees have a large variety of valve configurations and combinations of manual and/or actuated (hydraulic or pneumatic) valves. Examples are identified in API Specifications 6A and 17D.

A basic surface tree consists of two or three manual valves (usually gate valves because of their strength).

A typical sophisticated surface tree will have at least four or five valves, normally arranged in a crucifix type pattern (hence the endurance of the term "Christmas tree"). The two lower valves are called the master valves (upper and lower respectively) because they lie in the flow path, which well fluids must take to get to surface. The lower master valve will normally be manually operated, while the upper master valve is often hydraulically actuated, allowing it to be a means of well control while an actuated wing valve is normally the primary well remotely (from control room or control panel) controlled valve. Hydraulic tree wing valves are usually built to be fail safe closed, meaning they require active hydraulic pressure to stay open.

The right hand valve is often called the flow wing valve or the production wing valve, because it is in the flowpath the hydrocarbons take to production facilities (or the path water or gas will take from production to the well in the case of injection wells).

The left hand valve is often called the kill wing valve. It is primarily used for injection of fluids such as corrosion inhibitors or methanol to prevent hydrate formation. In the North Sea, it is called the non-active side arm (NASA). It is typically manually operated.

The valve at the top is called the swab valve and lies in the path used for well interventions like wireline and coiled tubing. For such operations, a lubricator is rigged up onto the top of the tree and the wire or coil is lowered through the lubricator, past the swab valve and into the well. This valve is typically manually operated.

Some trees have a second swab valve, the two arranged one on top of the other. The intention is to allow rigging down equipment from the top of the tree with the well flowing while still preserving the Two barrier rule. With only a single swab valve, the upper master valve is usually closed to act as the second barrier, forcing the well to be shut in for a day during rig down operations. However, avoiding delaying production for a day is usually too small a gain to be worth the extra expense of a having a christmas tree with a second swab valve.

Subsea trees are available in either vertical or horizontal configurations with further speciality available such as dual bore, monobore, concentric, drill-through, mudline, guidlineless or guideline. Subsea trees may range in size and weight from a few tons to approximately 70 tons for high pressure, deepwater (>3000 feet) guidelineless applications. Subsea trees contain many additional valves and accessories compared to Surface trees. Typically a subsea tree would have a choke (permits control of flow), a floline connection interface (hub, flange or other connection), subsea control interface (direct hydraulic, electro hydraulic, or electric) and sensors for gathering data such as pressure, temperature, sand flow, erosion, multiPhase flow, single phase flow such as water or gas.

Source:

Wikipedia